A major midlatitude hurricane in the Little Ice Age

Dickie, John; Wach, Grant

doi:https://doi.org/10.5194/egusphere-2023-902

CC1: 'Comment on egusphere-2023-902', Michael Chenoweth, 29 Jun 2023

AC1: 'Reply on CC1', John Dickie, 02 Jul 2023

Michael Chenowith’s important 2006 compilation assesses historical Atlantic tropical storms and hurricanes using records that include naval logs, yet appears to dismiss log entries and communications from career naval officers of two fleets (53 warships including 37 sail of the line from two naval powers) who described the 1757 storm as a ‘hurricane.’ Officers, at sea from the age of 12, were necessarily familiar with hurricanes given both navies’ longstanding Caribbean service, and equally aware that logs were legal records used in court martial inquiries.
Chenowith’s Table 1 (2006) lists two 1757 hurricanes. In it he accepts the 1757 ‘Florida to Boston’ hurricane as sufficiently supported by historical accounts and rejects a Barbados storm. The ‘Florida to Boston’ hurricane is noted in Ludlum (1963, Chronological Index p. 192) as affecting the ‘entire coast’ on September 22-24, 1757. Both authors refer to Warden’s (1819, p. 155) September 22-24 ‘Florida to Boston’ track which dovetails with the Boston Herald account (October 17, 1757) noting the storm passed offshore September 23-24, 1757. On September 22 rising SE winds were recorded 1000 km away at Louisbourg in the logs of French and British warships. The ships off Cape Breton noted southeasterly winds rising steadily on September 22 into near-hurricane force and reaching hurricane force on September 25. The logs were compared to land-based weather records that describe the storm.
This project characterizes the intensity of the 1757 hurricane by determining its spatial and temporal characteristics derived from wind vectors recorded simultaneously at multiple ship locations as the system approached Louisbourg. Correlating wind speeds in kph to Beaufort terms and engineering models to the Saffir-Simpson scale allowed us to estimate the wind speeds required to snap 3-foot diameter masts off ships, drive battleships onto their sides, build waves that threw 70 and 80-gun ships ashore, tear down fortress stone walls and flood a town under 21 feet of surge, we believe that even its historical descriptions show this was a hurricane of unprecedented intensity consistent with other Little Ice Age hurricanes. The 1757 storm was much more intense than any modern systems that struck Nova Scotia. Three examples and their metrics are presented in the paper. Even without our quantitative characterization, the intensity of this hurricane rivals any descriptions of storms described in Ludlum (1963) or Chenowith (2006), few if any of which provide an entire western Atlantic meteorological context for subjective observations lacking the metrics that define hurricanes.
Boston Herald, Oct. 17, 1757
Chenowith, M. A.: Reassessment of Historical Atlantic Basin Tropical Cyclone Activity, 562 1700–1855. Climatic Change Vol. 76, 169-240, 2006.
Ludlum, David. Early American Hurricanes 1492-1870. American Meteorological Society. 198 647 pp, 1963.
Warden, David: A statistical, political, and historical account of the United States of North 704 America from the period of their first colonization to the present day. Vol. 1 of 3, 552 705 pp, 1819.

Citation: https://doi.org/10.5194/egusphere-2023-902-AC1

RC1: 'Comment on egusphere-2023-902', William Gomez Pretel, 17 Jul 2023

I would like to congratulate the author on their impressive work. This research effectively utilizes primary sources, such as logbooks, to explore the crude navigation techniques of the time and potentially determine the storm's impact on the ships' locations. The study makes valuable contributions to weather reconstruction, tropical storm analysis, and engages archaeologists and maritime historians in meaningful discussions. I highly recommend that the editor consider publishing this work. However, the paper requires revisions to improve its readability, making it accessible to a broader audience and facilitating better comprehension.

Additionally, I suggest that the authors carefully consider the comments provided by Dr. Chenoweth (an expert in historical hurricanes), and seek additional sources to further support their claims. While logbooks offer valuable historical information, it is important to acknowledge that data on tropical storms before 1850 is often highly ambiguous. It is likely that a hurricane of this nature, as reported by the authors, would have been documented in various newspapers of the time.Furthermore, the results, maps, and references would benefit from improvement. In light of these suggestions, I would like to provide detailed comments on specific points:

The abstract should be improved to enhance readers' understanding of the methodology, avoiding the use of unexplained acronyms. Additionally, the results need clarification, specifically addressing the important result of the possible location of a ship impacted by the storm. It is also important to highlight that this tropical storm is one of the most intense ever recorded to engage the audience effectively.

To provide context for unaware readers, a brief explanation of the Seven Years' War should be included.

Acknowledging the ambiguity and uncertainty derived from the logbooks and other sources is crucial. Factors such as the use of compasses, poor state of hydrography, and navigational techniques contribute to the uncertainty of ship locations. This ambiguity should be clearly conveyed in the article.

To enhance readability, it is recommended to present a separate subsection or use tables to provide a structured and organized description of each ship and its corresponding logbook entries. This will help avoid confusion while reading.

The "Surge" section should not be a separate subtitle; instead, it should be briefly explained within the article.

It is important to compare temperatures with records from other regions around the world or at the same latitude to determine if it was an unusually high-temperature year as hurricanes of such Category are not common in those latitudes.

The authors' use of the Virot et al. theory of wind breaking trees is an interesting method. However, additional clarification is required regarding its application to ships and masts, given the inherent uncertainty involved. The concept of wind breaking trees and its adaptation to ships should be reviewed to provide a more accurate estimation of wind speed, as the structure of ships differ considering other catastrophic shipwreck events linked to hurricanes in the Caribbean where similar fleets were entirely lost.

The authors mention draft charts and a chart from 1776, which should be presented along with other relevant information. Moreover, it is essential to mention that the identified shipwreck location represents a possible location, as further archaeological processes are required to determine its identity. Regarding Image 5b, the authors should explain the methodology used for superimposition and specify the points of reference that allowed for this analysis. This data holds significant importance for the paper, as it not only provides historical insights into the surge but also offers potential locations for important military sunken vessels, attracting the attention of maritime historians and nautical archaeologists.

In the Discussion and Conclusions section, it is important to address certain points. The authors mention the Invincible located at the center of the storm, and it would be beneficial to include the ship's location on the map to enhance the discussion. Additionally, providing information about the distances between the study area and other major ports mentioned, such as Quebec or Labrador, would add context to the analysis.

The authors have possibly made an outstanding discovery by comparing the intensity of this tropical storm with present-day storms and finding it to be more severe in the past. Some studies on hurricanes and climate change have demonstrated that pre-industrial hurricanes are indeed more severe. It is recommended to emphasize this finding as an important result and highlight its uniqueness and significance. Is this possibly the strongest hurricane ever recorded in that area? Is there information available about other hurricanes with similar intensity?

Technical and typographical corrections:

Centuries should be presented without numbers. For example, "18th" should be "eighteenth century."

The term "plus" can be replaced with "in addition" or "furthermore."

When mentioning "navies," it is important to clarify which navies are being referred to and provide supporting information.

Some ports and cities need to be accompanied by their respective country of origin to provide clarity for readers who may not be familiar with these locations.

When reconstructing events, it is important to specify whether it is 7 am or 7 pm to avoid ambiguity.

The measurements of the ships and other relevant data should be standardized using an international system, rather than relying on measures limited to a few countries.

The references require major revisions according to the journal's standards.

While the article includes good references, I suggest incorporating and tracking more references from historic hurricane databases such as Poey, Millas, and additional newspaper sources to enhance the interpretation and reduce ambiguity regarding this phenomena as mentinned by Dr. Chenoweth.

The references need to be better specified, especially primary sources obtained from archives. They should include complete information, including the location and other relevant details. For example, when mentioning NOAA, proper citation should be provided, including the website from which the information was extracted or the agency presenting the information.

Overall, the references need a thorough review. Not all references follow the same format; some include full names while others use initials. Some references should include the link from where they were obtained, and those that include a website should have the date of access specified. The newspaper sources should be checked for clarity and availability for consultation. If they were accessed online, include the link, date of access, and provide more details about the source.

Citation: https://doi.org/10.5194/egusphere-2023-902-RC1

AC2:
'Reply on RC1', John Dickie, 25 Jul 2023

Dr. Pretel raises multiple excellent points to improve the paper's flow and readability. We offer clarity on a couple.
The location of Tilbury (wreck) was identified by Gilles Brisebois and Pierre Leclerc on July 14, 1986 through the ship’s (named) bell, guns, ballast and shot, and a range of artifacts including coins. Despite legal right of access (diving), the British High Commission’s engagement with Federal and provincial authorities required independent rediscovery.
Pretel rightly raises the inaccuracy of eighteenth-century navigation. We believe this was mitigated by the British ships being close to shore and sighting in on known (named) coastal landmarks with bearings and distances entered in the logs during the storm. Including the historical chart showing the wreck location is an easy addition.
While pre-1850 storms might, in general, be described by ambiguous data, this storm is not. It struck at the height of hurricane season to the day Fiona struck in 2022. Ship locations were fixed from landmarks on shore, and since wind speed and direction are vectors at each of these locations, a time-based geometric solution allowed us to determine that the shape of the storm was cyclonic and allowed us to estimate its center.
We have not discounted Dr. Chenowith’s work as it supports ours, and it seemed unnecessary to repeat Chenowith’s exhaustive analysis or doubt his conclusions and those of primary sources that this storm tracked north along the Atlantic seaboard to Nova Scotia, typical of tropical storms and hurricanes that enter Canadian waters at the height of hurricane season when sea surface temperatures are at their peak allowing tropical cyclones to track farther north.
Dr. Chenowith’s excellent compilation and reassessment of past compilations and analysis of primary sources interpreted this storm as a hurricane (storm #73 in Table IV which was #29 under Poey, #25 under Ludlum; the storm was not noted by Millas, Mock or Garcia Herrera). Finding additional mention of this storm may better constrain its path and early development, but in Chenowith's own work he accepts the available data as sufficient for his interpretation, with dates off New England dovetailing perfectly with dates and details we derived independently from an exceptional number of logs, ancillary naval records and land-based journal entries in New Brunswich and at Louisbourg.
There is no basis for assuming Dr. Chenowith's detailed work missed any records that would 'make or break' his interpretation. He already assessed other compilations including reassessing the works of Poey and Ludlum in his own work.
To this we added Knox’s diary describing the storm from land and 19 historical British sources including the logs of eight British warships from the ships’ lists and searching the National Archives UK. We also included translated log excerpts (and ancillary records) from French ships moored in Louisbourg Harbour. The Public Archives of Nova Scotia collections showed that no periodicals from that time have survived. While there may be additional sources missed by Chenowith, there is no reason to suspect this. Few observations include the number of sources and the level of detail at Louisbourg, most of which exist as or could readily be translated into numerical data.
Contextually, ENSO (El Nino Southern Oscillation) and NAO (North Atlantic Oscillation) calculated for 1757 indicate ENSO+ (La Nina; higher wind shear in the Pacific and lower wind shear in the North Atlantic increase the probability of Atlantic hurricane generation) and NAO-2 to NAO -3 increases the probability of more intense Atlantic hurricanes tracking up the eastern seaboard (e.g., Gurgis and Fowler 2009; Trouet et al. 2012).
A seminal climatology of Atlantic hurricane extratropical transition by Hart and Evans (2001) assessed a century of Atlantic hurricanes. They show that half of tropical systems transition to extratropical systems at higher midlatitudes, especially in New England and Nova Scotia. Sufficient historical detail is unlikely to exist to allow us to define to what degree this occurred in 1757. But even without this, basic calculations such as surge at La Grave Battery (Fortress Louisbourg) reveal a surge early in the storm that was double that of Fiona in 2022, a major hurricane entering Canadian waters, and nearly triple that of Hurricane Juan (Category 2), after backing out the tides. This indicates a storm of unprecedented power, and if it was extratropical by landfall, the parent storm would have been incrementally more powerful.
The engineering model used may actually be better suited to ship’s masts than to an ‘average’ tree in a forest since masts were made from single trees of superior structural integrity in 1757 with the test closer to being an analog to single ‘trees’ (masts) exposed to winds on the open sea. Structural reinforcing (shrouds and stays) transfer wind energy to the hull and minimize torsional and other forces on the masts. It suggests that critical speeds for unreinforced trees are minimal estimates.
As an analog, on September 26, 1818, the frigate USS (formerly HMS) Macedonian encountered a hurricane northeast of Bermuda at approximately 35 degrees latitude and 53 degrees longitude. The dates appear to coincide with Chenowith’s (2006) ‘Final Storm Number 253’ listed as a hurricane in Table IV). Damage to the ship closely parallels that described for the 1757 hurricane except that line of battle ships had a much heavier construction than a frigate. Saegesser (1970) provides a very detailed account based on the ship’s log and ancillary damage reports, and notes that in the same storm the Dutch brig De Hoope lost all topmasts and spars, the brig Ann from Nova Scotia was abandoned at sea, the brig Mary from Bristol was overturned, the ship Catherine Dawes from Philadelphia sank and a Baltimore schooner and a Nantucket whaler were both dismasted. The ships in the 1757 hurricane were much larger and of a heavier construction and, designed as floating gun platforms, had a wider beam, requiring greater wind force to do similar damage.
Dr. Pretel rightly notes the importance of tightening up parts of the manuscript for clarity, the addition of a table (hurricane timeline) and including reference points for readers unfamiliar with the region of the study, and ensuring references meet journal standards in a final draft. We thank him fro his positive review and constructive advice.

Gurgis, J. and Fowler, A. A history of ENSO events since A.D. 1525: implications for future climate change. Climatic Change. 2009. 92, pp. 343-387
Saegasser, Lee. The U.S.S. Macdedonian and the hurricane of 1818. Proceedings of the U.S. Naval Institute. January, 1970. file:///C:/Users/johnd/Desktop/The%20USS%20Macedonian %20and%20the%20Hurricane%20of%201818%20_%20Proceedings%20-%20January%201970%20Vol.%2096_1_803.html
Trouet, V., Scourse, J. and Raible C. North Atlantic storminess and Atlantic meridional overturning circulation during the last millennium: reconciling contradictory proxy records of NAO variability. Global and Planetary Change. 2012. 84-85, pp. 48-55.

Citation: https://doi.org/10.5194/egusphere-2023-902-AC2
- RC2:
  'Reply on AC2', William Gomez Pretel, 26 Jul 2023
  
  This is an exceptional work, very well supported, and I'm genuinely hopeful that the editor will accept it.
  The analysis of available hurricane data, as explained in the author's comments, reveals a highly valuable discovery firmly grounded in primary sources. An intriguing aspect is the lack of news coverage in the area following an extreme weather event.
  The comprehensive explanation of hurricanes' impact on vessels and ships is highly appreciated. While some authors contend that hurricanes are among the major causes of shipwrecks in the Caribbean Sea, these marine incidents are primarily influenced by the region's geomorphology and limited geographical knowledge of past centuries.
  Congratulations ! and I look forward to the publication of this article, incorporating the necessary corrections.
  
  Citation: https://doi.org/10.5194/egusphere-2023-902-RC2
  - AC3: 'Reply on RC2', John Dickie, 26 Jul 2023
    
    We, too, were curious why more was not written about this storm. It seems to have tracked offshore until landing in Nova Scotia. It is almost limited to the naval records of those who experienced it. The Nova Scotia archives collections showed that despite several newspapers existing at the time, none from that period have survived. Perhaps the quick action of the British government to capture Louisbourg in June-July, 1758, and this narrow turnaround to mobilize the army and fleet - only nine months after the hurricane - reveals that wartime priorities of reporting on successes rather than setbacks dominated the press. However, some British newspapers did cover the storm in some detail. Knox's Bristol Journal carried firsthand accounts that were truly terrifying. The paper maintained a running narrative over weeks that covered the shipwrecked being taken as prisoners, sea battles that reclaimed them and so on. I found five articles that focused on those lost or held as prisoners at Louibsourg. Subsequent communications regarding the potential capture of the entire British fleet had the French Admiral pursued them with his largely intact fleet could have seen all of the British ships taken as prizes, and with ships and 4000 troops at Louisbourg it would have been easy to capture a largely undefended Halifax and hold it to the end of the war. Louisbourg was the stepping stone to the fall of Quebec at the Plains of Abraham. The fall of Halifax would have changed the outcome of the war in North America. Thank you again for your interest and support.
    
    Citation: https://doi.org/10.5194/egusphere-2023-902-AC3

CC2: 'Comment on egusphere-2023-902', Cary Mock, 17 Jul 2023

AC4: 'Reply on CC2', John Dickie, 02 Aug 2023

Mock raises important points and we intend to explore his case studies as suggested. Nova Scotia is situated where tropical systems transition into post-tropical ones. The 1757 storm began as a tropical system and made landfall at the height of hurricane season. Hart and Evans’ (2001) seminal climatology for North Atlantic extratropical transition shows 50% of landfalling Atlantic Hurricanes between the end of the 19th C to the mid-late 20th C experienced extratropical transition, losing their tropical energy to become a cold-core system. Certain conditions result in the explosive release of thermal energy.
Fiona in 2022 was a major hurricane (Cat 3) entering Canadian waters and landed as a post-tropical system with wind speeds equivalent to a major hurricane. In 2003 Juan made landfall near Halifax as a Category 2 hurricane yet retained its tropical character. I recall the air onshore in the passing of both storms: after Fiona the air was very cool and dry and after Juan it remained hot and humid, observations upheld by Environment and Climate Change Canada records.
The 1757 storm (but not the storm itself) was succeeded by noticeably colder and drier air on land (Knox, 1769). This is consistent with the earliest records of this system which distinguish cooler drier air over the continent and tropical air of the hurricane following the Gulf Stream north. This unintentionally describes the dynamics behind midlatitude extratropical transition.
The challenge we face, given that hurricane season in the midlatitutudes coincides with the autumnal expansion of strongly baroclinic westerlies that serve as a trigger for transition as noted by Hart and Evans (2001), is where to draw the line? Storms can remain tropical over nearshore waters and transition at landfall. We do not know if warmer than usual ocean temperatures allowed the system to intensity well into the Scotian Shelf though we have descriptions of unusually hot weather in Halifax harbour that coincide with a heat wave in Europe with temperatures not surpassed until 2006. What we do know is that there was an unprecedented release of energy over a very short period of time in the coastal zone, reflected in the metrics we have attempted to recreate.
Sea surface temperatures off Nova Scotia at the height of hurricane season do not support a purely extratropical system like the 2000 storm I experienced at sea off Sable Island. It formed off Cape Hatteras. The rate of pressure drop allowed it to be classified as a ‘superbomb.’ We included its metrics in our comparison of systems passing over the same bathymetry to compare against the surge calculated at three locations at and close to Louisbourg.
Our records agree with Mock's work that shows New England experienced colder temperatures on land, just like the colder temperatures felt on land at Fort Cumberland after the passing of the 1757 storm. Conditions remained cold into October leading to frost and snow and a deeply cold winter. These are the continental conditions Hart and Evans (2001) model as seasonal expansion of baroclinic westerlies with the onset of fall that are a catalyst for extratropical transition. Our intent is to understand why there was such an exceptional release of energy that was undoubtedly tropical without getting caught in the debate as to whether it was a hurricane or a post-tropical storm. In most likelihood it was both since that is a typical evolution off Nova Scotia. Our question is why was this system so powerful? To even approach an answer we needed to attempt to quantify historical observations. The climatology of Hart and Evans (2001) explores the conditions that trigger explosicve transition, and inevitably require an exceptionally powerful 'parent' hurricane.
A broader atmospheric context for 1757 indicates ENSO (La Nina) and NAO index at 2-3+ support an active Atlantic hurricane year and a higher probability of more powerful storms tracking up the eastern seaboard of North America. Proxy data indicate a warmer North America and historical records for Halifax Harbour note an unusually hot day that was compared to the hottest weather the officers had experienced in the Mediterranean (Knox, 1769) and coincided with the unusual heat wave in Europe. A search of Nova Scotia and New Brunswick newspapers and journals at the Public Archives of Nova Scotia for the Atlantic region showed none have survived from that time.
We supplemented Dr. Chenowith’s historic analysis using nine Royal Navy logbooks based on the Ships List for the Nova Scotia Station in 1757, searched the National Archives (UK) and extracted the surviving logs for analysis. I also located Knox’s (1769) journal which contains weather records for land-based observations of the passing storm. We did rely on the English translation of French logbooks since McLennan’s research has long been accepted by scholars and researchers as being of exceptional quality. The French fleet was treated as a single location owing to the fact they were moored in harbour. The location of the ships in our diagram is based on historic charts showing historic fleet moorings.
Dr. Chenowith generously provided excerpts of HMS Winchelseas log showing it encountered the hurricane at 36 45N and 70 54 W on September 23, 1757. It places that vessel off the Carolinas and well into the Gulf Stream (comparison to NOAA sea surface temperature charts in September). That same day British and French ships 1357 km to the northeast recorded in their logs that winds had veered to SE and increased to gale force, rising to hurricane force winds until September 26 when the winds shifted to south, SW, W and finally NW. The logs note that in response to the rising SE winds on September 23 the British lowered topgallant masts and secured their ships in anticipation of heavy weather. At the same time in Louisbourg Harbour the French ships recorded that they prepared for a coming storm by setting four 2-ton anchors from the bow of each ship.
The justification in adopting Beaufort Wind Scale terms follows the important work of Wheeler and Wilkinson (2004) and Wheeler and Wilkinson (2005) and Wheeler et al. (2010) as well as the CLIWOC (2003) project. Beaufort reflects a Royal navy officer who systematized and simplified a terminology that was already enjoying convergent evolution and common usage by the mid 18th Century. There is virtually no divergence in terminology between the logs of ships under Holbourne’s command in 1757. Subtle differences in adjectives as modifiers to key terms like ‘gale’ followed the methodology of recognizing synonyms or diminutive terms to rank intensity that allows comparison to the modern scale and associated wind speeds.
We thank Dr. Mock for his inisight and suggestions and look forward to reviewing his papers. Perhaps part of our challenge is defining if this system was tropical or extratropical. Through their life cycle, hurricanes in Canadian waters are very often both.
Hart, R. and Evans, J. 2001. A climatology of extratropical transition of Atlantic tropical cyclones. Journal of Climate. Vol. 14, 546-564
Wheeler, D. and Wilkinson C. (2004) From calm to storm: the origins of the Beaufort Wind Scale. The Mariner's Mirror, 90:2, 187-201, DOI: 10.1080/00253359.2004.10656896
Wheeler, D. and Wilkinson, C. (2005) The determination of logbook wind force and weather terms: the English case. Climatic Change 73: 57–77 DOI: 10.1007/s10584-005-6949-1
Wheeler, D., Garcia-Herrera, R. and Wilkinson, C. (2010) Atmospheric circulation and storminess 709 derived from Royal Navy logbooks: 1685 to 1750. Climatic Change. Vol. 101, 257-280

Citation: https://doi.org/10.5194/egusphere-2023-902-AC4

CC3: 'Comment on egusphere-2023-902', Michael Chenoweth, 26 Jul 2023

I thank the authors in their earlier reply to my first note for reminding me of Ludlum's use of Warden's account.

Because Ludlum's publishers were generous in the amount of material he could present, it is safe to say that his

source was reliable even though I did not have access to it. I have some additional comments for the authors to

consider.

The authors claim that at least a Category 4 hurricane struck Nova Scotia in 1757. In part, they base their

assertion from their estimated winds and a comparison of past surge heights in hurricanes in Nova Scotia and

elsewhere in the North Atlantic. Please note that no storm of minimal Category 4 hurricane strength or greater

has been observed in the North Atlantic north of 30.2N latitude [Hurricane Michael in 2018 at 30.2N 85.4W

see: https://www.nhc.noaa.gov/data/tcr/AL142018_Michael.pdf], while at the latitude of 45N, just south of Canso,

the highest is 85 knots in post-tropical Hurricane Fiona in 2022. The authors are stressing the bounds of

climatology by claiming a Category 4 or higher intensity over 1,500 nautical miles further north than ever before

recorded.

The author's do not explain to the reader how a one-minute sustained wind measured at a standard height

of 10m is to be equated to wind force terms in the mid-18th century. For more on the history of the Beaufort

Scale, I highly recommend Blair Kinsman's article "Who Put the Wind Speeds in Admiral Beaufort's Wind Force Scale"

Pt 1: https://babel.hathitrust.org/cgi/pt?id=uc1.c049815166&view=1up&seq=280

Pt 2: https://babel.hathitrust.org/cgi/pt?id=osu.32435026806562&view=1up&seq=18

The typical logbook of that era will normally have three eight-hour time periods (first, middle, and latter parts)

with a single "prevailing" wind or a sequence of winds at unspecified times within the 24-hour period. In storms,

there may be additional wind force terms but over what time period the estimate is made is usually uncertain. A

logbook entry of hurricane force winds can only be estimated as a wind of at least 119 kph and stronger winds are

guess-work unless there are wind-induced damage reports on land to use (Chenoweth, 2007). Such reports from land

appear to be absent in their case study, only damage caused by the storm surge.

The author's use the definition of a squall from 3 different meteorological organizations and argue for

providing a quantitative estimate based on modern estimates from anemomemters. Again, squalls cover a wide range

of wind speeds and in the 18th century there is no direct way to assign a value or range of values to such a

gust of wind. The authors simply assert that any squall is one of 40 to 60kph. The authors write: "We place ‘hard

squalls’ at the upper end of the spectrum." I assume they will add 60kph to any such subjective description. We

now have 119 km/hr as a minimum estimate with up to 60kph added subjectively if systematically.

The author's rely on the Saffir-Simpson Hurricane Wind Scale (SSWS) which assesses damages to property on shore

relative to the sustained peak 1-minute wind. The SSWS states that on average the damage rises about a factor of four

for every category increase. There is no description in the SSWS of how such winds affects modern ship's of any size,

not that they would be comparable to a sailing ship of the line in the 18th century. The author's assert on page 2,

lines 31-32 that--the writing is very imprecise in their original--presumably the logbooks and post-storm report

descriptions "suggest was a major hurricane (Category 3+ on the Saffir-Simpson Hurricane Wind Scale)." What is the

baseline of damage reports from land and their associated wind force values that they are using to make this statement?

Apparently, it is Virot's estimates of wind force to snap trees on land is then their assumption to "measure"

the minimum wind allegedly required to dismast a line of battle ship. I don't see how trees rooted in the ground can

be compared with masts tied to one another on a ship rolling, pitching, and yawing in a fluid can be considered

comparable.

A minimal category 4 hurricane, or equivalent extratropical storm with winds of the same intensity, is 209kph.

If one takes the estimate of 170kph (about 92 knots) as approximately correct, which is not necessarily the case,

this is a Category 2 hurricane. This would not be defined as a Major Hurricane in modern records.

The authors also try and use storm surge to argue for the unprecedented intensity of the storm. They compare the

storm surges of Hurricane Juan (2003) and Hurricane Fiona (2022) to show the peak wind intensities and storm surges.

Below are the six-hourly positions and intensities of Hurricane Juan and Hurricane Fiona which both impacted the 1757

study area and are cited by the authors. Winds are in knots and pressure in millibars.

Juan 2003

YEAR MONTH DAY TIME(Z) LAT LON WIND PRESSURESTATUS

2003 9 28 1800 402 641 90 970 HU

2003 9 29 0000 428 639 85 972 HU

2003 9 29 0600 460 638 65 982 HU

2003 9 29 1200 498 624 45 995 TS

Fiona 2022

YEAR MONTH DAY TIME(Z) LAT LON WIND PRESSURESTATUS

2022 9 23 1800 378 617 110 940 MH

2022 9 24 0000 423 607 100 932 EX

2022 9 24 0600 450 612 85 931 EX

2022 9 24 1200 468 612 75 938 EX

2022 9 24 1800 479 608 65 952 EX

2022 9 25 0000 488 598 60 968 EX

In both instances, sustained winds of about 80 to 85 knots (148 to 157kph) are the estimated peak intensity when

nearest the latitude of Canso. Note that once these hurricanes both crossed the north wall of the Gulf Stream, they

inexorably weakened (in part) to the colder waters they crossed over; more apparent in Juan, less so with Fiona due

to its becoming extratropical at around the time it reached cooler waters and continued deepening in pressure to 45N.

Storm surge is extremely variable and subject to multiple variables as the author's are certainly aware. Appealing to

storm surges in the very different environments of the Gulf of Mexico and the Bahamas is unconvincing. It is better to

compare storms that struck Nova Scotia when trying to make qualitative comparisons.

Additionally, the authors assume a sudden deceleration in the storm's forward translation "may have enhanced storm

surge height" but this is based on inadequate synoptic analysis. This arises from studying too small an area and not

the larger region. In fact, additional data from the area indicates that the hurricane came from the southwest and

probably was moving over land when it crossed the Gulf of Canso, and the center would be further to the west than that

estimated by the author's as having made landfall on the Canso Peninsula, which would suggest a storm moving from the

south to north.

Since the publication of Chenoweth (2006) I have found information that I had not yet located when I published my

compilation. The frigate HMS Winchelsea (ADM 52 1105) sailing from New York for Charleston encountered the hurricane

at 5am local apparent time on 24 September in about 3645N 7054W. I have modernized and corrected spelling and

punctuation, but here is the logbook record for the seaman's day beginning Noon 23 September and ending Noon 24 September.

[START OF LOGBOOK EXTRACT]

The first part little wind and cloudy, middle part fresh gales, latter part almost a hurricane. At 10pm got down the

topgallant yard, flattering winds with thunder, lightning & rain; handed the topsails, winds variable, at 130am lay to

under main courses, at 5am it fell from a strong gale to quite calm, at 515am the main sail split by the wind flying

round all at once to NW which blew the mizzen to pieces, before we could get the main sail clewed up it blew a storm

little less than a hurricane with a great sea which was occasioned by a sudden shifting of the wind, did all we could

to put the ship before the wind but could not before we could get the lower yards down, she lay waterlogged and motionless,

about 6am the mainmast went over the side about 16 feet from deck and carried away the flying boom and broke its fall on

small bower anchor stock, we cleared ourselves of the wreck as fast as possible, cutting anything that had connection

with the masts, lost by this accident, one third of the deck poop awnings [and other damages]. Winds during the 24-hours

given as: Calm-E-ESE-S-round to NE & suddenly to NW. [END OF LOGBOOK EXTRACT]

Mapping this report along with other data from the region indicates that the storm took 30 hours to move from the point

it encountered HMS Winchelsea until the time it passed by Louisbourg Light after 11am 25 September, a distance of about

737 nautical miles. The average rate of movement is about 24.6 miles per hour. The hurricane likely made landfall between

Canso and Halifax while moving northeast or northeast by north. Although data is unavailable north and northeast of the

Canso region, a reasonable interpretation of the synoptic weather map would be that the low continued into the Labrador

Sea, perhaps not far off the Labrador coast, and would account for the westerly winds and improved weather in the following

24 hours.

The track suggested here might actually be one that accounts for the surge experienced in Canso Bay. Modelling studies

would be needed to assess what combination of winds and seas would best account for the observed weather. It is possible,

but within a large range of uncertainty, that the 1757 storm was one of the strongest on record in Nova Scotia, but it

is not necessary to be a Category 4 or 5 hurricane to establish such a record. The authors should consider how uncertain

their estimates actually are, and modulate their take-aways to minimize the dramatic headline, and attempt to show how

this might be a record-breaking storm even if only by a small amount, if at all. Regardless, I do not doubt it is one of

the most intense hurricanes in the region but it was not a Category 4 or stronger hurricane, nor even a major hurricane

(Category 3).

Chenoweth, Michael. "A Reassessment of Historical Atlantic Basin Tropical Cyclone Activity, 1700-1855", Climatic Change (2006)

76, 169-240.

Chenoweth, Michael. "Objective classification of historical tropical cyclone intensity." J. Geophys. Res., 112, D05101,

doi:10.1029/2006JD007211, (2007).

Citation: https://doi.org/10.5194/egusphere-2023-902-CC3

AC5:
'Reply on CC3', John Dickie, 11 Aug 2023

ENSO and NAO for 1757 (proxy research referenced in another response) indicate it was a la Nina year with NAO -2 to -3 index which is conducive to tropical cyclone development and storms that track along the eastern seaboard.
Hart and Evans (2001) at Penn State University’s Department of Meteorology studied the best fit tracks at 6 h intervals for all North Atlantic tropical cyclones between 1899 and 1996 to address the lack of a climatology model that characterizes the extratropical transition of North Atlantic hurricanes. They refined their model with the more accurate 1950-1996 and 1979-1993 data sets and infilled gaps using post-transition pressure change statistics from the European Centre for Medium Range Weather Forecasts (ECMWF).
Their analysis of all (841) tropical cyclones between 1899-1996 shows 355 (42%) transitioned to extratropical cyclones. In the data set for 1950-96 46% transitioned; 51% of landfalling cyclones. The probability of transition increases at higher latitudes over time, peaks in late September – October and then retreats south late season. Transition seasonality is based on the competing delayed warming of the Atlantic and slow continental cooling in fall which shifts tropical intensification northward to be juxtaposed against increasing baroclinicity with expanding westerlies during autumn continental cooling. The model shows that cyclones that form south of 20N have a more intense tropical character that remains tropical much farther north. The zone of tropical intensification (tropical regulator) can fall north of the extratropical regulator (strongly baroclinic zone) resulting in the explosive release of tropical energy and an intensification of the extratropical system that can be more powerful than the parent storm. The center of the zone most conducive to this process lies between 37-47N and 65-50W, south of Cape Breton. This model demonstrates that the storm in 1757 might well have continued to intensify and retain its tropical character into higher latitudes to result in an explosive release of energy manifested in the extreme conditions described at the time.
The 1757 hurricane was first identified off Florida at the height of hurricane season and remained tropical to at least the Carolinas where the latitude and longitude of HMS Winchelsea on September 23, 1757 places it over the Gulf Stream. It then passed New England on September 24 and struck southeastern Cape Breton on September 25. When it struck Winchelsea on September 23, the logs of the British and the French ships at Louisbourg, 1357 km to the northeast, recorded that the winds shifted to SE and increased to gale force and then to SE hurricane force winds into the 25th and did not vary during this time until the wind direction changed to westerlies. These log entries on the 23rd also describe the actions taken by the British ships off Cape Breton to prepare for the coming storm by lowering and securing topgallant masts while the French, commenting on the coming storm in their logs, prepared for it by setting four anchors at the bow of each warship moored in Louisbourg Harbour. This infers that the storm was of considerable size and might have prolonged its tropical phase with energy from the Gulf Stream.
Engineering models are universally accepted as a method for deriving forces behind the failure of materials. A vessel at sea is subjected to motion in three directions (heave, pitch and roll). In addition, yaw describes rotation around a point on the X-Z plane. The 1757 log of HMS Invincible shows its trajectory was highly oblique to wave crests orthogonal to wind direction. This allowed the ship to minimize pitch. Limiting vessel motion in the X-Y plane (fore and aft rig along the centerline of the ship) with wind force applied steadily from the Z direction ensured minimal variance in the incident angle of the wind force on the masts. Standing rigging held masts immobile and countered torsional forces that might contribute to failure. The flooding of the hold would also have reduced the ship’s righting moment, contributing to yet more stability of the masts relative to the direction of sustained wind force.
Dr. Chenowith correctly notes that there are many variables that affect surge. This is why we selected three modern analogs that passed over the same bathymetry with similar translation speeds. These two factors have the greatest impact on surge outside of sustained wind speed. These analogs had peak surge levels a fraction of the surge in 1757 calculated independently at: (1) Battery de la Grave mid storm, (2) Louisbourg Town peak storm, (3) St. Esprit – Tilbury shipwreck peak storm. To ensure we described storm surge and not storm tide, we deduced the timing of tides and backed the tidal influence out to leave surge.
Wind vectors from ship locations show these surge heights were tens of kms from the center of the storm which was estimated based on normal lines drawn to wind vectors at ship locations. These vessels were sufficiently near shore to triangulate their locations based on coastal landmarks. Hurricane Laura was not presented in the paper as a comparison for the 1757 storm surge. It was a modern example of the lateral distance over which surge height diminishes with distance from ground zero.
Hart and Evans’ (2001) climatology allows that the incrementally earlier arrival of seasonal colder, baroclinic continental autumn westerlies in the Little Ice Age (LIA) meeting with an intensifying tropical cyclone following the Gulf Stream to northern latitudes could trigger a more explosive release of tropical energy resulting in storm metrics exceeding those of any tropical or extratropical system to strike Nova Scotia in the ensuing 266 years.

Citation: https://doi.org/10.5194/egusphere-2023-902-AC5
- CC4:
  'Reply on AC5', Michael Chenoweth, 11 Aug 2023
  
  The climatology of Hart and Evans is interesting in its’ own rights but does not address this specific storm. Until they present a convincing synoptic weather account of the storm with the estimated range of uncertainties of intensity and status (hurricane, extratropical) appeals to Hart and Evans is just arm waving. Hart and Evans’ climatology also allows for storm metrics that might not exceed those of any system since 1757.
  A proper synoptic weather analysis would assist the author’s in making informed assumptions about the tropical to extratropical transition associated with this system.
  As I referenced in my earlier reply, the hurricane moved at an average speed of almost 25 nautical miles per hour from the time it was encountered by HMS Winchelsea and passed over the fleets about 11am 25 September. The strongest winds that began to dismast ships were felt starting about 3am on 25 September and continued for another 8 hours. This would give us a crude estimate of hurricane force winds extending nearly 200 miles northeast of the storm center when the outer edge of presumed hurricane force winds was first felt by the fleets. There are also weather reports from Annapolis Royal, on the west side of Nova Scotia, available for study at the UK National Archives. These reports will aid in setting limits on the larger wind field structure and in doing so assist in setting plausible constraints on the storm’s status as a hurricane undergoing transformation to an extratropical cyclone, or potentially interacting with non-tropical weather features while moving poleward.
  The timeline of the author’s Figure 3 is in error. They mark the start of each day from the seaman’s day that begins at Noon and ends 24 hours later at Noon. From their own accounts, the wind shift of the storm center reported by the Tonnant was at 11am on 25 September. For the average reader who is not familiar with logbooks, they should mark the actual civil dates and convert the figure to show civil local apparent time. As Figure 3 reads now, the labels should be labeled 24-25 September, not 25 September and so forth before and after for the other date labels on the figure. The strongest winds encountered by the fleets were in about an 8-hour period between 3am and 11am 25 September. The winds then rapidly weakened between 11am and the early afternoon hours of 25 September. This is at variance with their best-fit wind speed curve which weakens the winds from after 5am.
  The author’s have an interesting account to relate. But as the reviewers and others note, the paper needs to be completely restructured to make it readable. As I stated in my last reply, you may well have one of, perhaps even the most intense, such storms. Make it easier for a reader to follow your case and use all the necessary interpretive tools to assess it.
  
  Citation: https://doi.org/10.5194/egusphere-2023-902-CC4
  - AC7: 'Reply on CC4', John Dickie, 12 Sep 2023
    
    In order to overcome the limitations of historical records from the Eighteenth Century our analysis has been both consistently scientific and systematic. It has also been innovative.
    Aligning our research with an established climatology for Atlantic Tropical cyclones is hardly ‘arm waving.’ It was a reasoned approach that allowed us to deduce processes from historical records and compare them to an accepted working climatological model derived from a detailed scientific analysis of a century of modern hurricanes from the same region.
    This climatology also presents a reasonable explanation for the unusual intensity of Atlantic hurricanes in the midlatitudes late in the season. This served as a clue to why a Little Ice Age storm might likewise be more intense. Continental westerlies colder than today during the Little Ice Age intersected north-moving tropical cyclones to create the conditions for explosive transition, manifested in greater wave heights, stronger winds and high surge than modern systems. The climatology shows that the optimal location for this process coincides with the path of the 1757 hurricane at landfall.
    Our paper title considers the continuum of tropical and extratropical systems. It is consistent with the accepted climatology that many explosive transitions involve storms that retain their tropical characteristics much longer. We have presented sufficient reason why this might have been the case in 1757.
    
    Citation: https://doi.org/10.5194/egusphere-2023-902-AC7

RC3: 'Comment on egusphere-2023-902', Antoine Lachance, 28 Jul 2023

The need for pre-modern (before 1851) data on hurricane intensity cannot be overstated to improve our understanding of long-term storm trends. This study takes advantage of a historical event, the British naval blockade of France’s Fortress Louisbourg during the Seven Years’ War (1756-1763), to characterize the intensity of the Louisbourg Hurricane that occurred in 1757. The authors gathered valuable data, such as ship logs, and accomplished impressive work to provide a detailed quantitative description of an old storm. Ultimately, this study is a novel and relevant addition to the growing literature on storminess during the Little Ice Age in the North Atlantic, while shedding light on an important historical event.

I recommend to the editor to consider this manuscript for publication in Climate of the Past, provided that the authors make some changes to the manuscript. In particular, the manuscript proves confusing at time due to an insufficiently thorough methodology section, incoherence in certain sections’ content, and unclear statements about the main conclusions of the study.

I suggest that the authors follow the suggestion by Michael Chenoweth (26 July 2023) to include clear statements about the uncertainty of the results.

General comments:

The authors make the claim that the Louisbourg Hurricane was one of the most powerful storms ever recorded in the region, despite that it occurred during the Little Ice Age, a period of colder climate that should not promote hurricane formation. This is a major finding of the study and provide support to other studies such as Donnelly et al. 2015’s paleo-storm reconstruction from Mattapoisett Pond, Massachusetts, and Oliva et al. 2017’s paleo-storm reconstruction from Robinson Lake, Nova Scotia. Yet, this important finding should be highlighted more clearly in different sections of the manuscript (Abstract, Discussion, Conclusion), and introductory sections could be narrowed down to lay the ground for the latter discussion of this important result.

Certain parts of the text need more tightening to improve readability and coherence of the manuscript. Descriptions of different aspects of the Louisbourg Hurricane are scattered into different sections of the text (“The Little Ice Age”, “Historical Context”), despite having a dedicated section to describing the hurricane. Additionally, parts of the results (e.g., “Wave Height”, “Wind”) include important methodological notes that should be explained earlier in the text, in the methodology section.

Related to the above comment, the methodology could be more complete and should include a detailed description of the steps taken to estimate wave height, storm surge, wind strength, wind direction, and comparison with modern storm, in order to give the reader a sense of what is to come before getting into a detailed description of the results.

Specific comments:

Introduction

Figure 1: If possible, an indication of the locations of Fort Cumberland and the two coastal sites where storm surge was calculated would make the map more informative.
Line 24: The sentence “Three French naval squadons at Louisbourg and the blockading…” is unfinished and is not completed in the next paragraph (line 31) after Figure 1 (lines 26-30).
Lines 56-59: The sentence starting at line 56 and ending at line 59 would benefit from having references to studies that show the LIA to be a “stormier” period than usual (e.g., Donnelly et al. 2015; Oliva et al. 2017).
Line 62: It is unclear what the term “modern system” refers to. Is it modern hurricanes, modern climatology, modern weather station measurements?

Methodology

The methodology section would benefit from having a table outlining each historical document used in this study. The columns could include a brief description of the historical document, location of the record, information extracted from each source, and alterations made to the historical source (e.g., cursive text was transposed). This table would improve understanding about the data necessary to compile and aggregate the necessary information to characterize the storm intensity.
Information related to estimating wave height, wind speed, wind direction, and storm surge should be included in this section.

The Little Ice Age (LIA)

The title of this section is misleading, as the section does not focus solely on describing the Little Ice Age, but rather on storminess during the Little Ice Age. The title should be adjusted to reflect this.
This section is important to the manuscript, as it provides the climatological context to understand storminess during the Little Ice Age, and links directly into the main conclusion of the article. However, it is confusing to read, and its importance does not stand out to the reader. The different paragraphs could be organized in a more logical way, for example by first focusing on the general context of the LIA, followed by a description of examples of increased storminess during this period, followed by mechanisms or hypothesis of storm formation during the LIA, in general and in the region of interest, including the fact that annual proxy record may not record conditions capable of fueling hurricanes during that period.
For improved coherence, this section should not include information on the period before or after the LIA (for example, the information cited at lines 120-122), or information on the Louisbourg Storm (last paragraph), unless it is clearly linked to the context of the Little Ice Age.
Line 117: The claim that the Labrador Current plays an important role in extratropical transition could be more substantiated. One sentence explaining the theory that links the Labrador Current to extratropical transition would be enough.
Lines 118-120: The sentence starting at line 118 and finishing at line 120 is hard to understand and contains the verb “show” twice to describe first 1600 years of cold Labrador Current temperatures, and a sudden warming from 1850 onward. The sentence could be re-written for more clarity.

The Louisbourg Storm

This section thoroughly describes, moment by moment, the Louisbourg storm as described by the ships’ logs. However, the description proves, at times, confusing to read. This issue could easily be fixed by re-iterating the position of the French and British fleets in the text at the beginning of the section; by using sub-titles stating, if possible, the time and location of the observations, as seen in Figure 2; and by describing the observations by the French and British fleet separately. Ultimately, the information provided in this section should as much as possible mirror what is seen in Figure 2. The reader should easily go back and forth between Figure 2 and the text.
This section should contain more references to the historical documents.

Results (Wave Height, Wind, Surge, Modern Storms)

Figure 3 should be cited in the text when described (lines 272-279).
Line 278: Missing “t” to the word “meet”.
Line 382: The text refers to Figure 4a-b, but should be referring to Figure 5a-b instead.
Figure 5a: The yellow rectangle is labelled 4b, but should be 5b, as it references the bathymetry map in 5b.
This section would benefit from having a comparative table between the estimated parameters of the Louisbourg Hurricane, and the same parameters for the modern analogues. It would serve as a useful summary of the main results.

Discussion

The discussion should tie back with ideas discussed in the introductory sections and highlight the main results outlined in the abstract. Specifically, the author should mention that this storm was probably of higher intensity than any modern storm and discuss its cyclogenesis in relation to the LIA. Discussing hypothesis for intense storm formation during the LIA, considering the authors’ findings, would make this article more relevant to the broader literature on storminess during the LIA. Additionally, the discussion should include a section on the uncertainty inherent to the main results.

Donnelly, J. P., Hawkes, A. D., Lane, P., MacDonald, D., Shuman, B. N., Toomey, M. R., . . . Woodruff, J. D. (2015). Climate forcing of unprecedented intense‐hurricane activity in the last 2000 years. Earth's Future, 3(2), 49-65.

Oliva, F., Peros, M., & Viau, A. (2017). A review of the spatial distribution of and analytical techniques used in paleotempestological studies in the western North Atlantic Basin. Progress in Physical Geography, 41(2), 171-190.

Citation: https://doi.org/10.5194/egusphere-2023-902-RC3

AC6: 'Reply on RC3', John Dickie, 28 Aug 2023

The authors would like to thank Dr. LaChance for his thoughtful critique. His structured review is very helpful, beginning with general thoughts that the paper is a ‘novel and relevant addition to the growing literature on storminess during the Little Ice Age in the North Atlantic while shedding light on an important historical event.’
We agree that the uncertainty of the results relative to modern climate research should be reiterated in the manuscript. Our approach has been to demonstrate that, in this unique case, the large number of independent observations close to shore allowed certain uncertainties inherent to Eighteenth Century navigation to be minimized. One example is that, being so close to the coast, the logs note their position relative to known landmarks sighted in and used to triangulate location which is far more accurate than relying on sextants for position alone.
The uncertainty of wind speed is clear given the lack of modern methods used to characterize modern storm intensity. We rely instead on the time-independent metrics such as material strength (of masts made from trees) and the wind force required to trigger structural failure. It is necessarily an imperfect approach, but ships under reefed topsails on a consistent bearing allowed a consistent and sustained force to be applied that not only overcame the strength of the masts, but overcame the structural reinforcement by rigging designed to efficiently transfer those forces to the ship and minimize other forces (such as torsional forces) applied to the masts that might cause failure.
The use of engineering models is widely accepted but it is true that this only allows us to estimate of wind speed. That said, the preferential selection of trees lacking defects that cause structural weakness and the structural reinforcement of masts with rigging infers that our estimate is a minimum. The separation of sustained winds and squalls is clear in the log entries.
In this usage ‘squalls’ likely represent hurricane rain bands. Their frequency shown in Figure 3 is concentrated around the storm center (highest winds reflected in the greatest concentration of wind-derived ship damage) and is consistent with rain bands in modern tropical cyclones relative to the eye. We present a range of wind speeds associated with squall definitions from various meteorological authorities. The ephemeral nature of these squalls and their association with intense rainfall is implied in several of the ship records including that of Captain Palliser of HMS Eagle. Certainly, from a descriptive tone the observations in 1757 are consistent with these key elements of modern tropical cyclones. Unfortunately, historical records and descriptions do not provide the certainty of modern measurements, though they do combine to present a defensible argument of the intensity of wind-derived forces described at the time and the resulting structural failure which can be used to interpret the threshold (versus peak) wind speeds. The uncertainty of wind speed estimates noted by some reviewers is an unescapable reality and we agree that it needs to be noted. However, it does not prevent us from estimating wind force since failure occurs at a threshold rather than peak wind speed.
Dr. LaChance goes on to provide general comments that are very helpful, such as suggesting that we more clearly highlight the major findings of the study, tighten the structure of the paper and readability of specific sections to improve the clarity and flow, and include a more complete description of the methodology taken to estimate the different metrics used to characterize storm intensity.
One of the greatest challenges in working with historical records is deriving quantitative measurements from direct observations made at the time. A single set of observations carries considerable uncertainty, but the unique nature of a fleet is that multiple independent observations can be used to reduce uncertainty in derived storm metrics.
Surge may be the most reliable metric in this study. It is possible to quantify flooding because of descriptions of the height of water relative to specific locations, such as Battery de la Grave (known elevation relative to modern sea level corrected for historic sea level and tidal range). Estimating the surge needed to flood the streets of the town with seawater was based on the lowest elevation streets in historic Louisbourg to present a minimum surge estimate. The analysis of the surge required to float HMS Tilbury during the storm necessarily requires a surge similar to that calculated for Louisbourg.
Another example of how the metrics support each other is that descriptions of waves throwing 70- and 80-gun battleships ashore in the relative protection of Louisbourg Harbour requires a surge height comparable to that calculated separately for the town and the Tilbury site.
Dr. LaChance’s suggestion of including a table comparing the metrics of the modern analogs (storms) against those of the Louisbourg Storm is well taken. Multiple independent estimates of storm surge are then able to be compared to multiple, independent modern storms that tracked across the same bathymetry at comparable translation speeds.
He then takes the time to review the paper line by line and offers constructive suggestions to improve the text and structure in order to help us present a more concise and defensible argument. We appreciate that effort and the spirit in which it was undertaken and accept that this will improve the paper.
Part of our challenge in presenting this work has been its multidisciplinary nature (historic and scientific analysis) that relies heavily on description despite the requirement for brevity in journal articles. As such, specific suggestions that tighten up the manuscript’s readability and impact such as those provided by Dr. LaChance are most welcome.

Citation: https://doi.org/10.5194/egusphere-2023-902-AC6
AC8: 'Reply on RC3', John Dickie, 12 Sep 2023

Several suggestions were made regarding adding historical records. In our view we have already compiled sufficient historical records. More, even if they existed, would likely add little to what we have used in the paper. Instead, the climatological model for hurricanes in the North Atlantic entering higher latitudes late season models the atmospheric circulation between continental air (autumnal westerlies) and north tracking tropical cyclones. in some respects, our research serves as a test of that model, and it allows us to explain our characterization of extreme storm intensity with an incremental seasonal distinction inherent to the Little Ice Age.
This project used historical records to characterize the intensity of an unusually severe hurricane that struck Nova Scotia Canada in 1757. The descriptions of damage and the conditions required to create it exceeded by a wide margin the impact of any modern (post-1851) Atlantic cyclones striking the same region. This storm provides an opportunity to explore heightened storminess in the North Atlantic during the Little Ice Age (LIA) increasingly recognized in the literature, and appears to be consistent with the climatology for tropical cyclone development in the midlatitudes during Atlantic hurricane season. Greater storm intensity appears to have resulted from the heightened LIA contrast between deeper, sustained cold autumn continental westerlies than occur today, and Atlantic tropical cyclones that historical compilations show continued to form in the hurricane main development region and intensified into the midlatitudes in autumn. The increased probability of following a more easterly track (into the Atlantic) later in hurricane season also provides a greater probability for tropical systems to remain over the Gulf Stream when sea surface temperatures are at their peak. More detail is presented in Hart and Evans (2001) noted in other responses.
The unique opportunity presented by the ‘Louisbourg Storm’ of 1757 is that its path and timing coincided with a large concentration of warships during the British naval blockade of Fortress Louisbourg during the Seven Years War (1756-1763). The systematic nature of naval record keeping has been shown in the literature to be a reliable source of empirical weather data and is an important source of historic climate data spanning the rise of anthropogenic climate influence.
This study analyzed ships logs and ancillary naval records from vessels in port (French fleet, Louisbourg) and at sea (Great Britain fleet patrolling coastal Cape Breton Eastern Canada) preserved in national archives in Europe. In addition, land-based records in the form or reports and journals describe the passing tropical system and the clear descriptions of the arrival of a distinct air mass of cold continental air in its wake. The climatology for this part of the North Atlantic suggests this air mass is distinct from the hurricane proper.
This storm was assessed in compilations of historical storms. It was first reported off Florida at the height of Atlantic hurricane season. It followed the Gulf Stream through the Carolinas where records show it to have been over the Gulf Stream, suggesting it continued to be tropical to that point. No temperature records exist other than land-based observations of the contrast between the passing cyclone and the arrival of very cold air in its passing, suggesting that it was either tropical or undergoing extratropical transition while entering Nova Scotia waters. We located a large number of historical records from 1757 that provide an unbroken time sequence as the storm passed offshore New England to enter Canadian waters. The number of ships logs and ancillary documents consulted to assess the intensity of the storm at landfall was considerable and yet no compilation seems to have considered any of these sources. Our focus was not on whether the system was tropical or extratropical, but on its catastrophic impact indicated by the level of impact.
Its significance extends beyond climatology. The decision of the French Admiral at Louisbourg to not capture the stricken British ships prevented the French from doubling their naval force in Nova Scotia. Sailing south with 4000 troops sent to defend Louisbourg they could have captured a largely undefended Halifax. New France would have two ports in Nova Scotia to attack long supply lines to the American Colonies and prevented the Fall of Louisbourg, Quebec and Montreal. Success in North America would have eliminated France’s impetus to support the separation of the American Colonies, leaving Britain compelled to agree to petitions by the colonies for representation in parliament and to reinforce a resurgent New France on their northern border.
To provide a broader atmospheric circulation context, we determined that ENSO and NAO conditions (references cited in other responses) were highly favourable for North Atlantic tropical cyclone development in 1757. Regional proxy studies show summer temperatures across northern Canada were warm in 1757 and historical records showed such extremes of heat in Britain and Europe that summer that those records were not broken until the 21st Century. Unusual heat was recorded on one day in Halifax Harbour during that same heat wave. We do not have SST records outside of mean annual data but the considerable temperature variability certainly indicates that the conditions that drive tropical intensification could have developed.
It is not reasonable to expect the same scientific methods used in the modern period to quantify storms to have existed nearly three centuries ago. Instead, we examined descriptions of weather conditions, sea state and infrastructure and ship damage to deduce the forces behind storm waves whose height could be reasonably estimated, wind speeds whose force was deduced based on structural damage and examining not only sustained winds but ephemeral increases over sustained winds, and surge heights calculated from flooding described at specific geographic sites whose sea levels were recalculated for 1757 and whose tidal range was removed to provide accurate surge estimates. In all metrics, the attempt was to derive minimal values to direct any bias toward a lower storm intensity. Comparing these results tested consistency between metrics from multiple independent records, and comparing the results to modern analogs of known intensity allowed us to test conclusions regarding storm intensity.
The hurricane life cycle on the North American eastern seaboard renders the simple attribution of ‘tropical’ vs. ‘extratropical’ cyclone problematic. Winter extratropical cyclones such as the January 2000 example presented in the paper (‘Nor’easters’) differ from tropical cyclones that travel into the midlatitudes during hurricane season where they encounter seasonal conditions that trigger extratropical transition. Atmospheric circulation dominated by ENSO and NAO conditions in 1757 were favourable for North Atlantic hurricanes to generate and track along the Atlantic seaboard. The seasonality of the 1757 storm and its origin in the tropics shows it to be the latter, and the time of year coincides with a higher probability of tracking along the Gulf Stream and undergoing tropical intensification in to higher latitudes. What is unknown is how much of its tropical character was retained at landfall.
Hart and Evans’ (2001) note that the National Hurricane Center (NHC) relies on sea surface temperatures and satellite images to assess storm asymmetry which is an indicator of the level of transition. They note ‘the NHC declaration typically occurs early in the 1–2-day period of transition, when the storm is just beginning to lose its tropical characteristics.’ The Louisbourg Storm was recorded as being off the Carolinas (and the Gulf Stream) on September 23, at sea off New England on September 24 and striking Nova Scotia near the Canso Peninsula on September 25. The hurricane may have begun to lose its tropical energy prior to landfall, but the severity of the storm in coastal waters could indicate this is where transition took place. The storm was also large, influencing ships off Cape Breton on the same day it passed the Carolinas, 1350 km away, suggesting it may have continued to draw heat energy from the Gulf Stream much longer.
Additional land-based weather records for the region have not survived, based on a review of the collections of periodicals and historical journals for the period in question in provincial archives. Additional historic records in the United States were not located by other researchers, so the request for us to peruse the same sources and come up with different results is curious. Instead, the accepted seasonal controls on the climatology of mid latitude tropical cyclones explains the observations we have included in our research to sufficiently characterize this system. The degree to which it was ‘tropical’ or ‘extratropical’ is largely immaterial and may not be definable. It was necessarily both over its life since this is how tropical cyclones transfer equatorial heat energy northward. We have presented sufficient indications, from its large diameter to the enormous release of energy close to the coast, to the symmetry of wind vectors that clearly indicate its eye, to suggest it could have retained much of its tropical character sufficiently late that it meets the criteria used by the NHC to have been a hurricane at landfall, and is sufficiently aligned with the model of Hart and Evans (2001) such that its explosive energy suggests it was largely tropical quite late in its life.
The implications are twofold: (1) an intensifying tropical cyclone met with much colder LIA continental westerlies than we see today at the same time in autumn, triggering a much more intense storm than has occurred in the warming since the end of the LIA, and (2) a similar contrast may then occur given future ocean warming with greater tropical intensification reaching higher midlatitudes much later in hurricane season that would encounter strongly baroclinic conditions and triggering explosive extratropical transition.
We feel that there has been adequate historical analysis and that the established processes for North Atlantic hurricanes are more relevant to understanding why this LIA storm was so powerful during a supposedly colder climate period. The limitations of mean-annual data, descriptive historical records and the challenge associated with resolving seasonality in proxy studies speaks to some of the challenges in more accurately defining the LIA which on a mean-annual basis was a colder climate period, but which certainly exhibited strong seasonal variability that resulted in colder, more sustained winters but also permitted hot summers and conditions that created anomalously warm SSTs, even if over a much shorter period during hurricane season.
Several reviewers made specific suggestions that tighten the structure and readability of the paper and will drive improved clarity of argument and presentation is achieved. In particular, adding a table of the timeline of weather and damage faced by different ships at different times and their location has been suggested as a useful companion to Figure 3. Dr. LaChance recommends repositioning the methodology used to acquire storm metrics earlier in the text and adding a table that provides a clear comparison between the metrics of the 1757 storm and those of the analogs presented in the text. It has been suggested that the title is misleading. Since major hurricanes have entered Canadian waters since 1851 including Fiona in 2022 (Cat 3) it is not at all preposterous that a major hurricane might have, under the right conditions, transitioned close to or at the coast. Future considerations of hurricane intensity that are beyond the scope of this paper might consider if the seasonal constraints of the LIA created conditions that might favour fewer but more powerful tropical cyclones transferring equatorial heat northward.

Citation: https://doi.org/10.5194/egusphere-2023-902-AC8

AC9: 'Comment on egusphere-2023-902', John Dickie, 12 Sep 2023

This is intended to be a concluding comment. While we worked extensively with historical records it is also important to focus on modern climatology that considers seasonal atmospheric circulation patterns inherent to the North Atlantic in the fall. We felt this was superior to relying on the chance of finding additional historic records, considering our efforts to do so resulted in none in the region that have survived. However, since our focus is an attempt to understand processes, working with the climatology allows us to not only test the model with a colder climate period, but identify aspects of the model that could explain the intensity deduced in our research.

This project used historical records to characterize the intensity of an unusually severe hurricane that struck Nova Scotia Canada in 1757. The descriptions of damage and the conditions required to create it exceeded by a wide margin the impact of any modern (post-1851) Atlantic cyclones striking the same region. This storm provides an opportunity to explore heightened storminess in the North Atlantic during the Little Ice Age (LIA) increasingly recognized in the literature, and appears to be consistent with the climatology for tropical cyclone development in the midlatitudes during Atlantic hurricane season. Greater storm intensity appears to have resulted from the heightened LIA contrast between deeper, sustained cold autumn continental westerlies than occur today, and Atlantic tropical cyclones that historical compilations show continued to form in the hurricane main development region and intensified into the midlatitudes in autumn. The increased probability of following a more easterly track (into the Atlantic) later in hurricane season also provides a greater probability for tropical systems to remain over the Gulf Stream when sea surface temperatures are at their peak. More detail is presented in Hart and Evans (2001) noted in other responses.

The unique opportunity presented by the ‘Louisbourg Storm’ of 1757 is that its path and timing coincided with a large concentration of warships during the British naval blockade of Fortress Louisbourg during the Seven Years War (1756-1763). The systematic nature of naval record keeping has been shown in the literature to be a reliable source of empirical weather data and is an important source of historic climate data spanning the rise of anthropogenic climate influence.

This study analyzed ships logs and ancillary naval records from vessels in port (French fleet, Louisbourg) and at sea (Great Britain fleet patrolling coastal Cape Breton Eastern Canada) preserved in national archives in Europe. In addition, land-based records in the form or reports and journals describe the passing tropical system and the clear descriptions of the arrival of a distinct air mass of cold continental air in its wake. The climatology for this part of the North Atlantic suggests this air mass is distinct from the hurricane proper.

This storm was assessed in compilations of historical storms. It was first reported off Florida at the height of Atlantic hurricane season. It followed the Gulf Stream through the Carolinas where records show it to have been over the Gulf Stream, suggesting it continued to be tropical to that point. No temperature records exist other than land-based observations of the contrast between the passing cyclone and the arrival of very cold air in its passing, suggesting that it was either tropical or undergoing extratropical transition while entering Nova Scotia waters. We located a large number of historical records from 1757 that provide an unbroken time sequence as the storm passed offshore New England to enter Canadian waters. The number of ships logs and ancillary documents consulted to assess the intensity of the storm at landfall was considerable and yet no compilation seems to have considered any of these sources. Our focus was not on whether the system was tropical or extratropical, but on its catastrophic impact indicated by the level of impact.

Its significance extends beyond climatology. The decision of the French Admiral at Louisbourg to not capture the stricken British ships prevented the French from doubling their naval force in Nova Scotia. Sailing south with 4000 troops sent to defend Louisbourg they could have captured a largely undefended Halifax. New France would have two ports in Nova Scotia to attack long supply lines to the American Colonies and prevented the Fall of Louisbourg, Quebec and Montreal. Success in North America would have eliminated France’s impetus to support the separation of the American Colonies, leaving Britain compelled to agree to petitions by the colonies for representation in parliament and to reinforce a resurgent New France on their northern border.

To provide a broader atmospheric circulation context, we determined that ENSO and NAO conditions (references cited in other responses) were highly favourable for North Atlantic tropical cyclone development in 1757. Regional proxy studies show summer temperatures across northern Canada were warm in 1757 and historical records showed such extremes of heat in Britain and Europe that summer that those records were not broken until the 21st Century. Unusual heat was recorded on one day in Halifax Harbour during that same heat wave. We do not have SST records outside of mean annual data but the considerable temperature variability certainly indicates that the conditions that drive tropical intensification could have developed.

It is not reasonable to expect the same scientific methods used in the modern period to quantify storms to have existed nearly three centuries ago. Instead, we examined descriptions of weather conditions, sea state and infrastructure and ship damage to deduce the forces behind storm waves whose height could be reasonably estimated, wind speeds whose force was deduced based on structural damage and examining not only sustained winds but ephemeral increases over sustained winds, and surge heights calculated from flooding described at specific geographic sites whose sea levels were recalculated for 1757 and whose tidal range was removed to provide accurate surge estimates. In all metrics, the attempt was to derive minimal values to direct any bias toward a lower storm intensity. Comparing these results tested consistency between metrics from multiple independent records, and comparing the results to modern analogs of known intensity allowed us to test conclusions regarding storm intensity.

The hurricane life cycle on the North American eastern seaboard renders the simple attribution of ‘tropical’ vs. ‘extratropical’ cyclone problematic. Winter extratropical cyclones such as the January 2000 example presented in the paper (‘Nor’easters’) differ from tropical cyclones that travel into the midlatitudes during hurricane season where they encounter seasonal conditions that trigger extratropical transition. Atmospheric circulation dominated by ENSO and NAO conditions in 1757 were favourable for North Atlantic hurricanes to generate and track along the Atlantic seaboard. The seasonality of the 1757 storm and its origin in the tropics shows it to be the latter, and the time of year coincides with a higher probability of tracking along the Gulf Stream and undergoing tropical intensification in to higher latitudes. What is unknown is how much of its tropical character was retained at landfall.

Hart and Evans’ (2001) note that the National Hurricane Center (NHC) relies on sea surface temperatures and satellite images to assess storm asymmetry which is an indicator of the level of transition. They note ‘the NHC declaration typically occurs early in the 1–2-day period of transition, when the storm is just beginning to lose its tropical characteristics.’ The Louisbourg Storm was recorded as being off the Carolinas (and the Gulf Stream) on September 23, at sea off New England on September 24 and striking Nova Scotia near the Canso Peninsula on September 25. The hurricane may have begun to lose its tropical energy prior to landfall, but the severity of the storm in coastal waters could indicate this is where transition took place. The storm was also large, influencing ships off Cape Breton on the same day it passed the Carolinas, 1350 km away, suggesting it may have continued to draw heat energy from the Gulf Stream much longer.

Additional land-based weather records for the region have not survived, based on a review of the collections of periodicals and historical journals for the period in question in provincial archives. Additional historic records in the United States were not located by other researchers, so the request for us to peruse the same sources and come up with different results is curious. Instead, the accepted seasonal controls on the climatology of mid latitude tropical cyclones explains the observations we have included in our research to sufficiently characterize this system. The degree to which it was ‘tropical’ or ‘extratropical’ is largely immaterial and may not be definable. It was necessarily both over its life since this is how tropical cyclones transfer equatorial heat energy northward. We have presented sufficient indications, from its large diameter to the enormous release of energy close to the coast, to the symmetry of wind vectors that clearly indicate its eye, to suggest it could have retained much of its tropical character sufficiently late that it meets the criteria used by the NHC to have been a hurricane at landfall, and is sufficiently aligned with the model of Hart and Evans (2001) such that its explosive energy suggests it was largely tropical quite late in its life.

The implications are twofold: (1) an intensifying tropical cyclone met with much colder LIA continental westerlies than we see today at the same time in autumn, triggering a much more intense storm than has occurred in the warming since the end of the LIA, and (2) a similar contrast may then occur given future ocean warming with greater tropical intensification reaching higher midlatitudes much later in hurricane season that would encounter strongly baroclinic conditions and triggering explosive extratropical transition.

We feel that there has been adequate historical analysis and that the established processes for North Atlantic hurricanes are more relevant to understanding why this LIA storm was so powerful during a supposedly colder climate period. The limitations of mean-annual data, descriptive historical records and the challenge associated with resolving seasonality in proxy studies speaks to some of the challenges in more accurately defining the LIA which on a mean-annual basis was a colder climate period, but which certainly exhibited strong seasonal variability that resulted in colder, more sustained winters but also permitted hot summers and conditions that created anomalously warm SSTs, even if over a much shorter period during hurricane season.

Several reviewers made specific suggestions that tighten the structure and readability of the paper and will drive improved clarity of argument and presentation is achieved. In particular, adding a table of the timeline of weather and damage faced by different ships at different times and their location has been suggested as a useful companion to Figure 3. Dr. LaChance recommends repositioning the methodology used to acquire storm metrics earlier in the text and adding a table that provides a clear comparison between the metrics of the 1757 storm and those of the analogs presented in the text. It has been suggested that the title is misleading. Since major hurricanes have entered Canadian waters since 1851 including Fiona in 2022 (Cat 3) it is not at all preposterous that a major hurricane might have, under the right conditions, transitioned close to or at the coast. Future considerations of hurricane intensity that are beyond the scope of this paper might consider if the seasonal constraints of the LIA created conditions that might favour fewer but more powerful tropical cyclones transferring equatorial heat northward.

Citation: https://doi.org/10.5194/egusphere-2023-902-AC9


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A major midlatitude hurricane in the Little Ice Age

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