The answer is blowing in the wind: seasonal hydrography and mixing of the inner sea of Tierra del Fuego, Southern Patagonia

Castillo, Manuel I.; Zuñiga, Constanza; Barrios-Guzmán, Carmen; Cisternas, Natalia; Garces-Vargas, José; Landaeta, Mauricio F.; Piñones, Andrea; Rojas-Celis, Marcela; Guerrero, Alicia I.; Sepúlveda, Maritza

doi:10.5194/egusphere-2025-5692

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https://doi.org/10.5194/egusphere-2025-5692

Preprints

26 Nov 2025

| 26 Nov 2025

Status: this preprint is open for discussion and under review for Ocean Science (OS).

The answer is blowing in the wind: seasonal hydrography and mixing of the inner sea of Tierra del Fuego, Southern Patagonia

Manuel I. Castillo, Constanza Zuñiga, Carmen Barrios-Guzmán, Natalia Cisternas, José Garces-Vargas, Mauricio F. Landaeta, Andrea Piñones, Marcela Rojas-Celis, Alicia I. Guerrero, and Maritza Sepúlveda

Abstract. This study characterizes seasonal hydrography and mixing processes in Almirantazgo Fjord, a sensitive ecosystem in southern Chilean Patagonia. Although estuarine and tidal forcing conventionally explain fjord dynamics, wind stress effects remain less understood in this high-latitude region. The study analyses a comprehensive six-month dataset including a moored time-series of temperature, salinity, and dissolved oxygen, cross-fjord CTD transects, and hydrographic profiles derived from seal-deployed sensors. Observations indicate distinct seasonality, shifting from a stratified water column in summer – defined by low-salinity surface water from glacial melt – to a mixed winter state with significantly reduced vertical stability. The analysis identifies persistent, topographically channelled up-fjord winds as a primary physical driver. By applying the Wedderburn number (W_b) and mechanical energy balance calculations, we determined that wind stress perturb the pycnocline (W_b > 1); furthermore, wind power input rivals that of estuarine circulation during stratified summer periods. Under such conditions, wind forcing amplifies vertical mixing, modulates the pressure gradient, and supports deep-water oxygenation. First-order flushing time estimates suggest slow deep-basin renewal (greater than 5 months), signalling sensitivity to physical forcing. These results indicate that wind constitutes a primary mechanism regulating the hydrographic structure and biogeochemical function of the Tierra del Fuego inner sea.

Received: 17 Nov 2025 – Discussion started: 26 Nov 2025

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RC1:
'Comment on egusphere-2025-5692', Anonymous Referee #1, 13 Dec 2025 reply
Review of ‘The answer if blowing in the wind: seasonal hydrography and mixing of the inner sea of Tierra del Fuego, Southern Patagonia’ by Castillo et al.
The manuscript by Castillo et al. uses in situ, seal-collected, and moored oceanographic data along with wind data to explain physical processes in Almirantazgo Fjord, Southern Patagonia. While the observations are interesting and the manuscript generally well-written, I had many questions about the methods of data collection and interpretation. These questions will have to be addressed before I can recommend the manuscript for publication. Below I will detail my major and minor concerns.
Major concerns
My first major concern is the lack of discussion of different layers in fjords. Specifically, the authors didn’t introduce the different layers in the fjord and how these layers could potentially impact the results that they presented. Most literature suggests 3 to 4 layers in fjords (e.g. Farmer and Freeland, 1983 The physical oceanography of Fjords - ScienceDirect; Wan et al., 2017 Subtidal circulation in a deep‐silled fjord: Douglas Channel, British Columbia - Wan - 2017 - Journal of Geophysical Research: Oceans - Wiley Online Library) so a simplified 2 layer would impact the results. Recent papers have shown the importance of intermediate water in fjords (e.g. Kongsfjorden and Hornsund hydrography – comparative study based on a multiyear survey in fjords of west Spitsbergen - ScienceDirect) and so think they should be acknowledged. I suggest adding at least one paragraph in the introduction that outlines the different layers in fjords, which would set up the discussion about residence time nicely. In the discussion I suggest adding a paragraph about how using a simplified view of 2 layers could impact the results.
My second major concern centers around the map figures that were used. In particular, it is difficult to see the regions that the paper discusses and many names and areas aren’t on the maps. In addition, it would greatly help the reader to add known currents and also to make the station locations bigger and easier to see. Since this is such a complicated region, it is possible that 2 maps (1 regional, one local) are required.
My third major concern is related to instruments used and the apparent lack of CTD data processing. First, AML instruments don’t have software that processes the data so were the data post-processed? If so, how was this done? Post-processing is important for aligning the sensors, removing spikes, etc. and the data are usually not considered publishable until the data have been full processed (e.g. see GitHub - Sea-BirdScientific/seabirdscientific: The Sea-Bird Scientific Community Toolkit). This concern is specifically apparent in Figure 5, where there is a significant near-bottom salinity inversion. It is possible that the CTD hit the bottom to cause this because fjords are generally salt-stratified (see Farmer and Freeland, 1983). Similarly, how were the seal data processed? Figure 4 shows many strange features including salinity inversions that could be caused by processing errors or could be linked to the data averaging.

Minor concerns
Line 35 – Graphical abstract – again the intermediate water is absent here, which is misleading. I think that in this case the water column depth is 30 m (the depth of the bottom sensor and I think that should be make clear in the caption. Also, the box at the surface beside the ship doesn’t make sense to me and should be explained in the caption.

Lines 47-48 – How do you know that the primary mechanism for the physical dynamics is the along-fjord pressure gradient? The primary mechanism is likely fjord-dependent. Citations are needed to back this up.

Lines 60 to 63 – This is a weak paragraph and the point of it wasn’t clear to me.

Lines 69 to 70 – I suggest adding some examples of recent work in BC fjords since it is quite relevant to the work shown here (e.g. Bianucci et al., 2024 OS - Fjord circulation permits a persistent subsurface water mass in a long, deep mid-latitude inlet; Jackson et al., 2023 Winter Arctic Outflow Winds Cause Upper Ocean Cooling and Reoxygenation in a Temperate Canadian Fjord - Jackson - 2023 - Geophysical Research Letters - Wiley Online Library; Jackson et al., 2025 Why Deep‐Water Dissolved Oxygen Is Higher in G̱ngaay Xyangs (Juan Perez Sound), Haida Gwaii, Than Other British Columbia Fjords - Jackson - 2025 - Journal of Geophysical Research: Oceans - Wiley Online Library; Hannah et al., 2024 Oxygen dynamics in a deep‐silled fjord: Tight coupling to the open shelf - Hannah - 2024 - Limnology and Oceanography - Wiley Online Library)

Line 84 – I suggest adding another sentence here about how katabatic winds specifically impacts this region.

Lines 97 – 99 – I suggest adding a reference here

Line 103 – A reference is needed here after SPI

Lines 103-104 – Here is an example (South Pacific Current) of a current that should be added to an introductory figure

Line 106 – Here and elsewhere – I don’t think you can cite papers that are in review

Line 125 – Another example of map improvements – the full Magellan Strait isn’t seen in Figure 1 so the Pacific connection is difficult to see

Figure 1 – In addition to points above - Since the 3 different fjord zones are discussed, I suggested adding them to this introductory figure. Also I suggest adding the names of the meteorological stations to Figure 1

Line 140 – Patagonian Current not on Figure 1

Line 141 – Antarctic Circumpolar Current not on Figure 1

Line 143 – Punta Arenas and Carlos III Island not on Figure 1

Lines 147 – 148 – The sentence starting with ‘The DC’ doesn’t make sense to me

Line 148 – What gradient are the authors referring to?

Line 150 – Darwin Glacier not on Figure 1

Line 186 – What does CTDOF stand for?

Lines 211 – 212 – What reference salinity was used to calculate FWC? How was this reference salinity decided?

Lines 214-215 – I suggest adding a citation here to show that this is the definition of MSAAW

Line 224 – How were data from the WiSens NKE CTD processed? Also, some CTD sensors have interference with stainless steel frames that cause sensor drift (see https://docs.rbr-global.com/__attachments/16941886/0012918revA%20Proximity%20effects%20on%20conductivity%20measurements%20for%20the%202000dbar%20CT%20cell.pdf?inst-v=16a1419f-71a8-4d9a-9fa2-79f4a187420f#:~:text=As%20a%20point%20of%20reference,distances%20from%20the%20conductivity%20cell.) Is there documentation showing that stainless steel frames are not an issue for the WiSens CTDs?

Line 234 – Are the authors referring here to station A2 on the map? If so, please state

Line 235 – What do the authors mean by near-surface? Please state the actual depth

Lines 237 – 239 – I don’t recall that the authors examined the Hobo data. I suggest removing this sentence from here if the data weren’t used in this paper.

Section 3.1 – I’m not convinced that these regional scale winds are accurate in coastal waters, particularly fjords (see The influence of mesoscale land–sea breeze circulation on local wind climatology in the Svalbard fjords of Kongsfjorden and Hornsund - Kitowska - 2021 - International Journal of Climatology - Wiley Online Library) Have the authors explored the difference between the ERA5 and meteorological winds?

Line 277 – I don’t see S1 – could the authors explain?

Line 280 – Where is the location of the ERA5 station used. Please add to Figure 1

Line 286 – I don’t see Table 2

Line 287 – Gregorio station isn’t on Figure 1

Line 290 – This is the first time that form factor is mentioned. I suggest adding it to the methods and include citations

Figure 3 – It looks like the labels in the figure and captions are different for S2 and N2 constituents. Also I suggest specifying in the caption that the amplitude shown is from the model that was discussed in section 2.3

Lines 309 to 313 – I don’t see where low absolute salinity water was observed in deeper waters

Figure 4 – In general I didn’t see value in this figure as it is shown. The numerous salinity inversions made be question the accuracy of the data. And I’m not entirely sure how this picture fits with the larger story. If the authors use the data, I suggest calculating monthly or bi-monthly averages – I suspect that the inversions are a consequence of averaging a longer period of time. The length of time of the averages not the dates when the data were collected were not specified.

Lines 341-342 – I don’t think that the DO data are shown in Figure 4?

Figure 5 – As mentioned above, the large salinity inversion shown in b) and d) are strange. I don’t know why the near-bottom salinity inversion isn’t present in the bottom profile?

Line 352 – I suggest adding references to support the statement that thermal inversions are typical fjord structures

Lines 352 – DO is not shown in Figure 5

Lines 353-354 – I’m not familiar with conservative density. Please explain

Lines 354-355 – Again the authors state that the waters are salt-stratified yet Figure 5d shows a near-bottom salinity inversion

Figure 6 – Is this from data collected by the seals? How were summer and winter defined?

Lines 370 to 372 – How does this compare to other similar studies (e.g. Hauri et al., 2013 Wind‐driven mixing causes a reduction in the strength of the continental shelf carbon pump in the Chukchi Sea - Hauri - 2013 - Geophysical Research Letters - Wiley Online Library; Evans et al., 2019 Frontiers | Marine CO2 Patterns in the Northern Salish Sea)?

Line 387 – Winds blowing to the fjord head are not obvious in Figure 2

Figure 7 – What is the wind direction shown in a)? How was the seasonal trend for each time series estimated? How were warmer and colder events defined?

Line 425 – A negative trend of -1.212C per day isn’t correct

Line 426 – A seasonal trend of 557 g/kg per day isn’t correct

Figure 8 – I think there needs to be some explanation as to why the oxygen concentrations at the bottom are greater than the surface in Figure f) – ie why are the numbers all negative?

Line 629 – How does the plume gain heat?

Line 631 – What proof is there of mixing in the upper freshwater that increases DO?

Lines 692-694 – This is a bold statement. If the authors using data in 30 m of water near the head of the fjord to prove this statement?

Lines 696-697 – Were the seal data and wind-driven mixing examined together?

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Citation: https://doi.org/10.5194/egusphere-2025-5692-RC1

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Short summary

The study use several CTD measurement to describe the seasonality of the stratification and mixing of one of the southernmost Fjords of Patagonia: The Almirantazgo Fjord. In the region, typically tidal was quoted as the most important forcing to mix the inner-sea of Tierra del Fuego, here the study shows that the persistent along-fjord wind-stress could be as important as the freshwater input to the fjord.


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