the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Coastal and regional marine heatwaves and cold-spells in the Northeast Atlantic
Coline Poppeschi
Sandra Plecha
Guillaume Charria
Ana Russo
Abstract. The latest IPCC report describes an increase in the number and intensity of marine heatwaves (MHWs) and a decrease in marine cold-spells (MCSs) in the global ocean. However, these reported changes are not uniform on a regional to local basis and it remains unknown if coastal areas follow the open ocean trends. Ocean temperature measurements collected by satellites (from 1982–2022) and 13 coastal buoys (from 1990–2022) are analyzed in the Northeast Atlantic and three subregions: English Channel, Bay of Brest and Bay of Biscay. The activity metric, combining the number of events, intensity, duration and spatial extent, is used to evaluate the magnitude of these extreme events. The results from in situ and satellite datasets for each of the studied regions are quite in agreement, although the satellite dataset underestimates the amplitude of activity for both MHW and MCS. This supports the applicability of the method to both in situ and satellite data, albeit with caution on the amplitude of these events. Also, this localized study in European coastal Northeast Atlantic water highlights that similar changes are being seen in coastal and open oceans regarding extreme events of temperature, with MHWs being more frequent, longer, and extending over larger areas, while the opposite is seen for MCSs. These trends are explained by changes in both the mean and variance of sea-surface temperature. Besides, the pace of evolution and dynamics of marine extreme events differs among the subregions. The English Channel is the region experiencing the most drastic changes over the last four decades. Summer marine heatwaves were very active in the English Channel in 2022 due to long events, in the Bay of Biscay in 2018 due to intense events and in the Bay of Brest in 2017 due to a high occurrence of events. Winter MCSs were the largest in 1987 and 1986 due to long and intense events in the English Channel. Finally, our findings suggest that at an interannual time scale, having dominant high-pressure conditions over Northern Europe and a low off the Iberian Peninsula favor the generation of strong summer MHWs in the Northeast Atlantic, while the opposite situation in winter dominates for MCSs.
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Amélie Simon et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-430', Anonymous Referee #1, 24 Apr 2023
This study uses satellite data and coast mooring observations to detect summer marine heatwaves and winter cold spells in 3 coastal regions in the northeast Atlantic. Summer marine heatwaves are more frequent, longer, and extended over a larger area over the past decades and the marine cold spells have opposite trends. It is speculated that the high/low-pressure systems in the region are key drivers of extreme events, especially the marine heatwaves in 2003, 2018, and 2022 and their spatial distributions. In general, this is an interesting study to provide background information for a better understanding of the climate drivers and future climate projections of these extreme events. Here are some suggested revisions before the manuscript can be considered for publication.
The main comment is on the atmospheric patterns associated with the marine heatwaves and cold spells. It is nice to show the geopotential height anomalies associated with the events, however, it would be more informative to be quantitative about the drivers of the events. It is preferred to have a mixed-layer heat budget analysis, or at least show the different components of air-sea heat flux anomalies, which would provide some indication of the drivers of the events.
From your mooring observations, have you observed the vertical extent of the heatwave and cold spell signatures?
It would also be good to know how the upper ocean in the region would precondition the marine heatwaves in the summertime, in addition to the atmospheric forcing.
Here are some specific comments.
The writing needs to be improved. Here are just some examples:
Line 72: “significant difference” – not informative
Line 73: “more or less extensive” – not informative
Line 231-236: the equation is not clearly explained.
Line 316: “yearly constant” – wording
Line 319: “the first 10 years” – not actually. The smoothing distorted the decadal variations. The decline is during 1985-2002 from the yearly data.
Line 321: ”spatial dispersion” is not defined.
Line 323: how is the SST variance calculated?
Citation: https://doi.org/10.5194/egusphere-2023-430-RC1 -
RC2: 'Comment on egusphere-2023-430', Anonymous Referee #2, 04 May 2023
This is a very interesting and detailed study on changes in seawater temperatures and extreme events (heatwaves and cold spells) in the NE Atlantic. The results are clearly presented and the methods are robust and based on previous research. The characterization of extreme temperature events and their relationship with atmospheric patterns is also of interest for understanding larger-scale connections. I have some comments on a few aspects that are less clear or detailed in the preprint.
1.- (line 234): although the authors refer to Simmons et al., 2022, nor there nor in this preprint there is a detailed explanation about the determination of area for these marine extreme events.
2.- Figures 2(a) and 5 (a) depict the time series of the extreme events as yearly mean. There are two issues for this representation; the first is the number of data used for the calculation of that mean values, which is not given in the M&M section; and related to that, the graphs in figures 3 (upper right) and 6 (upper right) show the variance. Again, no indication of the number of data, and there are very significant variations, indicating that the dispersion of data can be very large in some cases, with variances even exceeding in two orders of magnitude the mean value.
3.- Also in the legends to figures 3 and 6, please change ‘middle’ by upper right, otherwise is a bit confusing.
4.- Figures 2b and 5b show the spatial distribution, along the whole domain, of the extreme events, but note that the ‘whole domain’, as depicted in figure 1 (left), does not include the central portion of the Bay of Biscay. If results have been interpolated, this should be clearly explained. The data points shown in figure 1(right) do not include this area.
5.- Figures 4 and 7 (both lower right), please eliminate decimal scale for the x axis (year).
Citation: https://doi.org/10.5194/egusphere-2023-430-RC2 -
EC1: 'Comment on egusphere-2023-430', Bernadette Sloyan, 25 May 2023
I encourage the authors to consider the reviewer comments and post a reply to these, including how you intent to revise your manuscript, in the public discussion.
Citation: https://doi.org/10.5194/egusphere-2023-430-EC1
Amélie Simon et al.
Amélie Simon et al.
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