the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Feb 2024
Marine Heatwaves in the Red Sea and their Relationship to Different Climate Modes: A Case Study of the 2010 Events in the Northern Red Sea
Abstract. In the context of climate change, the oceans are progressively warming, leading to an increase in the occurrence of marine heatwaves (MHWs). This warming trend is particularly striking in the Red Sea and has a significant impact on its ecosystem. The current study focuses on the characteristics of MHWs in the Red Sea in recent decades and examines their spatial patterns in the Red Sea sub-regions. In addition, the relationships between MHWs frequency and different climate modes are investigated. The extreme MHW events that occurred in the northern region in 2010 were analyzed. Through the analysis of satellite-derived sea surface temperatures (SST), a warming trend was observed that began in 1994 and has intensified significantly since 2016. This rise in temperature is accompanied by an increase in the frequency and total number of MHW days in the basin. In the last four decades (1982–2021), there have been 78 MHW events with a total of 1016 heat days. It is noteworthy that 46 % of the events and 58 % of the heat days occurred in the last decade. The spatial analysis of MHW characteristics in the Red Sea shows high variability, with longer and more intense MHWs occurring in the northern Red Sea (NRS), while they were more frequent in the southern Red Sea (SRS). The annual MHW frequency in the NRS peaked in 2010, 2018, 2019 and 2021, while it was highest in the SRS in 1998 and from 2017 to 2021. When comparing the annual mean values of atmospheric variables with the annual frequency of MHWs, a correlation was found. It was observed that years characterized by an increased frequency of MHWs coincided with anomalously high total heat fluxes and air temperatures, while exhibiting anomalously low wind speeds. This relationship was particularly pronounced in the NRS in 2010 and in the SRS in 1998. A link is then established between the SST anomaly, the MHW frequency and certain climate indices. The Atlantic Multidecadal Oscillation (AMO) and the Indian Ocean Dipole (IOD) showed a positive spatial correlation with the SSTA and the MHW frequency. In contrast, the East Atlantic/West Russian pattern (EATL/WRUS) showed a negative correlation with the SSTA and the MHW frequency, especially in the NRS. The MHWs of 2010 were further investigated as it was one of the warmest years in our study period, which had highly frequent MHWs with a different spatial distribution than the other warm years. It was also observed that the AMO and IOD were in a robust positive phase in 2010, while the EATL/WRUS and North Atlantic Oscillation (NAO) were in their most pronounced negative phase, which may have contributed to the increased occurrence of MHWs in that year. This study highlights the link between climate indices, atmospheric conditions and the occurrence of marine heatwaves in the Red Sea and provides valuable insights into this critical aspect of climate change.
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RC1: 'Comment on egusphere-2024-355', Anonymous Referee #1, 16 Feb 2024
Review of Marine Heatwaves in the Red Sea and their Relationship to Different Climate Modes: A Case Study of the 2010 Events in the Northern Red Sea
Main comment
In this paper, the authors have run an extensive analysis of SST and MHW events in the Red Sea and also at a regional level (North and South RS). They have characterized SST anomaly and MHWs and also assessed long-term trends. Additionally, an attempt to relate atmospheric variables in the region with MHWs was run and a winter event on 2010 was analysed with higher detail.
The work presented in the manuscript is certainly of interest, especially in an area as unique as the Red Sea but some concerns arise from the text. Regarding the title of the manuscript, the case study of 2010 seems to be a main aim but the discussion in the corresponding section is not as extensive as the reader could expect. I suggest changing the title or deepening the case study analysis. Besides, I could not find a proper justification of the event studied. Was it a record event? The most intense? Which is the interest of studying this event?
Another main concern is that the analysis of the relation between atmospheric variables and MHWs, or SSTA, seems too cursory despite the undoubted interest it may have. I recommend that the authors describe this analysis in more detail, as I assume that this is work that has already been done but it is not sufficiently highlighted in the text.
With this more detailed analysis and the other reviewers’ suggestions I would like to see a newly revised conclusions section to give some potentially interesting findings in the field of MHWs.
I would like to congratulate the authors on their work and encourage them to further develop it according to the indications received in this review process, prior to publication. My final decision is to review again after major revision but not because of problems in the methodology or conclusions but to clarify and deepen some of the analysis to improve the final result.
Minor comments
About Red Sea subregions, I could not find a justification for the spatial division between NRS and SRS. Please, indicate the spatial division criteria in the text.
Why do you consider winter months Jan-Feb-Mar? And summer months? It seems a little bit artificial division. Please better justify your selection or, better, consider different periods for winter and summer months.
In your analysis, you mostly describe winter and summer months. What happens to SST and MHWs in spring and summer months? Sometimes you refer to annual frequencies, MHWDs… Please, be consistent with the periods selection and analysis or better indicate why and when such periods are being analysed.
2010 event analysis it appears that is only for a winter event of the ten recorded during the year. Why?
Please, do not use bold type fonts for the axis labels in the figures and don’t use titles in plots if they can be explained in the caption.
Improve figure resolution for better readability. It’s maybe because of the pdf conversion but carefully check all figures and use large enough fonts, especially if the text is placed inside plots.
Please, consistently use acronyms throughout the text. Take care especially when using MHW and “marine heat waves”.
Please, carefully review the text as there are some typos or misspellings.
Line 172: an event can not be described by frequency, annual variability can.
Lines 172-173: Which is the difference between duration and total days. Maybe you are not referring to events but years?
Lines 178-179: Is there any threshold for cold/warm years? Only the pos/neg sign of SSTA?
Line 180: How do you define a MHWD? A single day exceeding 90 percentile or a day belonging to an MHW event?
Line 231: What does “non consistent” trend mean? Statistically? Spatially?
Lines 246-249: Can’t get the relevance of indicating the relatively cold/warm years in each period. It’s just variability.
Lines 275-282: The description of atmospheric variables in the case of MHW events deserve a more extensive and detailed analysis. I suggest the authors to properly rewrite this part.
Line 356: lake?
Citation: https://doi.org/10.5194/egusphere-2024-355-RC1 -
RC2: 'Comment on egusphere-2024-355', Anonymous Referee #2, 25 Apr 2024
Review of "Marine Heatwaves in the Red Sea and their Relationship to Different Climate Modes: A Case Study of the 2010 Events in the Northern Red Sea"
General Review
The authors have studied SST and MHWs in the Red Sea for the period 1982-2021. Mean MHW characteristics have been reported for the basin (and North/South sub-basins) and trends of SST anomaly and MHW frequency have been discussed over the study period. Potential links of SST anomaly and MHW frequency with certain climate indices have also been investigated. A more detailed analysis of selected sub-periods and groups of years has been shown, with a special focus on year 2010. The vertical extent and concurrent atmospheric conditions for a selected event that occurred within 2010 have also been investigated.
The topic of this work is important to address and I truly appreciate the efforts of the authors to provide a description of their results within a straightforward structure. There are however some parts that could be clarified and enriched to support some of the methodological choices made by the authors and substantiate their results:
A first concern is the criterion for detecting unusual behaviors of years and sub-periods. To my understanding, the definition of warm/cold years bears some issues, for which my concerns are detailed under specific comments (same concern for the “no-trend period”). In the same context, the choice for studying in detail the year 2010 has been slightly confusing while reading the text. It should be clear which specific findings justify the “exceptional character” of this year, thus motivating for a dedicated analysis.
In addition, the choice of MHW frequency (being a tricky MHW property especially for basins with high warming rates) should probably be better justified, as the authors use only this MHW parameter to investigate links between MHWs and climate indices.
A major concern is the interpretation of the atmospheric conditions and surface heat exchanges during the examined MHW. Causal links are reported in several parts within the text without sufficient analysis to support them (again, my concerns are detailed in the specific comments provided below).
Finally, some language issues would greatly improve the manuscript if corrected. Some have been detected and reported under specific lines but further checks should be made (e.g. syntax issues or typos in text and captions). Red Sea or RS (same for NRS, SRS) should be consistently used throughout the text. I also strongly suggest a more conservative approach in wording that suggests a cause-and-effect relationship where results are not sufficient to support such conclusions.
On these grounds, I would like to kindly encourage the authors to send a revised version of their work and perceive this review as constructive feedback towards improving their already interesting work and important contribution to the MHW literature.
Specific comments:
Abstract
Line 15: Why frequency?
Line 16: Why 2010?
Line 17: “...warming trend was observed that began in 1994 and has intensified significantly since 2016…” Looking at the time-series of Fig. 5 I cannot understand how this conclusion is obtained. This also contradicts what the authors call a “no-trend-period” beginning at 1992
Line 19: “heat days” here mean MHW days? Not clear
Line 23: “while it was highest” should better be rephrased
Line 23-24: could probably be removed from abstract to focus on results that come right next
Line 24: Why frequency? What about other properties?
Line 27: same as before
Introduction
Line 48: A smoother transition from the 1st to the 2nd paragraph would improve readability
Line 59-61: I would suggest: “However, only a few studies have investigated MHWs in the Red Sea (Genevier et al., 2019; Bawadekji et al., 2021; Mohamed et al., 2021) and up to date there have been no studies investigating the link between climate patterns and the occurrence of MHWs in the Red Sea region”
Line 67: Maybe better present separately any literature on future projections (now reported among results for past trends)
Line 77: marine heatwaves -> MHWs
Line 90-91: typos: un -> in, The -> the. Also use articles before each index consistently, or simply list indices, e.g., […] and their correlation with the following climate indices: AMO, IOD,…etc
Line 90-92: Maybe move this sentence after introducing all different indices
Line 119: why frequency?
Line 121: As in general comment, choose RS or Red Sea
Line 93-4: A reference should better be added here
2.1 Datasets
Line 116-118: please, rephrase
Line 118-119: “The study also aims to investigate the correlations between the different climate modes with the annual sea surface temperature anomaly (SSTA) and the annual frequency of MHWs in the Red Sea ” Why this parameter? Using only MHW frequency should better be justified.
2.2 Methods of analysis:
Line 165: please, rephrase sentence
Line 179: Define strong positive/negative, it is not clear
Line 190: What would be unusual for MHWs and climate modes? The motivation is not very clear here, and maybe it should be introduced earlier in the manuscript, justifying why year 2010 is a case study.
Line 191-2: Here you focus on events falling entirely within 2010? Or also partly e.g., the ones peaking within the year?
Line 191: Therefore probably does not fit here
Line 194: You mean when SST exceeds the threshold by #X (SSTthres-SSTclim), instead of #X (SSTthres), right? Also, simply exceeding the threshold shows that MHW occurs (of any category, not a moderate one). So please revise the sentence accordingly.
Line 194-5: Why these events?
3. Results
3.1
Line 209: […] in the RS.
Line 218: better write “with longer MHW durations”
Line 220: Here you mean the most intense?
Line 220-22: “Both the mean…spatial variability (Fig. 3e, f)” I cannot see a “similar pattern”. There is a north-south gradient in both fields but there are also non negligible differences (Gulfs of Suez/Aqaba, coastal areas in central and southern RS, Strait of Bab El-Mandeb).
Line 222: Figure 3e
Line 231: What does non consistent mean here? Please, revise for clarity.
Line 232: Specify the area geographically (in relation or not to its bathymetry, as you prefer) but rephrase, as the deep waters cannot exhibit SST trends :)
Line 235-37: This statement is probably too far-fetched. In addition, there areas ~17degN with high frequency trends in both figs. Most importantly, why is frequency the appropriate matric to derive such a conclusion? Why not compare trends of other parameters against SST trends?
3.2
Line 240: Are trends stat. significant?
Line 242-246: This separation into “distinct phases of variability” seems somewhat arbitrary. For instance, why the no-trend-period begins at 1992 and not 1996? Figure 5a shows from 1992 up to 1996 a clear warming trend (already present during the previous period).
Line 246: I have major concerns regarding the criterion used for detecting “unusually” cold (warm) years (i.e., colder (warmer) than the previous or following year) implies that SST variability is unusual.
Line 250-255: Try to follow clearly explained criteria for characterizing temporal periods instead of phrases “generally warm”, “generally high”, “particularly warm” mentioned in this paragraph. This comment applies for the entire manuscript (when characterizing years, sub-periods and trends).
Line 258-9: “In warm years, the SRS and the northern regions of the Gulfs of Suez and Aqaba had the highest SSTA and also the highest number of MHWDs (Fig. 7) ". This is not true, as Gulfs of Suez and Aqaba show minimum SSTA during the “warm” years, which is not the case for MHWDs in these regions.
Line 260: “2010 was a unique year…” How do we know this? To my understanding, comparing 2010 vs a warm-years average cannot support that 2010 shows an unusual behavior. I guess that other individual years may show similar spatial SSTA/MHWD patterns with 2010, or be very different from the warm-year average.
Line 263: I would rephrase: The inter-annual variations of MHWs frequency in the RS are shown in Fig. 9.
Line 272-4: “indicate that the rapid increase in SST in the RS has caused…” Maybe comment here (and/or later in discussion) on how your methodological choice for using a fixed climatological baseline affects the event detection (being dependent on both the warming trend of the RS and internal variability). Also: “…are expected to increase…” I would avoid this statement.
Line 279: “This high heat flux has caused...” Fig. S3 cannot support a causal link between Tair and heat flux.
Line 280: “Since the wind in these years was not sufficient to support the cooling of the SST (S4), this excess heat absorbed by the ocean from the atmosphere likely led to the formation of several MHW events in the above mentioned years” Is this an assumption or has it been examined also analyzing the heat flux components? Weak winds are potentially associated with reduced latent heat loss from the sea surface leading to increased net heat flux. Such an analysis should be presented if a conclusion is to be made on the role of low winds in causing/maintaining MHW conditions during specific periods. Importantly, S4 does not show a significant decrease in wind speed in the examined years/regions (2010 in the NRS and 1998 in the SRS).
3.3
Line 290: Not true for MHW frequency (Fig. 10b), please check
Line 294: “between 0.2-0.4” is for SSTA or freq?
Line 295-6: “…in the deepest part of the RS..” Not sure I understand the need to link with deep water formation areas in this sentence.
Line 297: were -> was
Line 298: in -> for , The later -> The latter
Line 305-6: “These are the same areas…” This is true for the western coast around 14-18degN, but areas do not coincide
Line 307-318 I suggest avoiding the term coincidence in this discussion. I have the impression that in some cases the actual relationship among the examined parameters (as shown from results) is not as strong as implied in certain parts of the text e.g. line 315.
3.4
Line 329-330 Please, rephrase for clarity
Line 337: “especially in the surface layers” Where do we see this?
Line 341: But at 110m the event did not take place at the peak of the surface event (as I see in Fig. 14)
Line 343: “indicating that…” I would remove this part as this is not clear from the data and is only based on a single event
Line 345: maybe say reaching locally 4deg?
Line 346: western coasts (not western Africa)
Line 347: “This increase in Tair led to…” Such a cause-effect relationship is not supported. See also my previous concern on Line 280
Line 355-6: Same concern as above. The interpretation of the increased net heat flux should be based on the bahavior of its components. The increase in Tair and total heat flux during the event cannot support that the event is “due to large amount of heat absorbed by the ocean”. There might be significantly suppressed heat loss (probably caused by the weakened winds) being responsible for the observed heat balance during the event. Reduced latent heat loss could also be associated with drier than usual air masses but all this should be investigated in order to support a causal link between the anomalously warm SST, heat flux and other physical parameters (e.g., wind).
Line 356: lake you mean lack?
4. Conclusions
Line 368: occur -> are
Line 370-2: See previous comments (eg Line 355-6)
Line 372: See previous comment (eg Line 260) on the exceptional character of 2010.
Line 374: “2010 breaks the trend with the highest values ever recorded in the NRS” This should be corrected as Fig. 5 shows that 2021 presents higher SSTA than 2010. It should be clear in terms of which characteristics the year 2010 shows an “exceptional” character (and on these grounds explain the motivation for this case study).
Line 380: “resulting in…” Please, see previous concerns on assuming causal links
Fig. 12 Is this plot based on mean basin (NRS) SST? Also, see previous comment on explaining the different categories based on threshold exceedance)
Fig. 14 Maps are for 2010 but this is not mentioned in caption. Also, the middle row is labeled as “event peak day” while caption says it corresponds to the average from 20 to 25 March. Please, revise accordingly
Fig. 13 Title of panel (a): extend -> extent
Supplement:
In S2, S3 and S4, captions state: “The red shaded areas represent the years with the highest MHW frequency in each basin.” Therefore, shouldn’t exactly the same periods be shaded in these 3 figures?
Citation: https://doi.org/10.5194/egusphere-2024-355-RC2
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