Dense water formation in the Eastern Mediterranean under global warming scenario
Abstract. Dense water formation in the Eastern Mediterranean (EMed) is essential in sustaining the Mediterranean overturning circulation. Changes in the sources of dense water in the EMed point to changes in the circulation and the water properties of the Mediterranean Sea. Here we examine with a regional climate system model the changes in the dense water formation in the EMed through the twenty-first century under the RCP8.5 emission scenario. Our results show a shift in the dominant source of Eastern Mediterranean Deep Water (EMDW) from the Adriatic Sea to the Aegean Sea at the first half of twenty-first century. The projected dense water formation reduces by 75 % for the Adriatic Sea, 84 % for the Aegean Sea and 83 % for the Levantine Sea by the end of the century. The reduction in the intensity of deep water formation is related to hydrographic changes of surface and intermediate water, that strengthen the vertical stratification hampering the vertical mixing and thus the convection. Those changes have an impact on the water that flows through the Sicilian Strait to the Western Mediterranean and therefore on the whole Mediterranean system.
Iván M. Parras-Berrocal et al.
Status: open (until 13 Apr 2023)
RC1: 'Comment on egusphere-2023-159', Anonymous Referee #1, 02 Mar 2023
- AC1: 'Reply on RC1', Iván Manuel Parras Berrocal, 30 Mar 2023 reply
Iván M. Parras-Berrocal et al.
ROM model data for Eastern Mediterranean Dense Water Formation https://doi.org/10.5281/zenodo.7594313
Iván M. Parras-Berrocal et al.
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Review of the article “Dense water formation in the Eastern Mediterranean under global warming scenario” by Parras-Berrocal et al.
In this paper the authors study the evolution of the dense water formation in the Eastern Mediterranean (EMED) along the 21st century, under RCP 8.5 greenhouse gases emission scenario, using the ROM Regional Climate Model (RCM). This RCM have been previously validated and used in several climate studies for the Mediterranean basin and sub-basins. They find a significant reduction of the deep water formation (DWF), between 75% and 85%, in the three regions were deep and intermediate convection take place (Adriatic, Aegean and Levantine basin) by the end of the century. The authors identify the increase in the water column stratification, due to the projected warming and salinization caused by the global warming, as the main factor driving this DWF reduction. They also predict a shift in the main Eastern Mediterranean Deep Water (EMDW) formation region, from the Adriatic to the Aegean Sea, similar to what occurred in the mid-90s with the so called Eastern Mediterranean Transient (EMT).
I find the paper very interesting. As the authors mention, this topic has not been studied in depth and it is very important to understand the expected changes in the Mediterranean thermohaline circulation as a consequence of the climate change. Although the use of a single model limits the robustness of the results, as the authors themselves point out, the results presented and the analysis of the mechanisms behind them are very relevant to the climate modeling community of the Mediterranean region. The manuscript is well written and organized, all the ideas are concisely and clearly stated, and well referenced. There are only a couple aspects that, in my opinion, need to be clarified before its publication in OS.
The first and more important one is the selection of the density of reference used to estimate the DWF rate in the model. As the authors themselves explain, the potential density they use in the model for the newly formed deep water in the Adriatic, Aegean and Levantine basins is slightly lower than the observed and reported in the literature. This is due to the lower salinity, and hence density, of the model respect to the observations in these regions (figure S1). Adjusting the reference density to the ‘model reality’ is a sound methodology, so this shouldn’t be a problem. It would be interesting, though, to identify these references (both for observations and model) in the profiles of figure S1. The selected reference corresponds with approximately 650 m depth in the historical period. However, the authors maintain the same reference densities in the future to compute the DWF rate evolution, which could have led to an underestimation. The profiles for the projections seem to show a general reduction of the density in the whole water column for the Adriatic and the Aegean regions (figures S2,3). This mean that the density of reference would correspond with a deeper layer, and that future deep water might be lighter than the present one. It is difficult to identify these differences in the figures, and very likely there will be no significant variations in the results, but in my opinion the authors should clarify this point in in the results or discussion sections. Would the DWF rate increase if a different density of reference is used for the future? Maybe including a third set of panels with the evolution of the density in figure 4 would be also of help.
My second concern is that the authors did not describe the limits of the regions of each sub-basin used to estimate the average profiles shown in the figures and the DWF rates. As they point out in the introduction, the areas of the Aegean and Adriatic where the deep convection take place are very specific. Are the profiles and DWF rates estimated in these specific areas or in the whole sub-basins? When computing the volume of deep water formed every year, do they account for all the volume of water with densities higher than the reference in a specific region, in the whole sub-basin or in the whole EMED (maybe considering the possible spread)? The region selected could also modify the results, so I think this should also be clarified in the MS. Maybe you could include the basins limits in figure 1 (the color scale is also missing).