Assessment of Red Sea temperatures in CMIP5 models for present and future climate

outputs from all the selected GCMs were not available at the time this study was launched, so whether the mechanisms behind the extreme events will change could not be assessed. Never- theless, recent studies have confirmed the results of [18] showing that the driver for MHW changes in most regions of the world is the change in the mean state, rather than changes in the intra-seasonal variability [7,38]. Therefore, here we focus on assessing how the changes in the mean state would modify the statistics of MHWs in the Red Sea and the Gulf of Aden. To analyse how the increase of the mean temperature would affect the MHW characteristics in the Red Sea, we generate a high frequency projection of Red Sea SST for the period 2065– 2100 by combining the observed present day intra-seasonal variability with the lower frequency components from the GCM outputs for the end of the century. Namely, we first compute daily maps of satellite SST anomalies for the present climate (1982–2017) removing the monthly aver- age from the satellite data. Then, we add to these anomalies the monthly SST maps from each one of the selected GCMs (2065–2100). In other words, we keep the observed intra-seasonal variability while imposing the low frequency variability projected by the GCMs (see S5 and S6 Figs). Then, we compare the statistics of MHWs for that period (ensemble average) with the sta- tistics of present day MHWs. It is worth noting that the intra-seasonal variability of the SST has a STD of 0.85˚C (see Skill Assessment Formulation in the SI), which is lower than the projected

increase in the mean temperature. This, although not conclusive, reinforces the idea that changes in the MHWs will probably be dominated by the changes in the mean temperature. In order to characterize MHWs at a particular location we follow the approach of [24], which is an adaptation for the marine environment of what is typically done to characterize atmospheric heat waves. Namely, a MHW is defined as a period of at least 5 consecutive days with temperatures above a certain percentile, computed from, at least, a 30-year period in the present climate. [24] proposes the use of the 90th percentile, while other studies use the 99th percentile [18,22,23]. Then, for the evaluation of future MHWs, the projected heat waves are defined following the same criterion, but using the threshold computed for the present climate. It must be noticed that the choice of any of those thresholds is somehow subjective and the MHWs characteristics could change depending on it. Therefore, we have tested the MWHs characteristics using different thresholds (90th,95th,98th and 99th), to assess the differences of the MHWs in the future with respect to present conditions. The results are qualitatively very similar (S7 Fig), so they will not be discussed here in detail. The results shown here are based on the definition of [28], which is an adaptation of the previous studies tuned to better repre- sent the conditions that induce coral bleaching in the Red Sea. In particular, we consider a MHW as a period of at least 7 consecutive days with temperatures above the 95th percentile of

the satellite SST computed for the period 1982–2017 at a particular location. This definition has been used to characterize the major events of coral bleaching in 1998, 2007, 2010, 2012 and 2015 [28]. The 95th percentile of present SST shows significant regional differences. In the Outer Region, the values are minimum (30˚C), while it increases to 31.5˚C in the western part, in the Gulf of Aden, less affected by the open sea dynamics and lateral mixing. The 95th percentile increases to 33˚C in the Southern Red Sea (18˚N) and decreases to 30˚C in the northern Red Sea (S8A Fig). With this threshold, the number of MHWs per year ranges between 0.6 and 1 events/year, with higher values in the northern part of the Red Sea and the Outer Region (S8B Fig). The mean intensity of the MHWs, defined as the average excess over the chosen threshold during the heat wave duration [24], tends to have larger values in the Outer Region and the Gulf of Aden, where intensities exceed 0.7˚C. In the rest of the domain, MHWs intensities are around 0.5˚C (S8C Fig). Our results project a mean increase of the summer SST of approximately 3.3 + 0.6˚C (1.6 ±> 0.3˚C) at the end of the 21st century under the RCP8.5 (RCP4.5) scenario. This increase PLOS ONE Assessment of Red Sea temperatures in CMIP5 models for present and future climate PLOS ONE | https://doi.org/10.1371/journal.pone.0255505 July 30, 2021 16 / 26