Annual maximum sea surface temperature across the Red Sea (1982-2015)

Ocean warming is a major consequence of climate change, with the surface of the ocean having warmed by 0.11 °C/decade over the last 50 years and is estimated to continue to warm by an additional 0.6-2.0 °C before the end of the century. However, there is considerable variability in the rates experienced by different ocean regions, so understanding regional trends is important to inform on possible stresses for marine organisms, particularly in warm seas where organisms may be already operating in the high end of their thermal tolerance. Although the Red Sea is one of the warmest ecosystems on earth, its historical warming trends and thermal evolution remain largely understudied. We characterized the Red Sea's thermal regimes at the basin scale, with a focus on the spatial distribution and changes over time of sea surface temperature maxima, using remotely sensed sea surface temperature data from 1982-2015. The overall rate of warming for the Red Sea is 0.17 ± 0.07 °C/decade, while the northern Red Sea is warming between 0.40 and 0.45 °C/decade, all exceeding the global rate. Our findings show that the Red Sea is fast warming, which may in the future challenge its organisms and communities

Spatial correlations between Chl-a, SST, wind regime, and IMI in the southern Red Sea during summer.

<p><b><i>a)</i></b> Chl-a and wind regime. <b><i>b)</i></b> Chl-a and IMI. <b><i>c)</i></b> Chl-a and SST. <b><i>d)</i></b> SST and wind regime. The wind regime is defined as the averaged wind speed in the southern Red Sea (below 17°N) and the western Gulf of Aden (west of 46°E). The critical correlation value for a p<0.05 significance level with 11 degrees of freedom is <i>r</i> = 0.55.</p

<i>In situ</i> nutrient an velocity observations and model outputs of temperature and salinity

<p><b><i>a)</i></b><i>In situ</i> measurements of nutrient concentrations and mean velocity vectors at ~66m depth during September 2011; reprinted and adapted with permission from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0168440#pone.0168440.ref018" target="_blank">18</a>]. <b><i>b)</i></b> <i>and</i> <b><i>c)</i></b> September climatological salinity and temperature at 65m depth, calculated from the MITgcm circulation model.</p

Statistical models of summer Chl-a averages over the southern Red Sea.

<p><b><i>a)</i></b> Linear regression model of weekly Chl-a using IMI with a one-week lag as predictor. <b><i>b)</i></b> Linear regression model of weekly Chl-a using the wind speed averaged in the Gulf of Aden (west of 46°E) as predictor. <b><i>c)</i></b> Multivariate linear regression surface (white plane) of weekly Chl-a with the final predictors of wind speed and IMI. Positive and negative model errors on the data are represented with red and blue lines respectively. The solid black circles represent the datapoints.</p

Vertical profiles of model outputs and <i>in situ</i> ship-borne observations, depicting the summer influx of colder, fresher, and nutrient-rich intermediate water masses (GAIW) into the southern Red Sea.

<p><b><i>a)</i></b> and <b><i>b)</i></b> Profiles of temperature and salinity climatologies during winter (Jan-Feb), obtained from the MITgcm circulation model, and averaged over the transect indicated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0168440#pone.0168440.g001" target="_blank">Fig 1A</a>. <b><i>c)</i></b> and <b><i>d)</i></b> Similar profiles for the summer period (Jul-Aug). <b><i>e)</i></b> and <b><i>f)</i></b> <i>In situ</i> measurements of nutrient concentrations and salinity during the summer 2011, aggregated in cells of one-degree width and centered at approximate depths of 5, 10, 25, 50, 75, 100 and 200 m (marked by black diamonds on the y-axis).</p

Southern Red Sea simulated minimum salinity between 0m and 100m depth.

<p><b><i>a)</i></b> September climatology of the lowest salinity values observed between 0m and 100m depth—calculated from the MITgcm circulation model, and <b><i>b)</i></b> the depth at which these minimum salinity values occur.</p

Spatiotemporal distribution of remotely-sensed OC-CCI Chlorophyll in the Southern Red Sea (2000–2012).

<p><b><i>a)</i></b> Monthly climatology of Chl-a averaged in the southern Red Sea (blue-box area average, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0168440#pone.0168440.g001" target="_blank">Fig 1A</a>). The red circular datapoints represent the individual months (2000–2012), while the grey shadow depicts the 90% confidence intervals of the climatology. <b><i>b)</i></b> Spatial distribution of Chl-a during summer; calculated based on monthly climatologies (May to August). <b><i>c)</i></b> Ratio of July to annual Chl-a observations, highlighting the higher concentrations of Chl-a in the southern Red Sea during July; computed from monthly composites between 2000 and 2012.</p

Summer SST climatology in the southern Red Sea, displaying evidences of cooling along the GAIW intrusion pathway.

<p><b><i>a)</i></b> Average SST difference between June and August (2000 to 2012), depicting a cooling of surface waters on the eastern shore of the southern Red Sea. <b><i>b)</i></b> SST monthly climatologies (solid lines) and individual monthly datapoints (solid circles), from 2000 to 2012 for the following regions (displayed in panel a): the rectangles represent areas in Farasan archipelago (red), central southern Red Sea (blue), and Dahlac archipelago (green).</p

Schematic representation of the Red Sea, and percentage data coverage of satellite Chl-a data.

<p><b><i>a)</i></b> Geographical locations in the Red Sea. Delimitations of the southern Red Sea (south of 17°N, blue dashed rectangle), and the western Gulf of Aden (west of 46°E, green dashed rectangle). The elongated pink rectangle represents the area over which the model outputs were averaged to obtain the vertical profiles (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0168440#pone.0168440.g002" target="_blank">Fig 2</a>). <b><i>b)</i></b> MODIS percentage data coverage in monthly composites for July (2002–2012). <b><i>c)</i></b> Similar to <b><i>b</i></b>, but for OC-CCI merged sensor product data.</p