The interannual variability of the exchange flow between the Red Sea and the Gulf of Aden through the Bab-al-Mandeb strait is examined based on a high-resolution, nonhydrostatic hindcast model simulation covering a 19-year period (1995–2013), using the MITgcm (MIT general circulation model). The model is validated against moored profiles and along-strait cruise observations collected during the period from June 1995 to November 1996 and 19-year sea surface temperature satellite observations. The model well reproduces the properties of the water masses at the strait over a wide range of spatiotemporal scales, including the typical two- and three-layer seasonal patterns and the related intraseasonal-to-interannual cycles. The seasonality of the exchange flow is predominately determined by the time-varying surface winds, with a higher correlation over the Gulf of Aden, reflecting the importance of local Gulf of Aden processes for the exchanges at the strait. The alternation of the two seasonal patterns is driven by a combination of the buoyancy-driven mean circulation with the wind-induced transport. The onset/offset of the two patterns is estimated to take place one-to-two weeks after the respective monsoon-driven wind reversal. Model results indicate that the onset dates and durations of both patterns exhibit a considerable interannual variability. Additionally, the duration of the summer (winter) exchange pattern presents a significant increasing (decreasing) trend of ~1.45 day/year (~1.22 day/year) over the 19-year period. Significant interannual variabilities and trends are observed in terms of the total volume of water, salt mass, and stored heat of the exchanges. Budget analysis of these trends suggests that the duration of the two exchange patterns is more important in determining the interannual variability and the related trends than the intensity of the exchange, or the variations in mean salinity or temperature of the exchanged water masses.

As the Earth's temperature continues to rise, coral bleaching events become more frequent. Some of the most affected reef ecosystems are located in poorly-monitored waters, and thus, the extent of the damage is unknown. We propose the use of Marine Heatwaves (MHWs) as a new approach for detecting coral reef zones susceptible to bleaching, using the Red Sea as a model system. Red Sea corals are exceptionally heat-resistant, yet bleaching events have increased in frequency. By applying a strict definition of MHWs on >30-year satellite-derived sea surface temperature observations (1985-2015), we provide an atlas of MHW hotspots over the Red Sea coral reef zones, which includes all MHWs that caused major coral bleaching. We found that: 1) if tuned to a specific set of conditions, MHWs identify all areas where coral bleaching has previously been reported; 2) those conditions extended farther and occurred more often than bleaching was reported; and 3) an emergent pattern of extreme warming events is evident in the northern Red Sea (since 1998), a region until now thought to be a thermal refuge for corals. We argue that bleaching in the Red Sea may be vastly underrepresented. Additionally, although northern Red Sea corals exhibit remarkably high thermal resistance, the rapidly rising incidence of MHWs of high intensity indicates this region may not remain a thermal refuge much longer. As our regionally-tuned MHW algorithm was capable of isolating all extreme warming events that have led to documented coral bleaching in the Red Sea, we propose that this approach could be used to reveal bleaching-prone regions in other data-limited tropical regions. It may thus prove a highly valuable tool for policy-makers to optimise the sustainable management of coastal economic zones. This article is protected by copyright. All rights reserved.

Mesoscale eddies are a dominant feature of the Red Sea circulation, yet their three-dimensional characteristics remain largely unexplored. This hinders our understanding of about eddy-induced transport in the basin. This study analyzes 14-year outputs from a high-resolution eddy-resolving model to investigate the three-dimensional signature of the Red Sea eddies, their contribution to the air-sea flux, and the eddy-induced transport of heat and salt. Eddies are mostly active and energetic in the central and northern Red Sea. Their associated variability explains ∼8% of the total variance in the surface heat flux, and particularly, ∼39% in the salt flux. The asymmetric eddy structure and meridional gradient drive significant transport of heat and salt across the basin. A negative feedback mechanism is identified that relates the eddy intensity and the meridional steepness of the mixed layer depth in the basin.