Abstract.The budget of eddy kinetic energy (EKE) in the Red Sea, including the sources, redistributions and sink, is examined using a high-resolution eddy-resolving ocean circulation model. A pronounced seasonally varying EKE is identified, with its maximum intensity occurring in winter, and the strongest EKE is captured mainly in the central and northern basins within the upper 200 m. Eddies acquire kinetic energy from conversion of eddy available potential energy (EPE), from transfer of mean kinetic energy (MKE), and from direct generation due to time-varying (turbulent) wind stress, the first of which contributes predominantly to the majority of the EKE. The EPEto-EKE conversion occurs almost in the entire basin, while the MKE-to-EKE transfer appears mainly along the shelf boundary of the basin (200 m isobath) where high horizontal shear interacts with topography. The EKE generated by the turbulent wind stress is relatively small and limited to the southern basin. All these processes are intensified during winter, when the rate of energy conversion is about four to five times larger than that in summer.The EKE is redistributed by the vertical and horizontal divergence of energy flux and the advection of the mean flow. As a main sink of EKE, dissipation processes is ubiquitously found in the basin. The seasonal variability of these energy conversion terms can explain the significant seasonality of eddy activities in the Red Sea.

Adjoint sensitivity analysis is applied to a set of eddies in the Red Sea using a high‐resolution Massachusetts Institute of Technology general circulation model and its adjoint model. Previous studies have reported several eddy events in the Red Sea, namely, a dipole captured on 17 August 2001 in the southern Red Sea, a cyclonic eddy in November 2011 in the northern Red Sea, and an anticyclonic eddy in April 2010 in the central Red Sea. Sensitivity analysis is applied here to investigate the governing factors that control the intensity and evolution of these eddies. The eddies are first reproduced by running the Massachusetts Institute of Technology general circulation model forward and their sensitivities to external atmospheric forcing and previous model states are then computed using the adjoint model. In the experiments, (relative) surface vorticity (curl of horizontal velocity) is defined as the objective function. The contributions of forcings and model states are quantified and investigated. The sensitivities to external forcings are distinct in different eddy events. The dipole in the central Red Sea is dominantly sensitive to the cross‐basin eastward wind jet. The anticyclonic eddy in the central Red Sea is most sensitive to the along‐basin wind stress. The cyclonic eddy in the northern Red Sea is sensitive to the net heat flux and to surface elevation perturbations even from the remote southern Red Sea, which is attributed to the propagation of baroclinic Kelvin waves along the coast. Analysis of the sensitivity to model state variables suggests that these eddies are also modulated by the boundary currents and the temperature profile distributions.

Capsule Summary An integrated, high resolution, data-driven regional modeling system has been recently developed for the Red Sea region and is being used for research and various environmental applications.

Mesoscale eddies are a dominant feature of the Red Sea circulation, yet their three‐dimensional characteristics remain largely unexplored. This hinders our understanding 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 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.

Satellite observations recently revealed trains of internal solitary waves (ISWs) in the off‐shelf region between 16.0°N and 16.5°N in the southern Red Sea. The generation mechanism of these waves is not entirely clear, though, as the observed generation sites are far away (50 km) from the shelf break and tidal currents are considered relatively weak in the Red Sea. Upon closer examination of the tide properties in the Red Sea and the unique geometry of the basin, it is argued that the steep bathymetry and a relatively strong tidal current in the southern Red Sea provide favorable conditions for the generation of ISWs. To test this hypothesis and further explore the evolution of ISWs in the basin, 2‐D numerical simulations with the nonhydrostatic MIT general circulation model (MITgcm) were conducted. The results are consistent with the satellite observations in regard to the generation sites, peak amplitudes and the speeds of first‐mode ISWs. Moreover, our simulations suggest that the generation process of ISWs in the southern Red Sea is similar to the tide‐topography interaction mechanism seen in the South China Sea. Specifically, instead of ISWs arising in the immediate vicinity of the shelf break via a hydraulic lee wave mechanism, a broad, energetic internal tide is first generated, which subsequently travels away from the shelf break and eventually breaks down into ISWs. Sensitivity runs suggest that ISW generation may also be possible under summer stratification conditions, characterized by an intermediate water intrusion from the strait of Bab el Mandeb.

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The baroclinic tides in the Red Sea are simulated using a three‐dimensional, nonhydrostatic, high‐resolution Massachusetts Institute of Technology general circulation model. Various observations have been used to validate the simulation results. A good match between the model results and observations from five tidal gauges has been obtained. Tidal amplitude and phase data from 21 tidal stations present high correlation coefficients and low deviations with the model results. Comparisons between model and Oregon State University Tidal Inversion Software data suggest consistent results, with only small discrepancies at the locations of the amphidromic points. Tidal currents from four mooring observations are in good agreement with the simulation results, with discrepancies appearing in shallow areas and those with complex bottom topography. Based on the simulation results, the basic characteristics of baroclinic tides in the Red Sea are analyzed. The properties of barotropic tides, and distribution of the forcing function parameter, indicate that the baroclinic tides are generated mainly in four areas: the Bab‐el‐Mandeb (BAM) Strait, the southern Red Sea, the Gulf of Suez, and the Strait of Tiran. This is confirmed by the spatial distributions of baroclinic tidal kinetic energy and energy flux. The properties of the conversion rate from barotropic tides to baroclinic tides, and the divergence of baroclinic energy flux, further reveal quantitatively that the southern Red Sea features the most of the generated baroclinic energy. The majority of the baroclinic energy disappears within the four areas, either dissipating due to friction and bottom drag or converting back to barotropic energy.

This study investigates the vertical eddy structure, eddy-induced transport, and eddy kinetic energy (EKE) budget in the Arabian Sea (AS) using an eddy-resolving reanalysis product. The EKE intensifies during summer in the western AS. Anticyclonic eddies (AEs) and cyclonic eddies (CEs) present warm-fresh and cold-salty cores, respectively, with interleaved salinity structures. The eddy-induced swirl transport is larger in the western AS and tends to compensate for heat transport by the mean flow. Zonal drift transport by AEs and CEs offset each other, and meridional transport is generally weaker. Eddies also produce notable upward heat flux during summer in the western AS, where ageostrophic circulations are induced to maintain a turbulent thermal wind balance. Plausible mechanisms for EKE production are governed by baroclinic and barotropic instabilities, which are enhanced in summer in the western basin, where signals are quantitatively one order larger than the turbulent wind inputs.