Sensitivity Studies of the Red Sea Eddies Using Adjoint Method

Abstract

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.

Impact of Atmospheric and Model Physics Perturbations On a High-Resolution Ensemble Data Assimilation System of the Red Sea

“…The outputs of this run have been extensively validated in previous studies (e.g. Gittings et al, 2018;Zhan et al, 2018 Observation error variances, which comprises errors due to instruments and unresolved scales and processes and interpolation, is an important element of the data assimilation system (e.g. Hoteit et al, 2010;Sivareddy et al, 2019).…”

“…The upper panels, corresponding to 12 th February, 2011, showcase the cyclonic eddy of the northern RS (e.g. Yao et al, 2014a), a permanent feature that is mainly influenced by thermohaline forcing (Zhan et al, 2018). The middle panels, showing SSH in the southern RS on 11 th August, 2011, correspond to the regime during which a dipole eddy feature, generated under the action of strong cross-basin Tokar winds, is prevalent (e.g.…”

“…The middle panels, showing SSH in the southern RS on 11 th August, 2011, correspond to the regime during which a dipole eddy feature, generated under the action of strong cross-basin Tokar winds, is prevalent (e.g. Zhai and Bower, 2013;Zhan et al, 2018).…”

Three‐Dimensional Signature of the Red Sea Eddies and Eddy‐Induced Transport

“…The model is forced by surface wind, air temperature, specific humidity, precipitation, and downward shortwave and longwave radiation of a 10-km Weather Research Forecast product (Viswanadhapalli et al, 2017) downscaled from ERA-Interim products of the European Centre for Medium-Range Weather Forecasts (Dee et al, 2011). In addition, the model outputs were validated against different observations and have been used in various studies, to name a few, the eddy energy budget , the adjoint sensitivity analysis of eddies (Zhan et al, 2018), and phytoplankton phenology in the southern (Dreano et al, 2016) and northern Red Sea (Gittings et al, 2018). This model is developed as an updated version of the one implemented by and Yao, Hoteit, Pratt, Bower, Zhai, et al (2014) to investigate the general and overturning circulation in the Red Sea.…”

“…These have been captured in various independent observations, including remotely sensed sea surface height (SSH) data (Bower & Farrar, 2015;Raitsos et al, 2017;Zhan et al, 2014), sea surface temperature (SST) data (Papadopoulos et al, 2015), chlorophyll-a data (Raitsos et al, 2013), synthetic aperture radar imagery (Karimova & Gade, 2014), and in situ observations (Sofianos & Johns, 2007;Zarokanellos et al, 2017;Zhai & Bower, 2013). Advances in high-performance computing resources now enable conducting high-resolution eddy-resolving numerical simulation of the Red Sea circulation with sufficient spatial and temporal resolution to investigate the three-dimensional characteristics of the Red Sea eddies (Zhan et al, , 2018. Nevertheless, the observations are sporadic and sparse in space and time.…”