Sensitivity Studies of the Red Sea Eddies Using Adjoint Method

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. Plain Language Summary This is the first quantitative sensitivity studies on the mechanisms of eddies in the Red Sea using adjoint method. The comprehensive discussion reveals the governing factors that control the intensity and evolution of these eddies.