Distinct Influence of Air–Sea Interactions Mediated by Mesoscale Sea Surface Temperature and Surface Current in the Arabian Sea

Abstract

During the southwest monsoons, the Arabian Sea (AS) develops highly energetic mesoscale variability associated with the Somali Current (SC), Great Whirl (GW), and cold filaments (CF). The resultant highamplitude anomalies and gradients of sea surface temperature (SST) and surface currents modify the wind stress, triggering the so-called mesoscale coupled feedbacks. This study uses a high-resolution regional coupled model with a novel coupling procedure that separates spatial scales of the air-sea coupling to show that SST and surface currents are coupled to the atmosphere at distinct spatial scales, exerting distinct dynamic influences. The effect of mesoscale SST-wind interaction is manifested most strongly in wind work and Ekman pumping over the GW, primarily affecting the position of GW and the separation latitude of the SC. If this effect is suppressed, enhanced wind work and a weakened Ekman pumping dipole cause the GW to extend northeastward, delaying the SC separation by 18. Current-wind interaction, in contrast, is related to the amount of wind energy input. When it is suppressed, especially as a result of background-scale currents, depth-integrated kinetic energy, both the mean and eddy, is significantly enhanced. Ekman pumping velocity over the GW is overly negative because of a lack of vorticity that offsets the wind stress curl, further invigorating the GW. Moreover, significant changes in time-mean SST and evaporation are generated in response to the current-wind interaction, accompanied by a noticeable southward shift in the Findlater Jet. The significant increase in moisture transport in the central AS implies that air-sea interaction mediated by the surface current is a potentially important process for simulation and prediction of the monsoon rainfall.

Recipes for How to Force Oceanic Model Dynamics

“…As in Seo () and (Renault, Masson, et al, ), the mesoscale anomalies are isolated from the large‐scale signal by using a spatial filter. The following filter description is derived from (Renault, Masson, et al, ) with minor modifications.…”

“…Another simulation has been carried out to briefly assess the mesoscale TFB impact on the mean and mesoscale oceanic circulations: In , the setup is the same than for C CFB except that the SST sent to WRF is spatially filtered (see description of the filter in Section ) in order to smooth out the thermal mesoscale structures. thus contains the CFB and the large‐scale TFB (as in, e.g., Seo, ). This simulation is used to briefly assess the mesoscale TFB impact on the mean and mesoscale oceanic circulations. …”

“…Small et al () provides a review of the different processes involved. The mesoscale TFB‐induced stress curl anomalies induce Ekman pumping that can have an influence on eddy propagation but not on eddy magnitude (Seo et al, ; Seo, ). Also, Ma et al () suggest that the mesoscale TFB, by causing wind velocity and turbulent heat flux anomalies, could regulate the Western Boundary Currents; however, in that study, a large spatial filter (with a spatial cutoff of more than 1000 km) is used.…”

Coupled ocean–atmosphere modeling and predictions

“…A new approach that separates the spatial scale of ocean-atmosphere coupling has been developed by comparing a fully coupled ocean-atmosphere model to the same model with an online 2-D spatial smoother that removes mesoscale SST fields seen by the atmosphere (Putrasahan et al 2013a,b;Seo et al 2016;Seo, 2017). The implementation of an interactive 2-D smoother allows large-scale SST coupling to be retained while removing the mesoscale eddy impact on the atmospheric boundary layer (ABL).…”

Influence of Mesoscale Features on Mixed Layer Dynamics in the Arabian Sea

“…The local eddy field strengthens during this time to reestablish stratification (Boccaletti et al, ). Additionally, the Somali Current advects waters with low potential vorticity northward, strengthening the lateral gradient in relative vorticity, generating barotropic instability, and providing another generation mechanism for the Great Whirl (Seo, ). For these reasons, the highest intraseasonal variability of MLDs in the Arabian Sea is during the southwest monsoon season.…”