Weakly-nonlinear Ekman pumping in the Sri Lanka Dome

In the Bay of Bengal, the Sri Lanka Dome (SLD) forms within a region of positive wind stress curl in the southwesterly wind shadow of Sri Lanka. Classic Ekman pumping ($w_{c}$) has been cited as a potential mechanism for introducing cool anomalies within the SLD and the Southwest Monsoon Current (SMC) system (often referred to as the Bay of Bengal cold pool). However, the region of maximum $w_{c}$ is positioned to the west of the SLD rather than being co-located with cool anomalies observed in the eastern SLD. Stern's 1965 theory adds a weakly-nonlinear correction ($w_{\zeta}$) to classic Ekman pumping; $w_{\zeta}$ depends on the wind stress ($\tau$) and lateral gradients in geostrophic vorticity ($\zeta$). Strong winds and vorticity gradients are often found within the SMC system, particularly along the east side of the SLD where $w_{\zeta}$ can exceed $w_{c}$. ERA5 wind data and AVISO geostrophic velocities are used to quantify the spatial distribution of weakly-nonlinear upwelling within the SLD, and the diagnosed upwelling regimes are then compared to cool signals from ERA5 SST. The internal structure, as measured from Argo floats, shows shallower mixed layer depths occur in regions of positive $w_{\zeta}$ relative to other regions with similar wind stress magnitude. A heat budget analysis shows that cool events often occur despite a net positive surface heat flux into the ocean, and suggests $w_{\zeta}$ impacts SST through vertical advection. $w_{\zeta}$ may also affect SST by shoaling the MLD and thus changing the timescale of response to net surface heat flux.