Graduate Thesis Or Dissertation
 

Wind-current Interactions of the Sri Lanka Dome and Tropical Indian Ocean

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  • The southwest monsoon, which may be idealized as the northward movement of the intertropical convergence zone in the Indian Ocean associated with differential heating of land and ocean, brings a seasonal reversal of currents and winds in the Bay of Bengal and the Arabian Sea. Atmospheric convection from monsoon activity in the Bay of Bengal drives precipitation over the Indian Subcontinent, which is crucial to agriculture and economic prosperity in the region. However, regional complex air-sea interaction, which is thought to regulate delivery of precipitation, is not fully understood. Ocean sea surface temperature is critical to atmospheric deep convection, but winds can modify the temperature through enhanced mixing, Ekman transport and pumping, and control on turbulent air-sea fluxes (latent and sensible heat). We examine how winds interact with currents during the southwest monsoon, and explore how this interaction feeds back on temperature and salinity evolution within the mixed layer. The Sri Lanka Dome (SLD) is an upwelling recirculation feature found in the Southwest Monsoon Current that may influence air-sea interaction in the Bay of Bengal. To quantify variability and understand the dynamics of the SLD, the twenty-three-year time series of AVISO satellite absolute dynamic topography (ADT) is used to track and measure the intensity of the SLD. The SLD shows both a strong seasonal cycle and considerable interannual variability. The dome typically forms in May to the east of Sri Lanka, intensifies through July and August, and migrates to the north and then east before dissipating in September off the coast of northeast Sri Lanka. SLD formation and dissipation, migration path, and magnitude display considerable interannual variability, with eastward movement significantly correlated with the strength of the Indian Ocean Dipole. We also quantify the SLD internal structure using the Argo float record. The SLD is associated with an elevated (15-40 m) pycnocline. The sea surface temperature response is complex as the subsurface temperature structure is not necessarily monotonic with height. We also address forcing by remote and local winds and their relation to the SLD. Strong wind stress curl from the wind jet south of Sri Lanka explains variability in Sri Lanka Dome during the first two months after formation until the SLD migrates north out of the wind jet's influence. Cool SST signals occur intermittently within the upwelling Sri Lanka Dome (SLD) but are strong enough to impact atmospheric processes during the Southwest Monsoon. Several SST cool event temperatures fall below 27.5°C, potentially disrupting organized atmospheric deep convection. Cool SST events are brief (1 week) compared to the seasonal 1-4 month lifespan of the SLD. And, while cool anomalies are more likely to occur during periods of strong upwelling within the Sri Lanka Dome, strong periods of upwelling often occur with no cooling of SST. Cool SST signals are often displaced southeastward of the ADT low. 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. However, the region of maximum w_c is west of the SLD rather than co-located with cool anomalies that are observed along its southeastern flank. 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 exist within the SMC system along the east side of the SLD. ERA5 wind data and AVISO geostrophic velocities are used to quantify the spatial distribution of weakly nonlinear upwelling within the SLD. We estimate that w_zeta can exceed w_c. The diagnosed upwelling regimes are then compared to cool signals from ERA5 SST. The internal structure (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. ECCO ocean state estimates monthly heat budget analysis shows that cool events often occur despite a net positive surface heat flux into the ocean, and indicates that w_zeta impacts SST through vertical advection and by shoaling the MLD, which changes the timescale of response to surface heat flux. Vertical advection and enhanced diffusivity are primary controls on the total heat tendency in the region. The Indian Ocean Dipole (IOD) is associated with increased east-west temperature gradients and basin-scale changes in the winds over the tropical Indian Ocean. Changes to wind forcing may impact the response of the surface mixed layer during the southwest monsoon. During the onset of the monsoon, positive IOD events strengthen the wind stress in the northern Bay of Bengal and the Arabian Sea, but decrease curl along 10°N. We use the ECCO ocean state estimates to quantify characteristics of the mixed layer during strong IOD events and contrast positive and negative modes. Heat and salt budgets show how wind forcing impacts the mixed layer under different dipole regimes (both by Ekman transport and vertical mixing). Although diffusion has the strongest seasonal impact on the heat tendency of the mixed layer, diffusion contributes less (relative to advection) to year-to-year differences in heat tendency between phases of the IOD. The largest contribution of diffusion to year-to-year differences occurs during the onset of the monsoon, which can vary in timing by several weeks. The salt tendency in the mixed layer is controlled by advection except in the Bay of Bengal. There, the diffusive salt tendency is comparable to both climatological salt tendency and variability during IOD events.
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  • Kerstin Cullen was supported through a NASA Earth and Space Science Graduate Fellowship. We also acknowledge the support of the Office of Naval Research (N00014-14-10236, N00014-15-12634, and N00014-15-12634) through the Air-Sea Interaction Research, Monsoon Intraseaonal Oscillations Research Initiatives, and Young Investigator programs.
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  • Pending Publication
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  • 2020-09-21 to 2021-04-22

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