Winter Convective Mixing in the Northern Arabian Sea during Contrasting Monsoons

Abstract

Along-track Argo observations in the northern Arabian Sea during 2017–19 showed by far the most contrasting winter convective mixing; 2017–18 was characterized by less intense convective mixing resulting in a mixed layer depth of 110 m, while 2018–19 experienced strong and prolonged convective mixing with the mixed layer deepening to 150 m. The response of the mixed layer to contrasting atmospheric forcing and the associated formation of Arabian Sea High Salinity Water (ASHSW) in the northeastern Arabian Sea are studied using a combination of Argo float observations, gridded observations, a data assimilative general circulation model, and a series of 1D model simulations. The 1D model experiments show that the response of winter mixed layer to atmospheric forcing is not only influenced by winter surface buoyancy loss, but also by a preconditioned response to freshwater fluxes and associated buoyancy gain by the ocean during the summer that is preceding the following winter. A shallower and short-lived winter mixed layer occurred during 2017–18 following the exceptionally high precipitation over evaporation during the summer monsoon in 2017. The precipitation-induced salinity stratification (a salinity anomaly of −0.7 psu) during summer inhibited convective mixing in the following winter, resulting in a shallow winter mixed layer (103 m). Combined with weak buoyancy loss due to weaker surface heat loss in the northeastern Arabian Sea, this caused an early termination of the convective mixing (26 February 2018). In contrast, the winter convective mixing during 2018–19 was deeper (143 m) and long-lived. The 2018 summer, by comparison, was characterized by normal or below normal precipitation which generated a weakly stratified ocean preconditioned to winter mixing. This combined with colder and drier air from the landmass to the north with low specific humidity led to strong buoyancy loss, and resulted in prolonged winter convective mixing through 25 March 2019. Along-track Argo observations in the northern Arabian Sea during 2017–19 showed by far the most contrasting winter convective mixing; 2017–18 was characterized by less intense convective mixing resulting in a mixed layer depth of 110 m, while 2018–19 experienced strong and prolonged convective mixing with the mixed layer deepening to 150 m. The response of the mixed layer to contrasting atmospheric forcing and the associated formation of Arabian Sea High Salinity Water (ASHSW) in the northeastern Arabian Sea are studied using a combination of Argo float observations, gridded observations, a data assimilative general circulation model, and a series of 1D model simulations. The 1D model experiments show that the response of winter mixed layer to atmospheric forcing is not only influenced by winter surface buoyancy loss, but also by a preconditioned response to freshwater fluxes and associated buoyancy gain by the ocean during the summer that is preceding the following winter. A shallower and short-lived winter mixed layer occurred during 2017–18 following the exceptionally high precipitation over evaporation during the summer monsoon in 2017. The precipitation-induced salinity stratification (a salinity anomaly of −0.7 psu) during summer inhibited convective mixing in the following winter, resulting in a shallow winter mixed layer (103 m). Combined with weak buoyancy loss due to weaker surface heat loss in the northeastern Arabian Sea, this caused an early termination of the convective mixing (26 February 2018). In contrast, the winter convective mixing during 2018–19 was deeper (143 m) and long-lived. The 2018 summer, by comparison, was characterized by normal or below normal precipitation which generated a weakly stratified ocean preconditioned to winter mixing. This combined with colder and drier air from the landmass to the north with low specific humidity led to strong buoyancy loss, and resulted in prolonged winter convective mixing through 25 March 2019.