Submesoscale temperature inversions in the Bay of Bengal during the winter monsoon

This study explores the formation of temperature inversions in the central Bay of Bengal (BoB) during winter using submesoscale-permitting simulations. Our simulations are motivated by a high-resolution frontal radiator survey in the BoB during the winter of 2013 which revealed numerous submesoscale temperature inversions in the top 40 m. The inversions have a lateral scale of O(1-10 km) and are O(10 m) thick, with the temperature increase across the inversion comparable to the lateral contrast in temperature near the surface. To study potential formation mechanisms for such inversions, we perform idealized three-dimensional simulations in a channel configuration with zonal, meridional and vertical dimensions of 72 km, 144 km and 400 m. The lateral resolution is 250 m. The initial condition is a jet in geostrophic balance with a density field whose lateral and vertical gradients are controlled by salinity in the top 50 m and by temperature at greater depths. After an initial phase of unforced evolution (3.4 inertial periods) to smooth any initial kinks in the density field, we turn on weak downfront forcing (4 inertial periods) for another 4 inertial periods. The downfront winds generate a submesoscale eddy field and pools of near-surface water with low Ertel potential vorticity (PV). We then turn off the winds to allow for subduction of the near-surface waters, resulting in the formation of submesoscale temperature inversions. The inversions occur in anti-cyclonic regions with anomalously low PV. The lateral and vertical scales of the inversions are in agreement with observations. We also contrast our model results with the output from a state-of-the-art coupled global simulation, gridded to a 10 km lateral resolution. Using this comparison, we examine to what extent the features in our idealized simulations are reproduced in the coupled simulations.