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Description: Diurnal Warm Layers (DWLs) play an important role in coupling the atmosphere and the ocean. However, observations of their lateral variability, especially in freshwater-dominated regions, remain limited. This study investigates lateral DWL gradients across diverse sub-basin length scales in the Bay of Bengal, employing a combination of remote sensing and in-situ observations. Comparison of surface DWL signatures over mesoscale lengths in 2015 exhibit O(1 C) differences over 100 km. Subsequent assessment using drifters from a July 2019 field campaign reveal differences of 0.4 C over 20 km and nearly 1.6 C over 100 km, highlighting lateral inhomogeneity in surface signatures of DWLs over mesoscale and smaller lengths. Two drifting meteorological buoys with profiling instruments deployed in the campaign, show lateral differences in subsurface DWL evolution over 30 km. Lateral gradients in background stratification and wind speeds lead to variations in observed DWL evolution, with a nearly 28 % difference in net heat content gained during the DWL evolution over the top 15 m. Equivalent difference in subsurface turbulent heat fluxes calculated using 1-D modeling are O(100 W/m2), driven mainly by lateral variability in stratification. Furthermore, differences in background processes, namely freshwater and warm-water advection result in O(10 m) differences in DWL depth. In summary, significant lateral variability in DWLs and hence upper-ocean heat content occurs over mesoscale and smaller length scales, especially in regions with high salinity gradients like the Bay. These differences can potentially influence air-sea interaction and must be parameterized for improved atmosphere-ocean coupling in models.

Location: SMAST East 101 and 102 and via Zoom