Abstract
Diurnal cycles of Sea Surface Temperature (SST) are important for ocean–atmosphere coupling. However, observations of their lateral variability, especially in freshwater-dominated regions and in presence of Diurnal Warm Layers (DWLs), remain limited. This study investigates the spatial differences in the diurnal SST amplitude (during DWL and non-DWL days) and subsurface DWL evolution in the Bay of Bengal using remote sensing, in-situ observations, and 1-D modeling. While satellite data reveal O(1 °C) differences in diurnal SST amplitude over 100 km, in-situ observations uncover finer-scale and more extreme variability, especially during DWL events. We observe that differences in diurnal SST amplitude over mesoscale and smaller lengths (< 100 km) are larger during DWL days (median: 0.2 °C, extreme: 1.4 °C) when compared to non-DWL days (median: 0.1 °C, extreme: 0.2 °C). Observations from drifters and complementary 1-D model simulations reveal that lateral differences in salinity-driven stratification leads to diurnal SST amplitude differences of about 0.2°C for shallow mixed layer scenarios (< 8 m). While stratification differences explain the median variability in diurnal SST amplitude, extreme differences in diurnal SST require additional contributions from spatial variations in surface forcing and optical properties. Observations also reveal that lateral differences in salinity stratification modify the DWL response, leading to O(10 m) differences in DWL depth, making it the same order as typical MLD scales in the Bay. These results highlight the critical role of small-scale differences in salinity-driven stratification (set by rainfall and mesoscale flow fields in the Bay) in causing diurnal SST and DWL response differences.