Observing and Modeling the variability of DWLs during the summer Monsoon in the Northern Indian Ocean

Diurnal Warm Layers (DWLs) play an important role in coupling the atmosphere and the ocean, but their observations in the freshwater dominated Northern Indian Ocean in summer Monsoons are rare. This study focuses on the following aspects of DWLs observed during a 5-day suppressed atmospheric convection phase of the southwest monsoon season in 2019: (i) DWL observations using innovative drifting flux profilers to simultaneously measure high resolution shear and stratification as well as the surface meteorological forcing variables to compute air-sea fluxes (ii) Observed spatial gradients of SST over 1-100 km scales and (iii) Modeling using the popular one-dimensional models increasing in complexity. These observations show regions of marginal shear instability at the DWL base in agreement with previous studies in the tropical Pacific. The commonly used constant stratification assumption within the DWL (e.g. Fairall et al. 1996) breaks down in scenarios with weaker winds and salinity-driven stratification. The vertical structure of DWLs is therefore explored using k-e turbulence closure scheme in General Ocean Turbulence Model (GOTM) framework. Insights from model-observation comparisons show that for days with similar wind speeds, the DWL response can vary based on whether warm water or freshwater advection plays a role. Notably, warm water advection leads to deeper DWLs, whereas the freshwater advection traps the DWL to shallower depths. Further, spatial differences of O(1 C) in diurnal cycles of SST are observed over O (1-100 km), showing remarkable lateral inhomogeneity in the evolution of DWLs.