Abstract
Monsoons over the Indian subcontinent deliver copious seasonal rainfall from June to November, yet their inherent Monsoon Intra-seasonal Oscillations (MISOs) remain poorly predicted. Errors in MISO predictions significantly affects regional and global weather forecasts. Improving MISO predictability requires a deeper understanding of ocean-atmosphere coupling and improved representation of upper-ocean stratification within the Northern Indian Ocean (NIO), particularly at mesoscale and submesoscale length scales. This dissertation investigate supper-ocean variability at these scales under two key meteorological regimes preceding MISO onset: calm, clear-sky conditions and tropical cyclone events. Chapters 2 and 3 of this dissertation examine the spatial inhomogeneity in sea surface temperature (SST) evolution over diurnal and intra-seasonal timescales, respectively. Both chapters focus on how unique freshwater-driven salinity stratification contributes to this variability, utilizing remote sensing, in-situ observations, and 1-D modeling. Chapter 2 reveals that while satellites show diurnal SST amplitude differences of O(1°C)over 100 km, in-situ observations capture finer-scale and more extreme variability. The upper-ocean’s response to diurnal heating is inhomogeneous over mesoscale and smaller lengths(< 100 km), particularly on days with Diurnal Warm Layer (DWL) presence compared to non-DWL days. Observations and complementary 1-D model simulations demonstrate that lateral differences in salinity stratification can account for up to 0.2°C differences in diurnal SST magnitudes for shallow mixed layer scenarios (< 8 m). Salinity stratification also modifies vertical DWL evolution at scales comparable to initial mixed layer depth. Chapter 3 extends this analysis to intra-seasonal timescales, demonstrating a nuanced role for salinity stratification in modulating spatial variability in SST evolution. Depending on the surface forcing and water clarity, enhanced salinity stratification can either increase or decrease surface warming, thereby driving spatial differences in SST of O(0.5°C) over 14-21 days. Higher daily mean net heat flux and turbid water conditions lead to stronger warming and density enhancement in salinity fronts. Conversely, warming is suppressed incases of lower heat flux, leading to partial density compensation, where temperature and salinity changes offset each other’s effects on density and thus reduce the gradient. An analytical threshold daily mean heat flux (𝑄𝑐𝑟𝑜𝑠𝑠) is derived to predict when stratification leads to stronger warming. This threshold typically falls between 103-130 Wm−2 in tropical open-ocean contexts, varying with initial and forcing conditions. These findings highlight a crucial interplay between salinity stratification, surface fluxes, and bio-optical feedbacks in shaping intra seasonal SST evolution and its spatial variability. Chapter 4 presents rare in-situ observations of the upper ocean following Cyclone Biparjoy in the NIO. The post-cyclone wake, nearly 30 km wide, exhibited asymmetric buoyancy gradients and vertical structures of temperature, salinity, and velocity at its edges. This asymmetry reflects the influence of submesoscale processes like Ekman Buoyancy Fluxes and Mixed Layer Eddies, with downfront (upfront) orientation relative to southwesterly monsoon winds at the edges of the wake. These unique observations highlight how interactions between monsoon winds and underlying three-dimensional submesoscale processes, in conjunction with surface heating, accelerate the recovery of a slow-moving cyclone wake. Collectively, the findings from this dissertation highlight the dynamic nature of upper ocean variability under contrasting meteorological conditions and offer physical insights that can guide improvements in MISO forecasting.