Department of Estuarine and Ocean Sciences
PhD Dissertation Defense
“Salinity Stratification and Lateral Variability in the Northern Indian Ocean: From Calm Diurnal Cycles to Cyclone-Induced Recovery”
By: Siddhant Kerhalkar
Advisor
Amit Tandon
Commonwealth Professor, Department of Estuarine and Ocean Sciences
UMass Dartmouth
Committee Members
Miles A Sundermeyer
Professor, Department of Estuarine and Ocean Sciences
UMass Dartmouth
Steven Lohrenz
Professor, Department of Estuarine and Ocean Sciences
UMass Dartmouth
J. Thomas Farrar
Senior Scientist, Physical Oceanography Department
Woods Hole Oceanographic Institution, Woods Hole, MA, USA
Kenneth Hughes
Senior Lecturer, School of Science
University of Waikato, Hamilton, New Zealand
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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 thesis investigates upper-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 thesis 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(1oC) over 100 km, in-situ observations capture finer-scale and more extreme variability. The upper ocean’s response to diurnal heating is inhomogeneous at 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 0.2oC 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(0.5oC) over 14-21 days. Higher daily mean net heat flux and turbid water conditions lead to stronger warming and density enhancement in salinity fronts, whereas lower heat flux may suppress warming, leading to density compensation. 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 intraseasonal 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 thesis highlight the dynamic nature of upper-ocean variability under contrasting meteorological conditions and offer physical insights that can guide improvements in MISO forecasting. |
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