Date of Award

Fall 2020

Document Type

Open Access Dissertation

Department

Marine Science

First Advisor

Subrahmanyam Bulusu

Abstract

The Indian Ocean and the monsoon system are dynamically complex. In the Bay of Bengal (BoB) and southeastern Arabian Sea (SEAS), surface circulation is strongly influenced by the monsoons and notable local eddying that modulates the East India Coastal Current (EICC). In this study, the role of freshwater transported from the BoB into the SEAS in determining both the timing of monsoon onset and the strength of the ensuing monsoon is examined. It is found that the long-term decrease in moisture flux from the sea surface and freshwater transport into the SEAS, along with a rise in upper ocean heat content (OHC) over a 15-year duration after 1994, contributed to the lack of strong monsoons in recent years; the prevailing interannual and interdecadal variability in these parameters associated with the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) events favored weaker and normal monsoons after 1994 until the strong monsoon of 2019. Both ENSO and the IOD are also found to modulate coastal Kelvin wave and Rossby wave propagation in the BoB, which impacts the mesoscale eddy field in the EICC region and further influences the precipitation associated with these eddies.

Intraseasonal oscillations (ISOs) significantly contribute to and modulate Indian monsoon rainfall. Satellite observations are used to monitor the 30-90-day, 10-20-day, and 3-7-day ISOs, and how they influence local atmosphere and ocean dynamics in the BoB. This research has shown the importance of using blended satellite altimetric observations and satellite-derived salinity to monitor ISOs in the BoB Indian Ocean. A multivariate analysis of the atmospheric 10-20-day mode is further conducted to both assess its overall characteristics in multiple oceanic variables and analyze how this atmospheric signal interacts with the underlying ocean. Temperature is notably found to have a strong biweekly signal in the central and southern BoB down to 250 m, with the mixed layer temperature showing a marked decrease after the 10-20-day precipitation maximum. This study has shown the 10-20-day mode in the atmosphere to heavily influence ocean dynamics, emphasizing the importance of continued modeling and satellite observation efforts for the monitoring and forecasting of these events.

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