Physical Controls of Coastal Hypoxia in the Indian Ocean Dipole

Abstract

The Indian Ocean Dipole (IOD) is an understudied physical phenomenon that involves a currently unpredictable reversal of temperature gradient across the Indian Ocean. In addition to increasing the risk of natural disasters such as tropical cyclones, the IOD influences the frequency and severity of coastal hypoxia events. Hypoxia refers to depleted oxygen levels operationally defined as an oxygen concentration below 60μmol/kg (Breitburg et al., 2018). Recurring and widespread coastal hypoxic events have been directly implicating the productivity of fisheries that sustain more than 2.49 billion livelihoods in the Indian Ocean littoral (NIC, 2013).

The overall mechanism of IOD propagation is well-established. However, existing literature lacks sufficient study of specific impacts of the IOD on different regions within the Indian Ocean basin. In our research, we analyzed the differing nature and extent of the influence of positive and negative IOD phases on a variety of locations across the Indian Ocean. We found that positive IOD phases increase the risk of coastal hypoxia in the Bay of Bengal while decreasing it in the Arabian Sea. The converse is true for a negative IOD year. Combining a theoretical understanding of the IOD, data from the biogeochemical MOM6 ocean model, and the Dipole Mode Index (DMI), an oxygen budget analysis formed the crux of our research, enabling the study of physical and biological mechanisms that influence coastal hypoxia in the Indian Ocean.