The Oxygen Minimum Zones (OMZs) in the Indian Ocean

Abstract. The Oxygen Minimum Zones (OMZs) in the northern Indian Ocean (i.e., the Arabian Sea and the Bay of Bengal) are among the most intense OMZs in the world’s oceans. While there is no clear evidence of a significant change in the Bay of Bengal (BoB) OMZ, the Arabian Sea (AS) OMZ followed the global trend and expanded in the last decades until 2013. Since then, however, this trend has reversed, and the AS OMZ seems to have shrunk. The stability of the BoB OMZ as well as the expansion and shrinkage of the AS OMZ in response to global warming is poorly understood. In this study we redefined the water masses and employed an extended Optimum Multiparameter (eOMP) Analysis to investigate changes in the oxygen supply due to mixing and biological oxygen consumption dynamics in these OMZs based on empirical field data from the Global Ocean Data Analysis Project version 2 (GLODAPv2) and a research cruise conducted with a German research vessel Sonne in 2024. Our findings reveal in line with previous studies a reversal in the expansion trend of the AS OMZ but also a shrinkage of the BoB OMZ between 1995 and 2016. In both regions this is due to an increased northward influx of oxygen-rich water masses from southern Indian Ocean, combined with a reduced contribution from relatively oxygen-poor local and equatorial water masses. However, we also observed that increased physical oxygen supply was accompanied by an increased biological oxygen consumption. These changes are likely linked to the slowdown of the global thermohaline circulation in the Indian Ocean. The slowdown is accompanied by a reduced inflow of the Indonesian Throughflow Water into the Indian Ocean and a lower output of Indian Ocean waters via the Agulhas Current/Leakage (at 32° S) into the Atlantic Ocean. A resulting increase in the residence time of water masses in the Indian Ocean is consistent with the detected biological oxygen consumption while the weaker zonal circulation seems have favored the meridional circulation which carried water from the southern Indian Ocean northwards. This implies a coupling between the OMZ in the Indian Ocean and climate change via the effect of the latter on the global thermohaline circulation as also seen in palaeoceanographic archives, whereas the drivers in past and today differs.