Oxygen minimum zones in the Indian Ocean and their response to changes in the Indian Ocean meridional circulation

Abstract

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 has been no clear evidence of a significant change in the Bay of Bengal OMZ (BoB OMZ), the Arabian Sea OMZ (AS OMZ) followed a global trend and expanded in the last decades until 2013. However, this trend has reversed, and the AS OMZ appears to have shrunk since 2013. The processes that stabilize the OMZ in the Bay of Bengal and regulate its expansion and contraction in the Arabian Sea are poorly understood. In this study, we redefined the source water types (SWTs) and employed an extended optimum multi-parameter (eOMP) analysis to investigate changes in the oxygen supply due to mixing and biological oxygen consumption in these OMZs based on empirical data from the Global Ocean Data Analysis Project version 2 (GLODAPv2) and data from a research cruise conducted with the German research vessel SONNE in 2024. In line with previous studies, our findings reveal a reversal in the expansion trend of the AS OMZ and an increase in oxygen supply to the BoB OMZ between 1995 and recent years. In both OMZs, this was due to an increased northward influx of oxygen-rich SWTs from the southern Indian Ocean, combined with a reduced contribution from relatively oxygen-poor local and equatorial SWTs. We also observed that the increased physical oxygen supply was accompanied by enhanced biological oxygen consumption, which partly offset the effect of this increase on the oxygen concentration in the OMZs. These changes are likely linked to a reorganization and slowdown of the shallow Indian Ocean meridional overturning circulation (IO-MOC), associated with the weakening of the Asian monsoon and the global thermohaline circulation (THC). This slowdown seems to have increased the residence time of water masses, which is consistent with the observed increase in biological oxygen consumption, whereas the weaker cross-equatorial circulation appears to have favored the northward meridional transport of oxygen-rich water from the southern Indian Ocean. This suggests a coupling between the OMZs in the Indian Ocean and climate-driven changes in the THC, as also indicated by palaeoceanographic records. However, the climate-related drivers differ between the past and present.