Energetic Bottom Current at the Equatorial Gap of the Ninety East Ridge in the Indian Ocean Based on Mooring Data

Bottom current at the equatorial gap of the Ninety East Ridge is revealed by long-term bottom mooring observations from 24 December 2019 to 2 April 2021. It is found that the observed bottom current has significant intraseasonal variability (ISV) with a dominant time scale of & SIM;33 days, which becomes enhanced in the boreal summer and winter. Variability of the bottom current was explored by using the Mercator Ocean outputs, which revealed that the enhancement of ISV of the bottom current is closely related to the westward propagation of Rossby waves originating from the eastern boundary, with their ray paths vertically propagating into the deep layer. Considering the importance of water mass transport through the gaps in the Ninety East Ridge to the renewal of deep water and modification of deep circulation in the Central Indian Basin (CIB) and West Australian Basin (WAB), our study provides in-situ evidence for previously unrevealed energetic variation of water mass exchange between the CIB and WAB, and it further helps us gain a comprehensive understanding of the pathway and variation of the deep meridional overturning circulation in the Indian Ocean. Plain Language Summary Water mass exchanges through the gaps in the Ninety East Ridge are critical to deep water properties in the Central Indian Basin (CIB) and West Australian Basin (WAB). Despite the importance of water mass exchanges through the gaps in the Ninety East Ridge, little is known about their temporal variability, as direct, long-term measurements are sparse. Based on long-ter m bottom mooring observations, we provide a description of the bottom current at the equatorial gap in the Ninety East Ridge and reveal its significant intraseasonal variability (ISV) with period of approximately 33 days. The Mercator Ocean model outputs reproduce the variability of the bottom current, and reveal the significant signals of westward propagation of Rossby wave, which are responsible for the enhancement of the ISV of the bottom current. As an important component of water mass exchange between the CIB and WAB and a potential component of the Indian Ocean deep meridional overturning circulation (DMOC), the bottom current revealed in this study will not only help us understand the renewal of the deep water and modification of the deep circulation in the CIB and WAB, but also provide us comprehensive knowledge of the pathway and variation of the DMOC.