A sea-level monopole in the equatorial Indian Ocean

Ravichandran M,Paolo De Luca,Erik Sahlée,Venugopal Thandlam,Ramakrishna S.S.V.S,Anna Rutgersson,Udaya Bhaskar T.V.S,Rahaman Hasibur

doi:10.1038/s41612-020-0127-z

Abstract

In this study, we show the relationship between sea-level anomalies (SLA) and upper-ocean parameters in the Equatorial Indian Ocean (EIO). This work also focuses on the variability of SLA obtained from satellite altimeter data in different spatial and temporal scales and its relationship with computed ocean heat content (OHC), dynamic height (DH), and thermocline depth (20 °C isotherm: D20) during 1993–2015. SLA showed low Pearson’s correlation coefficient (CC) with upper-ocean parameters over central EIO resembling a “Monopole” pattern. The Array for Real-time Geostrophic Oceanography (ARGO) in situ profile data in the central EIO also confirmed this. SLA over this monopole showed low correlations with all parameters as compared with eastern and western EIO. These findings show a clear signature of a persisting sea-level monopole in the central EIO. Oscillating SLA over western and eastern EIO during summer and winter monsoon months is found to be responsible for locking this monopole in the central EIO. Both SLA and OHC increased in EIO during 2006–2015 compared with 1993–2005. The month of January showed different east–west trends at different times. This trend during 1993–2015 is neutral, but it shifted from negative during 1993–2005 to positive during 2006–2015.

Highlights

The Equatorial Indian Ocean (EIO) is the second longest tropical ocean belt after the tropical Pacific
We studied the variability of sea-level anomalies (SLA) with upper-ocean parameters such as ocean heat content (OHC), dynamic height (DH), and D20 in the EIO
All the parameters (OHC, DH, and D20) from the observations SLA is observed during the onset of summer monsoon over the showed higher correlations over north and south of the equator as compared with the center

Summary

Introduction

The Equatorial Indian Ocean (EIO) is the second longest tropical ocean belt after the tropical Pacific. EIO plays an active role in global and regional climate change through ocean–atmosphere-coupled processes, and has a strong impact on many global and regional weather and climate patterns[1], including the precipitation patterns over the surrounding land mass[2]. The key feature of the EIO is the strong air–sea interactions and the strong air–sea coupling shows spatial and temporal variations from intraseasonal to interannual scales. The western and eastern EIO plays a key role in the formation of the Indian Ocean Dipole (IOD)[3,4]. It is well known that El Niño-Southern Oscillation (ENSO) has a global impact on climate[5]; in particular, the wind anomalies associated with ENSO extend up to the Indian Ocean, and in turn affect the basin-wide circulation[6]. Saji et al.[3] comprehensively described the IOD, and subsequent works recognized IOD and ENSO as the two dominant modes of interannual climate variability in the Indian

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