Intraseasonal variability of the equatorial intermediate current in the eastern Indian Ocean

Abstract

The intraseasonal variability (ISV; 30–105 days) in the equatorial Indian Ocean is important. However, the knowledge of the ISV of the intermediate current remains limited because of the especially sparse observations. This study focuses on the ISV of the Equatorial Intermediate Current (EIC; 200–1000 m) by taking advantage of three deep-sea moorings at (0°, 80°E), (0°, 85°E) and (0°, 93°E). The characteristics and the generation process of EIC ISV in the eastern Indian Ocean are shown in this study, with aid of reanalysis data and ocean model experiments. At the intermediate depths, the standard deviations (STDs) of the intraseasonal zonal current velocity at mooring sites are approximately 35%, 40%, and 59% of the STDs of the EIC velocity, respectively, suggesting the importance of the intraseasonal components in the EIC. The power spectral of observations shows a prominent peak at 90 days and secondary peaks at 30–60 days. The stronger 90-day variability results mainly from an equatorial basin resonance of the second baroclinic modes, with contributions from low-order modes three and four. Associated with the wave energy propagation ray, the EIC ISVs present obvious spatial distribution characteristics in both horizontal and vertical directions. They tend to intensify toward the west and have maximum amplitudes at depths of 400–600 m between 80 and 85°E. Generation of the EIC ISV is attributable to the combined contribution of the directly forced Kelvin and Rossby waves and the reflected Rossby waves driven by intraseasonal equatorial zonal wind forcing. It takes about 40 days to establish the intraseasonal EIC anomaly. This study provides observed evidence and dynamic analysis for the EIC ISV in the Indian Ocean, contributing to our understanding of the intermediate-deep circulation.