Origin of intraseasonal variability in the eastern equatorial Indian Ocean: intrinsic variability and local and remote wind stress forcings

Abstract

The eastern equatorial Indian Ocean (EEIO) experiences considerable intraseasonal variability (ISV) that arises from intrinsic variability in the ocean and/or is induced by atmospheric forcings. This study investigates the relative contributions to ISV from intrinsic variability and local and remote wind forcings using the Regional Ocean Modeling System. First, to evaluate the contributions of intrinsic variability and forced responses, we conducted three ensemble experiments with different initial conditions. The ensemble mean represents forced ISV whereas differences from the ensemble mean indicate intrinsic ISV. In the central EEIO, the forced ISV of temperature and zonal velocity is larger than the intrinsic ISV above 500 m of depth. This is strongly related to equatorial wave dynamics: the spatial and temporal variability of the forced ISV of temperature (zonal velocity) shows characteristics of equatorial Kelvin waves (Rossby waves). Second, to understand the roles of local and remote atmospheric forcings, especially those of wind stress, we conducted four regional forcing experiments in which intraseasonal wind forcings were retained in selected regions only and suppressed elsewhere. At the surface, local forcing dominates the ISV in the equatorial Indian Ocean. By contrast, in the subsurface, remote forcing also plays an important role in ISV. The deep penetration of oceanic ISV in the EEIO is caused by wind forcing in the western (west of 80°E) and middle-eastern (80°E‒100°E) regions. The influence of surface forcing penetrates eastward–downward for temperature and westward–downward for zonal velocity, consistent with Kelvin and Rossby waves, respectively.