[A-CG36] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG36-04] Second baroclinic mode Rossby waves in the south Indian Ocean

*Motoki Nagura¹, Satoshi Osafune¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:Rossby wave, meridional velocity variability, subsurface

Many previous studies about mid latitude Rossby waves examined satellite altimetry, which reflects variability near the surface above the pycnocline. Argo float observations provide hydrographic data in the upper 2000 m, which likely monitor subsurface variability below the pycnocline. This study examines variability in meridional velocity at mid latitudes and investigates Rossby waves in the south Indian Ocean using an ocean reanalysis generated by a four-dimensional variational method. Results show that there are two modes of variability. One of them is trapped near the surface and propagates to the west at a phase speed close to first baroclinic mode Rossby waves. This mode is representative of variability detected by satellite altimetry. The other mode has a local peak in amplitude at about 600 m depth and propagates to the west at a phase speed three times slower than the first baroclinic mode. Such slowly propagating signals are observed globally but largest in amplitude in the south Indian Ocean and consistent in phase speed with the second baroclinic mode. Results from numerical experiments show that the primary driver of slowly propagating signals in the south Indian Ocean is zonal winds in the tropical Pacific Ocean related to El Niño Southern Oscillation. Wind forcing in the tropical Pacific Ocean drives surface trapped jets, which propagate via the Indonesian Archipelago and excite subsurface variability in meridional velocity in the south Indian Ocean. In addition, surface heat flux and meridional winds near the west coast of Australia can drive subsurface variability.