Second Baroclinic Mode Rossby Waves in the South Indian Ocean

Abstract

Abstract Many previous studies of midlatitude Rossby waves have examined satellite altimetry data, which reflect 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 the variability in meridional velocity at midlatitudes and investigates Rossby waves in the southern Indian Ocean using an ocean reanalysis generated by a 4DVAR method. The results show two modes of variability. One is trapped near the surface and propagates to the west at a phase speed close to that of 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 ∼600-m depth and propagates to the west at a phase speed 3 times slower than the first baroclinic mode. Such slowly propagating signals are observed globally, but they are largest in amplitude in the southern Indian Ocean and consistent in phase speed with the second baroclinic mode. Results from numerical experiments using an OGCM show that zonal winds in the tropical Pacific Ocean related to ENSO are the primary driver of slowly propagating signals in the southern Indian Ocean. Wind forcing in the tropical Pacific Ocean drives a surface trapped jet that propagates via the Indonesian Archipelago and excites subsurface variability in meridional velocity in the southern Indian Ocean. In addition, surface heat flux and meridional winds near the west coast of Australia can drive subsurface variability. Significance Statement Many previous studies of midlatitude Rossby waves have used satellite altimetry measurements, which reflect variability in the upper few hundred meters of the ocean. Argo float observations have provided in situ hydrographic observations in the upper 2000 m, and these enable us to examine subsurface variability with high reliability. In this study, we used output from an ocean reanalysis, which assimilates in situ observations, and found that the meridional velocity below the surface (∼600-m depth) of the southern Indian Ocean propagates at a phase speed 3 times slower than that of surface variability. These slowly propagating signals can be of climatic importance because of their possible impact on meridional heat transport. We also discuss the driving force of these slowly propagating signals.