A Summer Mode of the North Indian Ocean-Atmosphere Interaction

Abstract:

Interannual variability in North Indian Ocean surface temperature is traditionally viewed as a passive response to El Nino/Southern Oscillation (ENSO) via the atmospheric bridge. This view is being challenged by new studies showing a double peak evolution of North Indian Ocean response to ENSO in sea surface temperature (SST). The first peak, concurrent with El Nino, is readily explained by the atmospheric bridge from the Pacific but the second peak in the post El Nino summer is due to local ocean-atmosphere interactions. First, El Nino excites downwelling ocean Rossby waves in the tropical South Indian Ocean, persisting the El Nino-induced warming. Second, the North Indian Ocean warming excites an anomalous atmospheric anticyclone spanning the tropical Northwest Pacific and the North Indian Ocean. The attendant anomalous easterlies in turn help sustain the ocean warming by reducing the prevailing southwest monsoon. The circular argument signifies the two-way interaction that persists El Nino anomalies through the summer over the Indo-Northwest Pacific. When initialized in spring, this coupled mode of summer is highly predictable. Numerical experiments and hindcasts with coupled models support this mode.

Analysis of a long ship track across the North Indian Ocean shows that this summer mode and its correlation with ENSO display pronounced interdecadal modulations, pronounced when ENSO amplitude is large (after the 1970s and at the turn of the 20th century). This coupled mode poses new problems of physical oceanography.