The Tropical Indian Ocean decadal sea level response to the Pacific Decadal Oscillation forcing

Abstract

The tropical Indian Ocean sea level displayed decadal variations in response to Pacific Decadal Oscillation (PDO). Contrasting patterns of decadal oscillation in sea level is found during the opposite phases of PDO especially in the thermocline ridge region of the Indian Ocean (TRIO; 50°E–80°E; 15°S–5°S). Epochal mean sea level rise is observed over the TRIO region during the cold phase of PDO (1958–1977), whereas epochal mean sea level fall is observed during the warm phase of PDO (1978–2002). Analysis reveals that the decadal variability in the sea level pattern in the TRIO region is in accordance with the PDO phase shifts and is primarily caused by changes in the surface forcing over the Indian Ocean as a response to PDO. The changes in the large scale Walker circulation over the tropical Indian Ocean region during the different phases of PDO support our hypothesis. The winds and wind stress curl variations associated with these large scale circulation changes are primarily inducing the observed regional decadal sea level variability over TRIO. The decadal forcing through Indonesian Through Flow (ITF; oceanic channel) however did not show any significant impact on the TRIO sea level variability. Further Ocean General Circulation Model (OGCM) sensitivity experiments are carried out to understand the mechanisms and the possible contribution of the Pacific Ocean through oceanic pathways, in the decadal variability of the TRIO sea level. It is noted that wave propagation from the Pacific Ocean to the Indian Ocean through ITF region has contributed to the sea level variations in the eastern Indian Ocean. But on the decadal time scale, tropical Indian Ocean/TRIO sea level is unaffected by decadal variability in the ITF. Moreover, the ITF contribution to the decadal sea level variability in the Indian Ocean is found to be significant only in the region south of 20°S.