A Coupled Air-Sea Biennial Mechanism in the Tropical Indian and Pacific Regions: Role of the Ocean

Abstract

Variations in the upper-ocean beat content are part of a mechanism to explain biennial signals in the tropical Indian and Pacific ocean regions. The mechanism involves modulations of the annual cycle of convection and processes of the dynamically coupled ocean and atmosphere system in the tropics. A critical element of that mechanism is persistent sea surface temperature anomalies on the time scale of one seasonal cycle. Analyses of composite vertical temperature profiles from hydrographic station data for various near-equatorial areas in the Indian and Pacific oceans show that variations in the ocean heat content depend on the depth of the thermocline in the warm-pool region (both eastern Indian and western Pacific) and temperatures in the upper-ocean mixed layer away from the warm pool (western Indian and eastern Pacific). The variations in thermocline depth in the Pacific are similar to those for El Niño-Southern Oscillation (ENSO) events and are present in the biennial mode as well. Apparently, similar sets of mechanisms operate on both ENSO and biennial time scales. These results suggest that changes in upper-ocean heat content contribute to the persistence of sea surface temperature anomalies important to the biennial mechanism, that both the Indian and Pacific oceans are actively involved in ENSO, and that ENSO could be an amplification of the biennial cycle.