Ocean Roles in the TBO Transitions of the Indian–Australian Monsoon System

Abstract

This study uses a series of coupled atmosphere-ocean general circulation model (CGCM) experiments to examine the roles of the Indian and Pacific Oceans in the transition phases of the tropospheric biennial oscillation (TBO) in the Indian-Australian monsoon system. In each of the three CGCM experiments, air-sea interactions are restricted to a certain portion of the Indo-Pacific Ocean by including only that portion of the ocean in the ocean model component of the CGCM. The results show that the in-phase TBO transition from a strong (weak) Indian summer monsoon to a strong (weak) Australian summer monsoon occurs more often in the CGCM experiments that include an interactive Pacific Ocean. The out-of-phase TBO transition from a strong (weak) Australian summer monsoon to a weak (strong) Indian summer monsoon occurs more often in the CGCM experiments that include an interactive Indian Ocean. The associated sea surface temperature (SST) anomalies are characterized by an ENSO-type pattern in the Pacific Ocean and basinwide warming/cooling in the Indian Ocean. The Pacific SST anomalies maintain large amplitude during the in-phase transition in the northern autumn and reverse their sign during the out-of-phase transition in the northern spring. On the other hand, the Indian Ocean SST anomalies maintain large amplitude during the out-of-phase monsoon transition and reverse their sign during the in-phase transition. These seasonally dependent evolutions of Indian and Pacific Ocean SST anomalies allow these two oceans to play different roles in the transition phases of the TBO in the Indian-Australian monsoon system.

Indian Ocean Sea Surface Temperature and El Niño–Southern Oscillation: A New Perspective

“…The above scenario of a more efficient wind-evaporation-SST feedback in the eastern Indian Ocean after the 1976-77 regime shift offers some clues as to how boreal winter SEIO SST anomalies may have helped to trigger El Niño events or ENSO transitions in recent decades (Yu et al, 2003). This favours the persistence of SST anomalies and the associated atmospheric center in the eastern Indian Ocean, which then acts as a persistent remote forcing on the whole Pacific system by promoting wind (Simmonds, 2003;Fauchereau et al, 2003;White et al, 2004) and these connections merit further study.…”

“…The evolution of Indian Ocean SSTs during the initiation and development of ENSO and its change over time around the 1976-1977 regime shift, illustrated in Figure 1, suggest that the Indian Ocean may play an active role in the transition phases of ENSO after 1977. This possibility should not be overlooked (Yu et al, 2003). In particular, the SEIO area in boreal winter emerges as a key precursor of ENSO evolution after the 1976-1977 regime shift.…”

“…The observational results reported here strongly support the hypothesis that the Indian Ocean now plays a fundamental role in the ENSO cycle (Yu et al, 2003;Wu and Kirtman, 2004). …”

Changes in the in-phase relationship between the Indian and subsequent Australian summer monsoons during the past five decades

“…It is obvious that the correlation is insignificant throughout most of the analysis period. This supports the suggestion of Yu et al (2003) that the Indian summer monsoon forces Indian Ocean SST anomalies into transition after the monsoon season thus reducing the amplitude and persistence of these anomalies.…”

“…Yu (jyyu@uci.edu) therein). Yu et al (2003) used a coupled atmosphere-ocean general circulation model (CGCM) to examine the relative importance of the Pacific and Indian Oceans to this in-phase transition. They showed that the in-phase monsoon events occur more often in CGCM experiments that include an interactive Pacific Ocean, but less frequently in the experiment that includes an interactive Indian Ocean.…”