GEOPHYSICAL MONOGRAPH SERIES, VOL. 147, PP. 365-377, 2004
Two different regimes of anomalous walker circulation over the Indian and Pacific Oceans before and after the late 1970s
Using the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data aided by
a coupled ocean-atmosphere model, we investigated two different regimes of anomalous Walker circulation system over the Pacific
and Indian Oceans before and after a climate shift, which occurred in the late 1970s. During the period before the climate
shift, an upper-level velocity potential anomaly systematically moves eastward from the tropical Indian Ocean to the warm
pool region of the western Pacific during the growth phase of El Niño-Southern Oscillation (ENSO). In the meantime, the activities
of South Asian and Australian summer monsoon systems are directly affected by the evolution of the anomalous Walker circulation.
During the period after the climate shift, in contrast, an upperlevel velocity potential anomaly in the vicinity of the Philippine
Sea and maritime continent is observed to expand westward into the northern Indian Ocean and South Asia during the decay phase
of ENSO. This feature is identified with a major precursory signal of an anomalous South Asian summer monsoon in the preceding
spring. The model captures a systematic eastward propagation similar to that observed prior to the late 1970s, but fails to
reproduce the westward extension of the velocity potential anomaly observed to prevail after the late 1970s. The model results
suggest that the cross-basin connection between the two oceans is a prerequisite for the turnabout of ENSO prior to the climate
shift, in terms of the occurrence of westerly wind bursts.
Citation: Kawamura, R.,