Abstract
Long-term hydrographic observations repeated by the Japan Meteorological Agency in the western Pacific have revealed not only the oceanic thermal variability associated with the ENSO events but also another interdecadal variability seemingly related to the global warming trend from the late 1970s. Since the background SST is high in the tropical western Pacific, even weak SST anomalies may affect strongly the atmospheric circulation including the Aleutian Low and the Asian winter monsoon. The extratropical atmospheric response to the interdecadal SST anomaly is global and looks quite different from that for the ENSO time scale. We note that the response is even reversed in the Asian monsoon region.
To the west of the date line the ocean behaves like a dynamical slave to the winter Asian monsoon as demonstrated using the ocean general circulation model. In particular, the winter monsoon and related easterly wind variations are responsible for maturity or immaturity of the cold Mindanao Dome off the Phillipine coast. However, the active (inactive) summer monsoon followed by the anomalous easterlies (westerlies) intensified over the tropical western Pacific from summer through winter appears to be responsible for the positive (negative) SST anomalies in the same area at least for the ENSO time scale. This suggests that an interesting positive feedback mechanism responsible for natural climate variabilities ranging from several years to decades may exist in the coupled ocean-atmosphere-land system in the western tropical Pacific.
