Summertime Synoptic-scale Variability over the Tropical Western Pacific: Role of Extratropical Forcing

The three-dimensional dispersion characteristics of the observed summertime synoptic-scale disturbances in the tropical western Pacific are studied. Emphasis is put on the region east of ~130oE off the equator, over which the strongest growth rate is found. In general wave activity flux is directed to the west, and its convergence contributes strongly to the amplification of the synoptic perturbations. Near the dateline where disturbances are initiated in the low levels, downward-directed wave activity is found in the 300-700 hPa layer. In the upper troposphere at the same geographical location, there is southward wave activity from the extratropics penetrating into the Tropics. These findings suggest that summertime synoptic-scale disturbances can be triggered by extratropical forcing.

The year-to-year variation of the western Pacific synoptic-scale (including tropical cyclone) activity is found to be sensitive to the strength of the Pacific jet stream. When the summer upper-level jet is anomalously strong, less extratropical wave activity is observed to reach the tropical latitudes, and the amount of synoptic-scale transients is reduced. This forcing mechanism leads to a new scenario for the western Pacific climate variability unrelated to ENSO.