Interannual and intraseasonal variations of convective activities in the tropical western Pacific during summer and their impact on the Northern Hemisphere circulation are investigated by using satellite cloud amount, sea surface temperature (SST) and geopotential data for 7 years (1978-1984).
During summers when SST in the tropical western Pacific is about 1.0°C warmer than normal, active convection regions consisting of a number of typhoons and tropical depressions are shifted northeastward from the normal position near Philippines to the subtropical western Pacific around 20°N and cloud amounts both in the middle latitudes and in the equatorial regions are greatly suppressed. A high pressure anomaly with little vertical tilt predominates in middle latitudes extending from East China, through Japan Islands to North Pacific during these summers.
Analyses of 5-day mean cloud amount reveal that the convective activity is largely modulated by the intraseasonal variations (ISV). The amplitude of ISV of convective activity in the Philippine Sea around 15°N-20°N is more intensified in warm SST summers than in cold SST summers resulting in stronger season mean convective activities in the former than in the latter.
Correlation computations between 5-day mean tropical cloud amount and 500mb geopotential height show that there exist wave trains of geopotential height emanating from the heat source region near Philippines to North America. Daily analyses of geopotential height indicate that these wave trains appear to be generated when convective activities in the Philippine Sea become intense and that the amplification occurs downstream from the western Pacific to the west coast of North America taking about 5 days.
It is concluded that Rossby waves are generated by the tropical heat source associated with ISV, and high pressure anomalies over East Asia and Northwest Pacific during warm SST summers can be understood as the results of frequent occurrence of Rossby wave generation.
Meteorological Society of Japan