Changes in summer TC activity related to thermal differences between land and the sea

Abstract

This study analyzed the effects of thermal differences between land and the sea on tropical cyclone (TC) activity. To this end, northern China in which thermal ridges appear in summer in the continent east of Asia was defined as “Land” and an area of the sea where temperatures are low in the tropical and subtropical western North Pacific was defined as “Sea” to analyze the time series of thermal differences between the land and the sea over the last 62 years. Change-point analysis was applied to these time series. According to the results, a significant climate regime shift existed in 1978. That is, positive values were distributed from 1951 to 1978 (5178) and negative values were distributed from 1979 to 2012 (7912). Thereafter, average differences between the 5178 period during which positive values were apparent and the 7912 period during which negative values were apparent was analyzed. With regard to TC genesis, TCs during the 7912 period showed a tendency of being mainly formed in the northwestern quadrant of the tropical and subtropical western North Pacific, and those during the 5178 period showed a tendency of being mainly formed in the southeastern quadrant. With regard to TC movements, whereas TCs during the 7912 period showed a pattern of moving west from the Philippines toward the Indochina peninsular and southern China, those during the 5178 period showed a pattern of moving north from the far southeastern sea of the Philippines to pass the East China Sea and go toward Korea and Japan. Therefore, the TCs during the 7912 period showed a tendency of being formed and moving in regions further west than those of the TCs during the 5178 period. With regard to TC intensities during the two periods, TCs during the 5178 period were more intense. Large-scale environments that affected these changes in TC activity between the two periods were analyzed. During the 7912 period, since temperature in the continent were lower than those in the sea, anomalous anticyclones were reinforced in the continent East of Asia and anomalous cyclones were reinforced in the sea. Consequently, anomalous northerlies were reinforced in a region ranging from the middle-latitude region in East Asia to the Indochina peninsular so that TCs could not move to the middle-latitude region in East Asia but moved west toward the Indochina peninsular and southern China during this period. Eventually, during the 7912 period, temperatures in the land were higher than those in the sea to show the west high east low anomalous pressure system patterns so that western North Pacific subtropical highs could not develop and thus the steering flows of anomalous southerlies that would move TCs to the middle-latitude region in East Asia were weak. These results were identified from 600 hPa relative humidity, 200–850 hPa vertical wind shears, and differences in sea surface temperatures between the two periods.