Environmental Factors Affecting Tropical Cyclone Activities

With the historical data, the dynamic features of the atmosphere and ocean in the different developing stages and the seasonal and interannual variation of tropical cyclones in the Northwest Pacific are analyzed in this work with the intention to understanding how the environment affects the activity of tropical cyclone in different time scales. Results indicate that the thermal structure of the ocean, the stability of the atmosphere and the vertical wind shear make important contributions to the change of tropical cyclone intensity in all time scales.As regard to the formation of tropical cyclone in the Northwest Pacific, it is found that the favorable conditions for tropical cyclogenesis can be summarized as1) warm ocean with the sea-surface temperature lying between28℃-30℃and the sea water in the upper65m warmer than26℃;2) vertical wind shear weaker than12.5m/s; and3) sufficient conditional available potential energy (CAPE, larger than1059J/kg). The intensification of the tropical cyclone usually happens when the sea-surface temperature is higher than26℃in the Northwest Pacific. Under the condition that the sea-surface temperature is higher than27.5℃while lower than29.5℃, the tropical cyclone usually experiences a rapid intensification. However, the tropical cyclone decays or only can maintain the intensity when the sea-surface temperature is lower than26℃. The tropical cyclone that intensifies in the Northwest Pacific usually experiences a low-to-moderate vertical wind shear with the values between2and10m/s. The larger the vertical wind shear is, the slower the tropical cyclone strengthens. In addition, the data analysis shows that the CAPE is much smaller in the intensifying stage than that in the forming stage of the tropical cyclone. In the case that the tropical cyclone undergoing a rapid intensification, i.e., the maximum wind speed increases20Knot per6hours, it is found that the changes of the sea-surface temperature, CAPE and vertical wind shear are usually small, which indicates that the rapid intensification of the tropical cyclones in the Northwest Pacific occurs under the condition that the evolution of the environment is relatively slow. The activity of the tropical cyclones exhibits considerably seasonal variation in the Northwest Pacific. Accordingly, the influences of the environmental factors on the tropical cyclones also demonstrate remarkable seasonal variations. At the beginning of the typhoon season, the low-level wind field plays a vital role in the formation and development of tropical cyclone and the area favorable for tropical cyclone formation expands from the west to the east gradually while, in the end of the typhoon season, the tropical cyclone activity is deeply influenced by the thermal factors and the vertical wind shear and the area favorable for tropical cyclone formation moves southwards.The long-term variation of the tropical cyclone is closely related to the upper-level thermal structure in the ocean in the Northwest Pacific. It is found that the interannual variations the tropical cyclone have negative relationship with the heat content of the upper-level ocean, which is opposite to that in the North Atlantic and East Pacific. This implies that the effects of the ocean on the tropical cyclone activity may be different in different areas. Therefore, the argument that the global warming leading to the enhancement of the tropical cyclone activity may not be reasonable.