An Analysis On Tropical Cyclones Induced By Easterly Waves In The Northwest Pacific Ocean

Based on the NCEP reanalysis data and cloud images from THORPEX-Pacific Asian Regional Campaign, the formation of tropical cyclone induced by the easterly wave in the Northwest Pacific Ocean is analyzed in this paper. It is found that, in the period from 2000-2009, there were 34 TCs induced by the easterly wave in the Northwest Pacific Ocean, which is 13.4% of the total number of TCs occurred in that area. According to the background flows, these 34 easterly-wave-related TCs can be divided to two categories. In Category I, TC was developed from the easterly waves embedded in the easterly flow. There are 22 cases (64.7% of total number) occurred in the years from 2000-2009. Category II denotes the TC is resulted from the interaction between the easterly waves and monsoon trough or ITCZ. There were 12 TCs can be identified as Category II in the years from 2000-2009 and is about 35.3% of the total TCs. Based on the analysis on the final stage of the tropical cyclogenesis, Category I can be further divided in two types, i.e., the precursor of TC is developed in the wave critical layer and the development of TC was controlled not only by the critical layer but also the scale contraction mechanisms.Via the detailed case-studies on the three TCs induced by the easterly wave, the forming processes of TCs related to easterly wave is investigated in this work. It is found that the wave critical layer is important to the formation of TCs. However, the formation of TCs related to easterly waves may not be only ascribed to the critical layer. The scale contraction resulted from the large-scale convergent flow also contribute to the tropical cyclogenesis together with critical layer mechanism as the easterly wave moved into the monsoon trough or ITCZ area.