The Inter-annual Variation Characteristics Of The Tropical Cyclone Intensity In The Western North Pacific And Mechanism Analysis

Based on the CMA-STI Tropical Cyclone Best Track Data from 1949 to 2007, the inter-annual variation characteristics of the tropical storm or typhoon (hereinafter referred to as TC) intensity in the Western North Pacific between July and October in 1949-1975,1976-1987,1988-2007 are respectively investigated in this article. By conducting a composite analysis on the atmosphere and ocean background in TC activity strong/weak years and a correlation analysis between Western North Pacific Subtropical High (hereinafter referred to as WNPSH), Tropical Easterly Jet (hereinafter referred to as TEJ), Monsoon Trough (hereinafter referred to as MT) indexes and the TC intensity, the mechanism of the inter-annual variation characteristics of the TC intensity in the Western North Pacific is investigated in this article. The results of the research are as follows:1. The spatio-temporal variation of the TC intensity in Western North PacificThere is an obvious inter-annual variation of the TC intensity in 1949-1975, 1976-1987,1988-2007 respectively. The TC intensity in 1949-1975 is larger than that in 1976-1987 and 1988-2007.The wavelet analysis shows that there exists a 2-4-year inter-annual variation period in TC intensity in July, August, September and October from 1949 to 2007. By the way of Mann-Kendall method, an obvious TC intensity jump happened in 1978. After the jump the TC intensity decreased. TC intensity maximum center are located in 15oN-25oN,125oE-135oE which is corresponded to the East of Taiwan and Philippines and 17oN-20oN,110oE-120oE which is corresponded to the South China Sea.2. The atmosphere circulation background characteristics in TC activity strong/weak yearsDuring the strong TC years, the WNPSH becomes weaker and is more northward and eastward than normal. Meanwhile, the 200hPa Eastern Asian Subtropical Westerly Jet (hereinafter referred to as EASWJ) locates more southward than normal. Besides, the 200hPa TEJ becomes stronger and locates more eastward than normal. Furthermore, the cold air in the southern hemisphere becomes stronger, and the cross-equatorial flow strengths, so that the MT becomes more active and locates more eastward, and vice versa.3. The ocean background characteristics in TC activity strong/weak yearsThe correlation analysis shows that there is a negative correlation between Western Pacific Warm Pool Sea Surface Temperature (hereinafter referred as to SST) and TC intensity, as well as a positive correlation between the equator central and eastern Pacific SST and TC intensity, and the correlation is durative through the whole year.In TC activity strong years, the Sea Surface Temperature Anomaly (hereinafter referred as to SSTA) in South China Sea, Western Pacific and the central of North Pacific is negative and the SSTA in the central and Eastern Pacific and the north of North Pacific positive in spring (March, April and May). While in July, August, September and October, the SSTA distribution type is similar to that in spring, but the north of North Pacific is negative SSTA, and the positive SSTA in the central and Eastern Pacific extends westward, and vice versa. The SSTA distribution type in TC activity strong/weak years is similar to that in El Nino/La Nina years.TC frequency and intensity in El Nino/La Nino years show different climatic features. Here the 17oN and140oE are used to divide the 0-50oN,100oE-160oE into 4 quadrants. In El Nino years, most of the TCs generate in the south of 17oN, and the frequency in Southeast quadrant is obviously more than that in Southwest quadrant. In La Nina years, TCs generate dispersedly and there is no obvious difference in the 4 quadrants. Further, the TC intensity in El Nino years is greater than normal years, while that in La Nino years is less than normal years. Comparing the TC intensity distribution in 4 quadrants, the difference of the TC intensity in El Nino/La Nina years mainly attributes to the difference in the Southeast quadrant (0-17oN,140oE-160oW).4. The mechanism of the TC intensity variationWhen the SST in the equator central and eastern Pacific increases (the El Nino), the TC genesis position locates more eastward, so that the updraft in the main TC genesis zone is stronger, which is in favor of the development of the TC.When the cross-equatorial flow strengths and the MT becomes more active, it's in favor of the development of the tropical depression in the TC generation area. Besides, the WNPSH becomes weaker and locates more eastward, and the TEJ locates eastward. In this way, the low level is convergent while the high level is divergent. Therefore the convergent updraft in the TC generation area and the divergent outflow in the high level are kept. Besides, the vertical wind shear is less than normal, which is also in favor of the development of the TC; and vice versa.