Variation Characteristics Of Tropical Cyclones Making Landfall Over China Under Global Warming And The Mechanism Of ENSO

Based on tropical cyclone(TC)data provided by the America Joint Typhoon Warning Center,this paper analyzes the variation characteristics of tropical cyclones making landfall over China during the 65-year period of 1951-2015 and their statistical relationship with the El Ni?o-Southern Oscillation(ENSO).To explore the possible mechanism of ENSO spatio-temporal changes in China's TC activities.At last,the potential risk of tropical cyclone in the Northwest Pacific is analyzed.The conclusions are as follows:(1)There are obvious interannual and decadal changes in the landing frequency,the landfall intensity,accumulated cyclone energy(ACE)and life history of tropical cyclones in China during the 65 years from 1951 to 2015.From the long-term change trend,the landing frequency presents a high-frequency and low-frequency oscillation with a period of 4-8 years,and landing frequency exists abrupt in 1988;The landfall intensity presented high frequency and low frequency oscillation with a period of 2-4 years,and the landfall intensity in 1966,1987 abrupt twice.From the long-term trend of change,the ACE shows a significant weakening trend,showing a 2-4 year cycle of high-frequency and low-frequency oscillation,and the ACE is likely to abrupt twice in 1972 and 1984.From the perspective of long-term change trend,the life history shows a significant enhancement trend,which is manifested as the high-frequency and low-frequency oscillation with a period of 2-4 years,and the life history shows likely to abrupt twice in 1985 and 1998.(2)Based on the landing frequency,the landfall intensity,ACE and life history of tropical cyclones associated with Ni?o 3.4 area sea surface temperature analysis found that: The landing frequency and Ni?o 3.4 area sea surface temperature has significant negative correlation,life history with Ni?o 3.4 area there is significant positively related to sea surface temperature,the landfall intensity,the ACE and Ni?o 3.4 area sea surface temperatures found no significant correlation.Through the sliding correlation analysis,it was found that the landing frequency was significantly negative correlation with the sea surface temperature in Ni?o 3.4,and the correlation coefficient peaked at-0.3.We used the sliding correlation method to analyze the interannual variation of correlation coefficient,and the overall sliding correlation coefficient was almost negative,and the significant negative correlation occurred in the 1990 s.Sliding t-test for sliding correlation coefficient can be concluded that the correlation between the two has undergone transitions in 1970,1987,and around 2000.The correlation coefficient between the landfall intensity and the surface temperature slide correlation coefficient of the area of Ni?o 3.4 is positively correlated with the negative correlation,and the stage is very obvious.Sliding t-test for sliding correlation coefficient can be used to determine the correlation between the two before and after 1999.The correlation coefficient between cyclone energy index and surface temperature sliding in Ni?o 3.4 area is positive from 1951 to the end of 1990 s,negative from the early 2000 s to 2015,and abrupt twice in 1967 and 1994.The correlation coefficient between the life history and the surface temperature sliding of Ni?o 3.4 area was positive from the late 1950 s to the early 2000 s,and negative from the late 2000 s,and the transition occurred around 1965,1982 and 1996.(3)Frequency of tropical cyclone distribution are mainly distributed in the two areas,one is in the northwestern Pacific(130-140°E,10-15°N)east of the Philippines,a northeast area in the South China Sea(15-20°N,115°E).And the South China Sea central value is slightly higher than the Northwest Pacific center value,which means that the most active area of tropical cyclone in China is in the South China Sea.The spatial distribution of EOF's first eigenvector is basically the same as that of multi-year spatial distribution.According to the spatial distribution of the second mode,the frequency of tropical cyclones in the Northwest Pacific Ocean and the South China Sea region of China often show the opposite trend.It was found that the time coefficient of the second mode was mutated around 1980,indicating that the opposite trend of the South China Sea and the Northwest Pacific began to weaken in the early 1980 s.The TC genesis locations move to the southeast in El Ni?o years and shift to the northeast in La Ni?a years.(4)TC is one of the main path from the tropical Pacific Ocean to the west to the Philippine sea and the South China Sea,another from the tropical Pacific Northwest arrived in South Korea,Japan,off the coast of east Asia continent area;The maximum frequency of tropical cyclones occurs at 115-125°E,and in the attachment area of the center of 15-25°N.The spatial distribution characteristics of the first eigenvector of the path frequency EOF are basically the same.The spatial distribution of the first eigenvector is the spatial distribution characteristic of the tropical cyclone path frequency.For the first time coefficient of the mode for sliding T test,found in 1986,and getting twice,in 1996 showed that the concentration distribution trend after 1986 shows weakening trend,concentrated distribution trend quickly increased by 1996.The spatial distribution of the second mode is positive and negative.This feature indicates that the frequency of the tropical cyclones in the northern part of Taiwan and the eastern part of the South China Sea is often reversed.The second mode of time was found to have mutated in the year 2000,indicating that the reverse trend of the north and east China searoutes in the South China Sea began to strengthen in the early 2000 s.TC frequency that influences the Chinese continental coast line in El Ni?o years is less than that which influences Japan.Meanwhile,in La Ni?a years,the TC frequency that influences Japan and its surrounding waters,the China Sea,the Bohai Sea,and the South China Sea,increases,while it decreases in the Taiwan Province.(5)Observed intensity is high value area is located in the southeast of the island waters(120-130°E,15-25°N),the high value area of the South China Sea area(110-115°E,15-20°N).The spatial distribution of the first eigenvector of the observed intensity EOF is basically the same as that of multi-year spatial distribution.For the first time coefficient of the mode for sliding T test,found in 1972 and getting twice,in 1982 showed that the concentration distribution trend after 1986 shows weakening trend,concentrated distribution trend quickly increased by 1996.The second mode,the spatial distribution of the positively upward-negative space distribution characteristics,this feature indicates that in the tropical cyclone in the north of the island and the south China sea east path frequency often can appear the opposite trend.Second time coefficient of the mode for sliding T test found mutations in around 2000,shows the island in the north and the South China Sea east path frequency opposite change tendency in the early 2000 s began to strengthen.The observed intensity of TC landfalls on the southern coast of China,Northeast Japan,the northeast part of the Japan Sea,and the east and west seas of the Philippines in El Ni?o years increases,while it decreases in the Bohai Sea,China Sea,the East China Sea,the Korean Peninsula,and the east and west seas of the Philippines.The observed intensity decreases on the Chinese coastline in La Ni?a years but increases in the Philippines.(6)The change in sea temperature(ENSO)in the equatorial Middle East Pacific can directly affect Walker circulation.Due to the increase(decrease)of the temperature in the Pacific Ocean in the middle of the equator,an abnormal rise(sinking)movement occurred in the area,causing the Walker circulation to weaken(increase);The abnormal enhancement(weakening)of the Southwest air flow of summer monsoon corresponding to the South China Sea region near El Nino(La Ni?a)at 120o E.At the same time,due to the El Ni?o(La Ni?a)year,the corresponding Asian continent's abnormal sinking(rising)movement strengthened(weakened)the sinking movement of the Hadly circulation here,and there was an anti-cyclone(cyclone)anomaly on the mid-latitude continent.The tropical cyclone path is more(less)inclined away from the mid-latitude continent.There are fewer(more)tropical cyclones landing in China.(7)The change of the local circulation in the Northwest Pacific makes the sea temperature increase,convection increase,middle relative humidity increase,wind vertical shear decrease,low vortex increase,and high dispersion increase in the southeastern part of the Northwest Pacific Ocean in El Ni?o year,which is conducive to tropical cyclones.Generate and develop here;At the same time,the wave disturbance activity of the lower troposphere weather scale also increased.The large-scale circulation conditions over the southeastern part of the Northwest Pacific have become more favorable for tropical cyclone generation.This adjustment of large-scale circulation provides favorable background for the Southeast movement of tropical cyclone generation position.The Southeast movement of the generating position makes the landing tropical cyclone activity have a longer moving distance and thus increases the history of life.(8)From the 1951-2015 Northwest Pacific TC generation frequency,Super typhoon frequency,annual maximum intensity,annual average strength,and life history,there are obvious characteristics of intergenerational changes from long-term trends.The annual maximum strength and annual average strength showed a significant weakening trend,and the trend after the mid-1970 s was more reliable because of the use of United States observation satellites in 1974,which recorded more accurate data.It is almost certain that after the mid-1970 period,the intensity of TC activities in the Northwest Pacific showed a weakening trend.(9)In the history of TC life,the linear changes in frequency,Path density,and observation intensity at different stages(generation,development,maturity,and decline)are in an upward trend,and they are close to the coastline of the East Asian continent in spatial distribution.That is,the activity of the Pacific Northwest TC is generally weakening,but from the perspective of the spatial distribution of linear change trends,the potential threat to the East Asian continent by TC is actually intensified.The increase in TC activity in eastern Asia is likely the result of a strengthened Walker circulation due to an increasing temperature gradient between the northwest Pacific Ocean and the central and eastern Pacific Ocean.The strengthening Walker circulation could increase the magnitude of the vertical wind shear,relative vorticity,and meridional wind shear of low-level easterlies near the equator in the tropical Northwest Pacific,which affects the spatial and temporal variations of TC activity in the Northwest Pacific.