Climatic Characteristics Of The Northwest Pacific Tropical Cyclone And Its Response Mechanism With The Thermal Status Of Ocean

Tropical Cyclone(TC)is a powerful weather system with strong impacts.It can bring disasters such as squally winds,heavy rains and storm surges to pose a great threat to human life and property safety.Therefore,it has become a hot issue in current research.The Northwest Pacific is one of the most concentrated areas of the global TC.Studying the formation process,structural changes and activity patterns of TC has great significance in disaster prevention and mitigation in China and other coastal countries.This paper is based on the best meteorological dataset of tropical cyclone of China Meteorological Administration.Using NOAA optimal interpolation 1/4 degree daily sea surface temperature analysis data,SODA heat content data and ENSO data,wavelet analysis,empirical orthogonal function decomposition(EOF),correlation analysis and T-test,I analyze the temporal and spatial variation characteristics of tropical cyclone in the Pacific Ocean in terms of frequency,intensity,duration,source and ACE.The effects of sea surface temperature,upper ocean heat content and two types of ENSO events on TC are discussed.The main conclusions are as follows: 1.Climatic characteristics of tropical cyclones in the Northwest Pacific(1)Tropical cyclones occur throughout the year.The high frequency period is from June to October.The frequency of occurrence of STY(Severe Typhoon)and TY(Typhoon)is higher than that of other grades.From the long-term trend,except the rising trend of tropical storm frequency,the frequency of other intensity tropical cyclones shows a downward trend.Except for STY,the frequency of tropical cyclones in all other levels shows a high value period in the mid-to-late 1980 s to the 1990 s,and a low value period since the 21 st century.(2)The tropical cyclone sources in the Northwest Pacific are mainly concentrated between 5°—22°N and 110°—155°E,and there are 3 obvious source areas,10°—22°N,110°—120°E,8°—20°N,125°—145°E and 5°—20°N,145°—155°E,which accounting for 27%,46%,and 21% of the total TC.As the intensity of tropical cyclones increasing,the source of tropical cyclones gradually shrinks and the high-frequency center gradually moves eastward.(3)Tropical cyclone ACE increases with the increasing of TC intensity.In the seasonal variation,the tropical cyclone ACE of each grade reached the maximum in August,and the ACE was small in January-April and December.The most active period of tropical cyclone is July-October,and the ACE was larger.On the whole,tropical cyclone ACE showed a downward trend.The ACE changes of different grades of tropical cyclones were different.The ACE of TS(Tropical Storm)and STY showed an upward trend,while the ACE of STY,TY and SuperTY(Super Typhoon)showed a downward trend.The duration of TD(Tropical depression),TS and SuperTY showed a downward trend,and the duration of STS(Severe tropical storm),TY and STY showed an upward trend.There are multiple time scales of periodic variation in the duration of tropical cyclones at each level.2.Variation characteristics of extreme temperature events in the Northwest Pacific and its relationship with ENSO(1)The extreme SST events in the Northwest Pacific from 1982 to 2014 have obvious inter-monthly,inter-annual and inter-decadal characteristics.The extreme high-temperature events mainly occur in June-October.The interannual and interdecadal changes showed that it showed a linear increase trend in 1982-1999,and the increase trend in 2000-2007 slowed down,then it showed a rapid upward trend in 2008-2014,especially after 2010.The low temperature events were mainly concentrated in January-March.In the interdecadal changes,they showed an increasing trend from 1982 to 1991,a downward trend from 1992 to 2014.(2)From 1982 to 2014,the frequency of extreme high temperature events in the Northwest Pacific increased significantly.The number of days of extreme high temperature was gradually increasing.The duration of single extreme high temperature events gradually increased,and the average temperature and maximum temperature also showed an upward trend.At the same time,the frequency of extreme low temperature events decreased significantly,and the number of days of it and the duration of single events decreased significantly.The average temperature and the lowest temperature of a single extreme low temperature event showed a linear upward trend.The changes of extreme SST events in the Northwest Pacific are characterized by multiple time scales.The extreme high temperature events show periodic changes of 1-4 years and 4-7 years.The low temperature events show multi-scale periodic changes,and the main period scale is 3-4 years.(3)The NINO 3.4 region has a significant negative correlation with the extreme high temperature in the Northwest Pacific in the second half of the year,and is significantly positively correlated with extreme low temperature events.This means that when the Pacific Ocean in the equator is in the cold phase of the ENSO cycle(La Ni?a event),extreme high temperature events in the Pacific Northwest are more likely to occur.Conversely,when the Pacific Ocean in the equator is in the warm phase of the ENSO cycle(the El Ni?o event),extreme cold events in the Pacific Northwest are prone to occur.3.Impacts of SST over the Northwest Pacific and SST anomalies on tropical cyclone activity(1)In the past 33 years,the SST of the whole Pacific Northwest has generally increased,and the temperature increase range is 0~1.6°C.The highest temperature increase is in the coastal area of the East China Sea,followed by the northwest Pacific warm pool area.The annual growth rate of SST is 0.019°C/a.(2)The TC source is concentrated in the high temperature sea area south of the 27 °C isotherm,and more than 87% of the TC is distributed in the sea area between the 28 and 29 °C isotherms.The stronger the TC intensity is,the more eastward the location of the source is generated.And the higher the TC level,the higher the sea surface temperature at which the source is generated.There was a significant correlation between SST and STY,SuperTY in the northwest Pacific Ocean.The main performance is that it has a significant negative correlation with the frequency,duration and ACE of STY and SuperTY,but not with other tropical cyclones.In the spatial distribution of tropical cyclone activity and SST correlation coefficient,the frequency,duration and ACE of tropical cyclone were negatively correlated with SST in the Northwest Pacific,especially negatively correlated with the South China Sea and the western and northern Pacific SST.(3)Under the background of warming in the Northwest Pacific,the frequency and ACE of tropical cyclones generally show a downward trend.From the comparison of various levels of tropical cyclones,the impact of extreme high temperature events on tropical cyclones is mainly reflected in STY and SuperTY,and there is a significant correlation between them.4.Impact of changes in heat content in the Northwest Pacific on tropical cyclone activities(1)The heat content of the upper 100 m in the Northwest Pacific has generally showed rising trend,especially in the eastern seas of the East China Sea,the South China Sea and the Philippine Islands.With 20°N as the axis,the seawater heat content shows a downward trend in the banded sea near 122°-155°E and the east and west of Kalimantan Island.Most of the 300 m seawater heat content in the Northwest Pacific is on the rise,and the heat content in the northern equatorial current is more obvious.The thermal content of 700 m in the upper layer is symmetrically distributed in the north and south with 15°N as the axis.The seawater heat content in the northern equatorial current region increases obviously,while the seawater heat content in the banded sea near 20°N decreases.(2)In the past 30 years,the seawater heat content in the Northwest Pacific has shown an upward trend.Especially since the late 1990 s,it shows a clear upward trend.By comparing the temperature increase trend of the upper layer of 100 m,300m and 700 m,it can be found that the upper layer 700 m heat content increases the most,the 300 m heat content increasing rate is the second,and the 100 m change range is the smallest.(3)At the interannual scale,the 100 m heat content in the upper Northwest Pacific Ocean is negatively correlated with tropical cyclone activity,and the high-grade tropical cyclone(STY and SuperTY)activity is significantly negatively correlated with heat content.The upper 300 m heat content is positively correlated with the frequency of tropical cyclones.Similarly,it is negatively correlated with tropical cyclone duration and ACE,and has a greater impact on high-grade tropical cyclones(STY and SuperTY)than low-grade tropical cyclones.The influence of the upper 700 m heat content on the tropical cyclone is similar to the upper 300 m heat content.By comparing the tropical cyclone and different depth heat contents of various grades,it can be found that the heat content of the upper 100 m,300m and 700 m has a consistent negative correlation with the tropical cyclone duration and ACE,and the correlation with the tropical cyclone frequency shows different characteristics.By analyzing the relationship between heat content and different levels of tropical cyclones,it is known that the upper ocean heat content has a greater impact on high-grade tropical cyclones(STY and SuperTY)than on low-grade tropical cyclones.(4)In terms of spatial distribution,the 100 m heat content in the upper layer of the western Pacific Ocean is negatively correlated with the tropical cyclone(Frequency,duration,ACE),and the negative correlation is most significant in the South China Sea and the Northwest Pacific warm pool.5.Effects of two types of ENSO events on tropical cyclone activity(1)The ENSO event has an important impact on the activities of TC in the Northwest Pacific.The frequency of TC generation in La Ni?a is higher than that in El Ni?o years,while the duration of TC and ACE in El Ni?o are greater than those in La Ni?a.From the comparison of TC activities of various levels,the impact of ENSO events on high-level TC(STY and SuperTY)is greater than that of low-level TC.(2)When the CP-ENSO event occurs,the frequency of TC generation in the Pacific Northwest is more,the duration is longer,and the ACE is larger.When the EP-ENSO event occurs,the frequency of TC generation in the Pacific Northwest is less,and the duration is shorter.And the ACE is small.But the rule of SuperTY activity is opposite.This shows that TC in the Northwest Pacific is affected by the CP-ENSO event,while the high-grade TC(SuperTY)is greatly affected by the EP-ENSO event.(3)By comparing and analyzing the spatial distribution differences of frequency and source of each level of TC under different phases of two kinds of ENSO events.In the EP/CP-El Ni?o year,the location of the TC in the northwestern Pacific will shift to the southeast,while in the EP/CP-La Ni?a year,the TC will be located to the northerly.The differences in the impact of the two types of ENSO events on the TC source areas of each level are mainly reflected in the differences between the northern part of the South China Sea and the southern part of the South China Sea,and the difference between the southeastern sea and the northwestern sea.