The Effect Of Westerly Wind Burst On The Evolution Of ENSO

This dissertation focuses on the development of El Ni?o under different phases of Pacific decadal oscillation(PDO),and investigates the similarities and differences of westerly wind burst(WWB)in El Ni?o's excitation mechanism and evolution mechanism.The role of westerly wind burst in the evolution of ENSO at different phases of PDO.This helps to improve the prediction skills of ENSO and provide a scientific basis for the prediction of seasonal climate in China.In this dissertation,ERA-Interim reanalysis data with the time resolution of four times a day from 1980 to 2017,GODAS data with the time resolution of once a day,and the monthly-mean sea surface temperature anomaly from ERSST are used to discuss the impact of westerly wind burst on the evolution of ENSO.The main conclusions obtained in this paper are as follows: Firstly,studies have pointed out that westerly wind bursts are stronger and more frequent in PDO positive phase and have a more significant effect on the evolution of El Ni?o;while in PDO negative phase,the situation is reversed.In PDO positive phase,the frequency,duration,and spatial range of westerly wind bursts are higher than those of PDO negative phase.At the same time,compared with the La Ni?a year and the normal year,westerly wind bursts ocurrmore frequent,last longer,and have a wider spatial range inthe El Ni?o year.During the initiation period of ENSO,the evolution of El Ni?o in PDO positive phase may be affected by the stronger,larger,and more frequent westerly wind bursts than that in PDO negative phase.The weather/climate system directly related to the westerly wind bursts has a wider spatial range in PDO positive phase.Since PDOchanged from positive phase to PDO negative phase,the weather/climate system related to the westerly wind burst moved westward,resultinginmore central El Ni?o during PDO negative phase.Secondly,a new index that is more suitable for describing westerly/easterly wind burst is defined,which can better describe the relationship between westerly wind burst and El Ni?o evolution.According to the activity characteristics of the sea level pressure anomaly field in the equatorial region from 1980 to 2017,a new index describing the equatorial oscillation activity in the equatorial region-the equatorial oscillation index(EOI)is proposed.By comparing the differences between the equatorial oscillation index and the southern oscillation index in describing westerly wind burst,easterly wind burst and Ni?o-3.4 SST anomalies,it is pointed out that the correlations of the EOI defined in this paper are superior to the classical SOI.On this basis,a quantitative analysis of the lead-lag relationship between each index and Ni?o-3.4 sea surface temperature anomaly was conducted.The correlation coefficient between SOI and Ni?o-3.4 sea surface temperature anomaly(SSTA)dropped sharply in May.It is related to the rapid change in sea level pressure in the subtropical South Pacific from April to May.Even when the SOI does not match the spatial distribution of the El Ni?o 's sea surface temperature anomaly,the EOI can still well match the spatial distribution of the El Ni?o 's sea surface temperature anomaly.Thirdly,according to the evolution characteristics of the sea temperature of each subsurface,the criterion of the equatorial Kelvin wave event is given on the pentad-month timescales,and the optimal statistical relationship between the westerly wind burst impacting Kelvin wave in the equatorial Pacific is analyzed.Using lead-lag analysis,we found the optimal statistical relationship between the westerly wind anomaly affecting the Kelvin wave in the equatorial Pacific: under the influence of the continuous westerly wind anomaly(60-day average),it can effectively cause a positive anomaly in the upper seawater temperature(60-day average)after 20 days in equatorial Pacific;under the influence of the continuous easterly wind anomaly(30-day average),it can effectively cause the negative anomaly of upper seawater temperature after 30 days(30-day average)in equatorial Pacific.In terms of spatial location,the westerly wind burst near the international dateline(150°E-170°W)has a good correspondence with the 20°Cisotherm depth anomaly in the equatorial Mid-East Pacific(160°W-100°W).From March to August,westerlywind anomalies in the western Pacific(west of 170°E)can cause changes in thermocline of 150°W,where the waves then propagate eastward to the eastern Pacific.During the ENSO mature period(September-February),westerly wind bursts near the international dateline(150°E-170°W)and sea surface temperature and thermocline in the eastern Pacific(near 120 ° W)formed Bjerkness positive feedback,so westerly wind anomalies in the equatorial western Pacific(near 170°E)and westerly wind anomalies in the equatorial eastern Pacific(near 110°W)have the greatest correlation at the same time.