Study Of The Structure And Evolution Characteristics Of Tropical SST Anomaly Gradient And Their Impact On Atmosphere

Tropical sea surface temperature(SST),which contains the strongest interannual variability signal ENSO(El Ni?o and Southern Oscillation),is the most important energy source in driving atmospheric motion and an important prediction source in short-term climate fluctuations.Previous studies have extensively analyzed the sea surface temperature anomaly(SSTA)modes in the tropical basins and their impact on the global climate.It is found that the ENSO signal has complex diversity in the SSTA structure,intensity,evolution process and climate influence,as well as significant interdecadal variation.And there are complex interactions between the ENSO phenomenon and SSTA modes in the tropical Indian and Atlantic basins,which makes the tropical ocean a unified whole with extensive connections.The influence of tropical ocean on atmosphere is not the effect of a single basin,but the result of the co-action of SSTA among different oceans.The complexity of ENSO and the concomitant variation of pan-tropics SST are becoming to the frontier scientific topics in international meteorology and climatology at present.It is necessary to further study the characteristics of tropical SSTA and their impacts from the view of large-scale spatial configuration and evolution.Firstly,the main categories of SSTA variability in boreal winter were analyzed worldwide.Then,by the EOF analysis of zonal anomaly field of SSTA,the main patterns of zonal SST anomaly gradient structure in boreal winter tropical Pacific,where the strongest interannual variability located,were extracted.Furthermore,two coupling patterns between the zonal gradient structure of SST anomaly and the Walker circulation in the whole tropical Pacific were investigated by EOF and SVD analysis.On the basis of two gradient patterns,a new division of Pacific SSTA spatial structure was established from the perspective of gradient components configuration for analyzing the complexity of ENSO phenomenon and its influence process.In addition,from the perspective of the integrality and evolution,the tropical ocean basins were connected and the accompanying evolution patterns of the pan-tropical SST anomaly structure from autumn to spring were analyzed and verified.The major conclusions of this dissertation can be briefly summarized as follows:(1)The main connections among regional SSTA variabilities in boreal winter25 regional SSTA indices in wintertime from 1951/52 to 2014/15 were selected to be clustered by systematic clustering analysis.The indices were divided into four categories with clear physical meanings.The indices of the first and second category have shown an obvious interannual oscillation characteristics.Category Ⅰ represents the accompanying variation of the Eastern pattern(EP)ENSO and the PDO.Category Ⅱ represents the reverse SST variations between the middle Pacific and the east/west sides in the Central pattern(CP)ENSO.The Category Ⅲ reflects the interdecadal SST warming abrupt of the Indo-Pacific warm Pool and the North Atlantic in the 1990 s.The variations cause by SSTA diffusion distribution under the action of equatorial wind current and western boundary current were gathered in Category Ⅳ,with the interannual variation weaker than Category Ⅰ and Ⅱ and a relatively slow interdecadal linear warming.Both the two strongest interannual variability types in global SSTA are located in the tropical Pacific Ocean.The Indo-Pacific warm Pool region holds the highest climate SST in the worldwide ocean while it has a relative weak interannual variation amplitude.(2)The coupling patterns between the wintertime tropical Pacific zonal SSTA gradient structure and the Walker circulationBy calculating the zonal anomaly field,the SSTA indicating the local anomaly were transformed into a form reflecting the relative anomaly in the zonal direction of the Pacific.The zonal anomaly field highlighted the relative cold and warm centers in the gradient structure of zonal SST anomaly,and balanced the contributions of the variabilities in western Pacific and the eastern Pacific to the gradient structure.The results of EOF analysis showed there are two main zonal gradient structures of SST anomaly in the tropical Pacific in wintertime,"east-west reverse type" and "zonal tripole type",which account for 93.7% of the total gradient structure variability.The EOF comparison and SVD analysis revealed that there is a strong spatio-temporal correlation between the zonal SST anomaly gradient and the abnormal Walker circulation.The coupling pattern also shows two coupling structures: the "east-west dipole mode" and the "zonal tripole mode",and the position of the relative cold and warm center is basically consistent with the position of the abnormal rising and sinking branches of the Walker circulation.The coupling pattern also showed two structures of "east-west dipole mode" and "zonal tripole mode" consistent with the above,and the positions of the relative cold and warm center were basically consistent with those of the abnormal rising and sinking branches of the Walker circulation.These two coupling pattern accounts 99.32% cumulative covariance square sum in SVD,representing the vast majority of coupling connections between SSTA gradient and Walker circulation.The responses of low-level wind field,SLP and Southern Oscillation index also showed there were obvious differences in the air-sea interaction process between the two gradient structures.It is proved that not only the local SST anomaly plays a role in the process of air-sea interaction,but also the zonal relative structure of SSTA has an important contribution.The traditional EP ENSO indices could precisely reflect the variation of the "east-west dipole mode".But CP ENSO index showed a similar relatively low relevance with both the two gradient structures.Due to the orthogonality in EOF,there were extremely significant correlations between the zonal SST anomaly gradient modes and the corresponding abnormal Walker circulation patterns,and there is almost no connection between the cross modes.It was beneficial to study the two gradient components with different air-sea coupling characteristics as two independent variables.(3)The influence of different zonal SST anomaly gradient structure components configuration on atmospheric circulationThe wintertime Pacific SST anomalies from 1979/80-2018/19 have been divided into "EOF1 independent anomaly type","EOF2 independent anomaly type","EOF1+2 mixed anomaly type I" and "EOF1+2 mixed anomaly type II" by different configurations of standardized time series of the two gradient structure components.The first three types with statistically significant number of cases since 1979 are analyzed in detail.The range and intensity of SSTA of "EOF1+2 mixed type I" was the widest and strongest,"EOF1 independent type" came second with a slightly weaker intensity,"EOF2 independent type" hold the narrowest range and weakest intensity.In the configuration of pan-Pacific SSTA,the "EOF1+2 mixed type I" is similar to the spatial structure in "Mega-ENSO"."EOF1 independent type" shown a significantly warm SSTA in Kuroshio area while the northeast side was cold.The North Pacific SSTA structure in "EOF2 independent type" was similar to the NPGO negative phase,but less obvious.The abnormal tropical convection activity caused by "EOF1 independent type" and "EOF1+2 mixed type I" could reach the upper troposphere,thus stimulating the wave propagation of atmospheric teleconnections to higher latitudes.The former one had strong circulation responses in the mid-high latitudes of both Eurasia and North America.In the latter one,the atmospheric response was more intense than former from the Northeast Pacific to North America but insignificant at mid-high latitudes of Eurasia.Both "EOF1 independent type" and "EOF1+2 mixed type I" could stimulate abnormal zonal wind waves from the North Pacific to North America and strengthen the North American subtropical westerly jet.The enhancement of subtropical jet in "EOF1 independent" was caused by the joint action of the abnormal circulation in subtropical Pacific and westerly anomaly long-distance transmission from the mid-high Eurasian,while it is mainly affected by the mid-low latitude Pacific Ocean in "EOF1+2 mixed type I".These differences were probably due to the different configuration of SSTA in the North Pacific.In the cases of "EOF2 independent type",the intensity of tropical deep convection and the atmospheric circulation response were much weaker.There was no obvious teleconnection wave propagation to the middle and high latitudes in "EOF2 independent type",and the impact is mainly shown as only the local effect in the tropical,and does not have the global scale influence like other two types.(4)The interannual variability of zonal SSTA gradient structures in tropical PacificThe further analysis of the SSTA gradient field since 1900 revealed that the interdecadal trends of the two gradient structures components were not monotonous.It is found a trough period of amplitudes in both two components from the late 1940 s to the early 1970 s.The amplitudes weakened before this period and increased after.The interdecadal change of ENSO intensity,frequency and SSTA center position since the late 1970 s can be explained as follows: The positive phase of "east-west reverse type" increased significantly in maximum amplitude but decreased in frequency after 1970 s,while the positive phase of "zonal tripole" increased obviously in both frequency and intensity in the same period.Compared with 1900-2018 period,the occurrence frequency of "EOF2 independent type" and "EOF1+2 mixed type I" increased by 29.5% and 27.6%,respectively after 1980,indicating that the contribution of zonal tripole component to SSTA structure increased significantly.The cases with no obvious zonal SSTA gradient appeared densely during the period from 1950 s to 1970 s and decreased significantly after the end of the 1970 s,representing that the obvious SSTA gradient structure in the Pacific was becoming more frequent.The increase of strong gradient leads to the fact that ENSO events become more and more intense and frequent in recent years.(5)Evolution of pan-tropical interannual SSTA variability and its connection with boreal summer atmospheric circulationsThe spatial patterns and temporal evolution modes of SSTAs in pan-tropical oceans are identified by applying an EEOF analysis of observed data from autumn to the following spring.The positive phase of first EEOF mode(EEOF1)displays an ENSO phase-locking evolution pattern in the tropical Pacific,with accompanying warming in the Indian and Atlantic oceans.EEOF2 is characterized by the development of ENSO in the eastern tropical Pacific and a phase transition in the Indian Ocean and the tropical North Atlantic.The most noticeable feature of EEOF3 is the development of the central Pacific ENSO.The first three EEOF modes account for 44.7% of the total variance of tropical SSTAs over the course of the three seasons.The existence of the accompanying evolution of the SSTA anomaly structure in each mode is verified by regional non-orthogonal projection.All the evolution modes have significant effects on the large-scale anomalies of potential height field in boreal summer.EEOF1 is closely related to the tropical atmospheric circulation,especially over the western Pacific region,reflecting the strengthening effect of the three tropical oceans on the anomalous anticyclone over the tropical Northwest Pacific during the decaying phases of El Ni?o.EEOF2 significantly impacts the circulation anomalies over the Indian Ocean and western Pacific.A linear regression model is established using the time series of first four EEOF modes to forecast the June to August 500 hPa geopotential height anomalies.The results reveal that the simulation effect could be significantly improved by using EEOF modes compared with the SST index model that isolated in space and time.