The Asymmetric Mechanism Of The El Nino And La Nina Structure

The positive and negative phases of ENSO,which are denoted as El Ni?o and La Ni?a show obvious spatial structure asymmetry characteristics.In this study,the spatial structure asymmetry between El Ni?o than La Ni?a are examined based on observational and climate model data.In addition,we also use the mixed layer heat budget analysis to diagnose the observed data in the equatorial central Pacific(5°N-5°S,160°E-170°W)and equatorial eastern Pacific(5°N-5°S,120°-90°W)and related physical mechanisms are explored.This study analyzes the spatial structural features of the strong El Ni?o and La Ni?a from 1958 to 2020,and explores the corresponding atmospheric circulation response features,as well as the main causes and physical mechanisms leading to the changes of these features.The conclusions are as follows:(1)Through composite analysis of SST anomalies in the observed data,it is found that the strong El Ni?o group is more inclined to the eastern equatorial Pacific with respect to the center of winter SST anomalies than the weak El Ni?o group,while the strong La Ni?a group is more inclined to the western equatorial Pacific with respect to the center of winter SST anomalies than the weak La Ni?a group.In the eastern equatorial Pacific,the amplitude of El Ni?o events is stronger than that of La Ni?a events,while in the central equatorial Pacific,the amplitude of El Ni?o events is weaker than that of La Ni?a events.In addition,most models are also able to simulate a negative SST tilt in the central Pacific and a positive SST tilt in the eastern Pacific.This indicates the generality of the spatial structure asymmetry characteristics of the strong El Ni?o and La Ni?a groups derived from the observational analysis.(2)A composite analysis of the depth-longitude profiles of the equatorial Pacific from the observational data reveals that the anomalous vertical velocity and precipitation are more skewed toward the eastern part of the equatorial Pacific in the strong El Ni?o event than in the weak El Ni?o event,while they are more skewed toward the western part of the equatorial Pacific in the strong La Ni?a event than in the weak La Ni?a event.In the lower level atmosphere,there is no significant difference between the two responses;while in the upper level atmosphere,their responses are more significant and show consistent spatial structure characteristics.(3)The heat budget analysis of the mixed layer in the observations reveals that the mixed layer temperature tendency is mainly contributed by the dynamic terms,while the net surface heat flux term plays a negative role in the development of ENSO.The linear part in the dynamic terms contribute positively to the development of ENSO;while the nonlinear part is negative in the equatorial central Pacific,suppressing the development of El Ni?o and promoting the development of La Ni?a,and positive in the equatorial eastern Pacific,promoting the development of El Ni?o and suppressing the development of La Ni?a.Specifically,the nonlinear zonal advection term contributes positively to this asymmetric feature in the equatorial central Pacific,while the nonlinear horizontal advection term contributes positively to this asymmetric feature in the equatorial eastern Pacific,while the nonlinear vertical advection term suppresses the development of ENSO.Therefore,the nonlinear horizontal advection is the main cause for the asymmetry of the spatial structure of the strong El Ni?o and La Ni?a groups.