Characteristics And Interdecadal Variability Of El Ni(?)o Diversity Based On The Onset Time

Research on El Ni(?)o diversity can be conducted from two perspectives:the spatial SSTA pattern and the onset time of El Ni(?)o.However,most related research is based on spatial classification at present.In fact,the mature phase of El Ni(?)o typically locks in boreal winter,hence different onset time of warm events may suggest different phase evolution under different dynamic processes.In this thesis,El Ni(?)o is classified into two categories:the spring(SP)type and the summer(SU)type based on the onset time.From both ocean and atmosphere aspects,different evolution and precursor characteristics,and different physical processes of the two El Ni(?)o flavors are revealed.Decadal variability of the two types of events is explored.Further,the simulations of global ocean temperature in multi-scenarios released by CMIP5 and CMIP6 are used to obtain the projected changes of the two types of events under global warming background.(1)The temporal and spatial evolution characteristics and dynamic mechanisms of the two types of El Ni(?)o.The SP El Ni(?)o is a basin-scale phenomenon.It goes through the earlier and stronger heat build-up process,and the earlier occurrence of westerlies in the equatorial Pacific,which can partly explain its earlier onset time.For SU El Ni(?)o,the anomalous signals are much weaker,which can be attributed to the weaker accumulation of warm water and shorter duration of positive Bjerknes feedback.During its peak phase,anomalous southeasterlies over the eastern Pacific enhance the WES feedback and impede the development of positive SSTAs there,and then lead to a west shift of SSTA center.Recharge/discharge processes exist in both types of events but are weaker in the SU type,which may be caused by the lack of meridional Sverdrup transports as a result of weak zonal wind anomalies.The relative importance of thermocline feedback(THF)and zonal advective feedback(ZAF)in two El Ni(?)o types is different.In SP El Ni(?)o,the THF is dominant compared to ZAF in both the GODAS and SODA datasets.In SU El Ni(?)o,however,these two terms are equally important in GODAS.(2)Decadal variability of the two types of El Ni(?)o in reanalysis.In the past 150 years,SU El Ni(?)o has occurred more frequently than SP El Ni(?)o.SU El Ni(?)o occurs more frequently during the PDO cold phase however the SP El Ni(?)o occurs more frequently during the PDO warm phase.The duration of both two types of El Ni(?)o events is longer and the amplitude is stronger during the PDO warm phase.Under global warming,the occurrence frequency of SP El Ni(?)o has increased,while that of SU has no obvious change.The occurrence ratio of SP/SU El Ni(?)o has increased but is still less than 1.The duration of both two types of events become shorter and the reduction of SP El Ni(?)o is greater.Amplitude of SP El Ni(?)o has increased,and that of SU El Ni(?)o has slightly weakened.The SSTA center of SP El Ni(?)o has shifted eastward,while the center of SU El Ni(?)o shifts westward obviously.(3)The projected changes of the two types of events under global warming.Eight indicators of the two types of El Ni(?)o events are examined to evaluate the simulating skill in CMIP5 and CMIP6.Results show that there are considerable differences in the performance of individual models,but in general,the CMIP5 is better than the CMIP6 in simulating SP and SU El Ni(?)o events.The 19 CMIP models that can better simulate the two types of events show that,under the RCP-8.5/SSP5-8.5 emission scenario,the occurrence frequency of SP El Ni(?)o is greater in the future,and that of SU El Ni(?)o may decrease.However,the ratio of SP/SU El Ni(?)o tends to be larger.Duration of SU El Ni(?)o show an increase,which is related to the increase in the SUs frequency.The amplitude of both two types of events increases in the future.Compared with the RCP-4.5/SSP2-4.5 scenario,under a higher emission(RCP-8.5/SSP5-8.5 scenario),the SP El Ni(?)o show a greater occurrence frequency,and the occurrence ratio of SP/SU El Ni(?)o is larger.Duration of both two types of events is longer.The amplitude of SP El Ni(?)o is stronger.