The Influence Of Initial Error On The Predictability Of The Two Types Of El Ni?o Event

Initial error can cause significant amunt of uncertainties in the predictions of two types of El Ni?o events.In this study,the conditional nonlinear optimal perturbation(CNOP)approach is applied in the Geophysical Fluid Dynamics Laboratory Climate Model version 2p1(GFDL CM2p1)through the principle component analysis based particle swarm optimization algorithm(PPSO).The optimal precursor(OPR)and the optimally growing initial error(OGE)of two types of El Ni?o are calculated to investigate the influence of initial error on the predictability of two types of El Ni?o.The conclusions are summarized as follows:(1)Evaluate the simulation capability of GFDL CM2p1.Firstly the climatology of the tropical Pacific in the model simulation is evaluated from 3 aspects: sea surface temperature wind and subsurface temperature.Although the model simulation shows westward shift of the cold tongue and stronger trade wind,it is capable of capturing the fundamental character of the tropical Pacific Ocean such as the warm pool and cold tongue lacated in the western and eastern equatorial Pacific respectively,the easterly wind and trade wind in the equatorial and subtropical region and the deepened thermocline depth from east to west.Then the composite of five Central Pacific(CP)and Eastern Pacific(EP)type of El Ni?o events are used to verify that GFDL CM2p1 model can simulate two different types of El Ni?o event based on sea surface temperature anomaly(SSTA)and subsurface temperature anomaly(STA).Though the simulation shows slightly westward shift of the SSTA center and is slightly stronger in relative to observation,it is capable of simulating different types of El Ni?o events with separated features.Therefore,GFDL CM2p1 model can be used to study the influence of initial error on the predictability of two types of El Ni?o events.(2)The OPR and OGE of EP type of El Ni?o are analyzed.By applying the PPSO algorithm,two OPRs and OGEs are obtained for EP type of El Ni?o event.The results suggest that both OPRs and their composite result show positive sea surface temperature perturbation(SSTP)and subsurface temperature perturbation(STP)in the western equatorial Pacific(2°N–2°S,135.5°E–165.5°E)and negative STP in the upper layers(0–85m)of eastern equatorial Pacific(2°N–2°S,79.5°W–109.5°W).The evolutions of the OPRs are listed as follows.The positive SSTP in the western equatorial Pacific induces westerly wind perturbation then triggers down-welling Kelvin wave,which then transports the positive STP eastward.As the positive STP reaches the eastern boundary of the Pacific,a positive SSTP emerges through thermocline feedback which in turn reduces the SST zonal gradient.The Walker Circulation is weakended and the westerly wind perturbation is intensified.Thus,Bjerknes positive feedback is established through SST wind and oceanic dynamic.Further analysis of the OGEs spatial patterns and evolutions also shows that the OGEs exhibit similar error structure in the western equatorial Pacific.In conclusion,for EP type of El Ni?o,the initial sea temperature perturbations in the western equatorial Pacific have a significant influence on it and the related initial errors can significantly lower the accuracy of the prediction of EP type of El Ni?o events.(3)The OPR and OGE of CP type of El Ni?o are analyzed.The parameters of the PPSO algorithm are modified based on the characteristics of the CP type of El Ni?o events in order to obtain the OPRs and OGEs for it.Analyzing the spatial pattern of the OPRs and their composite,the following conclusions can be drawn: the STP in the tropical Pacific is small comparing to the SSTP outside the tropical Pacific;the large SSTPs are mainly located in two separated area: the subtropical northern Pacific(20°N–40°N,175°E–140°W)and south of the 20°S.The importance of these two SSTPs is tested and the results show that the first one is crucial in the evolution of the CP type of El Ni?o event and the second one is less important.Further analysis of the evolution of the OPRs shows that the positive SSTP in the northern subtropical Pacific induces local atmospheric perturbation and triggers southwesterly wind perturbation.Then the magnitude of the trade wind decreases and evaporation is reduced resulting less heat transferred from the ocean to the atmosphere.Therefore,the positive SSTP moves equatorward.After the positive SSTP reaches the central equatorial Pacific it evolves locally through the zonal advection feedback.The positive SSTP in the central equatorial Pacific also triggers local westerly wind perturbation and induces the downwelling Kelvin wave which then induces positive SSTP in the eastern equatorial Pacific through the thermocline feedback.However,because the strength of the down-welling Kelvin wave is rather small,the positive SSTP in the east can not form a basin wide Bjerknes feedback and slowly dissipates.Eventually the OPRs evolve into a mature CP type of El Ni?o event.Analysis of the OGEs error structure shows that the magnitude of the sea surface temperature error(SSTE)decreases as the constraint reduces and the SSTE located in the northern subtropical Pacific shows a negative sign.The OGEs show similar evolution as the OPRs but with an opposite sign.According to the algorithm,the positive SSTE also can have large objective function value.Based on the result of the control experiment,there are two types of positive results: CP type and EP type.The EP type of positive results show similar error structure and evolution as the OPRs for the EP type of El Ni?o event while the CP type of positive results show similar evolution as the OPRs the CP type of El Ni?o event.In conclusion,the initial perturbation in the subtropical northern Pacific plays an important role in the evolution of the CP type of El Ni?o event and the related initial errors can significantly lower the accuracy of its prediction.In addition to that,the subsurface initial error in the western and central equatorial Pacific can also result in an unreliable prediction of the CP type of El Ni?o event.