Intraseasonal Tropical Atmospheric Variability Associated with the Two Flavors of El Nino
Résumé
The characteristics of intraseasonal tropical variability (ITV) associated with the two flavors of El Nino [i.e., the canonical or eastern Pacific (EP) El Nino and the Modoki or central Pacific (CP) El Nino] are documented using composite and regression analysis. Double space time Fourier analysis is applied to the NCEP-NCAR zonal wind at 850 hPa (U850) to separate the different components of the ITV in the tropical troposphere, which is then used to define indices of wave activity, and document the spatial pattern of the waves. It is shown that the ITV characteristics are altered during CP El Nino compared to the typical seasonal dependence of the ITV-ENSO relationship. In particular, while EP El Nino is characterized by enhanced MJO and equatorial Rossby (ER) wave activity during spring summer prior to the ENSO peak, during CP El Nino, the ITV activity is increased during the mature and decaying phases. It is suggested that ITV is more propitious to the triggering of the EP event: while during the CP event, it contributes mostly to the persistence of positive SST anomalies. The oceanic response of these ITV anomalous patterns is further investigated in the Simple Ocean Data Assimilation (SODA) reanalysis by documenting the seasonal evolution of the intraseasonal equatorial oceanic Kelvin wave (IEKW) activity during the two flavors of El Nino. It is shown that anomalous westerlies associated with ITV may generate the corresponding response in the ocean in the form of anomalous IEKW activity.
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