Abstract

The tropical Pacific variability has experienced changes in its characteristics over the last decades. In particular, there is some evidence of an increased occurrence of El Nino events in the central Pacific (a.k.a. 'Central Pacific El Nino' (CP El Nino) or 'El Nino Modoki'), in contrast with the cold tongue or Eastern Pacific (EP) El Nino which develops in the eastern Pacific. Here we show that the different flavours of El Nino imply a contrasted Equatorial Kelvin Wave (EKW) characteristic and that their rectification on the mean upwelling condition off Peru through oceanic teleconnection is changed when the CP El Nino frequency of occurrence increases. The Simple Ocean Data Assimilation (SODA) reanalysis product is first used to document the seasonal evolution of the EKW during CP and EP El Nino. It is shown that the strong positive asymmetry of ENSO (El Nino Southern Oscillation) is mostly reflected into the EKW activity of the EP El Nino whereas during CP El Nino, the EKW is negatively skewed in the eastern Pacific. Along with slightly cooler conditions off Peru (shallow thermocline) during CP El Nino, this is favourable for the accumulation of cooler SST anomalies along the coast by the remotely forced coastal Kelvin wave. Such a process is observed in a high-resolution regional model of the Humboldt Current system using the SODA outputs as boundary conditions. In particular the model simulates a cooling trend of the SST off Peru although the wind stress forcing has no trend. The model is further used to document the vertical structure along the coast during the two types of El Nino. It is suggested that the increased occurrence of the CP El Nino may also lead to a reduction of mesoscale activity off Peru. (c) 2012 Elsevier Ltd. All rights reserved.