Journal of Climate 2012 ; e-View
Mean Climate Controls on the Simulated Response of ENSO to Increasing Greenhouse Gases
Pedro N. DiNezio
International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HawaiiBen P. Kirtman
Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida, NOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, FloridaGabriel A. Vecchi
Climate model experiments are analyzed to elucidate if and how the changes in mean climate in response to doubling of atmospheric CO2 (2xCO2) influence ENSO. The processes involved the development, transition, and decay of simulated ENSO events are quantified through a multi-model heat budget analysis. The simulated changes in ENSO amplitude in response to 2xCO2 are directly related to changes in the anomalous ocean heat flux convergence during the development, transition, and decay of ENSO events. This consistency relationship results from the Bjerknes feedback and cannot be used to attribute the changes in ENSO. In order to avoid a circular argument, we compute the anomalous heat flux convergence due to the interaction of the ENSO anomalies in the pre-industrial climate with the 2xCO2 changes in mean climate. The weakening of the Walker circulation and the increased thermal stratification, both robust features of the mean climate response to 2xCO2, play opposing roles in ENSO - mean climate interactions. Weaker upwelling in response to a weaker Walker circulation drives a reduction in thermocline-driven ocean heat flux convergence (i.e., thermocline feedback), and thus reduces the ENSO amplitude. Conversely, a stronger zonal subsurface temperature gradient, associated with the increased thermal stratification, drives an increase in zonal current-induced ocean heat flux convergence (i.e., zonal advection feedback), and thus increases the ENSO amplitude. These opposing processes explain the lack of model agreement in whether ENSO is going to weaken or strengthen in response to increasing greenhouse gases, but also why ENSO appears to be relatively insensitive to 2xCO2 in most models.