The roles of radiation and dynamical processes in the El Niño-like response to global warming

Abstract

The current understanding of ENSO does not foreshadow how it might change as a consequence of global warming due to an increase in atmospheric greenhouse gas (GHG) concentration. A number of global coupled climate models simulate a "mean" El Nino-like change in tropical Pacific temperatures, precipitation, and winds but at least one model exhibits a La Nina-like pattern and others a more or less homogeneous warming in the tropics with little of either pattern. The mechanisms leading to a mean positive El Nino-like pattern (PEP) are studied in simulations with the Canadian Centre for Climate Modeling and Analysis (CCCma) coupled general circulation model. The changes associated with the PEP are compared with, and are shown to closely resemble, those observed for the positive El Nino phase of the ENSO oscillation in the current climate including the anomalies in SST, precipitation and atmospheric circulation, the changes of vertically integrated energy and heat transports in the atmosphere, and changes in the sign and magnitude of radiative energy balance terms. The PEP in the model is supported by changes in oceanic heat transport and surface longwave radiative flux in the face of solar radiative flux and evaporative flux changes which act to damp it away. There is negative cloud-radiation feedback associated with the PEP, as with the observed El Nino. Negative cloud feedback by itself does not, therefore, preclude the existence of a PEP response to GHG forcing. The climatological PEP does not exhibit an oceanic export of energy from the tropical Pacific, as inferred for the regular El Nino event, but rather an oceanic import of energy. Nevertheless the PEP provides an effective means of regulating climate warming and the energy budget in the tropical Pacific which is accomplished through energy transports out of the region by the atmosphere. The PEP is seen as a more or less straightforward manifestation of the feedback mechanism proposed by Bjerknes and as a physically plausible response to GHG-induced climate warming.