Impact of ENSO on SST Variability in the North Pacific and North Atlantic: Seasonal Dependence and Role of Extratropical Sea–Air Coupling

Abstract

The influences of El Niño–Southern Oscillation (ENSO) events in the tropical Pacific on interannual variability of the coupled ocean–atmosphere systems in the North Pacific and North Atlantic have been studied using a suite of experiments with a rhomboidal 30-wavenumber, 14-layer general circulation model (GCM). Observed month-to-month fluctuations of the sea surface temperature (SST) in the tropical Pacific during the 1950–95 period were prescribed as the lower boundary condition for the GCM. The SST conditions outside of the tropical Pacific were predicted by a simple ocean mixed layer model with a constant depth. Four independent integrations under this “Tropical Ocean–Global Atmosphere–Mixed Layer (TOGA-ML)” scenario were conducted. Both observational and model results indicate that the imposed ENSO forcing during midwinter is accompanied by prominent atmospheric circulation changes over the North Pacific and Atlantic. These teleconnection patterns in turn alter the heat exchange across the local sea–air interface. The extratropical SST anomalies generated by this “atmospheric bridge” mechanism typically attain maximum amplitudes in late winter or early spring. Detailed diagnoses of the monthly evolution of the surface heat budget during ENSO episodes at selected sites have been performed. The simulated SST response exhibits a 1–2 month delay relative to temperature changes in the overlying atmosphere. This lag relationship is associated with a polarity change of sea-to-air gradients in anomalous temperature and water vapor mixing ratio in late winter. From late autumn through midwinter, the action of the climatological wind on these gradients results in enhancement of the developing SST anomalies. In the months thereafter, the reversed gradients lead to attenuation of the SST signal. Shortwave radiative fluxes associated with variations in cloud cover play an important role in SST variability at some of the subtropical sites. The nature of sea–air feedbacks in the extratropics has been studied by contrasting the output from the TOGA-ML experiment and from another “TOGA” experiment in which two-way interactions between the atmosphere and ocean outside of the tropical Pacific were eliminated. Incorporation of sea–air coupling in TOGA-ML is seen to enhance the persistence of the ENSO-related atmospheric anomalies in the extratropics through late winter and early spring. Comparison with results from previous studies on midlatitude sea–air interactions suggests that part of the atmospheric signal in TOGA-ML may be attributed to forcing from extratropical SST anomalies produced by the atmospheric bridge mechanism.