The Impact of Extratropical Atmospheric Variability on ENSO: Testing the Seasonal Footprinting Mechanism Using Coupled Model Experiments

Abstract

Previous studies suggest that extratropical atmospheric variability influences the tropics via the seasonal footprinting mechanism (SFM), in which fluctuations in the North Pacific Oscillation (NPO) impact the ocean via surface heat fluxes during winter and the resulting springtime subtropical SST anomalies alter the atmosphere–ocean system over the tropics in the following summer, fall, and winter. Here, the authors test the SFM hypothesis by imposing NPO-related surface heat flux forcing in an atmospheric GCM coupled to a reduced gravity ocean model in the tropics and a slab ocean in the extratropics. The forcing is only imposed through the first winter, and then the model is free to evolve through the following winter. The evolution of the coupled model response to the forcing is consistent with the SFM hypothesis: the NPO-driven surface fluxes cause positive SST anomalies to form in the central and eastern subtropics during winter; these anomalies propagate toward the equator along with westerly wind anomalies during spring, reach the equator in summer, and then amplify, which leads to an ENSO event in the following winter. The anomalies reach the equator through a combination of thermodynamically coupled air–sea interactions, namely, the wind–evaporation–SST (WES) feedback and equatorial ocean dynamics. The initial off-equatorial anomaly propagates toward the equator through a relaxation of the climatological easterly winds south of the dominant SST anomalies, which leads to a reduction in upward latent heat flux. These westerly anomalies reach the equator during boreal summer, where they can excite downwelling equatorial Kelvin waves. The connection between off-equatorial variations and tropical ENSO-like conditions may also occur via the excitation of westward-propagating equatorial Rossby waves during spring, which reflect off of the western boundary as Kelvin waves, depressing the thermocline in the eastern Pacific during the following summer. NPO-related anomalies that form during the first winter in the tropical Pacific may also contribute to the development of an El Niño event in the following winter. The imposition of the NPO-related forcing caused warming in the ENSO region in ∼70% of the ensemble of 60 simulations; therefore, the response depends on the state of the tropical atmosphere–ocean system. For years where the control simulation was poised to develop into a neutral or negative ENSO event, the addition of the NPO heat fluxes tended to cause anomalous warming in the tropical Pacific in the following fall/winter; if the control was heading toward a warm ENSO event, the imposition of NPO forcing tends to reduce the amplitude of that event.