The Role of the “Atmospheric Bridge” in Linking Tropical Pacific ENSO Events to Extratropical SST Anomalies

The role of the atmospheric circulation as a “bridge” between sea surface temperature (SST) anomalies in the tropical Pacific and those in the midlatitude northern oceans is assessed. The key processes associated with this atmospheric bridge are described using output from four independent simulations with a general circulation model subjected to month to month SST variations observed in the tropical Pacific during the 1946–1988 period and to climatological SST conditions elsewhere (the “TOGA” runs). In episodes with prominent SST anomalies in the tropical Pacific, extratropical perturbations in the simulated atmospheric temperature, humidity, and wind fields induce changes in the latent and sensible heat fluxes across the air-sea interface of the midlatitude oceans. These anomalous fluxes in turn lead to extratropical SST changes. The relevance of the atmospheric bridge mechanism is evaluated by driving a motionless, 50-m deep oceanic negative mixed layer model at individual grid points with the local surface fluxes generated in the TOGA runs. The negative feedback of the mixed layer temperature anomalies on the imposed flux forcing is taken into account by introducing a linear damping term with a 5-mouth dissipative time scale. This simple system reproduces the basic spatial and temporal characteristics of the observed SST variability in the North Pacific and western North Atlantic. The two-way air-sea feedbacks associated with the atmospheric bridge are investigated by performing four additional 43-year runs of a modified version of the TOGA Experiment. These new “TOGA-ML” runs predict the ocean temperature outside the tropical Pacific by allowing the atmosphere to interact fully with the same mixed layer model mentioned above. The results support the notion that midlatitude ocean-atmosphere interaction can be modeled as a first-order Markov process, in which the red-noise response of mixed layer temperature is driven by white–noise atmospheric forcing in the presence of linear damping. The amplitude of near-surface atmospheric anomalies appearing in the TOGA-ML runs is higher than that in the TOGA runs. This finding implies that, in the TOGA-ML scenario, the midlatitude oceanic responses to atmospheric driving could exert positive feedbacks on the atmosphere, thereby reinforcing the air-sea coupling. The enhanced atmosphere-ocean interactions operating in TOGA-ML prolong the duration of persistent meteorological episodes in that experiment. A comprehensive survey is conducted of the persistence characteristics simulated in TOGA, TOGA- ML, and several other experiments subjected to prescribed SST forcing at various sites. Model scenarios in which observed tropical Pacific SST anomalies act in conjunction with SST perturbations in midlatitudes (either prescribed or predicted) are seen to produce the highest frequency of persistent events.