Air-sea interaction in the Pacific: the North Pacific Oscillation, heat fluxes, and ENSO (Invited)

The great expanse of the North Pacific is a natural place to look for air-sea interactions that drive important climate variability. This is especially so in light of the strong low-frequency variability that is observed in the North Pacific, as exemplified by the pronounced climate shift in the region that occurred in the late 1970s. Results from most modeling studies are consistent with the interpretation that the majority of this variability comes from simple thermodynamic forcing of the ocean by the atmosphere. A sensitive test of this paradigm is to see to what degree the observed relationships between the ocean and atmosphere can be forced by specifying oceanic conditions in model studies. We show the results of such a test, using the observed relationship between the North Pacific Oscillation (NPO), ENSO, and wintertime climate over North America as a diagnostic, to determine to what extent this relationship is conditioned by North Pacific ocean temperatures.

Modifications to the simple thermodynamic picture that arise from ocean dynamics are generally confined to the western boundary region. In particular, ocean anomalies in the region where the western boundary current separates from the coast are influenced by basin-wide processes. There is some model evidence that the atmosphere can be influenced by oceanic processes in this region, at least locally. In this case, it is modifications to the ocean-atmosphere heat flux that are most directly responsible for the atmospheric response, which suggests that specified heat-flux anomaly experiments are valuable for understanding this interaction. The results of such experiments are shown, and demonstrate that heat flux forcing in key regions can have effects that are not entirely captured by fixed-SST experiments.