Extratropical Air-Sea Interaction, Sea Surface Temperature Variability, and the Pacific Decadal Oscillation

We examine processes that influence North Pacific sea surface temperature (SST) anomalies including surface heat fluxes, upper ocean mixing, thermocline variability, ocean currents, and tropical-extratropical interactions via the atmosphere and ocean. The ocean integrates rapidly varying atmospheric heat flux and wind forcing, and thus a stochastic model of the climate system, where white noise forcing produces a red spectrum, appears to provide a baseline for SST variability even on decadal time scales. However, additional processes influence Pacific climate variability including the “reemergence mechanism,” where seasonal variability in mixed layer depth allows surface temperature anomalies to be stored at depth during summer and return to the surface in the following winter. Wind stress curl anomalies in the central/east Pacific drive thermocline variability that propagates to the west Pacific via baroclinic Rossby waves and influences SST by vertical mixing and the change in strength and position of the ocean gyres. Atmospheric changes associated with the El Niño–Southern Oscillation (ENSO) also influence North Pacific SST anomalies via the “atmospheric bridge.” The dominant pattern of North Pacific SST anomalies, the Pacific Decadal Oscillation (PDO), exhibits variability on interannual as well as decadal time scales. Unlike ENSO, the PDO does not appear to be a mode of the climate system, but rather it results from several different mechanisms including (1) stochastic heat flux forcing associated with random fluctuations in the Aleutian Low, (2) the atmospheric bridge augmented by the reemergence mechanism, and (3) wind-driven changes in the North Pacific gyres.