Decadal changes in the North Pacific oceanic frontal zones as revealed in ship and satellite observations

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Abstract
Influence of decadal climate changes upon the North Pacific subarctic and subtropical oceanic frontal zones (NP‐SAFZ and NP‐STFZ, respectively) is documented. Both satellite‐ and ship‐measured sea surface temperature (SST) data reveal the strong confinement of wintertime decadal SST anomalies to those frontal zones. The satellite data indicate that the wintertime NP‐SAFZ tended to be narrow and shift northward under the weakened surface westerlies during a warm event in the early 1990s. The opposite was the case under the enhanced westerlies during a cool event in the mid‐1980s. Decadal‐scale modulations in the frontal intensity are also evident in the satellite data. A diagnosis based on a simple linearized mixed layer model suggests that the observed anomalous frontal intensity was contributed by the sign reversal of anomalous wind stress curl through the Ekman processes and by an anomaly dipole of the net heat flux around the SAFZ. These decadal‐scale modulations in the NP‐SAFZ and their relationships with decadal fluctuations in the local surface westerlies are confirmed in the ship‐measured data over the 4 recent decades. It is found that the SAFZ happens to be a region where the decadal variability in the surface westerlies are unlikely to be masked by the higher‐frequency fluctuations, as it almost coincides with the ridge/trough line of the remote atmospheric response to the El Niño/Southern Oscillation.