Low-frequency Variability of Upper Ocean Heat Content Associated with Meridional Shifts of Western Boundary Current Extensions in the North Pacific

Generation and propagation processes of upper ocean heat content (OHC) are investigated using oceanic subsurface observations and an ocean general circulation model hindcast simulation. OHC anomalies are decomposed into two physically distinct components: (1) dynamical component　(OHCρ)　due to temperature anomalies that are associated with density anomalies and (2) spiciness component (OHCχ) due to temperature anomalies that are density-compensated with salinity. Analysis of the observational and the model data consistently shows that both dynamical and spiciness components contribute to interannual-to-decadal OHC variability with the latter component dominating in sub-polar regions in the North Pacific. OHCρ variability represents heaving of thermocline yielding westward propagation and intensification along the Kuroshio Extension as consistent with jet-trapped Rossby waves while OHCχ variability is eastward-propagating along subarctic frontal zone, suggesting advection by mean eastward currents. OHCχ variability tightly corresponds in space to horizontal mean spiciness gradient, whereas area-averaged OHCχ anomalies in the western subarctic frontal zone closely corresponds in time to meridional shifting of the subarctic frontal zone, the latter known to possibly influence on the atmospheric storm track and basin-scale circulations. Regression coefficient of the OHCχ time series on the frontal displacement anomalies quantitatively agree with the area-averaged mean spiciness gradient in the region, which confirms a hypothesis previously proposed by the authors on OHC generation mechanism via anomalous spiciness advection. These results suggest a crucial role of Western Boundary Current Extensions for decadal variability of OHC and its interaction with the atmosphere.