Decadal 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) in the North Pacific are investigated using oceanic subsurface observations and an eddy-resolving ocean general circulation model hindcast simulation. OHC anomalies are decomposed into physically distinct dynamical components (OHC rho) due to temperature anomalies that are associated with density anomalies and spiciness components (OHC chi) due to temperature anomalies that are density compensating with salinity. Analysis of the observational and model data consistently shows that both dynamical and spiciness components contribute to interannual-decadal OHC variability, with the former (latter) component dominating in the subtropical (subpolar) North Pacific. OHC rho variability represents heaving of thermocline, propagates westward, and intensifies along the Kuroshio Extension, consistent with jet-trapped Rossby waves, while OHC chi variability propagates eastward along the subarctic frontal zone, suggesting advection by mean eastward currents. OHC chi variability tightly corresponds in space to horizontal mean spiciness gradients. Meanwhile, area-averaged OHC chi anomalies in the western subarctic frontal zone closely correspond in time to meridional shifts of the subarctic frontal zone. Regression coefficient of the OHC chi time series on the frontal displacement anomalies quantitatively agree with the area-averaged mean spiciness gradient in the region, and suggest that OHC chi is generated via frontal variability in the subarctic frontal zone.