Origin of decadal-scale, eastward-propagating heat content anomalies in the North Pacific

Upper ocean heat content (OHC) is at the heart of natural climate variability on interannual-to-decadal time scales, providing climate memory and the source of decadal prediction skill. In the midlatitude North Pacific Ocean, OHC signals are often found to propagate eastward as opposed to the frequently observed westward propagation of sea surface height, another variable that represents the ocean subsurface state. This dichotomy is investigated using a 150-yr coupled GCMintegration. Simulated OHC signals are distinguished in terms of two processes that can support eastward propagation: higher baroclinic Rossby wave (RW) modes that are associated with density perturbation, and spiciness anomalies due to density-compensated temperature and salinity anomalies. The analysis herein suggests a unique role of the Kuroshio-Oyashio Extension (KOE) region as an origin of the spiciness and higher mode RWsignals.Wind-forced, westward-propagating equivalent barotropic RWs cause meridional shifts of the subarctic front in the KOE region. The associated anomalous circulation crossesmean temperature and salinity gradients and thereby generates spiciness anomalies. These anomalies are advected eastward by themean currents,while the associated surface temperature anomalies are damped by air- sea heat exchange. The accompanying surface buoyancy flux generates higher baroclinic, eastward-propagating RWs. The results suggest that the largeOHC variability in the western boundary currents and their extensions is associated with the spiciness gradients and axial variability of oceanic fronts.