Origin of Decadal-Scale, Eastward-Propagating Heat Content Anomalies in the North Pacific*

Abstract

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 GCM integration. 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 RW signals. Wind-forced, westward-propagating equivalent barotropic RWs cause meridional shifts of the subarctic front in the KOE region. The associated anomalous circulation crosses mean temperature and salinity gradients and thereby generates spiciness anomalies. These anomalies are advected eastward by the mean currents, while the associated surface temperature anomalies are damped by airsea heat exchange. The accompanying surface buoyancy flux generates higher baroclinic, eastward-propagating RWs. The results suggest that the large OHC variability in the western boundary currents and their extensions is associated with the spiciness gradients and axial variability of oceanic fronts.

Eastward salinity anomaly propagation in the intermediate layer of the North Pacific

“…These findings suggest that the trend and periodic fluctuations in oxygen might propagate from the east to the west on isopycnal surfaces. Furthermore, Taguchi and Schneider [] showed with long‐term data obtained by high‐resolution numerical modeling that spiciness variability corresponding to salinity anomalies on isopycnal surfaces generally corresponds to large anomalies of heat content in upper layers (above 400 m) along the subarctic front. Osafune and Yasuda [] showed by numerical experiments that layer thickness and temperature anomalies, which are induced by the 18.6 year tidal mixing change in their model, can propagate eastward in the subsurface layers along the subarctic front, and suggested that the low‐mode long Rossby wave corresponding to the thickness variability propagations plays a role in the temperature variability.…”

“…Thus, the eastward propagation of the variability might affect the climate variability in the North Pacific. Although the propagation of anomalies on isopycnal surfaces along the subarctic front has been inferred from the results of numerical models [e.g., Deutsch et al ., ; Osafune and Yasuda , ; Taguchi and Schneider , ] and climatological circulation patterns [e.g., Ueno and Yasuda , ; Masujima and Yasuda , ] described in previous studies, observations of actual anomalies and their propagation over broad areas, especially in deeper ocean layers, have been reported by only a few studies.…”

Impacts of regional mixing on the temperature structure of the equatorial Pacific Ocean. Part 1: Vertically uniform vertical diffusion

“…6a illustrates the striking differences that can occur in the propagation of dynamical (δ T SE ) and spiciness (δ T SE ) signals, in this case with the spiciness signal extending more prominently equatorward. Similar differences occur in our other regional solutions, and have been noted previously by Nonaka and Xie (2000) and Taguchi and Schneider (2014).…”

“…See Appendix A for a derivation of (6) and the definitions of δ T e and δ T e . Schneider (2004) and Taguchi and Schneider (2014) provide alternative derivations. Below the surface mixed layer, each component has a distinct physical interpretation, with δ T e arising primarily from wave adjustments and δ T e from advection (Section 3.2.3; Appendix A).…”

“…Then, provided that δρ e is small, we can write dication that it is associated with the vertical displacement of density surfaces. An alternative (but equivalent) formalism of the separation is presented by Schneider (2004) and the separation is extended to more general cases by Taguchi and Schneider (2014).…”