Zonally symmetric variability of the Northern midlatitude Oceanic Heat Content in CESM2

The characteristics of the simultaneous Ocean Heat Content (OHC) variability in the Northern hemisphere Western boundary current region (the Kuroshio and its extension in the North Pacific and the Gulf stream in the North Atlantic) are investigated. The monthly pre-industrial control experiment data of CESM2 is used to investigate the characteristics of internal variability in OHC. Empirical Orthogonal Functions (EOF) analysis is performed on the winter mean OHC integrated up to various depth levels in the domain of latitude (25°N ~65°N) for 1200 years.

We focus on the first leading mode, in which the OHC anomalies of the upper 1000m represent a synchronized variation in the northern midlatitude between the Pacific and the Atlantic. The variance explained by this mode increases as the integration includes deeper layers, or as the anomalies are low-pass-filtered. The principal component (PC) 1 exhibits a significant positive correlation at 0.32 when Arctic Oscillation (AO) precedes by 1 year. This mode resembles the one previously found in the observation (Kelly and Dong, 2004).

Associated with PC1, the wind stress curl in each domain exhibits significant positive wind stress curl 4~5 years in advance and shows the strongest negative anomalies when the WSC leads the PC1 by 1 year. No significant relationship is found between the PC1 and the lagging WSC. Similar lead-lag asymmetry is found the oceanic heat advection. Both the wind stress curl and oceanic lateral advection indicates the warm (cold) OHC associated with PC1 is driven by thickening (shoaling) of the upper layer due to Ekman downwelling (upwelling) and the warm (cold) advection via the responding geostrophic currents. Meanwhile, the net surface heat flux shows symmetric lead-lag relationship, with the strongest negative correlation with PC1 at lag 0, indicating that the primary role of the surface net heat flux in this region is damping excessive oceanic heat to the atmosphere.

A power spectral analysis on PC1 exhibits a red-noise type power spectrum with some notable peaks on decadal and multidecadal bands. The remaining question, whether the northern midlatitude synchronous oceanic variability between the Pacific and Atlantic is simply the integrated response to the random AO signal through ocean's dynamic pathways, is further investigated.