Mechanisms driving Subpolar North Atlantic Upper Ocean Heat Content Predictability in CMIP6 Decadal Prediction Systems

Previous work has indicated that the subpolar North Atlantic Ocean exhibits particularly high decadal predictability, influenced by both external forcing and predictable internal variability as a result of large-scale ocean processes. The mechanism driving subpolar North Atlantic (SPNA) upper ocean heat content (UOHC) predictive skill identified in the Decadal Prediction Large Ensemble of CESM (CESM-DPLE) is linked to predictable barotropic gyre and AMOC circulations, with the ocean memory linked to the Labrador Sea Water (LSW) thickness, further corroborated by other studies. Here, we investigate whether this mechanism holds in CMIP6 decadal prediction systems with variable SPNA UOHC skill by analysing lagged regressions between initial LSW deep density and AMOC, sea-surface height, the barotropic streamfunction, deep ocean density, and UOHC. We further investigate lagged regressions between the deep ocean density in the Irminger-Iceland Basins (IIB) and these same variables to determine whether some models show a stronger connection between the SPNA UOHC and the IIB density. We have determined that models with higher SPNA UOHC skill tend to exhibit stronger correlations between the SPNA UOHC at later years and the initial LSW density (i.e., the density at the first month after initialisation). However, high model predictive skill in this initial density is not necessarily associated with higher skill in the subsequent SPNA UOHC. In higher skill models, such as CESM2-DP, CESM1-DP and HadGEM3-GC31-MM, densification in the deep Labrador Sea (1000m-2500m) is associated with a near-simultaneous increase in the AMOC strength and spin up of the subpolar gyre (SPG) as well as a subsequent warming in the subpolar North Atlantic, which later spreads to the western SPG as well. In these models, deep density anomalies accumulate between 1000m-2500m and propagate eastwards at 45°N. In low-skill models, such as CanESM5, IPSL-CM6A-LR, FGOALS-f3-L or BCC-CSM2-MR, LSW densification exhibits either no link to AMOC strength or yields only a brief period of strong AMOC, and is not associated with a persistent warming pattern in the SPNA at later years in the simulations. In these models, density anomalies at depth at 45°N appear in the initial years, but dissipate rapidly and do not propagate eastwards.