Distinct Mechanisms of Ocean Heat Transport Into the Arctic Under Internal Variability and Climate Change

Northward ocean heat transport (OHT) plays a key role in Arctic climate variability and change. Unforced climate model simulations suggest that at decadal and longer timescales, strengthened Atlantic meridional overturning circulation (AMOC) is correlated with increased OHT into the Arctic. Yet greenhouse gas forced simulations predict increased Arctic OHT while AMOC weakens. Here we partition OHT changes into contributions from dynamic circulation changes and thermodynamic temperature advection, as well as meridional overturning and gyre changes. We find that under decadal-scale internal variability, strengthened AMOC converges heat in the subpolar gyre; anomalous heat is advected into the Arctic by both time mean circulations and strengthened gyre circulations. Under greenhouse gas forcing, weakened AMOC reduces subpolar gyre heat convergence; yet Arctic OHT increases as mean overturning and strengthened gyre circulations advect warmed surface waters. Thus, caution should be exercised when inferring Arctic OHT from AMOC, as the relationship between OHT and AMOC changes depends on whether they are internally generated or externally forced.