Comparing what drives ocean heat transport into the Arctic in pre-industrial control and greenhouse-gas forced climate model simulations
Key Points & Overview
- Mechanisms of ocean heat transport into the Arctic, that is, across 70°N, are distinct under internal variability and climate change
- Under decadal internal variability, increased northward ocean heat transport at 70°N is linked to strong midlatitude meridional overturning
- Under greenhouse gas forcing, midlatitude overturning weakens, yet northward ocean heat transport at 70°N increases due to regional circulations
Over the past few decades, the Arctic has been warming approximately twice as rapidly as globally-averaged temperatures. This Arctic amplification of greenhouse-gas (GHG) warming is also a robust feature of global climate model simulations. There is growing evidence that changes in northward ocean heat transport (OHT) into the Arctic from the North Atlantic play a key role in driving changes in Arctic climate both under internal variability and under climate forcing.
Previous studies have found that the strength of the Atlantic meridional overturning circulation (AMOC) — which transports warm salty water to the North Atlantic — is positively correlated with changes in ocean heat transport into the Arctic in model simulations representing the preindustrial climate. The apparent mechanism is that an enhanced AMOC leads to enhanced OHT throughout the North Atlantic and high latitudes. However, in GHG-forced simulations, AMOC weakens, reducing northward heat transport into the subpolar gyre, yet OHT into the Arctic increases, which is at odds with what was found under internal variability.
In our study we analyze data from a preindustrial control simulation and a CO2 quadrupling simulation in one climate model. For our internal variability analysis, we focus on periods in the preindustrial control simulation where OHT into the Arctic (i.e., at 70°N) is anomalously high and look at how much ‘dynamic’ changes in ocean circulation and ‘thermodynamic’ changes in temperature contribute to the Arctic OHT anomalies. For our CO2 quadrupling analysis, we focus on the response of the climate model by averaging over a 30-year period 100 years after the CO2 increase, permitting us to study the effect of climate change on AMOC. We find that under internal variability, a strong AMOC enhances heat convergence in the southern subpolar gyre. This anomalous heat is carried into the higher latitudes by gyre circulations. Under an abrupt CO2 quadrupling, AMOC weakens, reducing heat convergence in the subpolar gyre. Due to GHG forcing, surface water everywhere northward of 60°N has warmed, resulting in thermodynamic northward advection of heat by regional ocean circulations and heat convergence near 70°N. At 70°N, heat is anomalously carried northward both by thermodynamic advection by time mean overturning and dynamic heat redistribution by strengthened gyre circulations.
Our results indicate that knowledge of AMOC trends alone is insufficient to infer even the sign of trends in OHT into the Arctic, as it matters whether those trends are internally generated or externally forced. Caution should be exercised when applying the results of studies linking AMOC anomalies to changes in OHT at 70°N under internal variability to predicting how Arctic climate will change under anthropogenic warming.