Sea Ice and Clouds Mediate Compensation between Atmospheric and Oceanic Meridional Heat Transports
Bjerknes Compensation (BjC) is the process through which poleward ocean heat transport anomalies are compensated by opposing anomalies in atmospheric heat transport, hence reducing their net impact on the Arctic energy budget. Understanding the processes responsible for Bjerknes Compensation, and how they are represented in Earth system models, will help us better predict the response of the Arctic to expected increases in ocean heat transport into the Arctic.
Ocean heat transport into the Arctic Ocean is an important driver of the rapidly accelerating warming of the Arctic Earth system (Arctic Amplification). Recent increases in Atlantic and Pacific inflows into the Arctic have already led to significant changes in the Arctic marine system that is referred to as Borealization of the Arctic. However, the Earth system is known to compensate for increased ocean heat transport by reducing poleward atmospheric heat transport. This process, referred to as Bjerknes Compensation (BjC), is a negative feedback on Arctic Amplification. In this study, we study the processes leading to BjC in an ensemble of 18 CMIP6 models. We find that BjC depends strongly on the sensitivity of sea ice in a given model to OHT variability, as the ice-albedo feedback plays an important role in amplifying the heat content anomalies induced by the ocean. The model spread in sea cover hence explains the spread in BjC in CMIP6 models. Clouds have a secondary impact and generally tend to dampen BjC.
Bjerknes Compensation refers to the anticorrelation observed between atmospheric and oceanic heat transport (AHT/OHT) variability, particularly on decadal to longer time scales that may be important to the predictability of the climate system. This study investigates the spread in BjC across fully coupled simulations of phase 6 of the Coupled Model Intercomparison Project (CMIP6) forced by pre-industrial levels of greenhouse gases and critical processes—particularly related to sea ice and clouds—that may contribute to that spread. BjC on decadal to longer time scales is confirmed across all the simulations evaluated, and it is strongest in the Northern Hemisphere (NH) between 60° and 70°N. At these latitudes, BjC appears to be primarily driven by the exchange of turbulent fluxes (sensible and latent) in the Greenland, Iceland, and Barents Seas. Metrics to break down how sea ice and clouds uniquely modify the radiative balance of the polar atmosphere during anomalous OHT events are presented. These metrics quantify the impacts of sea ice and clouds on the surface and top of the atmosphere (latent, sensible, longwave, and shortwave radiative) energy fluxes. Cloud responses tend to counter the clear sky impacts over the Marginal Ice Zone (MIZ). It is further shown that the degree of BJC present in a given simulation at high latitudes is heavily influenced by the sensitivity of the sea ice to OHT, which is most influential over the MIZ. These results are qualitatively robust across models and explain the inter-model spread in NH BJC in the preindustrial control experiment.