Investigating Bjerknes Compensation under the abrupt-4xCO2 CMIP6 experiment

In 1964 J. Bjerknes postulated that, when an anomaly occurs in ocean heat transport (OHT), the atmosphere heat transport (AHT) exhibits an anomaly of opposite sign so that the top of the atmosphere (TOA) transport is approximately preserved. This phenomenon is now known as Bjerknes Compensation (BJC) and has been the object of many studies in the context of steady state climate simulations, on decadal and centennial time scales. Here, we examine BJC under extreme climate forcing, specifically under the quadrupling of atmospheric CO₂ in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The models exhibit a pronounced decline in the Atlantic Meridional Overturning Circulation (AMOC), to varying degrees, in response to melting sea ice and increased freshwater runoff. The OHT is reduced accordingly, and this can trigger an increase in AHT, particularly in the Northern Hemisphere. We examine the degree of BJC, in the context of model climate sensitivity. We also examine how changes in overturning in the Southern Hemisphere impact AHT there. The issue is significant, as increased AHT partially compensates for the cooling implied by reduced OHT.