Northern high latitude heat budget decomposition and transient 1 warming

6 Many climate models are unable to capture the magnitude of observed warming and sea ice 7 decline in high northern latitudes. To understand the factors affecting the simulated warming 8 we compare the response to increasing CO2 in two pairs of climate models by decomposing 9 the 40–90◦N region heat budget. Each pair includes a member with enhanced global and 10 northern high latitude surface temperature increase and smaller ocean heat uptake efficiency 11 compared with its counterpart member. Significant differences in sensitivity can be traced to 12 formulation differences between the model pairs. The difference in surface heat flux pertur13 bation between the models is the main forcing of the differences in temperature increase and 14 ocean heat uptake. Atmospheric heat transport and outgoing longwave radiation counteract 15 the model differences in northern high latitude warming while shortwave radiation differ16 ences enhance it. The surface flux perturbation difference is associated with a difference in 17 the North Atlantic ocean convection in the control climates: When Labrador Sea convection 18 is present in the control, it weakens in the perturbed climate leading to a larger reduction 19 in ocean overturning and heat transport to northern high latitudes, a relatively cooler high 20 latitude ocean surface, increased ocean heat uptake, and a reduction in high latitude atmo21 spheric warming compared to the model counterpart with stable Labrador Sea convection. 22 Because the 40–90◦N region accounts for up to 40% of the global ocean heat uptake the 23 process described here may substantially influence the global heat uptake efficiency. 24