Ocean Heat Transport and Water Vapor Greenhouse in a Warm Equable Climate: A New Look at the Low Gradient Paradox

The authors study the role of ocean heat transport (OHT) in the maintenance of a warm, equable, ice-free climate.An ensemble of idealized aquaplanetGCMcalculations is used to assess the equilibrium sensitivity of global mean surface temperature (T) and its equator-to-pole gradient (DT) to variations in OHT, prescribed through a simple analytical formula representing export out of the tropics and poleward convergence. Lowlatitude OHT warms the midto high latitudes without cooling the tropics; T increases by 18C and DT decreases by 2.68C for every 0.5-PW increase inOHTacross 308 latitude. This warming is relatively insensitive to the detailed meridional structure of OHT. It occurs in spite of near-perfect atmospheric compensation of large imposed variations in OHT: the total poleward heat transport is nearly fixed. The warming results from a convective adjustment of the extratropical troposphere. Increased OHT drives a shift from large-scale to convective precipitation in the midlatitude storm tracks. Warming arises primarily from enhanced greenhouse trapping associated with convective moistening of the upper troposphere. Warming extends to the poles by atmospheric processes even in the absence of high-latitude OHT. A new conceptual model for equable climates is proposed, in which OHT plays a key role by driving enhanced deep convection in the midlatitude storm tracks. In this view, the climatic impact of OHT depends on its effects on the greenhouse properties of the atmosphere, rather than its ability to increase the total poleward energy transport.