Thermal forces in the Southern Ocean upwelling

Morrison, Frölicher, and Sarmiento reply: William Kininmonth makes a number of unsubstantiated assertions, of which we address just two, due to space constraints. First, he posits that bottom water ascends to form equatorial surface water, being heated along the way. Second, he suggests that recent global warming may be partially caused by a slowing of the oceanic overturning circulation. We disagree with both of those unreferenced claims.

The central basis of our article is that the majority of the global upwelling occurs along predominantly adiabatic pathways—that is, no heat input occurs—in the Southern Ocean. That core idea is based on a large body of research. ^1–3 Rather than warming as it ascends as Kininmonth claims, the cold, dense water warms primarily at the surface as it travels northward out of the Southern Ocean.

The dynamics of the oceanic overturning circulation leads to a cooling, not warming, of the atmosphere in response to a slowdown of the overturning. The ocean circulation transports heat from the equator to the poles. Reducing the poleward heat transport leads to cooling of the ocean surface at high latitudes and enhanced sea-ice growth. Through an ice-albedo feedback, less incoming shortwave radiation is absorbed and the atmosphere cools. That dynamical sequence driven by changing ocean circulation has been extensively studied by climate scientists. ^{4 , 5}