On the Climatic Impact of Wind Stress

A climate model is used to study the climatic impact of the stress exerted on the ocean by the atmosphere. When this stress is set to zero everywhere, the climate becomes much colder, with global-mean near-surface air temperature dropping from 14.8° to 6.1°C. The largest temperature decrease occurs in high latitudes, where sea ice advances equatorward to 40° of latitude. Many of these changes are induced by the changes in the oceanic circulation. In particular, with momentum flux set to zero, the meridional transport of buoyancy in the ocean, including that fraction often associated with the buoyancy-driven circulation, essentially vanishes and, hence, so does much of the surface heat flux. Vertical transport of buoyancy in the ocean is also strongly affected. In addition, the model suggests that the flux of momentum to the ocean has a profound indirect influence on the transport of latent heat. However, the total radiative flux entering the planet at low and midlatitudes does not change much. Instead, the net energy transport across 40°S increases, whereas that across 40°N decreases. The poleward energy transport in the atmosphere increases at midlatitudes in both hemispheres, whereas the oceanic heat transport decreases most strongly in the Northern Hemisphere. The climate becomes colder in both hemispheres, which is not easy to infer from the meridional transport of energy either by the climate system as a whole or by its individual components. Furthermore, the model suggests that it is the wind stress driving the midlatitude oceans—that is, where the oceanic heat transport accounts for only a very tiny fraction of the total poleward energy transport by the climate system, which is of more importance for maintaining the mean position of sea ice edge and, hence, much of the global climate.