Remote Wind-Driven Overturning in the Absence of the Drake Passage Effect

Abstract

Zonal wind stress over the Southern Ocean may be responsible for a significant fraction of the meridional overturning associated with North Atlantic Deep Water. Numerical experiments by Tsujino and Suginohara imply that the zonal periodicity of the Southern Ocean is not necessary for midlatitude westerly winds to drive strong remote meridional overturning. Here, idealized numerical experiments examine the importance of zonal periodicity and other factors in setting the sensitivity of this overturning to the wind stress. These experiments support the conclusion that the wind can drive remote overturning in the absence of zonal periodicity. However, making the subpolar ocean zonally periodic roughly doubles the strength of the overturning induced by the wind there. Tsujino and Suginohara's experiments are especially sensitive to wind stress because their basin has a relatively small meridional range, which increases the Ekman transport associated with the wind stress. Depending on the stratification in the wind-forcing region, the heating associated with the westerly winds can occur almost exclusively near the surface or deeper in the thermocline as well. Subsurface cooling in the wind-forcing region reduces the remote effects and can occur through both vertical or horizontal diffusion. A scale analysis of the heat budget suggests that sufficiently strong subpolar westerlies produce remote overturning because there is no way for local cooling to balance wind-induced surface heating. Tsujino and Suginohara suggested that wind increases the overturning by enhancing the mixing-driven thermohaline circulation. However, an increase in thermohaline circulation is associated with increased conversion of turbulent kinetic energy to potential energy. This increase in the energy conversion is absent in the wind-driven case, indicating an important qualitative difference between mixing-driven thermohaline overturning and remote wind-driven overturning.