Impact of Westerly Wind Position on the Circulation of the Southern Ocean

 

By J.L. Russell, A. Gnanadesikan and J.R. Toggweiler

 

The location of the westerly winds in the Southern Hemisphere represents an important feature of global climate. The poleward shift in the Southern Hemisphere Westerlies represents one of the most obvious trends in the global climate system over the past 20 years. Equatorward shifts in the westerly winds have been proposed by paleoceanographers to explain climate shifts connected with the Last Glacial Maximum. However, very few model studies have been done to evaluate the impact of such shifts on global climate. A significant number of model studies have investigated the ocean's response to imposed variations in the magnitude of the zonal wind stress over the Southern Ocean (e.g., Toggweiler and Samuels, 1993, 1995; McDermott, 1996; Rahmstorf and England, 1997; Gnanadesikan, 1999; Gnanadesikan and Hallberg, 2000; Hallberg and Gnanadesikan, 2005). None of these studies, however, address the issue of how realistic changes in the location of the stress can affect the structure of the circulation.

The Southern Ocean circulation should be particularly sensitive to the strength and position of the Westerly Winds because the lack of topography within the latitudes of Drake Passage means that the effect of the mid-latitude westerlies must penetrate all the way to the bottom. One result of this is that some component of the upwelling produced by the divergent Ekman flow is relatively salty and comes from great depth (Gordon, 1971).  A component of this upwelled deep water is entrained into the Ekman layer and advected north towards the Subantarctic Front (Gordon, 1971). Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) form when Antarctic surface waters freshen, cool and mix with upper circumpolar deep water in deep winter mixed layers, creating the ubiquitous subsurface salinity minimum. The location of the wind would be expected to control the composition of this deep upwelling, as well as the fluxes of the watermasses with which it mixes to form AAIW and SAMW. However, it has been difficult to study such changes, as in ocean only models changes in winds also result in large changes in air-sea fluxes of heat and moisture.

In two coupled models recently developed at GFDL, the position of the Southern Hemisphere extratropical westerly jet differs substantially (the difference arises primarily from the use of differing numerical techniques employed in the atmosphere). The mid-latitude storm tracks in both hemispheres are shifted poleward in CM2.1 relative to CM2.0, with the larger shift (order 3°-4°) in the Southern Hemisphere. However, the zonally averaged heat and freshwater fluxes are very similar. We can therefore exploit the different climate simulations produced by the two atmospheric components of the GFDL coupled climate model to explore the impact of changes in the position of the Westerly Winds on the circulation of the Southern Ocean.

Figure 1: Wind stress in the GFDL coupled models compared with observations. Upper left: Zonal stress in NCEP reanalysis. Upper middle: Zonal stress in CM2.0. Upper right: Zonal stress in CM2.1. Lower left: Difference between CM2.1 and CM2.0. Lower middle: Error in zonal stress, CM2.0. Lower right: Error in Zonal stress, CM2.1. (Russell, Gnanadesikan and Toggweiler, subm. J. Climate)

 

 

Consistent with observed trends in the ocean (Gille, 2002; Thresher et al., 2004), a poleward shift of the surface expression of the Westerly Winds warms and salinizes SAMW, while simultaneously, freshening and cooling AAIW along the eastern boundaries of the basins. The poleward shift of the winds improves the injection of Subtropical Surface Water around Tasmania (and reduces the spurious injection by the Agulhas), thereby improving the character of both Subantarctic Mode Water and Antarctic Intermediate Water. This impact of wind shift has not been documented previously. The Southern Ocean deep waters are also affected by the shift: deep water upwelling intensifies around Antarctica when the divergence increases around the continent. The increased upwelling of saltier subsurface waters increases the efficiency and quality of AABW formation.

The sensitivity of the circulation to shifts in the location of the wind stress has important implications for climate. In particular, as discussed in a companion talk, the changes in ventilation produced by shifting the winds may have important implications for whether the ocean circulation represents a positive or negative feedback for changes in carbon dioxide concentration.