Global climate research

Ocean variability in the southern hemisphere resulting from the annular mode

Zonally-symmetric fluctuations of the mid-latitude westerly winds characterize the primary mode of atmospheric variability in the southern hemisphere during all seasons. This is true not only in observations, but also in an unforced 15,000 year integration of a coarse resolution (R15) coupled ocean-atmosphere model. Here it is documented how this mode of atmospheric variability, known as the Southern Annular Mode (SAM), generates ocean circulation and sea ice fluctuations in the model integration on interannual to centennial time scales that are tightly in phase with the SAM. The positive phase of the SAM is associated with an intensification of the surface westerlies over the circumpolar ocean (around 60^oS), and a weakening of the surface westerlies further north. This induces Ekman drift to the north at all longitudes of the circumpolar ocean, and Ekman drift to the south at around 30^oS. This figure gives mean simulated currents in the Southern Ocean, as well as the regression of annual-mean current onto the SAM index, showing the northward Ekman drift, and correlation of the northward component of the currents with SAM, which shows how tightly correlated simulated ocean circulation is with the SAM.

Through mass continuity, the Ekman drift generates anomalous upwelling along the margins of the Antarctic continent, and downwelling around 45^oS. The anomalous flow diverging from the Antarctic continent also increases the vertical tilt of the isopycnals in the Southern Ocean, so that a more intense circumpolar current is also associated with positive SAM. This is particularly true on interannual to decadal time scales, as demonstrated by this figure, which shows the spectra of circumpolar current intensity and the SAM (top), as well as the coherency between these two quantities (bottom). The coherency is generally greater than 0.5 on time scales longer than 20 years. (The large variability in the circumpolar current on centennial time scales seems to be a mode unrelated to the annular mode.) The anomalous divergent flow also advects sea ice further north, resulting in an increase in sea ice coverage. Finally, positive SAM drives anomalies in meridional heat transport; increases in poleward heat transport occur at about 30^oS, while decreases occur in the circumpolar region. Ocean and sea ice anomalies of the opposite sign occur when the SAM is negative.

The ocean and sea ice fluctuations associated with the SAM constitute a significant fraction of simulated ocean variability poleward of 30^oS all year round. This suggests that the SAM is likely an important source of large-scale variability in the real southern hemisphere ocean. This cartoon summarizes the simulated extratropical southern hemisphere variability associated with the SAM.

Download the publication (Hall and Visbeck 2002) describing these results in more detail.

Alex Hall and Martin Visbeck make up the team that did this work.