Potential Importance of Midlatitude SST Fronts for the Annular-Mode Variability, and Inter-Basin Differences in the Southern Annular-Mode Signature

Abstract:

Annular-mode variability in the extratropical atmosphere is manifested as latitudinal shifts of an eddy-driven polar front jet (PFJ) and associated stormtrack. Climatologically, they are both observed slightly poleward of a midlatitude oceanic front, a confluent region of warm and cool ocean currents, which maintains a surface baroclinic zone against poleward eddy heat transport. A set of “aqua-planet” AGCM experiments with zonally-uniform SST, which mimics the Southern Hemisphere, reveals certain sensitivity of the nodal latitude of anomalous westerlies associated with the annular mode to the latitude of frontal SST gradient. The sensitivity is particularly evident for its positive phase, where PFJ is situated systematically poleward of the SST front at whatever latitude it is located. Insensitively to the frontal latitude, by contrast, PFJ for the negative phase resides near 40° latitude, which nearly corresponds to the climatological PFJ axis that is realized without frontal SST gradient. The annular mode can therefore be interpreted as wobble of the atmospheric circulation system between a regime dominated by thermodynamic influence of frontal SST gradient and that by atmospheric internal dynamics. This notion is found useful for understanding inter-basin differences observed in the wintertime signature of the Southern Annular Mode (SAM) that are superimposed on the dominant zonally uniform signature. Compared to its negative phase, the axis of the surface westerlies observed in the positive phase of SAM exhibits stronger latitudinal excursions, which appears to reflect the inter-basin differences in the latitude of the oceanic frontal zone. Implications will be also discussed for the long-term trends in the westerlies over the midlatitude Southern Oceans.