Potential Importance of a Midlatitude Oceanic Frontal Zone for the Atmospheric Annular-Mode Variability: Inter-Basin Differences in the Annular-Mode Signature over the Southern Oceans

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 frontal zone, 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 evident for its positive phase, where PFJ is situated systematically poleward of the SST front wherever it is located. Insensitively to the frontal latitude, by contrast, PFJ for the negative phase resides around 40° latitude, which nearly corresponds to the latitude of the climatological PFJ 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 another 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 greater latitudinal excursions, which appears to reflect the inter-basin differences in the latitude of the oceanic frontal zone.