Baroclinic anomalies associated with the Southern Hemisphere Annular Mode: Roles of synoptic and low-frequency eddies

Using the ERA-40 reanalysis data, we study the different roles of synoptic and low-frequency eddies in sustaining the latitudinal shift of the low-level baroclinicity associated with the Southern Hemisphere Annular Mode. The eddy effect is assessed through the direct eddy thermal forcing via eddy heat flux and the indirect forcing via eddy-driven mean meridional circulation (MMC). We find that in addition to the synoptic eddy-induced MMC suggested by Robinson (2006), the direct eddy thermal forcing by low-frequency eddies is significant in driving the baroclinic anomalies. These two processes together prevail over the direct baroclinicity deduction by synoptic eddies. The different effects of synoptic and low-frequency eddies might be attributed to the distinct latitudinal distributions of their low-level eddy heat flux relative to the midlatitude jet. The different roles of the MMC induced by synoptic eddy momentum and heat flux are also emphasized, with the former leading the baroclinic anomalies and the latter acting to extend the baroclinic anomalies.