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

As the leading mode of extratropical intraseasonal variability of the Southern Hemisphere tropospheric zonal winds, the Southern Hemisphere Annular Mode (SAM) describes latitudinal fluctuations of the midlatitude jet. Observational and numerical studies suggested that there exists a baroclinic mechanism extending the persistence of SAM. In this mechanism, the shift of low-level baroclinicty with the jet is suggested plays a crucial role, which can cause a latitudinal shift of the eddy generation, results in anomalous eddy momentum forcing and thus enhances the anomalous zonal wind.

Using the ERA40 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 SAM. 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 enhance the baroclinic anomalies.