Two Types of Baroclinic Life Cycles during the Southern Hemisphere Summer

This study examines the dynamical processes that drive two types of baroclinic life cycles during the Southern Hemisphere summer. Composite analysis was performed for the years 1980 through 2004 with NCEP/NCAR Reanalysis and NOAA OLR data. The two types of life cycles are identified by the strength of the barotropic energy conversion at the beginning of the life cycles. For the weak barotropic (WB) life cycle, the barotropic conversion is anomalously weak at the start of the life cycle, and for the strong barotropic (SB) life cycle, this energy conversion is anomalously strong.

The WB (SB) life cycle is initiated by a strengthening (weakening) of the poleward wave activity flux out of the tropics. For the WB life cycle, the impact of the eddy-induced mean meridional circulation is to weaken and broaden the midlatitude jet, and for the SB life cycle, this flux strengthens and narrows the jet. For these zonal wind flow profiles, the baroclinic growth and maximum energy attained in the WB life cycles exceeds that for the SB life cycle. It was also found that the baroclinicity is weaker (stronger) at the beginning of the WB (SB) life cycle. This suggests that the impact of the barotropic governor and baroclinicity oppose each other with the former dominating.

It was observed that the WB (SB) life cycle is preceded by a strengthening (weakening) in tropical convection over Indonesia, which suggests that tropical convection is the ultimate driver of these two life cycles.