HR: 0800h
AN: A31A-0016    [Abstracts]
TI: Two types of baroclinic instability in Southern Hemisphere Summer
AU: Feldstein, S
EM: sbf@essc.psu.edu
AU: * Moon, W
EM: wum116@psu.edu
AB: The energetics of mid-latitude baroclinic eddy life cycles is investigated with NCEP/NCAR Reanalysis data. A composite analysis is performed for the Southern Hemisphere summer for the years 1980 through 2004. Individual life cycles are identified by local maxima in synoptic-scale eddy energy. Two types of baroclinic life cycles are examined, each defined by the strength of the barotropic energy conversion two days prior to the maximum baroclinic growth. For one life cycle, the barotropic conversion is anomalously weak before the maximum baroclinic growth, and for the other life cycle, the barotropic conversion is anomalously strong. These two life cycles are referred to as the weak barotropic (WB) and strong barotropic (SB) life cycles, respectively. The composite calculation for the WB (SB) life cycle finds that a strengthening (weakening) of the poleward wave activity flux relative to that of the climatology takes place before the initial growth of the synoptic scale eddies. The source region for these fluctuations in the wave activity flux is found to occur within the Southern Hemisphere tropics. For the WB (SB) life cycle, this change in the wave activity flux, and the corresponding eddy momentum flux, is shown to drive a weakening and broadening (strengthening and narrowing) of the midlatitude zonal mean jet. Consistent with the barotropic governor mechanism, where a reduction in the horizontal shear coincides with more rapidly growing baroclinic eddies, the baroclinic growth and the maximum energy attained for the WB life cycle exceeds that for the SB life cycle. Furthermore, it is found that the WB (SB) life arises when the baroclinicity is weaker (stronger) than that of the climatology. This suggests that the influence of the barotropic governor and baroclinicity oppose each other at the beginning of the life cycle, with the latter being dominant. Both the WB and SB life cycles coincide with an active Madden-Julian Oscillation (MJO). For the WB (SB) life cycle, the MJO convection corresponds to a strengthening (weakening) of the convection in the western tropical Pacific. These results suggest that the above changes in the wave activity flux are driven by the MJO. This, in turn, further suggests that the two types of baroclinic life cycles are ultimately triggered by MJO convection.
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 3337 Global climate models (1626, 4928)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting