Breaking Rossby Waves, Moisture Streamers, and the Southern Hemisphere Diagonal Convergence Zones

Abstract:

A combination of satellite observations (principally GPCP) and ECMWF reanalyses were used to examine the major diagonal convergence zones in the Southern Hemisphere. Time-lag height anomalies composited on high rainfall events in sub-tropical and mid-latitude sections of the climatological zones showed nearly equivalent barotropic wave packets propagating into the regions from higher latitudes, in a manner easily interpretable in terms of the structure of low-pass-filtered pv gradients. To examine the moisture field with which the wave vertical motion field interacted in producing precipitation, animations were first constructed from daily fields of PV on the 330-theta surface superposed on total column water (TCW). The animations showed frequent events of anticyclonic breaking of mid-latitude Rossby waves, coincidental with the development of streamers of total column water (TCW) drawn out of the tropics, into the subtropics and mid-latitudes just downstream of parallel PV streamers in the breaking waves. Vertical cross sections through selected streamers showed locally high levels of moisture extending through most of the troposphere, with dynamically consistent vertical motion fields. Although major streamer events with NW-SE tilt could be seen in the animations, TCW streamers were in fact seen with many orientations and in many locations around the hemisphere. An objective method was developed to classify TCW streamers by their horizontal tilt. When applied to observations from the 20 year period 1991-2012, the method showed that TCW streamers with NW-SE tilt preferentially occurred in the three climatological diagonal convergence zones regions during austral summer, but only in the SPCZ and SACZ regions during austral winter.