Behavior and occurrence of poleward Rossby wave breaking events on the boreal hemisphere

The behavior and occurrence of poleward Rossby wave breaking events on the boreal hemisphere are investigated as functions of the large-scale flow. For example, under winter conditions it can be shown that the zonal ozone distribution of the stratosphere has a strong influence on the strength and the position of the polar vortex as well as on the upper tropospheric large-scale flow characteristics. In the extra-tropics, the large-scale flow (or wave guide) determines the propagation of Rossby waves and the diffluence/ confluence of the flow defines regions of increasing Rossby wave amplitudes including the overturning of Ertel's potential vorticity (breaking) on isentropic layers. It is known that events of poleward breaking Rossby waves are often observed over the North Atlantic-European or Eastern Pacific region during wintertime. This study is focused on the seasonal influence of the observed large-scale flow in the upper troposphere on poleward Rossby wave breaking events and the link to precipitation and surface winds. The seasonal behavior of events of poleward breaking Rossby waves during the decade of the 1990s has been studied based on ECMWF Reanalysis data. We can show that the different, diffluent or confluent, large-scale flow structure determines the region and behavior of Rossby wave breaking events. These events are separated into four breaking types: poleward (up- and downstream) or equatorward (up- und downstream). The seasonal behavior of these four breaking types is studied in more details. We identified a strong seasonal dependence with major poleward breaking events in winter and minor in summer time over the North Atlantic-European region. Further, in some SGCM experiments we can show that the varying large-scale flow explains mainly the seasonal variation of poleward Rossby wave breaking events. In a composite study the impact of Rossby wave breaking events on mean precipitation is examined. Furthermore, the impact of Rossby wave breaking events on regional enhanced precipitation or surface storms is also studied