The impact of outflow branches from a tropical cyclone on the life-cycle of baroclinic-wave

The direction of vertically-integrated eddy momentum flux is investigated while a tropical cyclone is moving in the sub-tropical regions and an upper-level trough is penetrating equatorwards. It has been known that the outflow with low potential vorticity of the tropical cyclone weakens the upper-level trough and induces the reduction of vertical shear. However, it has not been studied about the role of outflow in modifying the direction of the eddy momentum fluxes that are determined by the characteristics of baroclinic wave life-cycle behavior. It is still difficult to predict the direction of the eddy momentum flux for a particular flow by a general rule. Some cases of tropical cyclones with different patterns of outflows are selected in the Australian Region. The vertically integrated eddy momentum fluxes are calculated using the re-analysis data sets from the European Centre for Medium-Range Weather Forecasts (ECMWF).

The sign of mean eddy momentum flux varies depending on the pattern of well-defined branches of outflow. If two branches of outflow are channeled in opposite directions, the sign of eddy momentum flux is the same and there is small difference in the magnitude of flux in both directions. On the other hand, flux tends to be mainly in one direction when there is only one channeled outflow branch. While tropical cyclones intensify, the excited outflow branches show transient movement. If an outflow branch locates at the equatorward side of entrance region of jet stream, it results in the intensification of jet but the jet stream becomes confined rather in meridional direction than zonal direction and zonal temperature gradient increases at surface. As the position of outflow branches is transient, the interaction with the jet can produce variable impacts on the behavior of baroclinic wave cycle.