The influence of ET on poleward Rossby Wave Breaking

The interaction between the outflow of a tropical cyclone, undergoing extratropical transition, and the midlatitude flow often results in a diffluent flow configuration in the midlatitudes. Studies have shown that the poleward anticyclonic breaking of Rossby waves (P2-RWB) and the subsequent formation of blocking events is observed frequently in a diffluent flow. This raises the question whether there exist a relationship between ET and P2-RWB events. In this work, we analyze if and how ET influences RWB and whether there is a tendency towards the P2 type.

For the analysis of said relationship, we use Ensemble data of the European Centre for Medium-Range Weather Forecasts (ECMWF), from which we selected TCs and corresponding forecasts. We only chose forecasts with diverging storm tracks for the TC which enables us to distinguish cases with and without ET. For this purpose, we employed an Empirical Orthogonal Functions analysis, followed by a fuzzy clustering before analyzing the structure of the Jetstream at 200 hPa and the RWB differences in the resulting scenarios. We also calculated the wave activity flux (WAF) to estimate the intensity of RWB for each potential outcome.

First results for Hurricane Noel (2007), based on a forecast from October 31, 2007, show three different main scenarios. In the first scenario, Noel rests in the tropics and dissipates, while in the upper troposphere there is a ridge without any tendency towards breaking. The second scenario shows the TC moving northwards, undergoing an ET, and amplifying a ridge which breaks in type P2. Finally, in the third scenario, Noel's track is comparable to the first scenario. Due to another extratropical system, though, there is also a P2 event as in the second scenario. The findings from this study shall be confirmed by investigating several other cases.