17.04.2019: FB1-Seminar

Mittwoch, 17. April 2019:

9:00 Uhr, Hörsaal, Düsternbrooker Weg 20

The last few winters continue to suggest that the winter NAO and hence winter average weather over large areas of Europe is predictable at seasonal lead times with a good level of skill. I will show mechanisms for this predictability and argue that it originates from predictable variations in tropical rainfall and initial conditions in the stratosphere. High predictability of tropical rainfall is first demonstrated for current prediction systems and this is shown to lead to predictable changes in seasonal vorticity sources. These are associated with stationary Rossby waves that propagate polewards and eastwards into the extratropics, including the Atlantic sector, where the connection to tropical rainfall can explain around half of the forecast variance in the NAO. We also show that initial atmospheric conditions are important for seasonal prediction of the NAO. Initial anomalies in stratospheric winds at the start of winter propagate downwards into the troposphere where they lead to anomalies in the winter mean surface conditions. Together, these mechanisms may explain the majority of forecast variance in the winter NAO. Finally, we discuss some of the remaining errors in these ensemble forecasts, including the unresolved signal-to-noise paradox which means that current seasonal forecasts are better at predicting the real world than they are at predicting their own forecast members.

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