Long Range Predictability of the Extratropics

Abstract:

We show some of the latest evidence and mechanisms for long range predictability of the extratropics. We now know that the winter North Atlantic Oscillation (NAO) is predictable on seasonal, interannual and even decadal timescales. On seasonal timescales, a significant proportion of this predictability originates in the tropics and also in the stratosphere. High predictability of tropical rainfall drives predictable stationary Rossby waves that propagate into the extratropics and the Atlantic sector where they project onto the NAO and initial atmospheric conditions in stratospheric winds can drive signals that propagate downwards into the troposphere on subseasonal timescales. On longer timescales, we show that fluctuations in axial atmospheric angular momentum, originating in the tropics and often triggered by ENSO, propagate slowly polewards into both hemispheres. These predictable signals are driven by the dissipation of transient eddies in the troposphere and they are skilful predictors of the extratropical jet streams in both the northern and southern hemisphere. Finally, I will discuss how all this reflects on the unresolved signal-to-noise paradox, whereby ensemble forecasts of the extratropics are better at predicting the real world than they are at predicting themselves.