Abstract

Skilful climate predictions of the winter North Atlantic Oscillation and Arctic Oscillation out to a few months ahead have recently been demonstrated but the source of this predictability remains largely unknown. Here we investigate the role of the tropics in this predictability. We show high levels of skill in tropical rainfall predictions, particularly over the Pacific but also the Indian and Atlantic Ocean basins. Rainfall fluctuations in these regions drive clear signatures in tropical and extratropical atmospheric circulation that are approximately symmetric about the equator in boreal winter. We show how these patterns can be explained as steady poleward propagating linear Rossby waves emanating from just a few key source regions. The sources of these waves are more or less active as tropical rainfall varies from winter to winter but they do not change position because the wave sources are anchored to regions of strong vorticity gradient associated with the extratropical jets. Finally, we show that predicted tropical rainfall explains a sizeable fraction of the predicted year to year variance of the winter North Atlantic Oscillation.