Tropical Forcing of Euro-Atlantic Weather Regime Transitions: Reanalysis and Predictions

Large-scale circulation regimes control much of the extreme weather in the Euro-Atlantic region during boreal winter. Precursors of transitions between these regimes are studied from ERA-Interim Reanalysis. Patterns of (planetary wave) tropical diabatic heating, geopotential height and the flux of Rossby wave activity preceding transitions for up to 20 days are identified, as well as the relationship of the tropical heating to the Madden-Julian Oscillation (MJO), the El-Nino Southern Oscillation (ENSO), and shifts in the intertropical Convergence Zone (ITCZ) in the Pacific and Atlantic Oceans. Indian Ocean tropical heating enhances the transition from the Scandinavian Blocking EA circulation regime to the NAO+ regime, while Pacific heating is seen ahead of transitions from all other regimes into the NAO- regime. The transition from the Blocking regime to the NAO- regime shows heating over the Indian, Pacific and Atlantic Oceans. The flux of Rossby wave activity shows that for some transitions, mid-latitudes play a role in forcing the tropical heating prior to transitions. Not all the precursor tropical heating was related to the phase of the MJO; Atlantic heating also plays a role, while the role of ENSO was minimal.

The role of tropical heating in the regime transitions is also studied in high resolution ensemble forecasts of the Integrated Forecast System coupled to the Nucleus for European Modelling of the Ocean model (25 ensemble members for 30 01Nov. start dates) in order to determine if the forecast model captures the sub-seasonal teleconnections.