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The impact of strong La Nina events on January-February North Atlantic mean sea level pressure is unknown, as these events have never been observed. Using large ensembles from the Met Office decadal prediction system, examples of strong La Nina events are simulated and the Atlantic response to these is found to be a positive North Atlantic Oscillation. This is very different to the wavelike response observed and simulated for strong El Nino events. The reason for this difference is traced to the fact that the December-January-February mean tropospheric teleconnection of El Nino-Southern Oscillation to the North Atlantic dominates for strong El Nino events, while the stratospheric teleconnection dominates for strong La Nina events. The strength of the tropospheric pathway grows linearly and symmetrically with El Nino-Southern Oscillation. The stratospheric pathway is the source of the asymmetry between January and February surface responses to strong El Nino and strong La Nina events.

Plain Language Summary The El Nino-Southern Oscillation has a large impact on January-February North Atlantic mean sea level pressure (MSLP), important for forecasting the mean climate over the United Kingdom and Northern Europe during these months. Simulations from the Met Office decadal prediction system are used to research the impact of strong negative (La Nina) events, not yet observed in nature. Really strong positive (El Nino) and negative (La Nina) events are found to have an asymmetric impact on the North Atlantic. This paper studies the reasons why. El Nino-Southern Oscillation impacts the North Atlantic via two routesone through the tropical Atlantic in the lower atmosphere (troposphere) and one through the middle atmosphere (stratosphere) in polar latitudes. It is found that the tropospheric pathway dominates for El Nino events, leading to a wavelike response in North Atlantic MSLP, characterized by a high pressure west of the United Kingdom. However, the stratospheric pathway dominates for La Nina events leading to a different MSLP pattern, characterized by high pressure over the Azores and low pressure over Iceland. Therefore, if a really strong La Nina event were ever to occur, the United Kingdom and Northern Europe might experience a very wet winter.