Ensemble-mean forecast maps

North Atlantic Oscillation

When examining the ensemble mean pressure field, it is often helpful to think in terms of the North Atlantic Oscillation (NAO), which affects Europe and North America and is predictable in winter (Scaife et al, 2014).   The NAO can be defined by the difference in Mean Sea Level Pressure (MSLP) anomaly between the Azores and Iceland.  A positive NAO is associated with stronger than average westerly winds and translates to milder and wetter winters in N.W. Europe and colder and drier in S.Europe. In contrast,  a negative NAO correponds to weaker westerly winds (or even easterly winds) and leads to colder and drier winters in N.W. Europe and milder and wetter winters in S.Europe.