Decadal changes in the link between El Nino and springtime North Atlantic oscillation and European-North African rainfall

The link between El Nino-southern oscillation (ENSO) variability in boreal winter (represented by the NINO3 index, i.e. East Pacific sea-surface temperature anomalies) and the large-scale circulation and weather conditions over Europe-northwest Africa in spring is explored, considering station reports of precipitation, sea-level pressure (SLP) anomalies and two North Atlantic oscillation (NAO) indices. It is found that these relations have undergone consistent and simultaneous changes in the 20th century. Three characteristic periods can be identified. During 1900-25 and 1962-87, positive NINO3 index values are associated with enhanced precipitation over central Europe and reduced rainfall in southern Europe and northern Africa. The ENSO influence on precipitation over Scotland and Norway is small. The rainfall anomalies can be explained from the advective and dynamical implications of a north-south dipole in SLP correlations (warm ENSO events followed by low pressure in northern Europe and high pressure over the Mediterranean Sea-North Africa). This dipole hardly projects on the commonly used NAO centres (Iceland and Azores/Gibraltar) and thus ENSO-NAO correlations are insignificant. During 1931 -56 the NINO3 index reveals little influence on precipitation over the Iberian Peninsula and Morocco, but there are large negative correlations with precipitation over Scotland and Norway. This is related to an alteration of the NINO3-SLP correlation pattern, which implies high pressure over northern Europe and low pressure over central Europe after warm events, and thus a virtually inverted dipole with respect to the other two periods. The large westward extension of the dipole leads to a significant NAO-NINO3 correlation of r = -0.5. These alterations were accompanied by substantial large-scale circulation changes during the period 1931 -56, as revealed by anomalously high pressure and dry conditions over central-western Europe, a change in precipitation-producing SLP patterns for Morocco and an anomalously low number of positive NAO and NINO3 index values. It is left for discussion as to whether the decadal variations described are due to a change in the physics of the teleconnection or to stochastic fluctuations. Copyright (C) 2003 Royal Meteorological Society.