United Kingdom and Ireland precipitation variability and the North Atlantic sea-level pressure field

The relationship between UK and Ireland (UK&I) precipitation variability an d the North Atlantic sea-level pressure (SLP) field is examined. Strong pos itive correlations between the North Atlantic Oscillation (NAO) and precipi tation in the northwest of the UK&I, particularly in winter, are confirmed but correlations are insignificant at the 0.05 level in the southeast durin g all months. This paper identifies new patterns of SLP associated with pre cipitation variability both for regions and months where precipitation vari ability is not strongly linked with the NAO and for patterns that appear to be more closely related to UK&I precipitation than the NAG. Two indices of monthly UK&I precipitation variability are calculated using empirical orthogonal functions (EOFs) of monthly UK&I precipitation anomali es. EOF1 represents precipitation variability for the UK&I as a whole and E OF2 the variability in the north-south precipitation gradient across the UK &I. Correlations between both these monthly EOF derived precipitation indic es and SLP show a north- south (sub-tropical/mid-latitude) dipole, which is particularly strong in winter. These correlation patterns are then used to construct new SLP indices, which necessarily relate more closely to UK&I p recipitation. The first index resembles the East Atlantic pattern from Sept ember to April. The second may be thought of as an alternative index of the NAG, such that it is optimized with respect to precipitation variability a nd is located northeast of those centres of action most commonly used to ca lculate the NAO index. Stepwise linear regression models, incorporating the two new indices and th e original NAOI, suggest that over 25% of UK&I precipitation variability th is century (1900-1994) in each month can be explained by a simple index rep resentation of the North Atlantic SLP field. This rises to over 40% of vari ance explained in nearly all regions of the UK&I. Copyright (C) 2001 Royal Meteorological Society.