Impact of the North Atlantic oscillation on winter convection: convective precipitation and cloud-to-ground lightning

摘要

The relationship between the North Atlantic oscillation (NAO) and winter (November to March) convection is analysed using two convective parameters: convective precipitation (CP) and cloud-to-ground (CG) lightning. Using winter CP values from 1950 to 2000 over the North Atlantic and western European area (30-70 °N, 60 °W-10 °E), positive correlations were found between the NAO index values and CP at latitudes higher than 50 °N, whereas negative correlations were found for latitudes lower than ～50 °N. The spatial pattern of correlation coefficients at latitudes higher (lower) than ～50 °N suggests that the advection of temperature (moisture) is the main factor contributing to the relationship between the NAO and CP. The NAO accounts for 47% (25%) of the first principal component of CP interannual variance at latitudes higher (lower) than 50 °N. The monthly winter CG lightning-flash density for 9 years (1992-2000) over the Iberian Peninsula was analysed in relation to the NAO on a small spatial scale (0.4° longitude × 0.4° latitude). The results were consistent with those found for the CP over the same zone. The correlation coefficient between the monthly NAO index value and the monthly CG flash number is r = -0.6, suggesting a relationship between them. Spatial variations in this relationship were also studied. The results indicated that the impact of the NAO on CG lightning is stronger on the western Iberian Peninsula and decreases eastwards.