Variations of precipitation and evaporation over the North Atlantic Ocean, 1958–1997

Abstract

The output from recent atmospheric reanalysis projects at the National Centers for Environmental Prediction and the European Centre for Medium‐Range Weather Forecasts is used to address regional and temporal variability of precipitation P and evaporation E over the North Atlantic. The difference between P and E is the net surface freshwater flux, which directly affects the salinity and hence the stratification of the ocean surface layer. Over North Atlantic subregions, spanning 10°–15° latitude and 30°–40° longitude, the area‐averaged P from the two reanalyses are highly correlated with each other outside of the tropics and are consistent with climatologies compiled from ship data. In the annual mean, P exceeds E in the North Atlantic poleward of 50°N; E exceeds P between 20°N and 50°N and also in the eastern tropical North Atlantic (10°N–35°N). Seasonality varies considerably among the subregions: P and E both peak during late autumn/early winter in the subpolar regions, and P has an autumn peak in the subtropics, where E shows little seasonality. The year‐to‐year variations of monthly and annual regional values are larger for P than for E, implying that the interannual variations of P minus E are driven primarily by P. These interannual variations are sufficiently large and persistent that they can account for substantial portions (0.1–1.0 salinity unit (SU) per year) of observed salinity variations in the North Atlantic surface waters. Atmospheric forcing of regional P shows consistent teleconnections throughout the year, although the patterns are strongest in winter and weakest in summer. The North Atlantic Oscillation (NAO) is the dominant pattern controlling P in the eastern and northern North Atlantic; P in the western North Atlantic shows associations with the atmospheric circulation as far away as the North Pacific. P anomalies of >3 cm month−1 are associated with multiyear periods of positive and negative NAO. The high‐NAO regime of the late 1980s and early 1990s has coincided with increased P over the northeastern North Atlantic and decreased P over the southwestern North Atlantic although the seasonality of the NAO impact varies widely among the regions.