An analysis of extra-tropical storms in the North Atlantic region as simulated in a control and 2 × CO₂ time-slice experiment with a high-resolution atmospheric model

Abstract

Climate simulations with an atmospheric model at T106 resolution are analysed to study the effect of greenhouse warming on the North Atlantic wind climatology. The analysis is based on winter data from 2 high-resolution numerical (time-slice) experiments: a control run and a 2 × CO₂ run. The storm track (500 hPa height variability), mean sea level pressure and average surface winds in the control experiment compare well with 5 years of analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) for the period 1991—1995. The mean sea level pressure field however differs markedly from a pressure climatology of 39 years (1955—1993) of analyses prepared by the Norwegian Meteorological Institute (DNMI) and from the ECMWF analysis for the period 1986—1990. It is argued that these differences are a manifestation of the low-frequency variability of the climate system in the North Atlantic region. The 2 × CO₂ experiment differs slightly from the control experiment in several aspects: (1) it has, relative to the control experiment, a negative pressure anomaly of 5 hPa over Scandinavia and a weaker positive anomaly over the central North Atlantic; (2) associated with this pressure anomaly pattern there is a wind direction anomaly over the eastern North Atlantic; (3) there is a small intensification of the variability of the 500 hPa height, which is most pronounced over the North Sea, the Bay of Biscay and central Europe; (4) the number of mid-latitude storm events decreases somewhat in most of the North Atlantic, particularly north of 55°N, but increases in the North Sea and the Bay of Biscay; (5) the overall frequency of deep depressions decreases slightly while the frequency of weak depressions increases. Considering the high level of observed interannual variability, it is not possible to say whether the anomalies in the 2 × CO₂ experiment are caused by the external greenhouse forcing or are just the result of natural climate variability. The model results suggest that the effect of a greenhouse-induced decrease in the meridional temperature gradient near the surface and the increase of this gradient in the upper troposphere balance approximately, in such a way that the net effect of CO₂ doubling on mid-latitude storminess is small.