AN ANALYSIS OF EXTRA-TROPICAL STORMS IN THE NORTH-ATLANTIC REGION AS SIMULATED IN A CONTROL AND 2XCO(2) TIME-SLICE EXPERIMENT WITH A HIGH-RESOLUTION ATMOSPHERIC MODEL

Climate simulations with an atmospheric model at T106 resolution are a nalysed to study the effect of greenhouse warming on the North Atlanti c wind climatology. The analysis is based on winter data from 2 high-r esolution numerical (time-slice) experiments: a control run and a 2 x CO2 run. The storm track (500 hPa height variability), mean sea level pressure and average surface winds in the control experiment compare w ell with 5 years of analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) for the period 1991-1995. The mean sea leve l pressure field however differs markedly from a pressure climatology of 39 years (1955-1993) of analyses prepared by the Norwegian Meteorol ogical Institute (DNMI) and from the ECMWF analysis for the period 198 6-1990. 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 x CO2 experiment differs slightly from the control expe riment in several aspects: (1) it has, relative to the control experim ent, a negative pressure anomaly of 5 hPa over Scandinavia and a weake r positive anomaly over the central North Atlantic; (2) associated wit h this pressure anomaly pattern there is a wind direction anomaly over the eastern North Atlantic; (3) there is a small intensification of t he variability of the 500 hPa height, which is most pronounced over th e North Sea, the Bay of Biscay and central Europe; (4) the number of m id-latitude storm events decreases somewhat in most of the North Atlan tic, particularly north of 55 degrees N, but increases in the North Se a 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 x CO2 experiment a re caused by the external greenhouse forcing or are just the result of natural climate variability. The model results suggest that the effec t of a greenhouse-induced decrease in the meridional temperature gradi ent near the surface and the increase of this gradient in the upper tr oposphere balance approximately, in such a way that the net effect of CO2 doubling on midlatitude storminess is small.