At KNMI we have studied the effect of increasing greenhouse gas concentrations on wind and waves in the North Atlantic region. In this study we used results from two high resolution climate simulations, made at the Max-Planck Institute in Hamburg with a numerical, atmospheric model (ECHAM3) at T106 resolution. One run was a so-called control run (with the present atmospheric CO2 concentration); the other was a run with doubled CO2. Each run had a limited duration of five years.
To have a check on the quality of the control run we began by comparing the results of this run with "observations". As is usually done in these situations we did not consider original observations, but instead we used the analyses made from available observations by the European Centre for Medium-range Weather Forecasts. The outcome of this comparison is illustrated - for the case of wind extremes - in figures 1 and 2, which give the 10% exceedence values of the wind speeds at a height of 10 m, for the analysis and the control run respectively. From this and other comparisons we concluded that the climate model is quite realistic in the way it represents the current wind climate.
Figure 1: The 10% exceedence value of winter (December to February) 10 m wind speeds over the north Atlantic area for the ECMWF analyses (1991 to 1995).
Figure 2: The 10% exceedence value of winter (December to February) 10 m wind speeds over the north Atlantic area for the control experiment (6 winters).
Next the 2xCO2 experiment was compared with the control experiment. We found a number of differences, but not a strong signal. This is illustrated in figures 3 (giving the 2xCO2 result for the 10 % exceedence of the 10 m wind speeds) and 4 (for the difference between the 2xCO2 run and the control run)
Figure 3: The 10% exceedence value of winter (December to February) 10 m wind speeds over the north Atlantic area for the 2xCO2 scenario (6 winters).
Figure 4: The difference of figures 3 and 2 (2xCO2 - control).
Similar comparisons were made for the mean sea level pressure, the mean 10 m winds, the variability of the 500 hPa geopotential heights and for the spatial distribution and frequency of the storms over the north Atlantic area. In general, differences were found to be small. We further qualified the magnitude of these variations by comparing them with the interannual variability found in the ECMWF analyses. This comparison (not shown) allowed us to conclude that the variations expected in the surface wind climatology in the north Atlantic region due to CO2 doubling are not larger than those that have occurred in the past decades. It is therefore impossible to say whether the anomalies in the 2xCO2 experiments are caused by the external greenhouse forcing or just the result of natural climate variability. The results suggest that the effect of the greenhouse-induced decrease in the meridional temperature gradient 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.The surface winds from the two climate model data sets were used to compute the resulting ocean wave characteristics. This was done with the help of the WAM model, a state-of-the-art numerical wave prediction model. Purpose was to see what could be the effect of CO2 doubling on the wave climatology. In particular we wanted to know whether the wave climatology was sensitive to the small differences in wind climatology described above. We studied the effect on the average and extreme wave heights and on the variability. The results are illustrated in figure 5 which gives the estimated change in the 10% exceedence value of the mean wave height due to CO2 doubling. As in the case of the winds, the differences are small. The largest (but still small) differences are found off the coast of the United States and in the North Sea.
Figure 5: The10% exceedence value of winter (December to February) total significant wave heights in the north Atlantic area (2xCO2 - control, 5 winters).
Confidence in regional climate predictions is low and the effects we find are small. Therefore, it is difficult to assess their significance. The main conclusion from our study seems to be a negative one: we do not find that CO2 doubling has a large impact on the wind and wave climatology of the north Atlantic region.
We would like to thank Lennart Bengtsson for making the ECHAM3 results available to us.
Beersma, J., K. Rider, G.J. Komen, E. Kaas and V. Kharin, 1997. An analysis of extra-tropical storms in the North Atlantic region as simulated in a control and 2xCO2 time-slice experiment with a high resolution atmospheric model. Tellus, 49A, 347 - 361.
Rider, Kathy M., Gerbrand J. Komen and Jules J. Beersma, 1996. Simulations of the response of the ocean waves in the North Atlantic and North Sea to CO2 doubling in the atmosphere. KNMI - Scientific Report, WR 96 - 05.
WASA group: J.C. Carretero, M. Gomez, I. Lozano, A. Ruiz de Elvira, O. Serrano, K. Iden, M. Reistad, H. Reichardt, V. Kharin, M. Stolley, H. von Storch, H. Gnther, A. Pfizenmayer, W. Rosenthal, M. Stawarz, T. Schmith, E. Kaas, T. Li, H. Alexandersson, J. Beersma, E. Bouws, G. Komen, K. Rider, R. Flather, J. Smith, W. Bijl, J. de Ronde, M. Mietus, E. Bauer, H. Schmidt and H. Langenberg, 1998. Changing waves and storms in the Northeast Atlantic? Bulletin American Met Soc, 79, 741 - 760.
For more information contact
Dr. Gerbrand Komen,
e-mail komen@knmi.nl, phone +31-
30-2206676.