Aerosols may affect the atmosphere and climatein different ways. Greenhouse gases, Carbondioxide for instance, tend to heat the atmosphere because of the greenhouse effect. Aerosol particles however such as sulphat, can cause cooling of the atmosphere due to their reflection of solar light. Black carbon may contribute to heating due to light absorption. Other particles, like organic Carbon and nitrate, contribute to cooling. In total, aerosols are believed to cool the atmosphere due to direct increased reflection of solar radiation. A further cooling produced indirectly through impacts of clouds. This is regarded to be far more uncertain than the direct effect (IPCC, 2001). This thesis is a study of anthropogenic contribution of sulphat- and Black carbon-particles to the direct effect and we emphasize regional aspects such as flowregimes. We use the geopotensial height in 1000 and 500 hPa of Northern Hemisphere as the diagnostic parameter.
Two experiments have been made with the CCM3-Oslo model coupled to aslab ocean. CCM3-Oslo is a version of the NCAR CCM3.2 extendedwith aerosol chemistry and physics. One with only natural aerosols ("control"), and the other with both natural and anthropogenic aerosols ("scenario"). In the analysis three analyses have been applied to the "control" and "scenario" results: (1) NAO-index to investigate if it shows a change in frequency in dry or mild winter, (2) Difference in the 500 hPa pressure field to confirm that the height of constant pressure surface is lowest in the "scenario", (3) Projection on empirical orthogonal functions (EOF) to relate to different circulation regimes.
The results show that the "scenario" undergoes a cooling effectwhen compared to the "control". However, it does not show any significant change in frequency of circulation regimes or NAO.The EOF's is not explaining all of the variance, and thisaffects the results. Blocking situations are not described by usingEOF-analysis and this has to be considered when the conclusion isformulated.