Changes in mid latitude variability due to increasing greenhouse gases and sulphate aerosols

Intra-seasonal variability in the Northern Hemisphere winter is investigated in ensembles of experiments using the Hadley Centre coupled ocean-atmosphere model. Synoptic techniques which identify low centres and blocking anticyclones have been used in preference to time-filtered variances of geopotential height as these may not uniquely identify true synoptic systems. Changes in mid-latitude variability are qualitatively similar in experiments that include changes in greenhouse gases only and those that also include the direct effects of sulphate aerosols. The presence of aerosols reduces the warming of the northern continents but the relatively larger warming of the land compared to the oceans remains in both experiments and at higher latitudes this leads to a reduced poleward transient flux of energy. There is an increase in the transient flux of energy at most latitudes, due to a greater transport of latent heat arising from the greater moisture availability. The total number of Northern Hemisphere storms decreases but there is a tendency towards deeper low centres. There are fewer storms in the North Pacific and North Atlantic source regions where the local baroclinicity is reduced. The climatological tracks from these regions are shorter with decreases at the northeastern ends of the tracks and increases in the regions with maximum counts towards the centre of the ocean basins. These changes are not generally statistically significant. The greatest changes in blocking anticyclones occur in the North Pacific where there is a downstream shift in the region of maximum activity. Changes in stationary waves show a strong increase in the PNA oscillation which may be influencing the changes in blocking as has been found by other authors. Possible mechanisms for forcing this response are speculated but it will need further experiments to unravel them.