ATMOSPHERIC CIRCULATION CLIMATE CHANGES

The role of the atmospheric circulation in climate change is examined. A review is given of the information available in the past record on the atmosheric circulation and its role in climate change, firstly at the surface via sea level pressure in both the northern and southern h emispheres and secondly for the free atmosphere. As with most climate information, the climate record is compromised by non-physical inhomog eneities arising from changes in observing and analyzing techniques an d changes in data coverage. Problems with and threats to the rawinsond e network are discussed. Global analyses produced by the operational c enters, U.S. National Meteorological Center (NMC) and the European Cen tre for Medium Range Weather Forecasts (ECMWF), for weather forecastin g purposes contain many discontinuous changes in the analyses arising from improvements in the system used to produce them. A discussion is given of the prospects for and motivation behind an activity known as 'reanalysis' in which the historical data are reanalyzed using a state -of-the-art system that is held constant for the entire record. The on ly sources of spurious change then are the changes in the observing sy stem, such as the introduction of space-based observations. Recommenda tions are made on needed actions for better understanding and monitori ng climate change. The role of the atmospheric circulation and the str ong links to other variables such as temperature, precipitation and wi nd are established and illustrated with a survey of decadal variabilit y, the evidence for it, and the way in which the observed atmospheric circulation is involved in the Pacific and Atlantic sectors. The impor tance of teleconnections is stressed, especially in the winter half ye ar, for understanding local climate change. The likelihood that change s will be manifested in the frequency and intensity of preferred modes of behavior in the atmosphere, such as the Fl Nino-Southern Oscillati on and Pacific-North American teleconnection patterns, rather than in changes in the modes is also emphasized. The recently observed climate changes and the tendency for an unprecedented prolonged El Nino are i nterpreted in this framework. The key coupled atmosphere-ocean charact er of decadal variability is noted with the atmosphere providing the s patial scales, the ocean the memory, but also with the need for collab orative, as opposed to destructive, interactions through the atmospher ic circulation.