Gregory J. McCabe, Jr. and Michael D. Dettinger
U.S. Geological Survey

General circulation model (GCM) simulations of atmospheric circulation are more reliable than GCM simulations of temperature and precipitation. Thus, some researchers are developing empirical relations between observed atmospheric circulation and observed temperature and precipitation to translate GCM estimates of future atmospheric circulation into estimates of future temperature and precipitation. Developing climate-change scenarios in this manner assumes, at least, that relationships between atmospheric circulation and surface climate variables, such as temperature and precipitation, are properly simulated by GCMs.

In this study, temporal correlations between 700hPa height anomalies over North America simulated by the Geophysical Fluid Dynamics Laboratory (GFDL) GCM and GFDL-GCM-simulated winter precipitation at eight locations in the conterminous United States are compared with corresponding correlations in observations. The objectives are to (i) characterize the relations between atmospheric circulation and winter precipitation simulated by the GFDL GCM for selected locations in the conterminous USA, (ii) determine whether these relations are similar to those found in observations of the actual climate system, and (iii) determine if GFDL-simulated precipitation is forced by the same circulation patterns as in the real atmosphere.

Results indicate that the GFDL GCM simulates relations between 700 hPa anomalies and local winter precipitation that are similar to relations found in observed data for most of the locations analyzed in this study. Results also indicate that at regional scales GFDL GCM simulations of the relations between 700 hPa anomalies and winter precipitation are most similar to observed relations for locations near oceanic sources of atmospheric moisture. These results suggest that the GFDL GCM may not adequately simulate variations in advection of atmospheric moisture into the interior parts of the USA and/or that this moisture is not adequately converted into precipitation in the interior parts of the country. This problem may be due, in part, to (i) the inadequate representation of topography in the GFDL GCM, (ii) stronger-than-observed mean winter zonal winds simulated by the GFDL GCM and the consequent more west-to-east paths of air flow and storm systems across North America, (iii) the relative weakness of important synoptic patterns in GFDL simulations, such as the Pacific North American circulation pattern, and (iv) the occurrence of `spectral rain'.

In addition, for some locations, the GFDL-simulated relationships between precipitation and 700 hPa anomalies can be quite different from observed relations. The differing relationships suggest that GFDL-simulated changes in regional precipitation in response to changes in atmospheric circulation could differ from changes that would occur in the actual climate system.

Abstract of article in
International Journal of Climatology 15, p. 625-638 (1995).