Relationships between 700 mb Circulation Variations and Great Plains Climate

The relationship between monthly midtropospheric circulation variations, occurring in the North American sector, and surface temperature and precipitation across the Great Plains is evaluated for the middle month of each season (January, April, July, and October). The results demonstrate that monthly Great Plains temperature variability is strongly associated with the majorpattems of midtropospheric circulation variation during all months considered Temporally, the strongest associations are observed during October. However, January, July, and April also exhibit spatially coherent regions of strong association. Spatially, the relationship tends to be strongest in the northern Plains, with decreasing association to the south. Precipitationmidtroposphere relationships are weaker than those for temperature during all months. The association between the midtroposphere and precipitation is relatively strong fiom late fall through late spring. However, the convective nature of precipitation in the region during the summer months limits any strong relationships in July. In a spatial sense, no preferred regions of precipitation explanation were indicated in the analysis. Monthly midtropospheric circulation variations are important in the specification of regional surface climates in many areas of the world (Lamb and Peppler 1987; Leathers et al. 1991). The recognition that preferred patterns of pressure variation exist in the atmosphere has led researchers to attempt to link temperature and precipitation changes at the earth's surface with these recurrent midtropospheric circulation variations. These monthly circulation variations and associations are described in a number of ways, the most popular of which is in the context of atmospheric teleconnections, the statistical association between climatic variables that are separated by large distances. Midtropospheric teleconnections thus represent atmospheric pressure patterns that tend to recur in data that are averaged over a specified period of time (days to seasons) and are associated with surface climate variations. The study of the relationship between flow patterns in the middle Circulation Variations 59 troposphere and surface climate is not new. This work, begun several decades ago (e.g., Klein 1948, 1%3; Stidd 1954; Friedman 1955), has continued to the present (e.g., Lanzante and Harnack 1982; Harnack and Lanzante 1985; Klein and Bloom 1987). The theory of global atmospheric teleconnections that was advanced by Wallace and Gutzler (1981) has led to a renewed interest in this field of climatological research. Whereas previous studies looked for the existence of statistical relationships between midatmospheric pressures and surface climate, without consideration of the controlling atmospheric dynamics, the use of atmospheric teleconnections allows the researcher to posit relationships in a physically coherent global perspective, because the teleconnection patterns can be explained in the framework of atmospheric physics. The existence of the major midtropospheric teleconnections is primarily a consequence of the geographical distribution of land and water and of orographic forcing by the earth's north-south mountain chains (Chen and Trenberth 1988a, 1988b). Because they are a result of invariant geographic forcing, the recurrent circulation anomalies are temporally and spatially robust, making them potentially important components in the study of past, present, and future climates. This study seeks to understand the relationship between monthly temperature and precipitation in the Great Plains region of the United States and 700 mb (millibar) teleconnection patterns in the North American sector (Fig. 1). This relationship will be specified through multiple regression analysis, regressing temperature and precipitation values from the months of January, April, July, and October against time series representing the strength and configuration of the major midtropospheric teleconnections for these same months. These four months were chosen for the analysis because they represent the midpoint of the four meteorological seasons. This study may also prove beneficial in the interpretation of climate modelling results. Recent evidence has shown that some general circulation models (GCMs), which are used for climate projections, poorly represent midtropospheric circulation variations. If the observed surface climate of an area is closely linked to midtropospheric teleconnections, and the models do not represent these patterns correctly, then projections by these models for such areas must be viewed with skepticism. Data and Methodology