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<br />Typical Northern Hemisphere winter circulations require a reservoir of cold air in the Arctic <br />regions, so one of the most noticeable aspects of global wanning would be shortened winters. <br />Modified winter patterns would prevail from about 15 December to about 15 February each <br />year, compared to 23 October to 8 March during the period from 1899 to 1969 (Kalnicky, <br />1987). In addition, the prevailing winter storm track across western North America would be <br />shifted northward to near the Canadian border. Therefore cyclonic activity in the central <br />Rockies and the Southwestern United States would be less frequent than at present. However, <br />the occasional storms that would occur, mainly in the form of cut-off lows, would provide <br />heavy precipitation. Overall, the southern and southwestern parts of the United States would <br />receive about 10 percent less winter precipitation and the northern third of the United States <br />would receive about 20 percent more winter precipitation than at present. On the basis of the <br />estimated temperature increases, the snowline in California and Arizona would rise, on the <br />average, by some 300 m, while the snowline in the northern Rocky Mountains would be <br />about 500 m higher than at present. The temperature change alone would have drastic <br />impacts upon snowpacks and the annual distribution of streamflow in western rivers (Gleick, <br />1987; Lettenmaier and Gan, 1990), although Karl and Riebsame (1989) have argued that its <br />impact upon total annual streamflow would not be great. <br /> <br />Modified summer patterns would prevail from 1 June until 15 September. The subtropical <br />high pressure belt would be centered about 370N., and the southern United States would be <br />subjected to increased easterly flows on the south side of it. As a result, the Gulf Coast <br />would be affected frequently by easterly waves, and tropical hurricanes would be more <br />common there than they are now. The summer monsoon rains in the Southwest would be <br />heavier and more widespread than at present, and would extend regularly as far north as <br />Colorado and Utah. The Southwest would receive perhaps 20 percent more summer rain than <br />at present, while the northern Rockies would be little changed. Overall, the Northem Great <br />Plains would receive as much summer rain as at present, but the distribution in time would be <br />changed. Increases in early sununer rain would be offset by decreases in the months of July <br />and August after the ground had dried out. Soil moisture would be scarce after July 1 <br />because of the high temperatures, and the scarcity of soil moisture would have a positive <br />feedback effect on temperature. The increase in monthly mean temperature would have its <br />maximum value of about 50C in the Dakotas or Minnesota during August and September. <br /> <br />4. MODELING OF PRECIPITATION UNDER DOUBLED-C02 CONDITIONS <br /> <br />The doubled-C02 scenario sununarized in Section 3 is not yet sufficiently quantitative to <br />generate the precipitation statistics needed for hydrological studies in GCCRP. The scenario <br />team will continue to examine climatological and hydrological data and output from <br />atmospheric models on various scales to make the scenario more quantitative. <br /> <br />Statistical models have shown only limited success at predicting streamflow on the basis of <br />weather map features. Typical correlation coefficients developed in such studies are in the <br />range of 0.4 to 0.6 (Cayan and Peterson, 1989). Karl and Riebsame (1989) have stated that <br /> <br />Printed January 18, 1991 <br />