Relationships between atmospheric circulation and snowpack in the Gunnison River basin, Colorado

Journal of

Hydrology ELSEVIER

Journal of Hydrology 157 (1994) 157-175

[1]

Relationships between atmospheric circulation and snowpack in the Gunnison River basin, Colorado G r e g o r y J. M c C a b e , Jr. US Geological Survey, Denver Federal Center, M S 412, Denver, CO 80225, USA

(Received 1 February 1993; revision accepted 15 November 1993)

Abstract In this study, winter mean 700 mbar height anomalies over the eastern North Pacific Ocean and the western USA are related to variability in snowpack accumulations measured on or about 1 April at 21 snowcourse stations within and near the Gunnison River basin in Colorado. Results indicate that lower than normal snowpack accumulations are primarily associated with positive 700 mbar height anomalies (anomalous anticyclonic circulation) over the western USA. Moist air from the Pacific Ocean is moved to the north of the western USA along the western margin of the anomalous anticyclonic circulation. In contrast, higher than normal snowpack accumulations are associated with negative 700 mbar height anomalies (anomalous cyclonic circulation) over the western USA and over most of the eastern North Pacific Ocean. The anomalous cyclonic circulation over the western USA enhances the movement of moisture from the Pacific Ocean into the southern and central parts of the West. Results also indicate that variability in winter mean 700 mbar height anomalies can explain over 50% of the variability in snowpack accumulations in the Gunnison River basin. The significant linear relationships between 700 mbar height anomalies and snowpack accumulations in the Gunnison River basin can be used in conjunction with general circulation model simulations of 700 mbar height anomalies for future climatic conditions to estimate future snowpack accumulations in the Gunnison River basin.

1. Introduction S n o w p a c k accumulations are an important source o f runoff and water supply in the western U S A ( G r a y and Male, 1981). Melt from snowpack accumulations accounts for a large p r o p o r t i o n o f annual runoff in m a n y river basins in the western USA, and snowpack accumulations are useful to model and forecast peak and total annual runoff (World Meteorological Organization, 1970; G r a y and Male, 1981). Scientists have estimated that increasing concentrations of" atmospheric c a r b o n dioxide m a y Elsevier Science B.V. SSD1 0022-1694(93)02426-X

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G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157-175

cause global warming and changes in temporal and spatial distributions of precipitation (Gammon et al., 1985; Bolin, 1986; Lins et al., 1988), and there is concern that global warming may adversely affect snowpack accumulations in the western USA and therefore have a negative effect on water supply. Scenarios of changes in temperature and especially precipitation are needed on a regional scale (104-106 km 2) to estimate the effects of increasing concentrations of atmospheric carbon dioxide on snowpack accumulations in the mountainous areas of the western USA. General circulation models (GCMs) are mathematical representations of the Earth-climate system and have been used to estimate the effects of increasing atmospheric carbon dioxide and other 'greenhouse' gases on global climate. Although GCMs are able to reproduce the general spatial and temporal distributions of most climatic variables on a global scale, GCM estimates of climate on regional scales vary markedly from observed values (Grotch and MacCraken, 1990). More uncertainty is introduced into such estimates of precipitation in mountainous areas because of the inadequate representation of topography in GCMs. Recent research suggests that GCM simulations of synoptic-scale weather patterns are more reliable than those of regional temperature or precipitation, because weather patterns are of a spatial scale that is compatible with the spatial resolution of most GCMs (McCabe and Legates, 1992; Hay et al., 1992). Empirical relationships between atmospheric circulation and surface weather variables can be used in conjunction with GCM estimates of future atmospheric circulation to provide more reliable estimates of future regional precipitation (Yarnal and Leathers, 1988; Hewitson and Crane, 1992a). Global atmospheric circulation can be described by negative and positive anomalies of atmospheric pressure which have dimensions of several hundreds to thousands of kilometers (Madden, 1979; Cayan and Peterson, 1989) and correlate with surface weather phenomena (Namias, 1975, 1981; Wallace and Gutzler, 1981; Blackmon et al., 1984; Knox and Lawford, 1990). Temporal and spatial variations in surface weather variables, such as temperature and precipitation, are related to variations in atmospheric circulation patterns (Muller and Wax, 1977; Knox, 1984; Knox and Lawford, 1990; Cayan et al., 1992). For example, systematic deviations in atmospheric circulation over the eastern North Pacific Ocean and the western USA create precipitation anomalies over the latter primarily by affecting the movement of storms and the advection of moisture into the western USA (Cayan and Peterson, 1989). Previous research results indicate that a statistically significant proportion of seasonal precipitation variability can be explained by time-averaged atmospheric circulation anomalies (Klein, 1963; Walsh et al., 1982; Weare and Hoeschele, 1983; Cayan and Roads, 1984; Klein and Bloom, 1987). In this study, the relationships between atmospheric circulation and snowpack measurements made on or about 1 April in the Gunnison River basin, located in SW Colorado, are examined as a first step in the development of regional climatic change scenarios for the basin. The objectives of the study are: (1) to describe the relationships between atmospheric circulation and snowpack accumulations in the Gunnison River basin; (2) to quantify the amount of variability in snowpack

G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157 175

159

accumulations in the Gunnison River basin attributable to variations in atmospheric circulation; (3) to develop regression equations to estimate snowpack accumulations from 700 mbar height anomaly data for use with GCM estimates of future atmospheric circulation.

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G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157 175

2. Methods

2.1. The study area The Gunnison River basin (Fig. l) was chosen for this study because it has similar attributes to other basins in the western USA, and because it is a major tributary of the Colorado River. The Gunnison River basin has a drainage area of approximately 20 534 km 2 and basin elevations are extremely variable, ranging from 1387 to 4359 m. The Gunnison River contributes approximately 42% of the streamflow of the Colorado River at the Colorado-Utah Stateline (Ugland et al., 1990). Water from the Colorado River is used by more than 12 million people and is used to irrigate approximately 10 000 km 2 of agricultural land (Mueller and Moody, 1984). In addition, the Colorado River basin has the greatest water deficiency (average precipitation minus potential evapotranspiration) of any basin in the conterminous USA; however, more water is exported from it than from any other basin in the USA (Dracup, 1977). The Gunnison River basin provides an opportunity to study the relationships between atmospheric circulation and one of the most important sources of water in the western USA.

2.2. Sources of climatological data Height anomalies for the 700 mbar level (in meters) were used to represent atmospheric circulation affecting the western USA. The 700 mbar atmospheric pressure surface is generally far enough aloft (approximately 3000 m above sea-level) to avoid most topographic influences, and provides a good representation of mid-tropospheric atmospheric circulation that controls seasonal weather variations (Cayan et al., 1992). Although some parts of the Gunnison River basin are at elevations above 3000 m, these topographic features have little effect on large-scale seasonal 700 mbar atmospheric circulation. In addition, several studies have indicated that 700 mbar atmospheric circulation is useful to identify relationships between atmospheric circulation and precipitation (Stidd, 1954; Klein, 1963; Klein and Bloom, 1987; Knox and Lawford, 1990; Cayan et al., 1992). The 700 mbar height anomaly data were generated from the gridded atmospheric pressure data produced by the Climate Analysis Center (CAC, Camp Springs, Maryland, USA). The anomalies were constructed by subtracting the 1950-1979 long-term mean 700 mbar height fields for each day from the observed daily 700 mbar height fields for the period of January 1947-June 1988 (the data were obtained from D. Cayan, Scripps Institute of Oceanography, La Jolla, CA). The 700 mbar height anomaly data for the area from 20°N to 70°N and from 90°W to 180°W were used to represent the atmospheric circulation primarily affecting the western USA (Fig. 2). This area includes 105 grid points from the CAC data set. Daily 700 mbar height anomalies for the months of November March were averaged to produce winter mean 700 mbar height anomalies for the winters from 1947-1948 to 1986-1987, for comparison with the snowcourse data measured on or about 1 April. The primary source of precipitation data for this study was snowcourse data (in

G.J. McCabe, Jr. / Journal o f Hydrology 157 (1994) 157-175

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water equivalent units) measured on or about 1 April during the winters 1947 1948 to 1986-1987 (40 winters) at 21 snowcourse stations located within and near the Gunnison River basin (Fig. 1). Cayan et al. (1992) suggested that snowpack accumulations in the western USA exhibit a high degree of regional spatial covariability. To examine the spatial variability of the snowcourse data among the 21 stations used in this study, a principal-components analysis was performed for winters during the 40 year record when all 21 stations reported 1 April snowcourse measurements (Johnston, 1980). Eighteen of the 40 winters were used in the principal-components analysis and included the winters from 1964-1965 to 1984-1985, excluding the winters 1966-1967, 1979-1980, and 1982 1983. The results of the principal-components analysis indicated that 81% of the variability in the snowcourse data for the 21 stations was explained by the first principal component (Table 1). In addition, each of Table 1 Percentage o f variance explained by the first six c o m p o n e n t s o f a p r i n c i p a l - c o m p o n e n t s analysis of snowcourse data m e a s u r e d on or about 1 April at 21 stations within and near the G u n n i s o n River basin Component

% Variance explained

C u m u l a t i v e explained variance

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81 8 3 3 2 1

81 89 92 94 96 97

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G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157-175

the 21 stations loaded highest on the first component (Table 2). These results indicate that a high degree of spatial covariability exists among the snowcourse data used in the analysis. Because the snowcourse data are highly spatially covariant, data from all 21 snowcourse stations were averaged for each of the 40 winters analyzed in the study to produce values of mean snowcourse data for the entire basin (Basin Mean Snowpack, BMS). For 34 of the 40 winters analyzed in this study, at least 18 of the 21 snowcourse stations reported snowpack measurements for 1 April (the smallest number of stations reporting data was 11 in the winter 1986 1987). Correlations between 1 April BMS and 1 April snowcourse measurements made at each of the 21 stations during the 40 winter period analyzed in this study ranged from 0.62 to 0.94, and were significant at a = 0.01, with 17 of the 21 correlations greater than or equal to 0.80 (Table 3). Correlations were calculated from at least 37 years of data for 18 of the 21 stations, and the smallest number of years used to calculate a correlation was 23, for Butte (Table 3). The 1 April BMS (in water equivalent units) for the winters 1947 1948 to 1986-1987 ranges from a minimum of 167 mm to a maximum of 662 ram, with a mean of 425 mm and a standard deviation of 106 ram, and is normally distributed. Winter temperatures are not included in this study because they have little effect on 1 April BMS in the Gunnison River basin. For example, the partial correlation of Table 2 Loadings of individual snowcourse station data on the first principal component of a principal components analysis of snowcourse data measured on or about 1 April at 21 snowcourse stations within and near the Gunnison River Basin Station

Loading on first principal component

Alexander Lake Cochetopa Pass Crested Butte Fourmile Park Independence Pass Ironton Park Lake City Lift McClure Pass Mesa Lakes Mineral Creek Monarch Pass North Lost Trail Park Cone Park Reservoir Porphyry Creek Red Mountain Telluride Butte Keystone Trickle Divide

0.83 0.78 0.92 0.86 0.83 0.85 0.92 0.88 0.95 0.91 0.90 0.92 0.95 0.91 0.84 0.95 0.94 0.92 0.95 0.96 0.87

G.J. McCabe, Jr. / Journal of Hydrolog.v 157 (1994) 157-175

163

Table 3 Correlations of basin mean snowpack in the Gunnison River basin measured on or about 1 April (1 April BMS) with snowcourse measurements made on or about 1 April at 21 stations within and near the Gunnison River basin during the winters 1947-1948 to 1986 1987. Station

Correlation to 1 April BMSa

No. of years of data used

Alexander Lake Cochetopa Pass Crested Butte Fourmile Park Independence Pass Ironton Park Lake City Lift McClure Pass Mesa Lakes Mineral Creek Monarch Pass North Lost Trail Park Cone Park Reservoir Porphyry Creek Red Mountain Telluride Butte Keystone Trickle Divide

0.88 0.62 0.89 0.77 0.77 0.80 0.79 0.84 0.92 0.87 0.90 0.86 0.94 0.83 0.87 0.88 0.92 0.81 0.91 0.92 0.87

38 38 38 38 38 38 39 31 38 39 37 39 38 38 40 40 37 38 23 27 37

All correlations are significant at a = 0.01. winter m e a n t e m p e r a t u r e s with 1 A p r i l B M S c o n t r o l l i n g for the effects o f winter p r e c i p i t a t i o n is - 0 . 1 7 , whereas the p a r t i a l c o r r e l a t i o n o f winter p r e c i p i t a t i o n with 1 A p r i l B M S c o n t r o l l i n g for the effects o f winter m e a n t e m p e r a t u r e s is 0.85 (winter m e a n t e m p e r a t u r e a n d winter p r e c i p i t a t i o n were c a l c u l a t e d f r o m d a t a for the m o n t h s o f N o v e m b e r - M a r c h for C l i m a t i c Division 2 in C o l o r a d o ) . These statistics illustrate the large effect o f winter p r e c i p i t a t i o n , a n d the relatively small effect o f winter m e a n t e m p e r a t u r e , on 1 A p r i l B M S in the G u n n i s o n River basin. 2.3. Data analysis Stidd (1954) a n d Klein (1963) e x a m i n e d the r e l a t i o n s h i p between p r e c i p i t a t i o n and 700 m b a r height a n o m a l i e s t h r o u g h the c o n s t r u c t i o n o f linear c o r r e l a t i o n fields between p r e c i p i t a t i o n at specified l o c a t i o n s a n d g r i d d e d values o f 700 m b a r height anomalies. These c o r r e l a t i o n fields are t e m p o r a l c o r r e l a t i o n s a n d in such field lines o f equal c o r r e l a t i o n are a n a l o g o u s to lines o f equal 700 m b a r height a n o m a l i e s associated with extreme values o f p r e c i p i t a t i o n (Stidd, 1954; Klein, 1963). On the basis o f this a n a l o g y , the relationship between 700 m b a r height a n o m a l y p a t t e r n s a n d p r e c i p i t a t i o n at a specified l o c a t i o n can be determined. In a study o f the relationship between 700 m b a r height a n o m a l i e s a n d m o n t h l y

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G.J. McCabe, Jr. / Journal o f Hydrology 157 (1994) 157-175

precipitation for various locations in the USA, Klein and Bloom (1987) found significant linear relationships between 700 mbar height anomalies and precipitation. They further concluded that the assumlStion of a linear relationship between monthly precipitation and 700 mbar height anomalies was useful for estimating monthly precipitation totals and the frequency of precipitation within a month. In this study, correlations between winter mean 700 mbar height anomalies at each of the 105 CAC grid points and 1 April BMS in the Gunnison River basin are examined to identify relationships between atmospheric circulation and snowpack accumulations in the Gunnison River basin. In addition to identifying general relationships between the height anomalies and 1 April BMS, linear regressions between these anomalies at each of the 105 CAC grid points and 1 April BMS were performed to quantify the amount of variability in 1 April BMS explained by variations in 700 mbar height anomalies. The linear regressions were developed according to the method of Klein (1963) and Klein and Bloom (1987), who used a stepwise multiple regression approach to derive multiple regressions with grid point values of 700 mbar height anomalies as the independent variables and precipitation at a specified location as the dependent variable. Klein and Bloom (1987) found that a large proportion (at least 42%) of the variability in monthly precipitation nationwide could be explained by 700 mbar height anomalies. The explanatory power of the regressions was evaluated by computing several goodness-of-fit statistics. In addition to the correlation coefficient (r), other statistics were calculated, such as the coefficient of determination (r2), the root mean square error (r.m.s.e.), and the index of agreement (D). The index of agreement is a measure of the relative error in model estimates and is given by

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L~= (le, - Ol + lOi - OI) where Pi is the model simulated value, Oi is the corresponding observed value, and O is the mean of the observed data. The index of agreement is dimensionless and ranges between 0.0 and 1.0, where 0.0 describes complete disagreement between the estimated and observed values and 1.0 indicates that they are identical. The index of agreement is used because the correlation coefficient and the coefficient of determination cannot account for additive differences or differences in proportionality (Willmott, 1981). The index of agreement is sensitive to differences between observed and estimated means as well as to certain changes in proportionality (Willmott, 1981).

3. Results and discussion

Correlations between winter mean 700 mbar height anomalies at each of the 105 CAC grid points and 1 April BMS for the winters 1947-1948 to 1986-1987 indicate

G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157 175

165

negative correlations over the western USA and a large part of the eastern North Pacific Ocean, and positive correlations over Alaska and the central North Pacific Ocean (Fig. 3). This pattern suggests that wet winters in the Gunnison River basin (large values of 1 April BMS) are associated with anomalous cyclonic atmospheric circulation over the western USA that produces an anomalous southwesterly flow of moist air from the North Pacific Ocean into the SW USA and Colorado. Fig. 3 also shows positive height anomalies centered over Alaska and the surrounding area, which indicate a weakened Aleutian Low. The Aleutian Low is a semipermanent low-pressure system that occurs during the winter in the North Pacific Ocean over the Aleutian Islands, and represents an area of cyclonic activity where numerous storms, moving from west to east, converge during winter. The positive height anomalies illustrated in Fig. 3 indicate decreased storm activity in this area. In contrast, the negative height anomalies over the western USA and eastern North Pacific Ocean suggest increased movement of storms into the western USA. The inverse of the pattern illustrated in Fig. 3 indicates that for dry winters in the Gunnison River basin, positive 700 mbar height anomalies dominate the western USA and most of the eastern North Pacific Ocean, and negative height anomalies are centered over Alaska. The negative height anomalies over Alaska indicate a stronger than normal Aleutian Low. The positive 700 mbar height anomalies over the western USA indicate anomalous anticyclonic circulation. The anomalous cyclonic circulation over Alaska and the surrounding area, and the anomalous anti60 °

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Fig. 3. Correlations of winter mean 700 mbar height anomalies with basin mean snowpack in the Gunnison River basin, Colorado, measured on or about I April for the winters 1947-1948 to 1986-1987. Solid line indicates positive correlations; dashed line indicates negative correlations; the asterisk indicates the location of the Gunnison River basin.

G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157-175

166

cyclonic circulation over the western USA steer the flow of moist air from the North Pacific Ocean to the north of the western USA. In addition, the positive height anomalies indicate enhanced subsidence of air over the western USA, which decreases the opportunity for precipitation. For comparison with the 700 mbar height anomaly pattern illustrated in Fig. 3, a composite-difference map of these anomalies derived from data for the 10 wettest and 10 driest winters in the Gunnison River basin, based on 1 April BMS, was constructed (Fig. 4). The composite difference was generated by subtracting mean winter 700 mbar height anomalies for the 10 driest winters (based on 1 April BMS) from those for the 10 wettest winters. The composite-difference map indicates the same relationships between the 700 mbar height anomalies and 1 April BMS in the Gunnison River basin as illustrated in Fig. 3. Fig. 4 shows that, for the wettest winters, negative 700 mbar height anomalies dominate the western USA and the anomalous cyclonic circulation produces anomalous westerly and southwesterly components of air flow over Colorado. The 700 mbar height anomaly patterns related to wet and dry winters in the Gunnison River basin, inferred from the correlation fields in Fig. 3 and from the composite-difference map in Fig. 4, are similar to those described by Cayan et al. (1992) in a study of the relationships between 700 mbar height anomalies and snowpack accumulations in the Sierra Nevada in California. Cayan et al. used 40 60 °

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G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157 175

167

years of N o v e m b e r - M a r c h precipitation and temperature data to divide the winter seasons into four categories: wet and warm; dry and warm; wet and cool; dry and cool. Composite 700 mbar anomaly maps then were constructed for each of the four categories. Both of the composites for the dry categories indicated positive height anomalies over the western USA, and both of those for the wet winter categories indicated negative height anomalies over the eastern North Pacific Ocean and the west coast of the USA. Cayan et al. concluded that annual variability in snowcourse measurements is well related to annual variations in 700 mbar height anomalies. Klein and Bloom (1987) performed a study of the relationships between monthly precipitation from 60 climatic divisions across the conterminous USA and 700 mbar height anomalies. As part of their study, Klein and Bloom produced composite maps of these anomalies for high and low winter precipitation frequencies in northern California, and the results of their study indicated that high frequencies of winter precipitation in northern California were associated with negative 700 mbar height anomalies centered in the eastern North Pacific Ocean, off the coast of the NW USA. In contrast, low frequencies of winter precipitation were related to positive 700 mbar height anomalies centered off the coast of the NW USA. The results of the studies by Cayan et al. (1992) and Klein and Bloom (1987) show some similarities to the results of this study. In all three studies, above-average winter precipitation is associated with the occurrence of negative winter mean 700 mbar height anomalies over the eastern North Pacific Ocean and over some part of the western USA. Although there are similarities between the results from the three studies, there also are some important differences. For example, the locations of the negative winter mean 700 mbar height anomalies associated with above-average frequencies of winter precipitation in northern California (Klein and Bloom, 1987) are centered in the eastern North Pacific Ocean, off the coast of the N W USA, whereas the negative winter mean 700 mbar height anomalies associated with above-average 1 April BMS in the Gunnison River basin are centered inland, over the central part of the western USA (Fig. 3). These differences indicate that winter precipitation in the Gunnison River basin and in northern California responds differently to variability in winter mean 700 mbar height anomalies.

3.1. Statistical relationships between 700 mbar height anomalies and snowpack accumulations Linear correlations between winter mean 700 mbar height anomalies at each of the 105 CAC grid points and 1 April BMS indicate that snowpack is most highly correlated with these anomalies at the CAC grid point located just west of Colorado (correlation coefficient -0.61, statistically significant at ~ = 0.01) (Fig. 3). For the 10 driest winters (10 smallest values of 1 April BMS) the mean height anomaly at the grid point located just west of Colorado was 10.3 m and - 8 . 3 m for the 10 wettest winters (10 largest values of 1 April BMS). Because of the highly statistically significant correlation between winter mean 700 mbar height anomalies at the CAC grid point located just west of Colorado and 1

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G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157-175

April BMS, they were linearly regressed against 1 April BMS to examine further the statistical relationships between these two variables. The regression produced the following regression equation: Pwin = 422.57 - 4.82P

(2)

where Pwinis 1 April BMS (mm) and P is winter mean 700 m b a r height anomalies (m) at the CAC grid point. Comparison of regression estimates with 1 April BMS produced an r 2 of 0.37 ( r - - 0 . 6 1 , significant at c~ = 0.001), an r.m.s.e, of 83 m m (20% of mean 1 April BMS for the 40 winters analyzed), and an index of agreement of 0.72 (Fig. 5). These results indicate that 37% of the variability in 1 April BMS in the Gunnison River basin can be explained by variations in the 700 m b a r height anomalies at this CAC grid point. Although these results indicate a statistically significant linear relationship between winter precipitation and these height anomalies, the regression estimates do not estimate the magnitude of 1 April BMS well, as indicated by the r.m.s.e, and the wide scatter of points in Fig. 5. Following the approach of Klein (1963) and Klein and Bloom (1987), a stepwise multiple regression was performed by using winter mean 700 m b a r height anomalies at each of the 105 CAC grid points for the 40 winters analyzed in this study as the independent variables and 1 April BMS as the dependent variable. Independent variables only were included in the resultant multiple regression if their significance

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Fig. 5. Comparison of winter mean 700 mbar anomalies for the Climate Analysis Center grid point located just west of Colorado and basin mean snowpack in the Gunnison River basin, Colorado, measured on or about 1 April for the winters 1947-1948 to 1986-1987.

G.J. McCabe, Jr. / Journal o f Hydrology 157 (1994) 157 175

169

level was equal to or exceeded 0.01. This equation included two of the 105 CAC grid points, and is of the following form: Pwin = 414.18 - 6.05P~ + 2.74Pz

(3)

where P~ is winter mean 700 mbar height anomaly (m) at Points P~ and Pz (Fig. 6). Comparing the regression estimates with 1 April BMS produced an r 2 of 0.54 (r = 0.74, significant at a = 0.001) with an r.m.s.e, of 70 m m and an index of agreement of 0.83 (Fig. 7). The r 2 increased from 0.37 to 0.54, the r.m.s.e, decreased from 83 mm to 70 ram, and the index of agreement increased from 0.72 to 0.83. These results indicate that 54% of the annual variability in 1 April BMS in the Gunnison River basin can be explained by variations in 700 mbar height anomalies at two of the CAC grid points used in this study. Partial correlations indicate the independent effects of winter mean 700 mbar height anomalies at Points PI and P~ on 1 April BMS. The partial correlation coefficient of the correlation of winter mean 700 mbar height anomalies at Point P~ with 1 April BMS, while controlling for the effects of these anomalies at Point P~, is -0.73 (significant at a = 0.01), whereas the partial correlation coefficient for the correlation between the winter mean 700 mbar height anomalies at Point P~, while controlling for the effects of these anomalies at Point P~, is 0.53 (significant at c~ = 0.01). The positive regression coefficient for Point P2 in Eq. (3) and the positive partial correlation coefficient between 1 April BMS and winter mean 700 m b a r height 60° ,~

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G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157-175

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anomalies at Point P2 indicate that 1 April BMS in the Gunnison River basin increases as the anomalies at Point P2 increase. To highlight the winter mean 700 mbar height anomaly pattern indexed by Point P2, correlation fields between the anomalies at each of the 105 CAC grid points and 1 April BMS were calculated for the winters when winter mean 700 mbar height anomalies were positive at Point P2 and 1 April BMS was above the 40 year mean, and for the winters when the anomalies at Point P2 were negative and 1 April BMS was below the 40 year mean (Fig. 8). The correlation field illustrated in Fig. 8 indicates that for the winters included in this analysis (25 of the 40 winters), winter mean 700 mbar height anomalies at Point P2 are highly correlated with 1 April BMS in the Gunnison River basin (r -- 0.70, significant at c~ -- 0.01). The spatial pattern generated by the correlations indicates a mid-latitude band of negative correlations oriented in a west-east direction, with positive height anomalies to the north. This pattern indicates a mid-latitude storm track into the central western USA which produces above-average winter precipitation (Fig. 8). The anomalous cyclonic circulation over the western USA, as indicated by the negative correlations, produces an anomalous southwesterly air flow over Colorado, and moist air and storms from the North Pacific Ocean readily intrude into the area.

G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157 175

171

60o

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172

G.J. McCabe,Jr./Journal of Hydrology 157 (1994) 157-175

3.2. Implications for climatic change scenario development Several studies have shown that GCM simulations of atmospheric circulation for current climatic conditions are reliable (Hewitson and Crane, 1992b; McCabe and Legates, 1992; Hay et al., 1992), whereas GCM simulations of regional precipitation have been shown to be unreliable (Grotch and MacCraken, 1990). Hewitson and Crane (1992b) showed that the Goddard Institute for Space Studies (GISS) GCM effectively simulates the synoptic-scale circulation over North America, and found that temporal and spatial distributions of GISS GCM simulations of sea-level pressures were statistically similar to temporal and spatial distributions of observed sea-level pressures. Similarly, McCabe and Legates (1992) found that the Geophysical Fluid Dynamics Laboratory (GFDL) GCM, as well as the GISS GCM, simulated temporal and spatial sea-level pressure variability similar to that found in observed sea-level pressures. These studies indicate that GCMs reliably simulate important components of atmospheric circulation and synoptic-scale circulation patterns. The significant relationships between winter mean 700 mbar atmospheric circulation and 1 April BMS in the Gunnison River basin may be useful in conjunction with GCM simulations of future atmospheric circulation to generate estimates of future 1 April BMS in the Gunnison River basin that are more reliable than current precipitation estimates from GCMs. Although GCM simulations of atmospheric circulation for current climatic conditions have been found to be reliable, it may be difficult for GCMs to simulate accurate estimates of atmospheric pressures for specific grid points, i.e. the two CAC grid points used in the regression equation Eq. (3). Therefore, more general relationships between winter mean 700 mbar height anomalies and 1 April BMS in the Gunnison River basin were examined to develop a regression model that is more compatible with the spatial scale (or grid system resolution) on which GCMs operate. To perform this task, winter mean 700 mbar height anomalies at several CAC grid points surrounding Points Pl and P2 were used to calculate average winter mean 700 mbar height anomalies for large areas (e.g. 10° of latitude by 10° of longitude) that approach the grid resolutions of most GCMs (Fig. 9). These mean values then were used as independent variables in a multiple regression against 1 April BMS to determine if average values of winter mean 700 mbar height anomalies from such large areas could be used to explain a large proportion of the variability in 1 April BMS comparable with that explained by Eq. (3). The resulting regression equation produced an r 2 of 0.48 (r = 0.70, significant at c~ = 0.01), an r.m.s.e, of 75 mm, and an index of agreement of 0.79. These statistics indicate that some statistical power was lost by using the large areal averages as compared with using winter mean 700 mbar height anomalies from specific grid points (i.e. P1 and P2). However, the average winter mean 700 mbar height anomalies from the two areas still explain almost 50% of the variability in 1 April BMS, and provide a regression equation that uses inputs from areas that are large enough to be compatible with the spatial resolution of most GCMs. Currently, GCM estimates of regional precipitation do not agree on the magnitude or direction of change in regional precipitation from current to future climatic

173

G.J. M c C a b e , Jr. / J o u r n a l o f H y d r o l o g y 157 ( 1 9 9 4 ) 157 175

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conditions. Although the regression equations presented in this study only explain approximately 50% of the variability in 1 April BMS in the Gunnison River basin, they do provide a method to combine the strength of GCMs (i.e. estimates of atmospheric circulation) with empirical relationships between atmospheric circulation and 1 April BMS. The use of these regression equations with G C M estimates of winter mean 700 mbar height anomalies for future climatic conditions should at least indicate the general direction of changes in 1 April BMS in the Gunnison River basin, which is an improvement over current G C M estimates of regional precipitation.

4. Summary and conclusion Correlation fields between winter mean 700 mbar height anomalies and 1 April BMS in the Gunnison River basin indicate that drier than normal winters occur when these anomalies are positive over the western USA. Moist air from the Pacific Ocean is moved to the north of the western USA along the western margins of the anomalous anticyclonic circulation. In contrast, wetter than normal winters are generally associated with negative winter mean 700 mbar height anomalies over the western USA and most of the eastern North Pacific Ocean. The anomalous cyclonic circulation produces an anomalous southwesterly air flow over the SW USA and Colorado that moves moist air from the North Pacific Ocean into Colorado. Linear regressions of winter mean 700 mbar height anomalies against 1 April BMS

174

G.J. McCabe, Jr. / Journal of Hydrology 157 (1994) 157-175

indicate that they account for over 50% of the variability in 1 April BMS. The significant linear relationships between these anomalies and 1 April BMS found in this study can be used in conjunction with GCM simulations of future 700 mbar height anomalies to generate estimates of changes in snowpack accumulations in the Gunnison River basin for future climatic conditions.

5. References Barry, R.G., Kiladis, G. and Bradley, R.S., 1981. Synoptic climatology of the western United States in relation to climatic fluctuations during the twentieth century. J. Climatol., I: 97-113. Blackmon, M.L., Lee, Y.H., Wallace, J.M. and Hsu, H.H., 1984. Time variation of 500 mb height fluctuations with long, intermediate and short time scales as deduced from lag-correlation statistics. J. Atmos. Sci., 41: 981-991. Bolin, B., 1986. How much CO 2 will remain in the atmosphere? In: B. Bolin, B.R. Doos, J. Jager and R. Warrick (Editors), The Greenhouse Effect, Climate Change, and the Ecosystem. Wiley, New York, pp. 93-155. Cayan, D.R. and Peterson, D.H., 1989. The influence of North Pacific atmospheric circulation on streamflow in the West. Geophys. Monogr. Am. Geophys. Union, 55: 375-397. Cayan, D.R. and Roads, J.O., 1984. Local relationships between United States west coast precipitation and monthly mean circulation parameters. Mon. Weather Rev., 112:1276-1282. Cayan, D.R., Riddle, L.G., Garen, D.C. and Aguado, E., 1992. Winter climate variability and snowpack in the West. In: K.T. Redmond (Editor), Pacific Climate (PACLIM) Workshop, 10-13 March 1991, Asilomar, CA, California Department of Water Resources Interagency Ecological Studies, Program Tech. Rep. 31, Sacramento, CA, pp. 125-134. Dracup, J.A., 1977. Impact on the Colorado River basin and Southwest water supply. Essay in water-resource design and practice. In: Climate, Climate Change, and Water Supply. National Academy of Sciences, Studies in Geophysics Series. National Academy of Sciences, Washington, DC, pp. 121-132. Gammon, R.H., Sundquist, E.T. and Fraser, P.J., 1985. History of carbon dioxide in the atmosphere. In: Atmospheric Carbon Dioxide and the Global Carbon Cycle. US Department of Energy, Washington, DC, pp. 25-62. Gray, D.M. and Male, D.H., 1981. Handbook of Snow. Pergamon, New York, 776 pp. Grotch, S.L. and MacCraken, M.C., 1990. The use of general circulation models to predict regional climatic change. J. Clim., 4: 286-303. Hay, L.E., McCabe, G.J., Wolock, D.M. and Ayers, M.A., 1992. Use of weather types to disaggregate general circulation model predictions. J. Geophys. Res., 97: 2781-2790. Hewitson, B.C. and Crane, R.G., 1992a. Large-scale atmospheric controls on local precipitation in tropical Mexico. Geophys. Res. Lett., 19:1835 1838. Hewitson, B.C. and Crane, R.G., 1992b. Regional climates in the GISS GCM: synoptic scale circulation. J. Clim., 5: 1002-1011. Johnston, R.J., 1980. Multivariate Statistical Analysis in Geography. Longmans, New York, 280 pp. Klein, W.H., 1963. Specification of precipitation from the 700-millibar circulation. Mon. Weather Rev., 91: 527-536. Klein, W.H. and Bloom, H.J., 1987, Specification of monthly precipitation over the United States from the surrounding 700 mb height field. Mon. Weather Rev., 115:2118 2132. Knox, J.C., 1984. Fluvial response to small scale climate changes. In: J.E. Costa and P.J. Fisher (Editors), Developments and Applications of Geomorphology. Springer, Berlin, pp. 318-342. Knox, J.L. and Lawford, R.G., 1990. The relationship between Canadian prairie dry and wet months and circulation anomalies in the mid-troposphere. Atmos. Ocean, 28: 189-215. Leathers, D.J., Yarnal, B. and Palecki, M.A., 1991. The Pacific/North American teleconnection and United States climate. Part 1: Regional temperature and precipitation associations. J. Clim., 4:517 528.

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Lins, H.F., Sundquist, E.T. and Ager, T.A., 1988. Information on selected climate and climate-change issues. US Geol. Surv. Open-File Rep., 88-718, 26 pp. Madden, R.A., 1979, Observations of large-scale traveling Rossby waves. Rev. Geophys. Space Phys., 17: 1935 1949. McCabe, G.J. and Legates, D.R. 1992. General-circulation-model simulations of winter and summer sealevel pressures over North America. Int. J. Climatol., 12: 815-827. Mueller, D.K. and Moody, C.D., 1984. Historical trends in concentration and load of major ions in the Colorado River system. In: R.H. French (Editor), Salinity in Watercourses and Reservoirs, Proc, 1983 Int. Symp. on the State-of-the-Art Control of Salinity, Salt Lake City, UT. Butterworth, Boston. MA, pp. 181 192. Muller, R.A. and Wax, C.L., 1977. A comparative synoptic climatic baseline for coastal Louisiana. Geosci. Man, 18: 121-129. Namias, J., 1975. Northern hemisphere seasonal sea level pressure and anomaly charts, 1947-1974. In: A. Fleminger (Editor), CalCOFI Atlas No. 22. Marine Life Research Program, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 243 pp. Namias, J., 1981. Teleconnections of 700 mb height anomalies for the northern hemisphere. In: S. Fleminger (Editor), CalCOFI Atlas No. 29. Marine Life Research Program, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 265 pp. Stidd, C.K., 1954. The use of correlation fields in relating precipitation to circulation. J. Meteorol., 11: 202 213. Ugland, R.C., Cochran, B.J., Hiner, M.M., Kretschman, R.G., Wilson, E.A. and Bennett, J.D., 1990. Water Resources Data for Colorado, Water Year 1990, Vol. 2, Colorado River Basin. US Geol. Surv. Water-Data Rep. CO-90-2, 344 pp. Wallace, J.M. and Gutzter, D.S, 1981. Teleconnections in the geopotential height field during northern hemisphere winter. Mon. Weather Rev., 109:784 812. Walsh, J.E., Richman, M.B. and Allen, D.W., 1982. Spatial coherence of monthly precipitation in the United States. Mon. Weather Rev., 110:272 286. Weare, B.C. and Hoeschele, M.A., 1983. Specification of monthly precipitation in the western United States from monthly mean circulation. J. Clim. Appl. Meteorol., 22: 1000-1007. Willmott, C.J., 1981. On the validation of models. Phys. Geogr., 2: 184-194. World Meteorological Organization, 1970. Guide to Hydrometeorological Practices, WMO 168.TP.82. WMO, Geneva, 323 pp. Yarnal, B. and Leathers, D.J., 1988. Relationships between interdecadal and interannual climatic variations and their effect on Pennsylvania climate. Ann. Assoc. Am. Geogr., 78: 624-641.