Visualizing Spatial Heterogeneity of Western U.S. Climate Variability

Monthly climatologies (1971-2000 monthly averages) for stations in the western United States, obtained from the NOAA/National Climatic Data Center (NCDC), are used to illustrate the spatial variations in the annual cycle of climate. Animated map sequences of temperature and precipitation, their average, intermonthly changes, and the local timing of annual maxima or minima provide a comprehensive spatiotemporal baseline of regional climate. The animated maps illustrate three scales of variation: 1) broadscale patterns related to the annual cycle of insolation and hemispheric-scale atmospheric circulation features; 2) mesoscale patterns related to location on the continent and the influence of specific regional circulation features like those associated with the North American monsoon; and 3) smaller-scale spatial variations, related to the mediation by local physiography of the influence of large-scale circulation. Although most western U.S. stations have temperature maxima in July, a delay occurs at stations along the West Coast and interior Washington, northern Idaho, and Montana. A seesaw pattern of precipitation maxima is evident between coastal areas (winter dominated) and the interior (summer dominated). Cluster analyses of the ratio of monthly-to-annual precipitation values for each station identify regions with similar annual cycles of precipitation. Regions of high spatial heterogeneity in the timing of when precipitation occurs include the northern Rocky Mountains, Utah, Arizona, and northwestern Montana. The superimposition of these three scales of spatial variability leads to steep gradients and, in some regions, considerable spatial heterogeneity in annual precipitation. The regional patterns of precipitation heterogeneity highlight vulnerability to drought, especially in regions of the interior west that do not have a dominant precipitation month or season.