A statistical analysis was undertaken to determine the nature and magnitude of the relationship of precipitation, temperature and streamflow in the western United States to large-scale atmospheric circulation patterns. The Southern Oscillation Index (SOB was used as an indicator of the El Nifio/Southern Oscillation (ENSO) and the PNA index as an indicator of the Pacific#North America pattern. These indices were correlated with surface climate data and split sample analyses were conducted to determine climate response during the extreme phases of each index. October-March precipitation was shown to be most strongly correlated with SOI averaged over the July-November period. The analysis showed that there are two centers of opposite association with the SOI. During low values of the SOI (ENSO events) precipitation is low in the Pacific northwest and high in the desert southwest. Correlations between SOI and temperature were greatest in the Pacific northwest. The split sample analysis also revealed statistically significant differences in precipitation occurring during extremes of the SOI. The PNA pattern was related to precipitation and temperature over a concurrent time period. Especially strong associations were noted in the Pacific northwest for both precipitation and temperature. Streamflow showed associations with SO I similar to precipitation. INTRODUCTIONClimate variability has direct social and economic impacts a•d has therefore been the subject of numerous studies at may space and time scales. Many of the most direct =pacts occur through the hydrologic cycle for which climate is the primary driving force. Of primary concern to s0c•ty are hydrologic extremes, for example, floods, result-'.• from large precipitation events, and droughts, which .rurally occur as a combination of low precipitation, and therefore streamflow, together with above normal temperat.m•es during the same periods. In the western United States '• effect of climate variability is particularly acute due in part to the dependence of the agricultural industry on both ;-dace and groundwater supplies for irrigation, which are each in turn related to precipitation. Climate sensitivity is froher amplified by growing populations in urban areas of the west, which are taxing available water for municipal and •ustria! uses. Power generation by hydroelectric facilities aed environmental concerns over instream water to preserve •aality and aquatic resources also relate to climate variability through amount and timing of both water availability and e•e•y and water requirements. These increasing demands occur in a semiarid environment where precipitation •0ants are, in general, small and highly variable both :seasonally and interannually when compared to the eastern Urnired States. In most of the western United States, water •'•r•ies are derived primarily from snowmelt runoff which is •r.a•.ally out of phase with demand. Thus large reservoirs '•ee required for redistribution of the water from periods of •h streamflow, usually occurring in the spring an...

Frequency distributions of daily precipitation in winter and daily stream flow from late winter to early summer, at several hundred sites in the western United States, exhibit strong and systematic responses to the two phases of ENSO. Most of the stream flows considered are driven by snowmelt. The Southern Oscillation index (SOI) is used as the ENSO phase indicator. Both modest (median) and larger (90th percentile) events were considered. In years with negative SOI values (El Niño), days with high daily precipitation and stream flow are more frequent than average over the Southwest and less frequent over the Northwest. During years with positive SOI values (La Niña), a nearly opposite pattern is seen. A more pronounced increase is seen in the number of days exceeding climatological 90th percentile values than in the number exceeding climatological 50th percentile values, for both precipitation and stream flow. Stream flow responses to ENSO extremes are accentuated over precipitation responses. Evidence suggests that the mechanism for this amplification involves ENSO-phase differences in the persistence and duration of wet episodes, affecting the efficiency of the process by which precipitation is converted to runoff. The SOI leads the precipitation events by several months, and hydrologic lags (mostly through snowmelt) delay the stream flow response by several more months. The combined 6-12-month predictive aspect of this relationship should be of significant benefit in responding to flood (or drought) risk and in improving overall water management in the western states.

Refugia have long been studied from paleontological and biogeographical perspectives to understand how populations persisted during past periods of unfavorable climate. Recently, researchers have applied the idea to contemporary landscapes to identify climate change refugia, here defined as areas relatively buffered from contemporary climate change over time that enable persistence of valued physical, ecological, and socio-cultural resources. We differentiate historical and contemporary views, and characterize physical and ecological processes that create and maintain climate change refugia. We then delineate how refugia can fit into existing decision support frameworks for climate adaptation and describe seven steps for managing them. Finally, we identify challenges and opportunities for operationalizing the concept of climate change refugia. Managing climate change refugia can be an important option for conservation in the face of ongoing climate change.

The state of knowledge regarding trends and an understanding of their causes is presented for a specific subset of extreme weather and climate types. For severe convective storms (tornadoes, hailstorms, and severe thunderstorms), differences in time and space of practices of collecting reports of events make using the reporting database to detect trends extremely difficult. Overall, changes in the frequency of environments favorable for severe thunderstorms have not been statistically significant. For extreme precipitation, there is strong evidence for a nationally averaged upward trend in the frequency and intensity of events. The causes of the observed trends have not been determined with certainty, although there is evidence that increasing atmospheric water vapor may be one factor. For hurricanes and typhoons, robust detection of trends in Atlantic and western North Pacific tropical cyclone (TC) activity is significantly constrained by data heterogeneity and deficient quantification of internal variability. Attribution of past TC changes is further challenged by a lack of consensus on the physical link- ages between climate forcing and TC activity. As a result, attribution of trends to anthropogenic forcing remains controversial. For severe snowstorms and ice storms, the number of severe regional snowstorms that occurred since 1960 was more than twice that of the preceding 60 years. There are no significant multidecadal trends in the areal percentage of the contiguous United States impacted by extreme seasonal snowfall amounts since 1900. There is no distinguishable trend in the frequency of ice storms for the United States as a whole since 1950.

[1] A newly available data set of daily precipitation observations was used to study the temporal variability of the frequency of short-duration extreme precipitation events for 1895 -2000 in the conterminous United States. Event durations of 1, 5, 10, and 30 day and return periods of 1, 5, and 20 year were analyzed. For all combinations of duration and return period, heavy precipitation frequencies were relatively high during the late 19th/early 20th Centuries, decreasing to a minimum in the 1920s and 30s, followed by a general increase into the 1990s. The frequencies at the beginning of the 20th Century were nearly as high as during the late 20th Century for some combinations of duration and return period, suggesting that natural variability cannot be discounted as an important contributor to the recent high values. Extensive quality control of data and Monte Carlo testing was performed to provide confidence in the reality of the early period high frequencies.