Thursday, 13 February 2003
Multi-Decadal Regime Shifts in United States Precipitation, Temperature and Streamflow at the end of the 20th Century
Intra- to multi-decadal variation in annual precipitation, temperature, and streamflow over the continental United States are evaluated here through the calculation of Mann-Whitney U statistics over running time windows of 6-30 years duration. When this analysis is conducted on time series of nationally averaged annual precipitation and mean temperature during 1896-2001, 8 of the 10 wettest years are found during the last 29 years of that 106 year period, and 6 of the 10 warmest years during the last 16. Both of these results indicate highly significant departures from long-term stationarity in U.S. climate at the end of the 20th century. The respective effects of increased wetness and warmth during those years are mainly evident west and east of the Rocky Mountains. Analysis of annual streamflow records across the United States during 1939-1998 shows broadly consistent effects. Initial evidence of the recent wet regime is most apparent in eastern streamflow, which shows a clear pattern of high ranked mean annual values during the 1970's. Over the Midwestern states a coherent pattern of high ranked annual flow is found during multi-decadal periods beginning during the late 1960's and early 1970's and ending in either 1997 or 1998. During the late 1980's and early 1990's a significant incidence of low ranked annual flow conditions throughout the west was roughly coincident with the onset of western warmth during the mid 1980's. Evidence of a recent and highly significant transition to wetter and warmer conditions nationally, and consistent variation in streamflow analyses, suggests that hydrological surplus in the central and eastern U.S. and hydrological deficit in the west may be representative of the initial stages of climate change over the continental United States.