Submitted to: Journal of Climate
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 13, 2003
Publication Date: December 1, 2003
Citation: Mauget, S.A. 2003. Multi-decadal regime shifts in united states streamflow, precipitation, and temperature at the end of the 20th century. Journal of Climate. 16(23): 3905-3916. Interpretive Summary: Statistical tests which evaluate the rankings of long-term records of yearly streamflow, precipitation, and temperature over the continental United States show interesting evidence of a shift to wetter and warmer conditions at the end of the 20th century. In records of nationally averaged annual precipitation and mean temperature during 1896-2001, it is found that eight of the ten wettest years occurred during the last 29 years of that 106 year period, and six of the ten warmest years during the last 16. Statistically, these results are highly inconsistent with the idea that U.S. climate has been basically unchanging during the 20th century. This change makes itself evident in increased rainfall and streamflow in the central and eastern U.S. after 1973, while evidence of warmth and decreased streamflow is found throughout the Rocky Mountain region and in the west after 1986. As these effects are consistent with the increased surface warming and strengthened hydrological cycle that are expected to accompany greenhouse warming, an interesting possibility to consider is that the initial effects of that warming are already apparent over the continental United States. Whereas average U.S. climate conditions can be described by relatively wet conditions east of the 100th meridian and more arid conditions in the west, it would also appear that those effects might be amplifying the most general feature of national climate. That is, producing wetter conditions east of the 100th meridian, and drier conditions to the west.
Technical Abstract: Intra- to multi-decadal variation in annual streamflow, precipitation, and temperature 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 method is demonstrated on time series of nationally averaged annual precipitation and mean temperature during 1896-2001, it reveals that eight of the ten wettest years occurred during the last 29 years of that 106 year period, and six of the ten 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 effects of increased wetness are primarily evident in the central and eastern U.S., while evidence of warmth is found throughout the Rocky Mountain region and in the west. 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 highly significant transitions to wetter and warmer conditions nationally and consistent variation in streamflow analyses suggests that increased hydrological surplus in the central and eastern U.S. and increased hydrological deficit in the west may be representative of the initial stages of climate change over the continental United States.