Identifying and Predicting Wet Dry Climate Regime Shifts in the Columbia River Basin

Identifying and Predicting Cool/Wet and Warm/Dry Climate Regime Shifts for the Columbia River Basin

Alan F. Hamlet

JISAO Climate Impacts Group

Department of Civil and Environmental Engineering

University of Washington

Revised 7/12/98

Multi- decade bimodal patterns of below and above average streamflow have been shown to occur in the Columbia River Basin. These climatic effects are associated with observed tendencies in the PDO climate index, however it is not clear if this is a cause-and-effect relationship, or whether the PDO index merely measures the symptoms associated with global climate dynamics that cause these trends.

In any case, knowledge of the dominant wet or dry pattern is extremely important for streamflow forecasting in the Columbia River Basin since observed streamflow inter-annual variability associated with ENSO events is biased up and down according to the decadal patterns. El Nino events during the wet decadal phase, for example, are rather ordinary in terms of streamflow outcome, whereas El Nino events in dry decadal periods are associated with extremely low streamflow and multi-year droughts. Unfortunately changes in these decadal patterns cannot be forecast easily in real time. Since about 1977 the climate for the PNW has been in a persistent dry phase, however observed streamflow in the past several years (and especially water year 1997) has been strongly above average. Does this mean that a shift towards a long-term wet cycle has occurred? It cannot be demonstrated conclusively that this is so.

To look for clues about the nature of these decadal shifts, the historic record of April-September average streamflow data from 1900-1996 were examined. Several interesting patterns emerged.

Firstly, April-September above or below average flows three years in a row in opposition to the apparent dominant decadal phase are observed only once in the record (1982, 1983, 1984). This is useful for confirming which phase is dominant. If there has been a recent event of 3 sequential years of above or below average streamflow, the observed pattern suggests (in the absence of evidence of a transition) that it is very likely that an associated wet or dry phase has established itself. This simple rule of thumb, while confirming that a shift has occurred does not, however, help to identify transitions very readily, since at least three years (and usually more) must pass before the trend can be identified.

Secondly, transitions between decadal-scale patterns appear to be strongly associated with extreme events in opposition to the dominant trend. The following picture identifies streamflow events more than 1.5 standard deviations above or below the mean in opposition to the dominant trend. With the exception of 1973-1974 in which the pattern appears to flip-flop from dry to wet, streamflow events of this magnitude in opposition to the established trend appear to "herald" the coming decadal shift. Another way to look at this is to consider the observed probability of a high flow event more than 1.5 std deviations above the mean during PDO positive epochs, and of a low flow event more than 1.5 std deviations below the mean during PDO negative epochs. From 1900-1996, the observed probability of the former event is 0.0, and of the latter event is 1/52 or 1.9%.

If these patterns are indeed general and repeating events for the Columbia Basin, it suggests that a regime shift from a dry cycle to a wet cycle has probably occurred around 1997 (an extremely high flow year similar to 1974), and that variability in Columbia Basin streamflow associated with ENSO events will be biased towards above average streamflow for the next several decades.