Climate Indexing Application to the Pacific Northwest G. Bothun and S. Ostrander Dept. of Physics, University of Oregon

Premise Climate represents seasonal weather patterns averaged over time. Such a representation comprises a quantitative climate waveform Departures from that waveform, if statistically significant, therefore comprise evidence of real climate change

(PNI), developed by at the UW by Ebbesmeyer and Strickland (1995) is a composite index that characterizes Pacific Northwest climate. The PNI uses three parameters: 1) air temperature at Olga; 2) annual precipitation measured at Cedar Lake WA; 3) snowpack depth at Paradise on Mount Rainier on March 15 of each year. Statistical Z-scores are computed for each year and the data is smoothed with a 5 year boxcar to yield a waveform that indicates multi-decadal periods of cool/wet (blue) and warm dry (red) climates.

Our study uses hundreds of climate sites in Western Washington to reform the PNI. Our new PNI, hereafter called the SHI (Stephanie’s Harmonic Index), is formed from the 20 best sites in the data. The index generated from that data (GD in the figure below) compares quite favorably with the PNI indicating that the PNI is a robust waveform and is not a fluke of using only 3 data points.

Weight 2 = Rainfall The original PNI is un-weighted. Below are example departures from the SHI that occur when either temperature data (weight 1) or precipitation data (weight 2) Is given more weight. The detailed waveform seems more sensitive to precipitation than it is to temperature. Work is in progress on weighted seasonal representations of the SHI.

The yellow waveform below represents an attempt to game the system by selecting those sites (starting in 1935) that will yield the most amount of positive signal (i.e. to bias the data towards warm/dry periods). As can be seen, the only significant departure occurs in the period. No attempt to maximize the post 1975 signal by preferential site selection produced a waveform that is any different than the SHI. This suggests the SHI is representative.

The final step now involves fitting an approximate spherical harmonic function to the data in the last century (in this case ) in order to predict future climate based on last centuries wave form. While the model is not a perfect fit to the data by any means, it is a reasonable approximation which clearly shows that, as of 2000, the PNW should be entering another extended cool/wet period. The data are in sharp disagreement with this prediction and therefore provide an indicator that the current period of warm/dry weather is likely to extend into the indefinite future.

Conclusions The PNI indexing system is robust The detailed climate waveform is more sensitive to precipitation than temperature Preliminary fits to the waveform suggest that the current period of warm/dry Western Washington climate that started around 1975 will continue into the indefinite future. This implies a) reduced hydropower, b) truncated ski season, c) possibly extended forest fire seasons.