Atmospheric pressure patterns,climatic change and winter wheat yields in North America

Winter wheat yields over a large area of the United States Great Plains are described as functions of the monthly surface atmospheric pressure pattern over North America. Seventy-eight years of pressure data were spatially decomposed with principal components analysis, and linear combinations of the resulting eigenvectors were used to fit a time series of five sensible climatic variables that are generally considered to be important to wheat yields. This surrogate data set was then used to fit the yield time series. The yield data were initially detrended with a simple linear estimator and adjusted with base constants specific for each crop reporting district.Interannual variation in the pressure field explains approximately 40% of the remaining variance in the yield data over the states of Colorado, Kansas, Oklahoma, Nebraska and Texas. When a reduced model was tested on five years of data simultaneously withheld, a similar amount of the variance was explained. Twentyone eigenvectors are consistently associated with the twelve sensible climatic parameters to which yields are most sensitive. Of these, five were found to be significantly changing (at between the 95 and 99% levels), in either linear or quadratic fashion, over the length of the pressure record. It is concluded that long-term change can and does take place in features of the general circulation that are important determinants of large area crop yields.