Depiction of the Variations of Great Plains Precipitation and Its Relationship with Tropical Central-Eastern Pacific SST

Abstract

Several advanced analysis tools are applied to depict the time?frequency characteristics of the variations of Great Plains (GP) precipitation and its relationship with tropical central-eastern Pacific Ocean sea surface temperature (SST). These tools are advantageous because they reveal the detailed features of the dominant time scales of precipitation variations, the combined effects of multiscale oscillating signals on the intensity of precipitation, and the variations of SST?precipitation relationships in time and frequency domains. The variability of GP precipitation is characterized by strong annual and semiannual signals, which have the most stable oscillating frequencies and the largest amplitudes. However, nonseasonal signals, which are less oscillatory and have smaller amplitudes and more variable frequencies with time, also contribute significantly to precipitation variability and may modify the seasonal cycle of GP precipitation. The phase of these nonseasonal signals is in phase (out of phase) with that of seasonal signals during the periods of heavy (deficient) precipitation. Significant correlations exist between GP precipitation and Niño-3.4 SST, and the strongest relationship appears when the SST leads the precipitation by 1 month. The GP precipitation increases (decreases) during El Niño (La Niña) episodes. Significant relationships appear on semiannual and annual time scales in the 1950s and on interannual time scales in the 1910s, 1940s, and 1980s. A particularly significant relationship appears on biennial time scales in the 1980s. The revealed SST?precipitation relationship is strongly seasonally dependent, with the greatest significance in summer. Warming of tropical central-eastern Pacific SST weakens the overlying easterly trade winds and strengthens the northward moisture supply from Central America through the Gulf of Mexico to the Great Plains. This dominant SST influence prevails in all seasons. However, the moisture transport from the southwest coast and the Gulf of California also contributes to the variability of GP precipitation in September?November, December?February, and March?May. In June?August, the increase in GP precipitation is caused by convergence between anomalous northerly flow over the northern plains, associated with the warming in the northeastern Pacific, and southerly flow over the southern plains, associated with the warming in the tropical central-eastern Pacific.