AN EXAMINATION OF EL-NINO LA NINA-RELATED PRECIPITATION AND TEMPERATURE ANOMALIES ACROSS THE NORTHERN PLAINS

Monthly total precipitation and mean temperature data records extendin g from the late nineteenth century to 1990 were collected for 147 stat ions in South Dakota, North Dakota, and portions of adjacent states an d provinces. This region, defined as the Northern Plains region (NPR), was examined for patterns associated with the warm phase (ENSO) and t he cold phase (LNSO) of the Southern Oscillation to elucidate some of the debate concerning a signal in this area. Based on a correlation an alysis, the NPR was treated as having one spatial degree of freedom. U sing Monte Carlo simulations of the Student's t-test statistic, four s easons with significant changes in mean precipitation or temperature d uring either ENSO or LNSO were identified. A highly significant signal was evident during the ENSO April to October season for precipitation , where the mean precipitation increased 7.21 cm for the 23 events stu died. Here 20 of these 23 ENSO events exhibited precipitation above th e median value, and 14 of the 23 events were in the upper quartile. In contrast, a strong signal for decreased LNSO precipitation was noted where May to August precipitation averaged 3.91 cm lower during the 17 events, with similar significance values. Complementing the enhanced ENSO warm season precipitation, the August to October temperature decr eased by 2.17 degrees C, with a significant number of events in both t he lowest half and lowest quartile. Finally, temperature averaged 4.67 degrees C cooler during LNSO winters. These results will be useful fo r limited-season prediction of precipitation and temperature tendencie s across the NPR. It is interesting to note that the initial ENSO year s did not reveal a significant temperature increase during the NPR win ter, which is in contrast to similar studies. However, by slightly mod ifying the years that were classified as ENSO years, a significant win ter temperature response was indicated. This suggests that there is a tendency for warmer NPR winters during ENSO; however, this was not sta tistically significant.