An examination of El Nino La Nina-related precipitation and temperature anomalies across the Northern Plains

Monthly total precipitation and mean temperature data records extending from the late nineteenth century to 1990 were collected for 147 stations in South Dakota, North Dakota, and portions of adjacent states and provinces. This region, defined as the Northern Plains region (NPR), was examined for patterns associated with the warm phase (ENSO) and the cold phase (LNSO) of the Southern Oscillation to elucidate some of the debate concerning a signal in this area. Based on a correlation analysis, the NPR was treated as having one spatial degree of freedom. Using Monte Carlo simulations of the Student's t-test statistic, four seasons with significant changes in mean precipitation or temperature during 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 studied. Here 20 of these 23 ENSO events exhibited precipitation above the 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 decreased by 2.17 degrees C, with a significant number of events in both the lowest half and lowest quartile. Finally, temperature averaged 4.67 degrees C cooler during LNSO winters. These results will be useful for limited-season prediction of precipitation and temperature tendencies across the NPR. It is interesting to note that the initial ENSO years did not reveal a significant temperature increase during the NPR winter, which is in contrast to similar studies. However, by slightly modifying the years that were classified as ENSO years, a significant winter temperature response was indicated. This suggests that there is a tendency for warmer NPR winters during ENSO; however, this was not statistically significant.