The distribution of wintertime precipitation over the midwest during the 1997-98 El Nino and 1998-99 La Nina

A diagnostic study of wintertime precipitation and associated circulation anomaly pattern (e.g., jet stream locations) is presented in the Midwest for the months of January and February during the 1997-98 El Niño and 1998-99 La Niña. Long term climatological records for the state of Indiana show that expected wintertime conditions during El Niños are warmer than normal by 2°F/day and drier than normal (i.e., 70% of normal precipitation). Statistics were not readily available for La Niña events. Using West Lafayette, Indiana as an example, and focusing on precipitation, we found that January 1998 was anomalously wet, with 168% of normal precipitation. More importantly, because of the extremely warm temperatures, nearly all of the month's total was due to rain. February 1998, on the other hand, was much closer to expected precipitation amounts for an El Niño winter month, with 80% of normal. During the La Niña months of January and February 1999, the precipitation amounts at West Lafayette were 201% and 177% of normal, respectively. The first half of January was colder than normal and all of the precipitation consisted of snow, whereas the latter half of January and most of February was considerably warmer than normal and nearly all precipitation was in the form of rain. The purpose of this paper is to examine some case studies during precipitation episodes, including at least one rain and one snow episode. The emphasis will be to diagnose differences in jet stream flow patterns between the monthly mean circulations for each of our four El Niño and La Niña months and the circulation patterns when individual precipitation episodes occurred. As an example of one such episode, we cite the period, 4-8 January 1998, which is when nearly all of the precipitation (all rain) for this anomalously wet El Niño month fell. In summary, this paper addresses the conference themes of "understanding natural climate variability" and "diagnostic studies of regional climate systems, including cold season and high latitude areas".