Tuesday, 15 January 2002: 4:29 PM
Wet Versus Dry Periods in the Midwest During January 1998 (El Nino) and January 1999 (La Nina)
During the winter season of an El Niņo, which usually represents the peak of this event, the expected conditions for the State of Indiana are 2°F warmer than usual and 70% of normal precipitation. The Midwest, as a whole, has similar conditions. January 1998, however, was anomalous in that extremely warm temperatures and 150% of normal precipitation occurred across the Midwest. The latter was considerably more than predicted. This paper examines reasons for the greater-than-expected "rainfall" amounts in January 1998, and compares circulation patterns during the month's major wet episode (3-8 January) to the more expected warm and dry period (25-31 January). Major differences in the large-scale flow features, as well as in the temperature and moisture conditions, were seen to take place over the Midwest between these two periods. Somewhat surprisingly, both periods seem to exhibit 500 hPa height anomalies similar to those of the Pacific-North American (PNA) pattern. The wet period closely resembled a negative PNA, while the dry period nearly resembled a positive PNA. Additionally, it appeared that a strong positive North Atlantic-type oscillation (NAO) was present during the wet period, whereas a negative NAO phase occurred during the dry period. The former provided a flow of warm moist air from the eastern Atlantic and Gulf of Mexico toward the Midwest.
For comparison with the January 1998 El Niņo results, a similar set of analyses was performed for the month of January 1999, near the peak of the winter La Niņa event. As for January 1998, the month of January 1999 contained a wet period (2-13), which was cooler than normal and a warm dry period (24-30). In contrast to the January 1998 results, the circulation patterns for the wet and dry periods of January 1999 showed no obvious connection to either a PNA or NAO.
In addition to the analyses described above, statistical correlations between Midwest temperatures and sea surface temperatures (SSTs) in the Niņo 3.4 region were compiled for the most recent El Niņo and La Niņa events. These will be discussed if time permits. Finally, no correlation between Midwest precipitation and SSTs was observed.