83rd Annual

Thursday, 13 February 2003
The Influence of PNA and NAO Patterns on Temperature Anomalies in the Midwest During Four Recent El Nino Events: A Statistical Study
Dayton Vincent, Purdue University, West Lafayette, IN; and S. Lashley, S. O'Connor, M. Skipper, B. O'Hara, T. Reaugh, and G. Lamberty
Poster PDF (397.4 kB)
The purpose of this paper is to determine if there is any statistical relationship between the weather (specifically, the temperature anomalies) over the Midwestern USA and the circulation patterns over the North Pacific and/or Atlantic Oceans during four El Niņo events. The former region is examined using the Pacific North American (PNA) index, while the latter uses the North Atlantic Oscillation (NAO) index. The four El Niņos occurred in 1982-83, 1986-88, 1991-92 and 1997-98. Three-day averages were calculated for each index, as well as for temperature anomalies. Ten stations were selected to represent the Midwest and their average temperature anomalies were used for all of the results presented herein.

PNA values were generally strong positive during the winter and early spring of each El Niņo event. This implies a teleconnection pattern of higher-than-normal geopotential heights at 500 hPa over Hawaii and Pacific Northwest and lower-than-normal heights over the North Pacific and eastern port of the United States (including the Midwest). During the remainder of the year, the PNA values were weak and consisted of alternating signs. The NAO index showed that positive values dominated in the late fall to early winter period for all but the 1997-98 El Niņo event. A positive value implies lower-than-normal heights over Iceland and higher-than-normal heights over the Azores, which intensifies the westerly jet.

Statistical analyses showed very little or no correlation between midwestern temperature anomalies and either the PNA or NAO index for the total period of all four El Niņos. There was also no correlation during the winter or summer half of the year, nor was there any correlation where a variety of lags or leads were considered. When the winter season was partitioned into November-December (ND) versus January-February-March (JFM), however, correlations improved considerably, but only for the PNA index. The significance of the latter results will be discussed at the conference, as well as the implications of all our results. Finally, neither the PNA nor the NAO showed any consistent periodic behavior during El Niņo events: total, winter or summer periods.

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