Rossby Wave Propagation and the Rapid Development of Upper-Level Anomalous Anticyclones during the 1988 U.S. Drought

The upper-tropospheric circulation is investigated for the three months of April, May, and June 1988 during which the Great Plains region of the United States experienced one of its most severe droughts in history. It is found that during this period the April–June (AMJ) seasonal-mean anomaly was not representative of the variability of 10-day low-pass anomalies. Rather, over North America large fluctuations on monthly and shorter timescales occurred, with the dominant streamfunction anomalies not strongly anticyclonic until June. In fact, the AMJ anomaly was dominated by two episodes of rapidly developing, intense anomalous anticyclones in early and late June. Examination of the daily 10-day low-pass streamfunction anomalies at 300 mb suggests that propagating Rossby waves originating in the west Pacific played a dominant role in the initiation of these intense anomalous anticyclones. Numerical experiments with a linear, time-dependent, barotropic model also support this hypothesis. These results suggest that the AMJ anomaly, which has been characterized as a wave train seemingly forced in the east Pacific, may not provide a useful picture of the circulation associated with the drought. Instead, the drought may be better studied not as a single seasonal event, but rather as a succession of events that together produced a serious hydrological deficit.