This analysis further refines Ropelewski & Halpert's (1987; Mon Wea Rev 115:1606-1626 analysis which investigates the relationship between El Niño-La Niña/Southern Oscillation events and southern United States precipitation. Comparisons are made between eigenvector-derived mid-tropospheric (500 mb) flow patterns over North America during extreme El Niño and La Niña months and a base climatology. In addition, the patterns are correlated to regional precipitation anomalies for the southern United States to determine mean surface responses. Cool season (November to March) months are divided into all winter months (AWM), positive anomaly months (PAM), and negative anomaly months (NAM). The extreme anomaly months were determined as any month with a Southern Oscillation Index (SOI) ± 1 standard deviation from the standardized mean. Therefore, the PAM and NAM anomalies represent the La Niña and El Niño extreme phases of the SOI, respectively. Results suggest that the positive (La Niña) SOI phase elicits a greater surface precipitation response than the El Niño phase. This is caused by substantial changes in the primary longwave flow during opposite SOI phases. During AWMs and NAMs, similar flow patterns, dominated by the Pacific/North American (PNA) teleconnection, prevail which induce similar regional precipitation responses. During PAMs, the mid-tropospheric flow shifts to a hybrid flow pattern which is between the PNA and the Tropical Northern Hemisphere teleconnections. Such displacement in the longwave flow variation centers ultimately affects jet stream flow and precipitation forcing, resulting in negative precipitation anomalies across the southern United States.
Precipitation event attributes, defined as the amount, duration, and frequency of individual rainstorms, vary considerably from place to place and season to season. Event variability is caused by a combination of large-scale and local climate processes. Correlations of seasonal time series show that the Pacificmorth American (PNA) teleconnection pattern and local rainfall event attributes are significantly related in specific areas of the south-eastern USA in some seasons. Shifts in the zonal/meridional flow over North America affect both the number of events and the event size. The total number of events in some regions changes by as many as 10 to 12 events per season with shifts in continental-scale circulation. An analysis of attribute distributions shows that the percentage of total events falling into selected intensity categories may also change by as much as 10 per cent between flow regimes. Relationships have both temporal and spatial patterns. Shifts in continental-scale circulation affect event attributes most strongly in winter, with more events occurring in zonal years. Events with greater amounts and durations are found east of the Appalachians in meridional years and west of the mountains in zonal years.
A precipitation event climatology is defined as the frequency distributions of the number, duration, and precipitation amounts of individual rainfall events at an observing station. A preliminary analysis of hourly precipitation records for 27 stations in the south-east United States of America for the period indicates that the operational definition of an event is a period of continuously recorded precipitation separated from other such periods by a dry interval of 2 h or more. The choice of the 2-h separation interval influences the climatological frequency distributions slightly, but maintains a distinction between convective and cyclonic systems. All south-eastern stations displayed similar results, the average number of events varying from near 30 in autumn to about 40 in summer. Most events lasted less than 3 h and gave less than 0.2 in. of precipitation. The seasonal and spatial variations in results suggest close connections between the event climatology and the atmospheric circulation. Short-duration storms suggestive of convective activity dominated in all seasons in Florida. To the north the winter maximum in durations and amounts indicated the importance of depression passage, while increased durations in all seasons over the Appalachian mountains reflected a topographic influence. A general region-wide relationship between the number of events per season, the seasonal total precipitation, and the seasonal number of rain days was obtained, but the results varied greatly from station to station.
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