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This analysis further refines Ropelewski & Halpert's (1987; Mon Wea Rev 115:1606-1626) analysis which investigates the relationship between El Nino-La Nina/Southern Oscillation events and southern United States precipitation. Comparisons are made between eigenvector-derived mid-tropos- pheric (500 mb) flow patterns over North America during extreme El Nino and La Nina 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 South- ern Oscillation Index (SOI) ± 1 standard deviation from the standardized mean. Therefore, the PAM and NAM anomalies represent the La Nina and El Nino extreme phases of the SOI, respectively. Results suggest that the positive (La Nina) SOI phase elicits a greater surface precipitation response than the El Nino 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.

The Gulf of Mexico mid-tropospheric response to El Niño and La Niña forcing