THE GULF-OF-MEXICO MIDTROPOSPHERIC RESPONSE TO EL-NINO AND LA NINA FORCING

This analysis further refines Ropelewski & Halpert's (1987; Mon Wea Re v 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;t ropospheric (500 mb) flow patterns over North America during extreme E l Nino and La Nina months and a base climatology. In addition, the pat terns are correlated to regional precipitation anomalies for the south ern United States to determine mean surface responses. Cool season (No vember to March) months are divided into all winter months (AWM), posi tive anomaly months (PAM), and negative anomaly months (NAM). The extr eme anomaly months were determined as any month with a Southern Oscill ation Index (SOI) +/- 1 standard deviation from the standardized mean. Therefore, the PAM and NAM anomalies represent the La Nina and El Nin o extreme phases of the SOI, respectively. Results suggest that the po sitive (La Nina) SOI phase elicits a greater surface precipitation res ponse 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 precipitat ion responses. During PAMs, the mid-tropospheric flow shifts to a hybr id flow pattern which is between the PNA and the Tropical Northern Hem isphere teleconnections. Such displacement in the longwave flow variat ion centers ultimately affects jet stream flow and precipitation forci ng, resulting in negative precipitation anomalies across the southern United States.