OBSERVED NONLINEARITIES OF MONTHLY TELECONNECTIONS BETWEEN TROPICAL PACIFIC SEA-SURFACE TEMPERATURE ANOMALIES AND CENTRAL AND EASTERN NORTH-AMERICAN PRECIPITATION

Most investigations of relationships between tropical Pacific sea surf ace temperature anomaly (SSTA) events and regional climate patterns ha ve assumed the teleconnections to be linear, whereby the climate patte rns associated with cold SSTA events are considered to be similar in s tructure and morphology but opposite in sign to those linked to warm S STA events. In contrast, and motivated by early evidence of nonlineari ty in the above regard, this study identifies characteristic (i.e., co mposite) calendar monthly central and eastern North American precipita tion patterns separately for warm and cold SSTA events indifferent reg ions of the tropical Pacific (central, eastern, west-central ''horsesh oe,'' far western) identified through principal component analysis. Th e precipitation anomaly patterns are computed from an approximately 1 degrees lat-long set of monthly station data for 1950-92. Their robust ness and nonlinearity are established using local, regional, and field statistical significance tests and a variance analysis. This combinat ion of unique SSTA analyses, resulting composite selection, and charac teristic precipitation anomaly determination from a fine-resolution da taset increases our understanding of tropical Pacific-North American p recipitation teleconnections in several respects. First. significant l inkages to the two SSTA modes related to traditional warm and cold eve nts (central and eastern tropical Pacific) are identified for all mont hs except September and October, with all exhibiting some nonlinear ch aracteristics. The most important of chose nonlinearities involve asso ciations with eastern tropical Pacific SSTAs, which affect precipitati on near the southern Atlantic and Gulf of Mexico coasts (dry for cold Novembers), around the Great Lakes and in the Ohio River valley (dry, warm, January-February), in the southeastern United Stares (dry, warm, July-August), and across the Northern Great Plains (dry. warm, Novemb er-January). Conversely,the regions confirmed to have essentially line ar associations with traditional warm and cold events include the Gulf of Mexico coast (positive relation with eastern tropical Pacific, Jan uary-March), Ohio River valley (negative, central tropical Pacific, Fe bruary), and mid-Atlantic coast (negative, eastern tropical Pacific, J uly-August). However, only nonlinear precipitation teleconnections are associated with SSTAs in tropical Pacific regions largely unrelated t o ENSO. These principally involve anomalously dry conditions in much o f the eastern half of the United States during January-March and in th e central United Stales in July-October (warm SSTAs in west-central tr opical Pacific horseshoe), and in a strip from Texas to New England in January and along the central gulf coast and lower Mississippi valley in April (warm SSTAs in far western tropical Pacific). The results th us demonstrate the sensitivity of central and eastern North American p recipitation teleconnections to the location and extent of tropical Pa cific SSTAs. in the appendix, the present results are also compared to the observed climate anomalies during the 1997-98 EI Nino event.