On the definition of El Nino and associated weather anomalies

El Nino has been associated with seasonal weather anomalies across the globe that influence many aspects of the forest environment. A new definition of El Nino adopted by the World Meteorological Organization Region IV (North America) identifies a number of additional El Nino seasons beyond those conventionally agreed. These additional seasons are characterized by SST anomalies primarily in the western central equatorial Pacific. We show that the seasonal weather anomalies over the U.S. associated with these additional Dateline El Nino seasons are substantially different from those associated with conventional El Nino seasons. Although some regions have similar associated anomalies, most of the major regional anomalies are quite different. Treating the two as a single phenomenon yields weaker overall seasonal weather associations and does not take advantage of the stronger associations available when the two are treated separately. Care must be taken when using conventionally identified El Nino associations using the new definition.