North American Precipitation and Temperature Patterns Associated with the El Niño/Southern Oscillation (ENSO)

Abstract

This paper describes an investigation of the “typical” North American precipitation and temperature patterns associated with the El Niño/Southern Oscillation (ENSO). Monthly surface temperature and precipitation data are analyzed using a method designed to identify regions of the globe that have responses associated with ENSO. Monthly composites, covering idealized two-year ENSO episodes, are computed for temperature and precipitation at all stations with data spanning seven or more ENSO events. The firm harmonic is extracted from the 24 monthly composite values and plotted in the form of a two-year harmonic dial vector. When plotted on a map of North America, these vectors reveal both the regions of coherent response and the phase of the responses with respect to the evolution of the ENSO episode. Time series of temperature and precipitation for the regions identified in the harmonic vector maps are examined to determine the magnitudes of the responses and the percentage of the time that the identified responses actually occurred in association with the ENSO events. The temperature anomalies are expressed in terms of standardized departures, while precipitation departures are expressed as percentiles of the appropriate gamma distributions, fitted to the entire data record. The analysis shows that above normal precipitation was associated with ENSO in 18 out of 22 cases (81%)in the “season” starting with October of the ENSO year to March of the following year for an area of North America that includes parts of the southeastern United States and northern Mexico. Above normal precipitation was also observed in the Great Basin area of the western United States in 9 out of 11 cases (81%) for the April through October “season” during ENSO years. No high latitude precipitation signals were indicated by this analysis. Areas of Alaska and western Canada experienced positive temperature anomalies in 17 out of 21 ENSO episodes (81%) during the “season” defined by December of the ENSO year through the following March. Parts of the southeastern United States near the Gulf of Mexico were found to have negative temperature anomalies associated with 20 out of 25 ENSO episodes (80%) for the “season” October of the ENSO year through the following March.