Midlatitude surface temperature variability and teleconnection associated with warm and cold phases of El Niño/Southern Oscillation

This study describes a comprehensive investigation of the contiguous US temperature patterns in relation to the warm and cold phases of El Niño/Southern Oscillation (ENSO) using a set of empirical and statistical analyses, such as harmonic analysis, annual cycle composites, and cross-correlation analysis. Monthly temperature composites for the first harmonic, covering 24-month ENSO events, are formed for all climate divisions over the USA spanning up to 29 ENSO episodes. From the harmonic vectorial maps plotted on the study area, each vector reveals both intensity and temporal phase of the ENSO-related temperature teleconnection, and the corresponding candidate and core regions are determined using a machine learning technique of a Gaussian mixture model (GMM) based on the magnitude and temporal phase of climate signal and Köppen climate classification. As a result of vectorial mapping, four core regions were designated as North-West/Central region (NWC), South-West region (SW), South-East region (SE), and North-East region (NE). During fall (0) to spring ( +) seasons, the results of this analysis show positive (negative) temperature response to the El Niño events at the NWC region (SW, SE, and NE regions), while the opposite patterns are detected for the cold phase of ENSO. The temporal consistency values were 0.66 to 0.83 (0.77 to 0.86), and spatial coherence values ranged from 0.93 to 0.99 (0.95 to 0.99) for the El Niño (La Niña) events. Comparative analyses of temperature responses to both warm and cold ENSO events reveal the high significance level of the ENSO-temperature correlation with an opposite tendency in monthly temperature anomalies. Below normal temperature, anomalies during the El Niño thermal forcing are more significant than above normal temperature departures during the La Niña events. Consequently, middle-latitude temperature responses to the El Niño and La Niña phenomena are detectable over the contiguous USA.