Torques Responsible for Evolution of Atmospheric Angular Momentum during the 1982–83 El Niño

Abstract

Atmospheric angular momentum (AAM) reached extremely high values during the large 1982?83 El Niño event. The mechanisms responsible for the anomalously high AAM are examined using mountain torque (τm) and friction torque (τf) time series computed from the National Centers for Environmental Prediction?National Center for Atmospheric Research reanalyses. AAM anomalies, defined with respect to a 29-yr climatology (1968?96), are mostly positive from mid-1982 onward, but notably they double in amplitude over a 2-week period in early 1983. Analysis of the torque series reveals that this sharp AAM increase is mostly related to anomalies in τm, primarily associated with American and Eurasian orography. After reaching its peak value in January, AAM anomalies decay slowly to near-normal values over the next three months, with anomalies in τf, especially over the subtropical North Pacific, playing a dominant role in this downturn. The relevant anomalies in τm and τf are discussed in relation to rapid synoptic-scale variability and longer-term, large-scale anomalous patterns in surface pressure and winds that characterized this El Niño event.