Angular Momentum and Torques in a Simulation of the Atmosphere's Response to the 1982-83 El Niño

Abstract

Anomalies in the angular momentum of the atmosphere (M) during the 1982-83 El Niño event and the torques responsible for these anomalies are investigated using output from the Canadian Climate Centre general circulation model. Model values of M during the year of the event are generally larger than those for the model climatology, thereby capturing the observed tendency toward higher values of M during El Niñto. Differences exist between the model and observations in the timing and amplitude of the largest anomalies, but these differences may he due to natural variability and not necessarily directly associated with the 1982-83 El Niño conditions. In late September and October 1982, the model atmosphere acquires momentum more rapidly than usual, leading to the development of the largest deviations from mean conditions at the end of this period, mostly associated with strong westerly momentum signals centered at 25°N. Large, sustained positive anomalies in tangential stress torques over the northern tropics are the major mechanism responsible for the modeled increase in M, but mountain torque anomalies centered at 35°N are also important at the end of October. A secondary maximum in the departure from mean M values occurs in January 1983 and is related to a general strengthening of westerly momentum anomalies over the model's tropical and midlatitude regions. Both mountain and tangential stress torques are involved in this episode, but no particular mechanism or region dominates the anomalous exchange of momentum.