Atmospheric torques during the winter of 1989: Impact of ENSO and NAO positive phases

The axial component of atmospheric angular momentum (AAM) reached a decadal minimum during the winter of 1989, when the cool (La Nina) phase of the ENSO cycle and the positive phases of the North Atlantic Oscillation (NAO) and the related Arctic Oscillation (AO) were active. Here, we examine the global atmospheric dynamics associated with this event from the torque point of view. It will be shown how mountain torques on North America, South America and Europe related to these oscillations led to the AAM minimum, partly compensated by a positive ENSO-related mountain torque over Asia. The friction torque had more nearly equal positive and negative anomalies associated with ENSO and NAO forcing, respectively, and made a smaller, offsetting contribution to the global AAM anomaly during this episode.