Abrupt atmospheric torque changes and their role in the 1976–1977 climate regime shift

During the climate regime/Pacific Decadal Oscillation (PDO) phase shift of 1976–1977, changes of up to several Hadleys occurred in the friction, mountain, and gravity wave torques that maintain the axial angular momentum (AAM) balance of the atmosphere. As required to produce a substantially different but stable climate regime, however, the total torque showed little net change (0.2 Hadley), while the AAM showed a modest increase, between the pretransition and posttransition periods in our data (1968–1975 and 1978–1997). The east‐to‐west transfer of atmospheric mass that occurs during the cold‐to‐warm phase shift of the PDO produced positive mountain torque anomalies on the atmosphere resulting from lower (higher) surface pressure to the west (east) of the Andes and Rockies (Himalayas), which were largely compensated by negative friction torque anomalies generated over the equatorial Pacific. The timing of events during the 1976–1977 transition window suggests that changes in low‐latitude (Pacific friction and Andes mountains) torques and the PDO index in early to mid‐1976 preceded changes in the extratropical torques during 1977, consistent with the poleward propagation of atmospheric zonal flow anomalies documented in previous studies. An increase in the global mean surface temperature and its rate of rise after the regime shift suggest that the negative friction torque anomalies that developed during the transition may have acted to sustain the warmer posttransition regime by suppressing cold‐water upwelling in the eastern equatorial Pacific and/or equatorial thermocline heat content recharge processes in the central and western Pacific.