The Atmospheric Angular Momentum Cycle Associated with a Madden–Julian Oscillation

Abstract

The period 1 December 1984 to 3 February 1985 was associated with strong intraseasonal fluctuations in both the global atmospheric angular momentum (AAM) and tropical convection. Consistent changes were observed in the length of day. The AAM budget for the 65-day period is examined here using circulation data from the National Meteorological Center. Surprisingly well-balanced global and zonal budgets are obtained for the vertically integrated AAM. This enables a closer examination of regional changes, to assess how they might be responsible for the changes in the global AAM. Both friction and mountain torques are important in the global AAM budget. The increase of AAM is associated first with a positive friction torque, then with a positive mountain torque. The subsequent decrease of AAM results from a negative friction torque. The accompanying regional changes are mostly confined to the Northern Hemisphere, with high global AAM associated with a stronger and southward-displaced subtropical jet. In the zonal budget, meridional AAM fluxes by the zonally asymmetric eddies are important and appear to lead the torques by a few days. The increase of AAM begins with a shift of the tropical convection from the east Indian to the west Pacific Ocean. The consequent enhancement of the trades east of the Philippines gives a positive friction torque. The friction torque also has a contribution from enhanced trades over Central America and the tropical Atlantic Ocean, which appear to be linked to an equatorward propagating upper-tropospheric wave over the region. A persistent high pressure anomaly subsequently develops to the east of the Himalayas, giving a positive mountain torque. The global AAM rises in response to these torques, but as the circumpolar vortex expands the trades are weakened, causing a negative friction torque and the final reduction of the AAM. Interestingly, no coherent signals are seen in the weak zonal-mean convection anomalies accompanying these AAM changes. Rather, the AAM budget suggests that the tropical Madden?Julian oscillation and the global AAM are linked through the interaction of Rossby waves generated by the tropical heating with a zonally varying ambient flow and with mountains. The surface stresses have both a local component related to the convection and a remote component induced by upper-tropospheric AAM fluxes.