The Atmospheric Angular-Momentum Cycle during the Tropical Madden–Julian Oscillation

Abstract

Global-scale, Wind and outgoing longwave radiation anomalies are documented during the atmospheric angular-momentum (AAM) cycle associated with the Madden?Julian oscillation. Cross-spectral and compositing techniques are applied to 150- and 850-mb National Meteorological Center wind data and outgoing longwave radiation (OLR) data during ten northern winter and summer seasons. Coherent wind and OLR signals are found in the tropics and in the winter hemisphere and are dominated by zonal wavenumbers 0?2. At the time of zero AAM anomaly, anomalous zonal winds are found over the western hemisphere tropics (during northern winter), while convection anomalies tend to be collocated with the seasonal mean Australasian convection. Zonal-mean OLR anomalies are small. As the AAM anomaly increases, the anomalous zonal winds move eastward and poleward, while dipole convection anomalies become established over the oceanic warm pool and result in an east-west shift of the Australasian monsoon. Zonal-mean OLR anomalies tend to be large when the AAM anomalies are large. During northern winter, the inferred frictional torques peak near the time of maximum AAM anomalies suggesting that pressure torques may also help force the angular-momentum changes.