Length of Day Changes Associated with the Madden-Julian Oscillation

Abstract

The previously reported spectral peak near 50 days in time series of length of day (LOD) is shown to occur in conjunction with episodes of tropical convective activity associated with the Madden-Julian oscillation (MJO). When the convective signal of the MJO is absent, LOD exhibits a red spectrum at intraseasonal time-scales. LOD is shown to be in phase with the convective anomaly due to the MJO over the date line and out of phase with the convective anomaly over the Indian Ocean. A composite angular momentum budget, made relative to the convective signal of the MJO, reveals that the zonal surface stress only partially accounts for the observed tendency of LOD. Not only is the amplitude some 50% too weak, the phase is shifted ahead of the LOD tendency by about 1/8 cycle. Hence, in order to balance the angular momentum budget, an additional mountain torque is postulated to occur. This additional torque is required to lag the frictional torque by about 1/4 of a cycle, but be of similar amplitude. The composite surface stress anomalies appear to result predominantly from zonal mean zonal wind anomalies. An important role for the zonally symmetric convective anomaly due to the MJO is suggested. The surface zonal wind anomalies at low latitudes, which exhibit a high degree of equatorial symmetry with zero amplitude on the equator, appear to be accounted for as the linear response to zonal mean convective heating in the presence of strong dissipation. The upper-tropospheric zonal wind anomalies, which mimic the angular momentum anomalies, are not accounted for by simple linear momentum balance. In particular, maximum zonal wind anomaly occurs on the equator, which suggests an important role for eddy fluxes of momentum during the life cycle of the MJO.