Effect of Advection by Upper Tropospheric Background Zonal Wind on MJO Phase Speed

Abstract

A robust linear regression algorithm is applied to estimate 95% confidence intervals on the background wind associated with Madden Julian oscillation (MJO) upper tropospheric atmospheric circulation signals characterized by different phase speeds. Data reconstructed from the ERA5 reanalysis to represent advection by the upper tropospheric background flow and MJO-associated zonal wind anomalies, together with satellite outgoing longwave radiation anomalies, all in the equatorial plane, are regressed against advection data filtered for zonal wavenumber 2 and phase speeds of 3, 4, 5, and 7 ms–1. The regressed advection by the background flow is then divided by the negative of the zonal gradient of regressed zonal wind across the central Indian Ocean base longitude at 80ºE, to estimate the associated background wind that leads to the given advection. The median estimates of background wind associated with these phase speeds are 13.4, 11.2, 10.5, and 10.3 ms–1 easterly. The differences between estimated values at neighboring speeds suggests that advection acts most strongly in slow MJO events, indicating that the slowest events happen to be slow because they experience stronger easterly advection by the upper tropospheric background wind.