Impact of the MJO on the west African monsoon, African easterly waves, and Atlantic tropical cyclones

The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics. During boreal summer, the MJO has significant implications for the formation and strengthening of African easterly waves in a “north African region” bounded by 50W, 40E, 35N, and 0N. Some African easterly waves form into tropical cyclones (TCs) upon their emergence into the Atlantic Ocean, suggesting a potential link for how the MJO modulates Atlantic TCs.

The MJO is partitioned into a cycle consisting of eight phases, with each phase exerting a unique effect on the basic state over north Africa. By focusing on significant MJO events, which are determined by taking cases with significant amplitude computed from the first two principal components from a combined EOF analysis, composites of various basic state fields are created as a function of MJO phase. Variations of zonal wind, brightness temperature, total precipitable water, and easterly wave activity are examined as a function of MJO phase. Eddy kinetic energy, convection, zonal wind, and precipitable water all exhibit coherent fluctuations as a function of MJO phase, suggesting that the MJO may impact African easterly wave activity through both dynamic and thermodynamic variables. MJO-related fluctuations in the west African potential vorticity field are also examined and consequences for easterly wave formation are discussed.

Dynamical fields during different MJO phases are used as initial conditions in a primitive equation model in order to investigate the relationship between MJO phase and African easterly wave development. The impact of MJO-related basic state fluctuations on easterly wave activity is assessed relative to impacts generated by the MJO's ability to force strong precursor disturbances over the Darfur region to seed easterly wave growth. Understanding the regulation of easterly waves by the MJO relationship could prove to be paramount to the forecast of TCs in the Atlantic Ocean.