Mechanism Studies of Madden-Julian Oscillation Coupling Into the Mesosphere/Lower Thermosphere Tides Using SABER,...

The Madden-Julian Oscillation (MJO), an eastward-moving disturbance near the equator (±30°) that typically recurs every ∼30-90 days in tropical winds and clouds, is the dominant mode of intraseasonal variability in tropical convection and circulation and has been extensively studied due to its importance for medium-range weather forecasting. A previous statistical diagnostic of SABER/TIMED observations and the MJO index showed that the migrating diurnal (DW1) and the important nonmigrating diurnal (DE3) tide modulates on MJO-timescale in the mesosphere/lower thermosphere (MLT) by about 20%-30%, depending on the MJO phase. In this study, we address the physics of the underlying coupling mechanisms using SABER, MERRA-2 reanalysis, and SD-WACCMX. Our emphasis was on the 2008-2010 time period when several strong MJO events occurred. SD-WACCMX and SABER tides show characteristically similar MJO-signal in the MLT region. The tides largely respond to the MJO in the tropospheric tidal forcing and less so to the MJO in tropospheric/stratospheric background winds. We further quantify the MJO response in the MLT region in the SD-WACCMX zonal and meridional momentum forcing by separating the relative contributions of classical (Coriolis force and pressure gradient) and nonclassical forcing (advection and gravity wave drag [GWD]) which transport the MJO-signal into the upper atmosphere. Interestingly, the tidal MJO-response is larger in summer due to larger momentum forcing in the MLT region despite the MJO being most active in winter. We find that tidal advection and GWD forcing in MLT can work together or against each other depending on their phase relationship to the MJO-phases.