Mechanism Studies of Madden-Julian Oscillation Coupling Into the Mesosphere/Lower Thermosphere Tides Using SABER, MERRA-2, and SD-WACCMX

The Madden-Julian Oscillation (MJO), an eastward-moving disturbance near the equator (30 degrees) that typically recurs every similar to 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. Key Points SABER and SD-WACCMX diurnal temperature tides show a statistically similar response connected to the tropospheric Madden-Julian Oscillation Tropospheric radiative and latent heating is more important in forcing the tidal MJO-response than tropo/stratospheric wind-filtering Coriolis, pressure gradient, advection, and gravity wave drag forcing are the important mechanisms in the MLT region for tidal MJO-response

Automated summary

The study investigates the impact mechanisms of the Madden-Julian Oscillation on tides in the mesosphere/lower thermosphere using observations and reanalysis models, finding that tropospheric radiative and latent heating play a more significant role in forcing the tidal MJO response than wind-filtering in the troposphere/stratosphere. Coriolis, pressure gradient, advection, and gravity wave drag forces are identified as important mechanisms for tidal MJO response in the MLT region.