Towards predicting the weather of the mesosphere and lower thermosphere

Our knowledge of the dynamics and chemistry of the mesosphere and lower thermosphere (MLT) has increased greatly over the last several decades, aided by extensive satellite and ground-based observations and advances in numerical models. Together they provide estimates of the climatology of the MLT and how it varies with season and over decadal timescales. However, we have limited capability in predicting MLT day-to-day variations, i.e., its weather. Empirical models that take as input the day of year and solar flux / geomagnetic activity indices remain the standard tool for predicting such things as the drag on space debris in low earth orbit.  Such models can disagree on the state of the atmosphere by a factor of two.  Using the specified dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM) we explore MLT weather variations in a simulation that covers the period 2005 to 2015. Here we focus on variations near the mesopause at representative equatorial and high-latitude sites. After removing seasonal variations, we find that the majority of day-to-day weather arises from changes in the amplitude and phase of atmospheric tides. Moreover, it is typical that at most 5 tidal modes are sufficient to capture most of the short-term variability. Using wavelet analysis, we show that tidal variations can be linked to both external forcing (e.g., solar flux) and variability that propagates from below. We confirm prior studies that have shown a link to sudden stratospheric warmings but also see variations correlated to the North Atlantic Oscillation, the El Niño-Southern Oscillation, and the Quasi-Biennial Oscillation. Additionally, we explore if persistence of tidal variability can be used to improve prediction of near term MLT dynamics and demonstrate improvements over climatological approaches. Taken together these finding provide a gateway to improved MLT weather prediction, with the potential to reduce uncertainty in targeted re-entry, collision avoidance and disruptions to radio communications and global positioning systems.