Analyzing the Unprecedented QBO "Disruption" of Early 2016

The high predictability of the quasi-biennial oscillation of zonal winds in the equatorial stratosphere makes it a boon to extended-range forecasting. However, in early 2016 the regularity of the QBO was temporarily disrupted when an anomalous easterly jet developed within the prevailing westerlies around 40 hPa. This QBO “disruption” affords us a unique opportunity to study the mechanics of the oscillation and their representation in global NWP models. To this end, we have employed an array of diagnostics to analyze the QBO in early 2016 and a similar, “normal” QBO in early 2014 using a high-altitude version of the Navy Global Environmental Model (NAVGEM) analysis and forecast systems.

Wave spectra and the zonal mean zonal momentum budget both indicate that vertically-propagating equatorial waves could not explain the disruption event, in contrast to classical QBO theory. Unusually strong meridional momentum transport from the Northern Hemisphere (NH) was coincident with the development of the aberrant equatorial easterlies at 40 hPa. The NH stratospheric subtropical jet, which normally accompanies the westerly QBO phase and shields the deep tropics from southward-propagating planetary waves, was absent leading up to the disruption. The momentum budget analysis further demonstrates that the disruption was primarily initiated by large-scale motion rather than anomalous subgrid processes, and in some cases, parameterized diffusion hindered the development of the easterly intrusion in forecasts.