Rebooting the Quasi-Biennial Oscillation (QBO)
The unusual tropical stratospheric winds of 2016
Quasi-Biennial Oscillation (QBO): The tropical winds in the lower stratosphere (~17 to 37 km and 10°S to 10°N) change direction from easterly to westerly with a period of ~28 months. This has been documented since 1953 (over 60 years) by tropical weather balloons (radiosondes). The QBO is known to be driven by upward propagating tropical waves that originate in the tropical troposphere. The NASA MERRA-2 reanalysis includes the most recent 15 cycles of the QBO, the first 14 cycles of which illustrated the typical QBO behavior, however, the last, most recent cycle evolved in an unexpected and unprecedented fashion.
The first figure shows the MERRA-2 tropical zonally averaged zonal (west to east) winds (at the 44 hPa level, ~22 km in altitude) for different cycles of the QBO. The wind curves for each cycle have been shifted in time to match when the winds change from easterly (negative) to westerly (positive). While there is variability between cycles they all show the westerlies persisting for about one year (top axis shows relative time) with amplitudes between 5 and 10 ms-1, except for the current cycle (bottom axis, red curve) which switches back to easterlies at a much earlier time. The short westerly phase is unprecedented, even when radiosonde observations extend the time record back to 1953 (28 cycles total).
The winds associated with the QBO do not change direction at the same time at all levels in the lower stratosphere. Instead the QBO change in wind propagates downward at a typical rate of ~ 1 km month-1, producing a distinctive sloping pattern in time vs altitude cross sections (second figure). The first panel shows the zonally averaged zonal (west to east) tropical winds from MERRA-2 for 2014 through early 2016, while the second panel shows the average of the 14 previous QBO cycles. Unlike the steady downward propagation of the westerly winds expected from past QBO’s, the 2016 pattern shows that the actual QBO deviated from its typical behavior throughout the lower stratosphere altitudes shown.
A full understanding of the cause of the disruption of the QBO is still under investigation. While the QBO is generally driven by upward propagating tropical waves, analysis of the MERRA-2 and the GMAO forward processing (FP) system show that the 2015-16 Northern Hemisphere winter demonstrated record high propagation of mid-latitude waves into the tropics and that these waves were responsible for the usual QBO wind signature. Once the 2015-16 NH winter ended, the winter season mid-latitude waves ceased disturbing the tropics and the “rebooted” anomalous QBO winds are proceeding to develop their typical 1 km month-1 downward propagation signature. The explanation of the stronger mid-latitude wave forcing for 2015-16 is unknown at this time, however there is a possibility that it may be related to the extreme El Niño that occurred concurrently.
NASA Feature: http://www.nasa.gov/feature/goddard/2016/a-strange-thing-happened-in-the-stratosphere
AGU EOS Feature https://eos.org/research-spotlights/mysterious-anomaly-interrupts-stratospheric-wind-pattern
Reference
Newman, P. A., L. Coy, S. Pawson, and L. R. Lait (2016), The anomalous change in the QBO in 2015–2016, Geophys. Res. Lett., 43, doi:10.1002/2016GL070373.
