Weakened QBO Due to Stronger Mean Tropical Upwelling

1. Weakened QBO Due to Stronger Mean Tropical Upwelling • Y. Kawatani1 and K. Hamilton2 • 1JAMSTEC Research Institute for Global Change, 2IPRC • Nature, 497, 478-481 (2013) Zonal wind QBO amplitude at 70 hPa near the equator. Top: Observed values from radiosonde winds during 1953–2012. Bottom: Results from the MPI-ESM-MR global coupled model driven with observed concentrations of greenhouse gases from 1900 to 2005 and then with projected increases through 2100. The quasi-biennial oscillation (QBO) dominates the circulation in the tropical stratosphere and impacts the large-scale seasonal mean climate in both the stratosphere and the troposphere. Evidence for long-term trends in the QBO have until now been equivocal, raising questions about the extent of stratospheric circulation changes in a global warming context. A new analysis of near-equatorial radiosonde observations for 1953–2012 reveals a long-term decrease in the zonal wind QBO amplitudes, which is particularly notable at the 70 hPa level where the QBO amplitudes dropped by roughly one-third over the period. This trend is also apparent in the global warming simulations of the four models in CMIP5 that realistically simulate the QBO. In the lowermost stratosphere, the vertical structure of the QBO is strongly linked to the mean upwelling, which itself is a key factor in determining stratospheric composition. The reduced QBO amplitudes are likely due to stronger mean flow of air from the tropical troposphere into the stratosphere. Time–height section of the amplitude of the simulated equatorial QBO in four global coupled models run with observed concentrations of greenhouse gases from 1900 to 2005 and then with projected increases through 2100. (a) MPI-ESM-MR, (b) HadGEM2-CC, (c) MIROC-ESM-CHEM, (d) MIROC-ESM. Contour labels in m/s.