The quasibiennial oscillation (QBO) of the prevailing winds in the tropical stratosphere between strong easterlies and strong westerlies represents the most repeatable aspect of the circulation in the atmosphere, other than the astronomically forced annual and daily cycles. The QBO is believed to be internally forced in the tropics via the interaction of the prevailing flow with vertically propagating waves.
Although rooted in low latitudes, the QBO has global impacts. The QBO is the “heartbeat” of the stratosphere, greatly influencing interannual variability in circulation and composition.
The QBO also affects the circulation at the Earth’s surface and is an important consideration in extended-range weather forecasts. Unfortunately, the QBO itself is not well represented in computer models used for weather predictions. Thus, any potential that the QBO offers for long-range weather predictions is not being well exploited by current forecasting systems.
The international QBO initiative (QBOi) is an effort among scientists to stimulate and coordinate computer modeling research on the QBO. The first QBOi workshop held in mid-March attracted 32 scientists from seven countries who reviewed achievements and remaining challenges in understanding and modeling the QBO.
What Do We Know About the QBO?
The workshop began with a review of the basic dynamical considerations underlying current theories about the QBO. Attendees agreed that the current paradigm, in which the QBO is maintained by the evolution and dissipation of vertically propagating waves, is sound but that questions remain about the dynamics behind the actual switching of the prevailing wind direction between westerly and easterly in the lowermost stratosphere. Although this switching mechanism can be clearly understood in very simplified models of the QBO that artificially constrain winds near the tropopause, meeting participants felt that in the real world this fundamental aspect of the QBO remains mysterious.
The workshop continued with a review of results from the few global models that have produced “QBO-like” winds in the equatorial stratosphere. This review revealed common deficiencies in all current simulations, notably with QBO winds being unrealistically weak in the lowermost stratosphere and having unrealistically small cycle-to-cycle variability in the model simulations. The review of existing capabilities also highlighted rapidly growing efforts at many centers to better represent the upper atmosphere in global models used for operational weather forecasting and long-term climate projections.
Developing a Framework for Testing Models
Participants agreed on a framework for coordinated computational experimentation at the many research centers now involved in the QBOi. For the initial phase, planned for the next 12–18 months, meeting attendees identified two major paths for experimentation.
First, groups will run long simulations with versions of their models that produce the most realistic mean flow oscillations. The groups will then repeat these runs under significantly perturbed global climate conditions. Participants decided that there should be a common set of model fields saved at frequent intervals so that assessors can diagnose the wave–mean flow interactions in these experiments.
As its second focus, the QBOi will also specify ensemble forecast experiments to be run for seasonal time scales from realistic initial states. Following the workshop, the QBOi executive committee created a blog to host ongoing discussion of QBOi issues.
Lesley Grey and James Anstey provided help key to achieving the workshop goals. We gratefully acknowledge funding from the UK Natural Environment Research Council and the Stratosphere-troposphere Processes and their Role in Climate project and local support from the Canadian Centre for Climate Modelling and Analysis. Sharon deCarlo helped produce the figure.
—Kevin Hamilton, International Pacific Research Center, University of Hawai`i, Honolulu, email: [email protected]; Scott Osprey, Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK; and Neal Butchart, Met Office Hadley Centre, Exeter, UK
Citation: Hamilton, K., S. Osprey, and N. Butchart (2015), Modeling the stratosphere’s “heartbeat,” Eos, 96, doi:10.1029/2015EO032301. Published on 2 July 2015.