Mechanisms for variability of the Quasi-Biennial Oscillation in laboratory and numerical models

Mechanisms for variability of the Quasi-Biennial Oscillation in laboratory and numerical models

To explore dynamical mechanisms that may influence the variability, and hence the predictability, of the QBO, e.g. through nonlinear interactions with the seasonal cycle and other cyclic oscillations in the atmosphere. What is the relationship of the QBO to the rest of the climate system in a changing climate?

The Quasi-Biennial Oscillation (QBO) is a cyclic reversal of the zonal winds in the middle and lower tropical stratosphere on a timescale of roughly two years. It dominates the climate of the tropical stratosphere, influencing the long range transport of momentum, heat and chemical constituents. It is also thought to play an important role in influencing the predictability of various features, such as the Madden-Julian Oscillation (MJO) and other phenomena at higher latitudes and in the troposphere, so understanding what determines its variability in space and time is important for a range of problems in seasonal climate prediction.

Although the basic mechanisms that drive the QBO are reasonably well understood, arising from the nonlinear interaction of upward-propagating internal gravity and planetary waves (generated in the troposphere) with the zonal flow, its detailed variability is complex, chaotic and much less well understood. Fluctuations in the wave sources, tropical upwelling and associated feedbacks are probably significant, while partial synchronization with the seasonal cycle also seems to play a major role as well as interactions with other phenomena at high latitudes. Progress is difficult, however, because the phenomenon is notoriously difficult to capture realistically in global climate models, typically requiring carefully tuned parameterizations of the wave interactions and feedbacks which are often quite ad hoc. Hence, the atmosphere often surprises modelers with events such as the recent “stalling” of the QBO that their models failed to predict. The likely impact of future global climate change on the QBO is also quite controversial and uncertain.

In this project, we propose to study a number of mechanisms that might influence the behavior of the QBO using a combination of simplified numerical models and a laboratory analogue of the QBO, in which factors such as the wave forcing and other parameters and feedbacks can be closely controlled and varied. The laboratory experiment (whose design and construction is nearing completion) is an extension of the well known configuration studied by McEwan & Plumb (1978), in which internal waves are launched into a salt-stratified fluid in an annular channel by oscillating flexible membranes in the bottom of the tank. In contrast to McEwan & Plumb, however, each segment of the membrane in the new experiment can be separately controlled by computer to enable varying spectra of internal waves to be excited and for the amplitude of the waves to be varied in time (thereby emulating the seasonal cycle and other modulations). The response of the fluid to this forcing in the form of time varying velocity fields will then be measured by optical particle imaging techniques.

The experiments will be complemented by a series of numerical model simulations (a) to achieve direct numerical simulation of some of the laboratory flows themselves for comparison and validation with experimental measurements and (b) to explore idealized simulations of QBO-like phenomena in simplified global circulation models. The numerical models will make use of Met Office codes such as ENDGAME to maximize opportunities to transfer benefits of this research directly to Met Office researchers. A version of ENDGAME is already being developed under an existing CASE studentship in Oxford to enable simulation of flows of a Boussinesq fluid in a cylindrical domain. The student will adapt code for this project through direct collaboration with Dr Shipway and colleagues via regular visits to the Met Office.

The aim of the experiments is to explore the impact of varying factors such as the spectrum of waves forcing the flow and temporal modulations of wave forcing etc. on the variability of zonal flow reversals, hopefully to capture some of the features observed in the stratosphere itself. The aim of the model simulations will be to enhance understanding of the laboratory flows, test and validate the modeling approach and to evaluate how such phenomena might manifest themselves in the full complexity of the real atmosphere. Such an approach would complement existing research in the Met Office on both observational studies and efforts to evaluate parameterizations and modeling strategies for seasonal prediction that involve the QBO. Its use of idealized modeling techniques is also entirely consistent with the approaches being promoted by international initiatives such as QBOi within the SPARC community.

Aims of the project

To study the influence of cyclic modulations of wave forcing, upwelling and other processes over a wide range of timescales on the variability of QBO analogues in laboratory experiments and numerical simulations.

Methods to be used

Laboratory fluid flow experiments, in which upward-propagating internal waves are launched into a density stratified fluid and the resulting flow measured by particle image velocimetry; 3D time-dependent numerical simulation of fluid motion in both laboratory domains and simple atmospheric circulation models.

Specialised Skills

Strong background in mathematics, theoretical physics or related subjects; skills in large-scale computing and data analysis; some aptitude for experimental design and measurement.

If interested please contact Peter Read peter.read@physics.ox.ac.uk, Alfonso Castrejon-Pita mailto:alfonso.castrejon-pita@wadham.ox.ac.ukand Scott Osprey scott.osprey@physics.ox.ac.uk

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