Internal interannual variability of the extratropical stratospheric circulation: The low-latitude flywheel

A new mechanism for internal interannual variability of the extratropical stratospheric circulation is described. The variability is internal in the sense that it arises in a stratosphere-only model without any interannual variability being imposed externally. In particular the wave forcing in the model, representing the effect of the tropospheric circulation, is kept constant from year-to-year and there is no imposed quasi-biennial variability in the equatorial winds. It is argued that the internal variability arises because of the longer 'memory' of the stratospheric flow at low latitudes. The smaller Coriolis parameter at low latitudes means that a given wind signal has associated with it a smaller temperature signal and is therefore less affected by radiative damping than a corresponding signal in the extratropics. The circulation at low latitudes can therefore act as a large flywheel, retaining memory of zonal mean quantities on an annual time-scale or longer.

The mechanism for the variability is studied using a 'mechanistic' primitive-equation model of the stratosphere. Waves are forced by a constant wavenumber one perturbation to the geopotential height field at the lower boundary and a seasonal cycle is prescribed through Newtonian cooling towards a time-dependent temperature field. Interannual variability is found within a certain range of forcing amplitude. The variability typically takes the form of a biennial oscillation, but more complex behaviour is also found under some circumstances. Diagnostics suggest that zonal flow anomalies in the subtropics persisting from the end of one winter to the beginning of the next are responsible for the interannual variability. Further experiments, in which the zonal flow at low latitudes is constrained in a particular configuration, provide further evidence that the role of the subtropical flow is crucial since constraining the subtropical flow reduces or eliminates the forcing range for which interannual variability is obtained.

The importance of the contrast between high latitudes and low latitudes for such internal modes of variability is consistent with their absence in stratospheric models such as the Holton-Mass model with a single degree of freedom for representing latitudinal structure. To illustrate the mechanism for the interannual variability further, an extended version of the Holton-Mass model is formulated that differentiates between high- and low-latitude regions. As in the case with full latitudinal resolution, interannual variability is obtained when the amplitude of the wave forcing lies within a certain range.