INTERNAL INTERANNUAL VARIABILITY OF THE EXTRATROPICAL STRATOSPHERIC CIRCULATION - THE LOW-LATITUDE FLYWHEEL

A new mechanism for internal interannual variability of the extratropi cal stratospheric circulation is described. The variability is interna l in the sense that it arises in a stratosphere-only model without any interannual variability being imposed externally. In particular the w ave forcing in the model, representing the effect of the tropospheric circulation, is kept constant from year-to-year and there is no impose d quasi-biennial variability in the equatorial winds. It is argued tha t the internal variability arises because of the longer 'memory' of th e 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 radiat ive damping than a corresponding signal in the extratropics. The circu lation at low latitudes can therefore act as a large flywheel, retaini ng memory of zonal mean quantities on an annual time-scale or longer. The mechanism for the variability is studied using a 'mechanistic' pri mitive-equation model of the stratosphere. Waves are forced by a const ant wavenumber one perturbation to the geopotential height field at th e lower boundary and a seasonal cycle is prescribed through Newtonian cooling towards a time-dependent temperature field. Interannual variab ility is found within a certain range of forcing amplitude. The variab ility typically takes the form of a biennial oscillation, but more com plex behaviour is also found under some circumstances. Diagnostics sug gest that zonal flow anomalies in the subtropics persisting from the e nd 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 sin ce constraining the subtropical flow reduces or eliminates the forcing range for which interannual variability is obtained. The importance o f the contrast between high latitudes and low latitudes for such inter nal modes of variability is consistent with their absence in stratosph eric models such as the Holton-Mass model with a single degree of free dom for representing latitudinal structure. To illustrate the mechanis m 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 fo rcing lies within a certain range.