CLIMATE
VARIABILITY
RESEARCH
Stratospheric Variability
Standard theory links stratospheric variability, such as sudden warmings, to the development of planetary waves in the troposphere (e.g. Matsuno 1971). However, this theory is inadequate in explaining many observed stratospheric events: many sudden warmings seem to be unconnected to the growth of disturbances in the troposphere.
To see whether tropospheric transience is a necessary requirement for realistic stratospheric events we have constructed a new numerical model of the stratosphere in which the tropospheric planetary wave forcing can be held constant. By integrating the model through many annual cycles, each with a different amplitude or zonal-harmonic of forcing, we can see what aspects of the observed stratospheric variability can be captured without tropospheric transience.
The Model
In Newsletters 12 and 14, John Thuburn described a new shallow water model based on the advection of potential vorticity (PV) on a hexagonal-icosahedral grid. We have since extended the model to three dimensions using the hydrostatic primitive equations and isentropic vertical coordinates. It currently has an artificial bottom boundary given by the 414K isentrope (close to an altitude of 16km) where the important influence of the troposphere is represented by specifying the Montgomery potential.
Results
Here we will show results from an integration with constant zonal-wavenumber 1 forcing of 300m. During winter the stratosphere goes through a series of minor warmings associated with the generation of eastward travelling anticyclones in the upper stratosphere. These anticyclones often merge with a second quasi-stationary anticyclone located near 180E. Figure 1 shows the PV on the 1439K isentrope (close to 40km altitude) during one such merger in February.
Anticyclone mergers are an important mechanism by which observed quasi-stationary anticyclones can strengthen and a strong sudden warming occur. For example anticyclone mergers qualitatively similar to the model's occurred during the wave 1 warmings of January 1992 (O'Neill et al. 1994) and January 1987.
Although the model's warming is weaker than can be observed, the experiment shows that such warmings can be generated as a result of the stratosphere's internal variability and need not be linked with the development of tropospheric waves.
References
Matsuno, T. (1971). A dynamical model of the stratospheric sudden warming. J. Atmos. Sci., 28, 1479-1494.
O'Neill, A. et al. (1994). Evolution of the stratosphere during northern winter 1991/92 as diagnosed by U.K. Meteorological Office Analyses. J. Atmos. Sci., 51, 1479-1494.
Andrew Gregory, John Thuburn, Alan O'Neill
CGAM, University of Reading
A.R.Gregory@reading.ac.uk
