Simulations of the stratosphere with a steady troposphere

Andrew Gregory, John Thuburn and Alan O'Neill

Department of Meteorology, University of Reading.

 

A new stratosphere-mesosphere model has been designed with many dynamical and numerical advantages, notably the use of:

Potential vorticity (PV) as a conserved prognostic variable.

Isentropic vertical coordinates.

A hexagonal-icosahedral horizontal grid.

Figure 1: The 4th grid. It has 12 pentagons and 630 hexagons.

It has often been assumed that stratospheric sudden warmings are initiated by the development of wave amplitudes in the upper troposphere. The model has been used to test the following hypothesis:

Warmings can occur without any tropospheric transience as a result of the stratosphere's non-linear response to steady tropospheric forcing.

Many model integrations have been performed forcing the model with the annual solar cycle and constant amplitudes of zonal wavenumbers 1 and 2 at its artificial boundary near the tropopause. These long waves have the greatest potential to influence the stratosphere.

Wave 1 Results

Despite the constant forcing, for large enough amplitudes sudden warmings occur (fig. 2). With 200m forcing there is a spring warming as is observed in the southern hemisphere. With 300m there are winter warmings as often occurs in the northern hemisphere. Vacillations with periods of 1-2 weeks are present in both integrations.

Figure 2: Time series of polar temperatures on the 1147K isentrope (near 37km) for selected wave 1 forced integrations. Note the sudden warmings and the short timescale vacillations.

The vacillations are associated with the generation of successive travelling anticyclones in the upper stratosphere (fig. 3). These help maintain a quasi-stationary anticyclone by the polewards advection of low PV. The stationary anticyclone acts to reduce the strength of the polar cyclone during the warming period. The behaviour is very similar to that observed during real stratospheric warmings.

Figure 3: Stereographic maps of parcels on the 1439K isentrope (near 42km) during the 300m wave 1 experiment. The parcels are colour coded by their PV value as calculated from back trajectories to January 13th using model wind and radiative heating fields. Red parcels are associated with the polar cyclone and anomalously blue regions are associated with anticyclones. The anticyclone in the bottom right of panel a travels east and merges with a stationary anticyclone (panels b-c). Meanwhile a new anticyclone has formed (see bottom of panels c-d). Note the movement of the cyclone away from the pole and its reduction in size as it is pulled around the anticyclone.

Wave 2 Results

Again sudden warmings occur for stronger forcing (fig. 4). For 150m there is a final warming, while for 300m there are two weak winter warmings. By 350m the winter warmings are very strong.

Figure 4: as figure 2 but for selected wave 2 forced integrations.

The PV maps of the wave 2 experiments show a different evolution to the wave 1 results. There are no successive travelling anticyclones in the wave 2 case. Two anticyclones form and split the polar cyclone into two as they merge over the pole (fig. 5). Warmings of this type are also observed in the northern hemisphere.

Figure 5: As figure 4 but for the 350m wave 2 experiment on the 1064K isentrope (near 36km). Two anticyclones form simultaneously as low latitude air is brought polewards (panel a). The anticyclones move polewards splitting the polar cyclone in two (panel b). The anticyclones merge over the pole (panel c) and the polar cyclone (red parcels) is stretched out.

Conclusions

The hypothesis has been proved correct - sudden warmings can occur without tropospheric transience provided wave amplitudes are large enough. The synoptic events are similar to real warmings.

At lower amplitude forcing (e.g. 200m) there are spring warmings similar to the observed behaviour in the southern hemisphere, while at larger amplitudes (e.g. 300-350m) there are strong winter warmings as observed in some northern hemisphere winters.

The warmings forced by wave 1 and wave 2 are different in character. The wave 1 forced warmings are associated with the generation of a series of travelling anticyclones which continually strengthen a quasi-stationary anticyclone. Only two anticyclones form in the warmings forced by wave 2.