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The climatology of a stratosphere-resolving version of the Met Office's climate model is studied and validated against ECMWF reanalysis data. Ensemble integrations are carried out at two different horizontal resolutions. Along with a realistic climatology and annual cycle in zonal mean zonal wind and temperature, several physical effects are noted in the model. The time of final warming of the winter polar vortex is found to descend monotonically in the Southern Hemisphere, as would be expected for purely radiative forcing. In the Northern Hemisphere, however, the time of final warming is driven largely by dynamical effects in the lower stratosphere and radiative effects in the upper stratosphere, leading to the earliest transition to westward winds being seen in the midstratosphere. A realistic annual cycle in stratospheric water vapor concentrations--the tropical 'tape recorder'--is captured. Tropical variability in the zonal mean zonal wind is found to be in better agreement with the reanalysis for the model run at higher horizontal resolution because the simulated quasibiennial oscillation has a more realistic amplitude. Unexpectedly, variability in the extratropics becomes less realistic under increased resolution because of reduced resolved wave drag and increased orographic gravity wave drag. Overall, the differences in climatology between the simulations at high and moderate horizontal resolution are found to be small. DOI: 10.1175/2009JAS3337.1

The conditions that lead to the major warming over Antarctica in late September 2002 are examined. In many respects, the warming resembled wave-2 warmings seen in the Northern Hemisphere; the winter cyclonic circulation was split into two smaller cyclones by a large amplitude planetary wave disturbance that appeared to propagate upward from the troposphere. However, in addition to this classic warming mechanism, distinctive stratospheric vacillations occurred throughout the preceding winter months. These vacillations in wave amplitude, Eliassen-Palm fluxes, and zonal-mean zonal winds are examined. By comparison with a numerical model experiment, it is shown that the vacillation is accompanied by a systematic weakening of the westerly winds over the season. This preconditions the Antarctic circulation, and it is argued that it allows anomalously strong vertical propagation of planetary waves from the troposphere into the stratosphere. By contrast, a survey of previous winters shows that stratospheric westerlies usually vary much more gradually, with vacillations only occurring for short periods of time, if at all, in a given winter. Similar vacillations in a numerical model of the stratosphere only occur if the forcing amplitude is above a certain value. However, the level of winter-mean wave activity entering the stratosphere during 2002 is not unprecedented, and there is still some uncertainty over the cause of the onset and persistence of the vacillation and, ultimately, the major warming.

The impact of a parameterized spectrum of gravity waves on the simulation of the stratosphere in the Met Office Unified Model (UM) is investigated. In the extratropical mesosphere, the gravity wave forcing acts against the mean zonal wind and it dominates over the resolved wave forcing. In the extratropical stratosphere, the gravity wave forcing gives a small acceleration in the direction of the mean zonal wind. Both summer and winter stratospheric jets have improved maximum strength and tilt with height when the parameterized gravity wave forcing is included, although the southern winter jet is still more vertically aligned than in observational analyses. The timing of the seasonal breakdown of the southern winter vortex is also improved by the addition of gravity wave forcing. In the Tropics, the most obvious impact is that the model reproduces the quasi-biennial oscillation (QBO) with a realistic mean and range of periods. It also reproduces most of the observed asymmetries between the easterly and westerly phases of the oscillation. The sensitivity of this modeled QBO to horizontal diffusion parameters is investigated and it is shown that diffusion set to damp out grid-length disturbances can also attenuate the QBO due to its long period, particularly in the narrower westerly phase. (Manuscript received 20 June 2001, in final form 24 September 2001)

Sudden stratospheric warmings (SSWs) are impressive temperature rises in the winter polar stratosphere driven by the breaking of planetary waves propagating up from the troposphere. In extreme cases, the sudden [...]

In this paper results are presented from an improved version of the troposphere-stratosphere configuration of the Met Office Unified Model (UM). The new version incorporates a number of changes, including new radiation and orographic gravity wave parameterization schemes, an interannually varying sea surface temperature and sea ice climatology, and the inclusion of convective momentum transport. The UM climatology is compared with assimilated data and with results from a previous version of the UM. It is shown that the model cold biases in the January winter stratosphere and the January and July summer stratosphere are reduced, chiefly because the new radiation scheme is more accurate. The separation between subtropical and polar night jets in July is also better simulated. In addition, in the current version stratospheric planetary wave amplitudes in southern winter are less than half those in northern winter, which is in much better agreement with observations than the previous model version. Despite these improvements, the model still has a cold bias in the winter polar stratosphere, which suggests that the model representation of gravity wave drag is inadequate. Sensitivity tests were carried out and showed that the improved simulation of the separation of subtropical and polar night jets in July is due both to the different sea ice climatology and to the inclusion of convective momentum transport. The improved simulation of stationary wave amplitudes in July cannot be attributed to an individual model change, although it seems to be related to changed wave propagation and dissipation within the stratosphere rather than changes in the tropospheric forcing.