ALBERT

Description: How warm, wet, and stormy will the next decade be? This question and how to answer it – decadal climate prediction – is currently generating a large amount of interest in the research community. The interest stems from the growing awareness that climate varies naturally on decadal time scales, both regionally and globally, with large socio-economic consequences, and has the potential to temporarily offset or exacerbate anthropogenic global warming. The aim here is to discuss the current status of decadal prediction and highlight areas where the stratosphere may play an important role.

Description: In this study the dynamics of the coupled troposphere/stratosphere system was investigated using the atmospheric circulation model MAECHAM5/ocean mixed layer coupled model. The focus was on the impact of the stratosphere on the troposphere. The January climatology of the model agrees well with the observations, especially in the extratropical region and below the uppermost model levels (where the sponging effect is strong). The dynamics that maintains the middle atmospheric circulation in MAECHAM5 is consistent with the middle atmospheric dynamics known from previous studies. The three-dimensional structure of the Northern Annular Mode (NAM) in MAECHMA5 agrees well with that observed. The NAM dominates not only the variability in but also the covariability between troposphere and stratosphere north of 20°N. A lag composite analysis based on the 50hPa NAM-Index (NAMI), a measure of the strength of the 'stratospheric mode', shows that strongly positive stratospheric NAM-phases are associated with strong anomalous westerlies in high latitudes and a temperature quadrupole between troposphere and lower mesosphere. The lower part of this quadrupole shows a dipole structure with an anomalous cold (warm) polar (mid-latitude) troposphere and stratosphere and is associated with a positive shear of the anomalous westerlies there. This dipole strengthens (weakens), together with the anomalous westerlies, in the NAM-increase (decrease) phase. The upper part of the temperature quadrupole shows a coherent dipole with an anomalous warm (cold) polar (mid-latitude) upper stratosphere and lowermost mesosphere and is associated with a negative shear of the anomalous zonal flow there. The warm pole in mesosphere and upper stratosphere strengthens and propagates downward together with anomalous easterlies, especially in the NAM-decrease phase. The dynamics during strong vortex is analyzed by computing the wave, residual Coriolis, and non-resolved (residual) forcing in the Transformed Eulerian Mean formulation (TEM). NAM temperature, zonal flow and geopotential height patterns and their downward propagation can be explained by the response of the stretching vorticity to residual Coriolis forcing changes, which are caused by the resolved and non-resolved wave forcing, and by quasigeostrophic adjustment of the troposphere to these changes. The persistence, increase, and downward propagation of the NAM-patterns is enhanced mainly by the planetary wave-vortex feedback that accelerates the westerlies within the coupled troposphere/stratosphere system. Thereby the strong vortex is strengthened further by the equatorward propagation of planetary waves and vice versa. The decrease and modulation of the strong NAM-anomalies is enhanced by two negative feedbacks that decelerate the westerlies in the upper and lower boundaries of the coupled troposphere/stratosphere system. The first feedback is a vortex-residual forcing feedback acting in upper stratosphere and lower mesosphere and can be explained by the filtering of gravity waves by the stratospheric jet. The second feedback acts near the lower boundary and is driven by both residual and planetary wave forcing. A lag composite analysis based on the 1000hPa-NAMI, a measure of the strength of the 'tropospheric mode', showed that tropospheric and stratospheric NAM are two different modes driven by two different forcings. In the tropospheric mode the tropospheric westerlies are induced by a strong internal wave forcing, which develops inside the troposphere on a synoptical time scale. This results in stronger tropospheric westerlies in the tropospheric mode. The stratosphere is also important in the tropospheric mode: It provides the upper troposphere with a low frequency westerly wind and wind shear and thus with enhanced baroclinicity that are associated with an amplification of the upper tropospheric wave, Coriolis and residual forcing. This forcing amplification leads then to an increase of the tropospheric circulation anomalies, which results in anomalous ascent and cooling in the Arctic and thus in a strong vortex that reaches its maximum 4-5 days later. The strong vortex contributes, through the stretching vorticity mechanism and the vortex-planetary waves forcing, to the persistence of both tropospheric and stratospheric westerlies and cooling. The stratospheric westerlies are much weaker in the tropospheric than in the stratospheric mode due to the much lower contribution of the planetary waves to the stratospheric wind anomalies in the tropospheric mode.