The Vertical Propagation of Tropospheric Rossby Waves and its Variation with Climate
Abstract
Rossby wave propagation is theoretically affected by zonal wind gradients, in particular the second derivative of the zonal wind shear in the vertical and meridional directions. Vertical propagation is hindered by the strong winds in the upper troposphere, particularly for short wavelength waves, or by critical levels where the background wind velocity equals the wave phase velocity. In practice, tropospheric winds and their gradients are altered in climate change situations, and therefore so is Rossby wave propagation. Experiments conducted with several generations of the GISS Global Climate Middle Atmosphere Model are used to assess this dependency, including the effect on the Arctic Oscillation. In general, with significant tropical warming, a west wind tendency arises at 0-30o latitude in the upper troposphere/lower stratosphere, and vertical propagating waves show relative equatorward propagation (with relative poleward propagation favored when significant tropical cooling arises). The latitudinal gradient of tropical warming is less important than the absolute magnitude of the effect, due to the tendency for tropical sea surface temperature changes (but not subtropical changes) to be translated relatively effectively to the upper troposphere. This result appears robust across the range of models used at GISS. The resultant 30oN-30oS wave energy flux convergences generate an intensified residual circulation, with greater upward flow through the tropical tropopause. In contrast, the high latitude refraction pattern of tropospheric waves, and the subsequent effect on the Arctic Oscillation phase, depends on the latitudinal gradient of tropospheric temperature changes, and is very model-dependent. When the latitudinal gradient of warming increases, in the tropics, at mid-latitudes, or high latitudes, positive west wind tendencies encourage relative equatorward wave propagation in the extratropics. This by itself would favor an indirect residual circulation change at high latitudes, and lower pressure over the pole (the positive phase of the AO). However, when large climate changes are involved, latitudinal variations in diabatic effects (such as latent heat release) can also produce changes in residual circulation patterns, sometimes at variance with those induced by wave transport changes, and little vertical consistency is seen in the AO phase change at levels within the troposphere or between the troposphere and stratosphere. The importance of the stratosphere in driving the high latitude effects depends upon the magnitude of the climate forcing; it becomes less important as the tropospheric climate change becomes larger (e.g.,2xCO2 warming or even strong Little Ice Age cooling). Solar forcing acting in conjunction with its effect on ozone, or in conjunction with the QBO, can alter tropospheric wave propagation into the stratosphere, which then appears to influence tropospheric meteorological fields. Transient effects due to volcanoes, or the beginnings of greenhouse gas warming, also primarily utilize lower stratospheric wind changes to influence the AO phase.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- Bibcode:
- 2002AGUSM.A22C..03R
- Keywords:
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- 3319 General circulation;
- 3334 Middle atmosphere dynamics (0341;
- 0342);
- 3362 Stratosphere/troposphere interactions;
- 3384 Waves and tides;
- 1620 Climate dynamics (3309)