Climate response to a variable gravity-wave source
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
An investigation examining the response of numerical models containing one of several gravity wave parameterisations to changes in a prescribed tropospheric gravity wave source has been completed. It has been found that the unique interaction between orographic waves and those comprising a broad spectrum, exhibit tell-tale features in an offline environment. The response to these from within the confines of a mechanistic computer model persist and show a significant effect about the southern winter stratosphere. Offline comparisons of the three parameterisations has highlighted significant differences between two of the schemes (Doppler Spread Parameterisation, Medvedev and Klaassen) and the Ultra-Simple Spectral Parameterisation. These are due to; (1) the way in which the latter models wave dissipation and (2) the makeup of the source used. These cannot be resolved by an adjustment of tunable parameters. Comparisons inside a mechanistic model indicate the shortcomings of these offline analyses, as those schemes which showed little difference previously, now differed significantly. The modelled climatic response to changes in the boundary source of gravity waves was largely predictable; warmer/cooler winter/summer polar mesosphere with a reduction in the stratospheric wind jets during times of solstice. These were attributable to circulation changes caused by differing amounts of mesospheric wave drag. However, the extent of the sensitivity of the southern hemisphere winter circulation was unexpected. Other dynamical differences seen included changes in resolved large-scale wave propagation, which in turn affected the nature of sudden warmings and the onset of final warmings. The modulation of a source of vertically propagating gravity waves by stationary planetary scale winds was seen to force similarly sized planetary scale winds within the mesosphere. The modulation of this tropospheric source of gravity waves appears linked with the Tibetan Low during the Asian monsoon season. Similar anomalous winds have been seen in observations and just such a mechanism has been proposed to help explain the existence of these. This has been the first study where such a result has been forced without the introduction of a contrived signal in the source below.