Of warming nights and shifting winds




Stone, Dáithí Alastar

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The attribution of recent global warming to anthropogenic emissions is now well established. However, the relation of recent changes in other properties of the climate system to human activities is not as clearly understood. The aim of this thesis is to improve our understanding of this relation in the case of two of these properties, namely the diurnal temperature range (DTR) and modes of tropospheric variability. The DTR, the difference between daily maximum and minimum temperatures, has decreased over global land areas at a rate comparable to the mean warming. Model simulations including the effects of human emissions produce a comparable change, albeit of smaller magnitude. This decrease results from increased reflection of solar radiation by clouds moderated by decreasing soil moisture, mostly through its effect on the ground heat capacity. Recent trends in indices of some modes of atmospheric variability suggest the possibility that forced climate change may manifest itself through a projection onto these pre-existing modes. Model simulations indicate that this is plausible in the case of sea level pressure, but only partly so in the case of surface air temperature. On the interannual time scale examined in this thesis, these projections are consistent with a linear interpretation, rather than a nonlinear one. These results are, however, sensitive to the representation of small scale processes in the models. For instance, the DTR response depends strongly on the representation of cloud and land surface processes. Further examination of the response of one of the tropospheric modes, namely the Southern Annular Mode which represents the meridional shift of the mid latitude jet in the Southern Hemisphere, indicates that it is sensitive to the parametrisation of sub-grid scale mixing in the ocean. Nevertheless, these results suggest that the recent changes are consistent with enhanced greenhouse warming, and indicate that they are likely to continue into the foreseeable future.



Carbon cycle (Biogeochemistry), North Pacific Ocean, Global warming, Global temperature changes