Intraseasonal atmospheric variability under different climate trends

Abstract

Low order climate models can play an impor- tant role in understanding low frequency vari- ability in the atmospheric circulation and how it can be affected by trends in the forcing consistent with climate change. A conceptual model of the midlatitudes zonally symmetric atmospheric circulation in geostrophic equilib- rium is studied from the perspective of dynam- ical systems theory. The model was first in- troduced by Lorenz in 1984. A study of the steady states and a bifurcation analysis are car- ried out in order to identify intervals of differ- ent behaviours in the parameter space. In the first place, the focus is put on the study of the nonautonomous system where the cross latitu- dinal heat contrast varies seasonally, accord- ingly to changes in insolation. The snapshot attractor of the nonautonomous system season- ally forced is compared with the attractor of the autonomous one for two distinct moments of the year (summer and winter). In both cases the effects of the time dependent forcing are reflected in a clear change of shape of the at- tractor. Predictability is lost in both cases: the summer attractor loses its periodicity when the forcing is seasonal. The winter one favours en- ergy transport through one of the two wave components included in the model. In addi- tion, the forcing is subjected to climate trends (both positive and negative). The analysis of the snapshot attractor of the system under cli- mate trends suggests that the model does not follow the geostrophic assumption in certain ranges of the forcing as the average wind flow does not always show a positive dependence on the equator to pole heat contrast. On the other hand, the energy transported by the eddies fol- lows the sign of the climatic trend. Overall, dif- ferent effects are observed. A chaotic behaviour can be completely suppressed in favour of a reg- ular periodic one and vice-versa. At the same time, circulation patterns can change, suddenly disappear and rebuild. In general, the use of the snapshot attractor proved to be a robust tool to study the internal variability of the cli- mate as well as the changing arising from cli- mate trends. Present and future perspectives on this work also include a spectral analysis to extensively understand the effects of climate trends on low frequency variability.