Response of the large‐scale structure of the atmosphere to global warming

Abstract

This paper discusses the possible response of the large-scale atmospheric structure to a warmer climate. Using integrations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) in conjunction with physical arguments, we try to identify what changes are likely to be robust and what the underlying mechanisms might be. We focus on the large-scale zonallyaveraged circulation, in particular on height of the tropopause, the strength and position of the surface westerlies and the strength and extent of the Hadley Cell. We present analytic arguments and numerical calculations that suggest that under global warming the height of the tropopause will increase in both the transient response and final equilibrium state, and an increase is clearly found in all the comprehensive models in CMIP5. Upper stratospheric cooling is also found in the comprehensive models, and this too can be explained by a radiative argument. Regarding the circulation, most models show a slight expansion and weakening of the Hadley Cell, depending on season and hemisphere. The expansion is small and largely confined to winter but with some expansion in Southern Hemisphere summer. The weakening occurs principally in Northern Hemisphere but the intermodel scatter is large. There is also a general polewards shift in surface westerlies, but the changes are small and again are little larger than the inter-model variability in the change. This shift is positively correlated with the Hadley Cell expansion to a degree that depends somewhat on the metric chosen for the latter. There is a robust strengthening in the Southern Hemisphere surface winds across seasons. In the Northern Hemisphere there is a slight strengthening in the westerlies in most models in winter but a consistent weakening of the westerlies in summer. We present various physical arguments concerning these circulation changes but none that are both demonstrably correct and that account for the model results. We conclude that the above-mentioned large-scale thermodynamic/radiative changes in the large-scale atmospheric structure are generally robust, in the sense of being both well understood and consistently reproduced by comprehensive models. In that sense the dynamical changes are less robust given the current state of knowledge and simulation, although one cannot conclude that they are, in principle, unknowable or less predictable.

Convergence of broad-scale migration strategies in terrestrial birds

“…Research of this kind contains particular relevance when considering the implications of global climate change [57]. For example, the effect of climate change on seasonal patterns of atmospheric circulation over the Gulf of Mexico [58] or North Atlantic [59] may create additional costs or hazards during transoceanic migration. Climate change may also alter the quality of ecological resources on the breeding grounds through the influence of mid-latitude climate extremes [60] or on the wintering grounds through the influence of global warming [61].…”

Changes in the width of the tropical belt due to simple radiative forcing changes in the GeoMIP simulations

“…Some studies have begun to bridge this gap by examining the response of comprehensive models to idealized and more realistic greenhouse gas forcings. While Grise and Polvani (2014) found evidence that Southern Hemisphere Hadley cell expansion scales with climate sensitivity, Vallis et al (2015) found little relationship between the transient climate response and Hadley cell expansion. Studies have also found evidence of a seasonality (Polvani et al, 2011b) and a lack of seasonality (McLandress et al, 2011) in Southern Hemisphere expansion.…”

Increased light, moderate, and severe clear-air turbulence in response to climate change

“…The strengthening of these thermal winds occurs because, despite polar amplification of warming in the lower troposphere, increased carbon dioxide (CO 2 ) is enhancing the column-averaged pole-to-equator temperature gradient in the midlatitudes, through the combined effect of tropospheric warming and stratospheric cooling (Shine et al, 2003;Delcambre et al, 2013;Goessling and Bathiany, 2016). The increase in the magnitude of the meridional temperature gradient at aircraft cruising altitudes stems from robust thermodynamic effects and is unlikely to be significantly abated by feedbacks from the dynamics (Vallis et al, 2015). In the lower troposphere, the result of the temperature changes is weaker zonal winds, which may cause more extreme weather (Francis and Vavrus, 2012).…”