The Changing Width of Earth's Tropical Belt
Prof. Dr. Thomas Birner (LMU München, Meteorologisches Institut)

Earth’s tropical belt can be defined by the band of rainy equatorial regions bordered by the arid subtropics to the north and the south. Because of the strong latitudinal gradients in temperature and precipitation at the edges of the tropical belt, any shift in its edges could drive major local changes in surface climate. Theoretical arguments and experiments with climate models suggest that increasing greenhouse gas concentrations should lead to a widening of the tropical belt alongside a poleward shift of the mid-latitude jet stream. However, observationally-based estimates of changes in tropical width have resulted in disparate rates of expansion, some of which much higher than those expected based on experiments with climate models.

In this talk, I will first discuss the morphology of the tropical belt in terms of its thermodynamic and circulation characteristics, and the resulting metrics that can be used to define its edges. By studying the interrelationships across different metrics and accounting for methodological differences, the tropics are found to have widened by about 2 degrees of latitude over the last four decades. However, it is too early to detect robust anthropogenically induced widening imprints due to large unforced variability. I will then discuss the coupling between large-scale atmospheric disturbances originating along the mid-latitude jet and the tropical overturning circulation (the Hadley cell), which gives rise to year-to-year variability in tropical edge latitudes and is fundamental for the tropical width response to increased greenhouse gas concentrations.