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Diagnostics of variability and trends in the latitudinal extent of the tropics

PI: Thomas Reichler
Institution: University of Utah
Abstract:
The goal of this study is to analyze the latitudinal extent of the tropical Hadley cell by determining the position of the steep gradient between the high tropical and low extratropical tropopause. Our preliminary analysis using reanalysis as well as radiosonde data indicates that the Hadley cell and thus the tropics have been gradually widening by about 4 latitude over the past 40 years. An expansion of the tropics represents an extremely important aspect of climate change, which implies major changes in the general circulation. Current theory is unable to explain the magnitude of the observed trend, but various tropical, extratropical, or stratospheric mechanisms are undoubtedly at play. The importance of the Hadley cell for climate raises the question how well this trend is captured by model simulations of perturbed climates.

Based on an analysis of tropopause heights, we propose to analyze the latitudinal position of the Hadley cell from the latest IPCC simulations. The various results will be compared with each other as well as with observational data. Furthermore, we will analyze how the results are related to the simulation of other important parameters like the annular modes, tropical SSTs, the temperature structure in the UT/LS region, and the strength and position of the extratropical jets.

The realization of this projects requires monthly-mean three-dimensional fields of temperature, geopotential, and zonal velocity on all available pressure levels, and monthly mean SSTs. We intend to analyze the output of all participating models.
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