Robust projections of vertical wind shear changes for the 21st Century

We explore the changes in tropical vertical wind-shear projected for the 21st Century in response to increased CO2, using a set of 23 climate model experiments performed for the IPCC-AR4. Many features of the shear changes are robust across the various models, in particular an increase on wind-shear in the tropical north Atlantic region. This region is the only one in the world exhibiting a robust increase in shear in the local summer season, the models show robust decreases in wind shear over much of the tropical oceans. The increase in Atlantic wind shear appears related to teleconnections from global-warming-induced reduction in the intensity of large-scale atmospheric circulation, which occurs preferentially in the zonally-asymmetric (i.e., Walker) component of the tropical Pacific circulation - i.e. "El Nino-like" atmospheric changes. Although the mechanisms behind the Pacific changes are distinct from those of El Niño (and are reproduced in both mixed-layer and full ocean dynamics coupled climate models), aspects of climate teleconnections resemble those associated with El Niño. The large-scale shears show a pronounced and robust weakening over the Indian and western tropical Pacific Oceans. The magnitude of the ensemble-mean changes is on the order of 0.5-1 m/s per degree warming, and the robust signals are evident in over 20 of the 23 models. Effort should be undertaken to understand the extent to which these robust changes in large-scale wind shear may impact hurricane activity, and they should be considered in discussions of projected changes to hurricane intensity and frequency.