On the relationship between uncertainties in tropical divergence and the hydrological cycle in global models

A survey of tropical divergence from three GCMs, three global reanalyses and four in-situ soundings from field campaigns shows the existence of large uncertainties in the ubiquity of shallow divergent circulation as well as the depth and strength of the deep divergent circulation. More specifically, only two out of the three GCMs and three global reanalyses show significant shallow divergent circulation, which is present in all in-situ soundings, and of the three GCMs and three global reanalyses, only two global reanalyses have deep divergence profiles that lie within the range of uncertainty of the soundings. The relationships of uncertainties in the shallow and deep divergent circulation to uncertainties in present day and projected strength of the hydrological cycle from the GCMs are assessed. In the tropics and subtropics, deep divergent circulation is the largest contributor to moisture convergence that balances the net precipitation, and inter-model differences in the present day simulations carry over onto the future projections. In comparison to the soundings and reanalyses, the GCMs are found to have deeper and stronger divergent circulation. While these two characteristics of GCM divergence affect the strength of the hydrological cycle, they tend to compensate for each other so that their combined effect is relatively modest.