Climate Model Biases in the Width of the Tropical Belt

Thursday, 18 December 2014
Nicholas Davis, Colorado State University, Fort Collins, CO, United States and Thomas Birner, CO State Univ-Atmospheric Sci, Fort Collins, CO, United States
The edges of the Hadley circulation and the tropical belt define the subtropical dry zones that separate the humid tropics from the stormy midlatitudes. Differences in surface climate, precipitation, and circulation in climate models may depend on where they situate this boundary.

In this study, climate model grid size is found to bias the mean width of the tropical belt, with finer horizontal resolution leading to a narrower tropical belt width. Three objective diagnostics – based on the Hadley circulation, the subtropical jet streams, and the subtropical tropopause break – are used to assess this bias. Horizontal resolution explains up to 50% of the intermodel variation in the tropical belt width in 25 Coupled Model Intercomparison Project Phase 5 (CMIP5) and 17 Chemistry-Climate Model Validation Activity 2 (CCMVal-2) models, with evidence that the relationship is strongest in high-top models and models with interactive chemistry. An analysis of the CCMVal-2 models reveals that an equatorward shift in the eddy momentum flux divergence at the poleward flanks of the Hadley cells is associated with both finer horizontal resolution and a narrower tropical belt. This suggests changes to the eddy fluxes and their balanced meridional flow as a possible mechanism for this grid size bias, and illustrates how a basic property of a model's dynamical core can influence its mean climate.

No other tropical belt width biases were found, suggesting that the processes biasing the poleward shift of the midlatitude jets in climate models do not similarly influence the Hadley cells or subtropical jets.