A Climate Model Grid Size Bias in the Width of the Tropical Belt

Abstract:

The aridity of the subtropics is driven by subsidence in the Hadley cells at the poleward edges of the tropical belt. Our understanding of earth's past, present, and future climates is contingent on understanding the dynamics influencing this region. While observations and reanalyses have been used to study the historical variability of the tropical belt, climate models are the primary means of studying the projected expansion of the tropical belt and its impacts due to increasing greenhouse gas concentrations and other forcings.

An important but unanswered question is how realistically climate models reproduce the mean state of the tropical belt. This study augments the existing literature documenting tropical widening in climate models by examining the mean width of the tropical belt in models from the CMIP5 and CCMVal-2 experiments.

Climate models tend to overestimate the tropical belt width based on the latitudes of the subtropical jet cores and underestimate the width based on the latitudes of the tropopause breaks compared to COSMIC GPS radio occultation observations. Additionally, up to 50% of the total intermodel variation in mean tropical belt width can be attributed to model horizontal resolution, with finer resolution leading to a narrower tropical belt. In the CCMVal-2 models, which supply eddy fluxes as model output, increased wave activity and an equatorward shift and intensification of subtropical eddy momentum fluxes is associated with finer resolution. Via the Coriolis torque this shift explains essentially all of the grid size bias and a large fraction of the total intermodel variation in Hadley cell width. Tropical eddy fluxes explain the remaining fraction, suggesting that the dominant mechanism of intermodel variation in the Hadley cells are eddy fluxes, and not the thermally-direct part of the circulation.