A climate model grid size bias in the width of the tropical belt

Earth's arid subtropics are situated at the edges of the tropical belt, which encircles the planet along the equator and covers half of its surface area. The climate of the tropical belt is strongly influenced by the Hadley cells, with their subsidence and easterly trade winds both sustaining the aridity at the belt's edges. Our understanding of earth's past, present, and future climates is contingent on understanding the dynamics influencing this region. An important but unanswered question is how realistically climate models reproduce the mean state of the tropical belt, and whether and why any latent biases exist within the models.

Up to 50% of the intermodel variation in mean tropical belt width in the CMIP5 and CCMVal-2 models can be attributed to model horizontal resolution, with finer resolution leading to a narrower tropical belt. In the CCMVal-2 models, an enhancement of subtropical wave breaking and an equatorward shift of the accompanying eddy momentum fluxes is associated with finer resolution. Via the Coriolis torque, this resolution bias in the eddy fluxes explains essentially all of the grid size bias and a large fraction of the total intermodel variation in Hadley cell width. Tropical wave fluxes explain the remaining fraction, suggesting that virtually all of the intermodel variation in Hadley cell width is due to the intermodel variation in eddy momentum fluxes.