The Effect of Increasing Model Resolution on the Northern Hemisphere Winter Midlatitude Storm Track: An Equatorward Shift due to Contraction of the Hadley Cell

We examine how changes in horizontal resolution impact the Northern Hemisphere winter midlatitude climatological storm-track position using the historical runs of six fully coupled climate models from the High-Resolution Model Intercomparison Project (HighResMIP). Each model has a low- and high-resolution version, with atmospheric resolutions from ∼100–200 to ∼25–50 km, respectively, and four of the six models also increase oceanic resolution from 1° to 0.25°. In all models, the storm-track position is more equatorward as resolution increases. This is associated with an intensification and narrowing of precipitation around the equator and a contraction of the Hadley cell. This shifts the subtropical jet equatorward, increasing the baroclinicity at lower latitudes, leading to more favorable conditions for storm genesis at these latitudes. The contraction of the Hadley cell with resolution is similar to that caused by El Niño on interannual time scales. Four of the six models, including those which increase atmospheric resolution only, show a corresponding El Niño–like sea surface temperature signature at high resolution. The Hadley cell contraction with resolution increase is not seen in models with prescribed sea surface temperatures. The increase in oceanic resolution drives the Hadley cell contraction in some models. In others, however, the increase in atmospheric resolution is the dominant driver, but only when the atmosphere is dynamically coupled with the ocean. These results show that increasing resolution alone could exacerbate the existing equatorward storm-track bias seen in CMIP6 models, although it is possible that this could be mitigated by optimization of parameterization settings.