A sensitivity study of changes in Earth's rotation rate with an atmospheric general circulation model

A series of general circulation model simulations in which Earth's rotation rate has been increased is presented using the community climate model version l (CCM1) of the National Center for Atmospheric Research (NCAR). The rotation rate has been altered in order to simulate day-lengths of 24-, 22-, 20-, 18-, 16- and 14-h Earth days. This is a plausible range of Earth's day-length during the last 4 billion years. In an earlier study with a simple energy balance ocean, which does not store heat (sometimes referred to as a swamp), it was shown that reducing the day-length to a 14-h day caused a 20% reduction in the global mean cloud fraction. In this study however, using fixed sea surface temperatures (SSTs) with January solar forcing, a slight increase in clouds occurs with faster rotation, although changes in relative humidity are similar to the earlier study. Furthermore, as in an earlier study, there is more sinking in the mid-latitudes. This sinking is most prominent over the Pacific and Atlantic storm tracks, indicating that the baroclinic eddies have been weakened. With faster rotation rates, the storm tracks are defined by shorter waves as compared to the control simulation. A significant change in the large-scale zonally averaged circulation occurs when the day-length is reduced to a values less than 18-hours. The effects of faster rotation rates on stationary eddy heat transport may help to explain high latitude glaciation of the Ordovician some 440 Ma years ago.