A SENSITIVITY STUDY OF CHANGES IN EARTHS ROTATION RATE WITH AN ATMOSPHERIC GENERAL-CIRCULATION MODEL

A series of general circulation model simulations in which Earth's rot ation rate has been increased is presented using the community climate model version 1 (CCM1) of the National Center for Atmospheric Researc h (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 plau sible 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 st ore heat (sometimes referred to as a swamp), it was shown that reducin g the day-length to a 14-h day caused a 20% reduction in the global me an cloud fraction. In this study however, using fixed sea surface temp eratures (SSTs) with January solar forcing, a slight increase in cloud s occurs with faster rotation, although changes in relative humidity a re similar to the earlier study. Furthermore, as in an earlier study, there is more sinking in the mid-latitudes. This sinking is most promi nent over the Pacific and Atlantic storm tracks, indicating that the b aroclinic eddies have been weakened. With faster rotation rates, the s torm tracks are defined by shorter waves as compared to the control si mulation. A significant change in the large-scale zonally averaged cir culation occurs when the day-length is reduced to a values less than 1 8-hours. The effects of faster rotation rates on stationary eddy heat transport may help to explain high latitude glaciation of the Ordovici an some 440 Ma years ago.