Evidence for control of Atlantic subtropical humidity by large scale advection

The interplay between large scale dynamics and tropospheric moisture is investigated. A simple conceptual model of the sources and sinks of humidity is used to reconstruct, using a backward Lagrangian trajectory technique, the water vapor distribution in the tropical and subtropical free troposphere. Satellite data in the water vapor channel from both Meteosat-5 and Meteosat-4 satellites are then used to validate the model following a model-to-satellite approach over the whole Atlantic ocean. There is excellent agreement between simulations and observations in the drier regions, but the simulated brightness temperature exhibits a warm bias within and near moist, convective regions. This bias is most probably due to the neglect of cloud effects in reconstructing the simulated brightness temperature, rather than to a dry bias in the simulation. A second advective simulation, performed with monthly mean rather than full transient winds, led to a substantially drier subtropics. This calculation demonstrates the importance of synoptic scale transient eddies in determining the humidity of the subtropical dry zones. It is speculated on this basis that discontinuous changes in synoptic eddy activity could provide a mechanism for rapid global climate changes.