Global Energy Transports and the Influence of Clouds on Transport Requirements—A Satellite Analysis

Abstract

This study examines the impact of differential net radiative heating on two-dimensional energy transports within the atmosphere-ocean system and the role of clouds on this process. Nimbus-7 earth radiation budget data show basic energy surpluses over the tropical oceans and relative or absolute energy deficits over low-latitude continental regions. The two-dimensional mean energy transports, in response to zonal and meridional gradients in the net radiation field, exhibit an east-west coupled dipole structure in which the west Pacific acts as the major energy source and North Africa as the major energy sink. It is shown that the dipole is embedded in the secondary energy transports arising mainly from the differential heating between land and means in the tropics in which the tropical cast-west (zonal) transports are up to 30% of the tropical north-south (meridional) transports. Thus, any perturbations to this dipole on an interannual basis due to regionally induced fluctuations of the net radiation balance give rise to low-latitude energy transport variations. In turn, the tropical variations lead to extratropical responses through alterations of requirements on both zonal and meridional transports at all positions on the globe. Cloud-induced transports, obtained by differentiating the cloud-free portion from the total transport field, indicate that year-to-year cloud amount changes are contributing to fluctuations of the global climate system through these mechanisms. Increased cloudiness increases zonal available potential energy, thus increasing the intensity of the north-south transports while slightly weakening the dipole intensity. It would thus appear that the basic role of cloudiness is to diminish the role of differential heating between continents and oceans and force the globe toward a more meridionally distributed energy imbalance. This implies the radiative feedback effects of clouds, regardless of factors determining cloud amount variability, reduce the radiative decoupling of land and ocean. This conclusion cannot be arrived at heuristically because it pertains to the specific optical properties of continental and oceanic cloud systems and additional factors governing cloud amount variability over the landmasses and oceans themselves.