Hadley circulations and their rôles in the global angular momentum budget in two- and three-dimensional models

Abstract

Differences between the general circulations calculated in an axisymmetric 2-dimensional model (2D) and a non-axisymmetric 3-dimensional model (3D) are examined, particularly in terms of meridional circulations, angular momentum budget and zonal wind distributions. Circulations of the two models have similar properties in the low-latitudes. The Hadley cell exists in both cases, and their widths and intensities are comparable, though the cell is slightly stronger in the 3D model than in the 2D model especially in the terrestrial standard case. The surface winds in the Hadley regions are easterly, so that angular momentum is pumped from the surface to the atmosphere and transported to the upper layers and to the mid-latitudes. The most distinct difference is in cell shapes: the cell widths are broader at higher altitudes in 2D, while they do not depend on height in 3D. Circulations in the mid-latitudes are essentially different. In 2D, a symmetric multiple cellar structure exists. The cells are systematically moving equatorward, and they transport the angular momentum downward. The time averaged surface zonal winds almost vanish and a strong westerly shear appears in the mid-latitudes. In 3D, on the other hand, zonal mean winds in the mid-latitudes are westerly from the surface to the upper troposphere and the westerly shear is relatively weaker because of the existence of non-axisymmetric modes. This suggests that baroclinic waves in 3D efficiently transport angular momentum downward in comparison to the symmetric cells in 2D. Dependencies on the rotation rates show that the width of the Hadley cell becomes smaller as the rotation rate becomes faster. In the case of a faster rotation rate than the terrestrial rotation rate, the steady easterlies and westerlies alternately exist in the zonal mean surface winds of the mid- and high-latitudes, while a multiple jet structure exists at the tropopause level.