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On Mean Meridional Circulations in the Tropics

Stefan L. HastenrathDepartment of Meteorology, The University of Wisconsin, Madison

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Abstract

Various models of the mean meridional circulations are examined in terms of the vorticity equation. These include: 1) the traditional model of two Hadley cells joining in a region of ascending motion near the equator; 2) a model with two “equatorial cells” enclosed between the traditional Hadley cells, implying subsidence over the equator; and 3) a model with a single “equatorial cell” enclosed between the two Hadley cells, with ascending motion on one side, and subsidence on the other side of the equator. All three models are considered compatible with the vorticity equation, under specific conditions of latitudinal and vertical distributions of absolute vorticity, frictional force, and vertical and zonal wind components. The existence of twin equatorial cells requires the eastward directed frictional force in the lower layers to decrease from the kinematical equator poleward, within the subsiding portion of either of the two equatorial cells. It is suggested that this may be related to the existence of (eastward directed) equatorial undercurrents in the Pacific and Atlantic Oceans. The requirements for twin equatorial cells do not seem to be met over the interior of the tropical continents.

Abstract

Various models of the mean meridional circulations are examined in terms of the vorticity equation. These include: 1) the traditional model of two Hadley cells joining in a region of ascending motion near the equator; 2) a model with two “equatorial cells” enclosed between the traditional Hadley cells, implying subsidence over the equator; and 3) a model with a single “equatorial cell” enclosed between the two Hadley cells, with ascending motion on one side, and subsidence on the other side of the equator. All three models are considered compatible with the vorticity equation, under specific conditions of latitudinal and vertical distributions of absolute vorticity, frictional force, and vertical and zonal wind components. The existence of twin equatorial cells requires the eastward directed frictional force in the lower layers to decrease from the kinematical equator poleward, within the subsiding portion of either of the two equatorial cells. It is suggested that this may be related to the existence of (eastward directed) equatorial undercurrents in the Pacific and Atlantic Oceans. The requirements for twin equatorial cells do not seem to be met over the interior of the tropical continents.

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