Double ITCZs and Double Hadley Circulations

The concept of a thermally driven tropical mean meridional overturning circulation, known as the Hadley circulation, is central to our understanding of the general circulation of the atmosphere. The rising branch of the Hadley circulation is where the tropical rain belt lies; the great deserts of the present climate lie along its descending branches. The latitude where the northern and southern Hadley cells meet, characterized by strong convergence near the surface and divergence aloft, is termed the Intertropical convergence zone (ITCZ) and is synonymous with the latitude of peak tropical precipitation. The Single-Hadley circulation paradigm therefore goes hand in hand with a Single-ITCZ paradigm.

In the present and past climates, the tropics host a dominant Double-ITCZ pattern which challenges the canonical concept of a Single ITCZ and hence a Single-Hadley circulation. Here I present a novel general circulation paradigm, whereby the mean tropical circulation can be described as one of three idealized states: 1) a Single ITCZ and a Single-Hadley circulation, 2) a Double ITCZ and a Double-Hadley circulation, and 3) a bifurcated ITCZ pattern with coexistent Single- and Double-ITCZ branches which go along with matching regional Single- and Double-Hadley circulations. Idealized solutions using simple models of the tropical circulation provide insight onto the origins of these three circulation patterns.

תאריך 18/02/2018
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