Thermally Driven Motions in an Equatorial β-Plane: Hadley and Walker Circulations During the Winter Monsoon

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Abstract
The linearized shallow-water equatorial β-plane equation was solved for a subset of approximate solutions applicable to thermally driven large-scale tropical circulation. In particular, the heat-induced monsoon circulations during Southeast Asian northeasterly cold surges are investigated. It was demonstrated that the response of the tropical atmosphere to a localized heat source consists of forced Rossby waves propagating westward and Kelvin waves eastward along the equator away from the region of forcing. In general, for any source/sink distribution, the heat-induced motion can have the characteristics of a Walker-type (ν = 0 at the equator) or a Hadley-type (u = 0 at the equator) response or a combination of both, depending on the latitudinal location of the forcing. Away from the equator, a forcing corresponding to the sudden imposition of mass at the lower layer, or equivalently in our model a rapid cooling of the lower troposphere, produces a sudden local surface pressure rise and strong anticyclonic flow to the west of the forcing. Strong NE-SW till in the axis of the anticyclone is observed and can be understood in terms of the dispersion of the various wave modes excited. The low-latitude response is, as expected, dominated by Kelvin and the gravest Rossby wave modes. Coupled with an equatorial heat source, the sudden cooling of the lower troposphere over a localized area in the subtropics gives rise to a northeasterly wind surge and large-scale Walker and Hadley circulations reminiscent of periods of strong cold surges over East Asia. Finally, the effect of the presence of a mean wind is shown to modify the spatial extent of the equatorial circulation with a mean easterly wind favoring the formation of equatorially trapped Walker cells.

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