Steady-state circulations forced by diabatic heating and wind stress in the intertropical convergence zone
Date
2011
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Abstract
A number of studies have shown the importance of using idealized models to gain insight into large-scale atmospheric circulations in the tropics, especially when investigating phenomena that are not well understood. The recent discovery of the Shallow Meridional Circulation (SMC) in the tropical East Pacific and West Africa is a perfect example of a phenomenon that is not well understood (Zhang et al., 2004). The vertical structure of the SMC is similar to the Hadley circulation, but its return flow is located at the top of the boundary layer. The current theory of the SMC is entirely different dynamically than the Hadley circulation because it has been thought of as a large-scale "sea-breeze" circulation rather a geostrophic balance in the meridional momentum equation. The SMC is a vital aspect of the general circulation since it can transport more moisture than the traditional deep Hadley circulation. Climate models often misrepresent the SMC, making many model simulations incomplete (Zhang et al. 2004; Nolan et al. 2007). We aim to better understand the dynamics near the Intertropical Convergence Zone (ITCZ) that involve both deep and shallow circulations using a steady-state linearized model on the equatorial β-plane that is solved analytically. The model is forced by prescribed diabatic heating and boundary layer wind stress curl. The circulations that arise from deep diabatic heating profiles suggest that both the Hadley and Walker circulations are always present, with the Hadley circulation being more prevalent as the deep heating is elongated in the zonal direction, similar to the ITCZ in the East Pacific. The Hadley circulation strengthens because the horizontal surface convergence increases in the meridional direction. Also, the zonal and meridional surface wind anomalies enhance as the deep heating is displaced farther from the equator. The surface wind field associated with this deep heating also forces a significant wind stress curl north of the equator. The atmosphere responds to the wind stress curl by opposing the initial dynamical fields, and generating Ekman pumping in the boundary layer. For example, the surface consists of anomalous negative vorticity in a region that previously contained positively vorticity. This is often referred to as spin down. The Ekman pumping in the boundary layer forces shallow circulations when the frictional forcing is zonally-elongated and sufficiently displaced off of the equator. This shallow circulation makes sense in the East Pacific, where the ITCZ is always north of the equator and is often zonally-elongated. There are two SMCs that develop, one north of the Ekman pumping, and the other to its south. The cross-equatorial SMC is shallower and is stretched in the meridional direction compared to the SMC north of the Ekman pumping since the Rossby length is very large near the equator. It turns out that the frictional forcing does not provide enough vertical or meridional motion to be seen when deep diabatic heating is also present using our simple model. Since the ITCZ is a transient phenomenon and the frictional forcing is more steady, there are days where this Ekman pumping can be seen when deep convection is suppressed. Future research should concentrate on better understanding the effect of the wind stress and surface temperatures on the buildup of subsequent convection using idealized models.
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Subject
atmospheric dynamics
Ekman pumping
equatorial waves
fluid dynamics
ITCZ
tropical meteorology