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Center for Nonlinear Studies |
The intertropical convergence zone (ITCZ), an east-west oriented line of intense vertical motion, clouds, and precipitation near the equator, is one remarkable feature of Earth’s atmosphere. At times its convection can span the entire Pacific Ocean centered at nearly the same latitude. Other times it can be active north of the equator in the eastern Pacific while it is active south of the equator in the western Pacific. Together with the Hadley circulation, they are fundamental features of the atmospheric general circulation. Yet, there are still many open questions about the ITCZ, such as, Why is the ITCZ never observed poleward of 30 degrees? Also, Why is the ITCZ narrow in eastern ocean basins? There are number of factors that play a role in ITCZ variations such as air-sea interactions, influences from nearby land and topography, radiative-convective-cloud complexities, and boundary layer processes. In this talk, my main focus is on boundary layer dynamics, which can be quite complex even when moisture is not directly considered. For example, when the boundary layer meridional inflow surrounding the ITCZ is large enough, the meridional advective terms v(du/dy) and v(dv/dy) play an important role in ITCZ formation. The latter term, in particular, is part of an embedded Burger’s equation in the meridional momentum equation, which can produce intense meridional gradients in the boundary layer and thus, intense vertical motion and vorticity. The sharpening of meridional gradients is simulated using an idealized sub-cloud boundary layer model. The model is what can be thought of as the simplest model necessary to reproduce the ITCZ while still retaining the meridional advective terms. Model experiments are conducted at a range of horizontal resolutions, from 1 degree to 1 km, to gain insight into how the role of horizontal advection changes with horizontal resolution. The boundary layer horizontal flow is forced by the monthly and boundary layer averaged pressure gradient field from the Year of Tropical Convection (YOTC) reanalysis. The numerical model solutions are generally in agreement with the YOTC dynamical fields; the main differences are in regions of sharp gradients where it is apparent that horizontal advection and Coriolis effects are crucial to understanding the preferential location and intensity of the ITCZ. If anyone would like to meet with Alex please email Balu Nadiga.
Host: Balu Nadiga