Bifurcated Tropical Convergence Zones in Matsuno–Gill-Type Models

A Matsuno--Gill-type model is used to study bifurcated intertropical convergence zones (ITCZs). Consistent with observations, solutions in the limit of strong damping are explored. The model includes an idealized Bjerknes feedback which consists of a relaxation to zonally asymmetric surface pressure, and idealized coupling of surface winds and sea surface temperature via oceanic Ekman balance. In addition, the model is forced by relaxation to hemispherically symmetric and antisymemtric surface pressure, and convective coupling of vertical winds and diabatic heating. Two ITCZ bifurcation mechanisms are identified. First, equatorial ocean upwelling increases equatorial surface pressure, which reduces vertical winds near the equator and can lead to precipitation peaks that straddle the equator in regions of equatorial ascent. Second, in the viscous limit ITCZs form along negative anomalies of the local Rossby number near the surface. Negative anomalies of the local Rossby number tend to occur near the equator for equatorial low pressure and off the equator for equatorial high pressure, leading to a single ITCZ in the rising branch of zonal overturning circulations and a double ITCZ that straddles the equator in the descending branch. When the northern hemisphere is differentially heated, a pronounced ITCZ forms north of the equator and a less elongated ITCZ forms south of the equator, mimicking annual-mean Pacific ITCZs. Changes in the zonal orientation of ITCZs are modulated in the idealized model by the strength of the ocean-atmosphere Ekman coupling, in a manner consistent with observed seasonal variations.