133 Convergence Zone and Hadley Cell Dependence on Rotation Rate and Seasonal Cycle Characteristics in Dry Atmospheres

Tuesday, 27 June 2017
Salon A-E (Marriott Portland Downtown Waterfront)
Spencer A Hill, UCLA & Caltech, Los Angeles, CA; and S. Bordoni and J. L. Mitchell

Though the seasonal migrations of Earth's Intertropical Convergence Zone are commonly believed to be limited by the length of the insolation annual cycle, a recent study using an idealized moist GCM indicates that the convergence zone does not extend beyond the tropics even under perpetual solsticial forcing. This is insensitive to the presence of mid-latitude eddies but is sensitive to planetary rotation rate, with seasonal migrations of the ascending branch of the Hadley cells (and consequently the convergence zone) extending farther poleward the slower the planet is rotating.

We use an idealized dry model to further refine the theoretical understanding of these phenomena, via simulations analogous to and extending the aforementioned moist cases. The importance of planetary rotation and lack thereof for both thermal inertia and baroclinic eddies emerge in the dry simulations also, implying root causes involving simpler steady-state, solsticial, axisymmetric, dry dynamics. Even for imposed thermal forcings that maximize on the summer pole, the cross-equatorial Hadley cell always terminates within the planet's "tropical regime", although this regime grows in size as planetary rotation rate is decreased. We explain these and other features to the extent possible based on existing theoretical frameworks, and we discuss the potential implications for planetary atmospheres, including those of Earth and Titan.

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