The emergence of a summer hemisphere jet in planetary atmospheres

Abstract

Zonal jets are common in planetary atmospheres. Their character, structure, and seasonal variability depend on the planetary parameters. During solstice on Earth and Mars, there is a strong westerly jet in the winter hemisphere and weak, low-level westerlies in the ascending regions of the Hadley cell in the summer hemisphere. This summer jet has been less explored in a broad planetary context, both due to the dominance of the winter jet and since the balances controlling it are more complex, and understanding them requires exploring a broader parameter regime. To better understand the jet characteristics on terrestrial planets and the transition between winter-and summer-dominated jet regimes, we explore the jet's dependence on rotation rate and obliquity. Across a significant portion of the parameter space, the dominant jet is in the winter hemisphere, and the summer jet is weaker and restricted to the boundary layer. However, we show that for slow rotation rates and high obliquities, the strongest jet is in the summer rather than the winter hemisphere. Analysis of the summer jet's momentum balance reveals that the balance is not simply cyclostrophic and that both boundary layer drag and vertical advection are essential. At high obliquities and slow rotation rates, the cross-equatorial winter cell is wide and strong. The returning poleward flow in the summer hemisphere is balanced by low-level westerlies through an Ekman balance and momentum is advected upwards close to the ascending branch, resulting in a mid-troposphere summer jet.