Equatorial anti-rotating day side wind flow in WASP-43b elicited by deep wind jets?

Abstract

We present WASP-43b climate simulations with deep wind jets (down to 700~bar) that are linked to retrograde (westward) flow at the equatorial day side for $p<0.1$~bar. Retrograde flow inhibits efficient eastward heat transport and naturally explains the small hotspot shift and large day-night-side gradient of WASP-43b ($P_{\text{orb}}=P_{\text{rot}}=0.8135$~days) observed with Spitzer. We find that deep wind jets are mainly associated with very fast rotations ($P_{\text{rot}}=P_{\text{orb}}\leq 1.5$~days) which correspond to the Rhines length smaller than $2$ planetary radii. We also diagnose wave activity that likely gives rise to deviations from superrotation. Further, we show that we can achieve full steady state in our climate simulations by imposing a deep forcing regime for $p>10$~bar: convergence time scale $\tau_{\text{conv}}=10^6-10^8$~s to a common adiabat, as well as linear drag at depth ($p\geq 200$~bar), which mimics to first order magnetic drag. Lower boundary stability and the deep forcing assumptions were also tested with climate simulations for HD~209458b ($P_{\text{orb}}=P_{\text{rot}}=3.5$~days). HD~209458b simulations always show shallow wind jets (never deeper than 100~bar) and unperturbed superrotation. If we impose a fast rotation ($P_{\text{orb}}=P_{\text{rot}}=0.8135$~days), also the HD~209458b-like simulation shows equatorial retrograde flow at the day side. We conclude that the placement of the lower boundary at $p=200$~bar is justified for slow rotators like HD~209458b, but we suggest that it has to be placed deeper for fast-rotating, dense hot Jupiters ($P_{\text{orb}}\leq 1.5$~days) like WASP-43b. Our study highlights that the deep atmosphere may have a strong influence on the observable atmospheric flow in some hot Jupiters.