Vertical wind shear in Neptune’s upper atmosphere explained with a modified thermal wind equation

Abstract

We present observations of Neptune taken in H-(1.4–1.8 µm) and K’-(2.0–2.4 µm) bands on the nights of July 3, 2013 and August 20, 2014 from the 10-m W.M. Keck II Telescope using NIRC2 coupled to the Adaptive Optics (AO) system. We track the positions of ∼100 bright atmospheric features over a 4–5 h window on each night to derive zonal velocities and wind profiles.Our results deviate from the smooth Voyager zonal wind profile from Sromovsky et al. (1993), often by 100–200 m/s, and often by 3–10 times their estimated uncertainties. Besides what appears to be a random dispersion, probably due to a mix of unaccounted for measurement errors, eddy motions, vertical wind shear, and wave-generated features that do not follow the mass flow, there is also a systematic deviation that is wavelength dependent. The H-band profile is best described with a 73–106 m/s shift towards the east for a retrograde flow (i.e., a lessening of the retrograding velocities) from the Voyager profile at the equator. The K’-band profile is consistent with Voyager on both nights.Comparing H and K’ contribution functions and K’/H intensities suggests equatorial H-band features are, on average, deeper than K’-band features. The H-band equatorial features also have greater eastward (less negative) velocities than K’-band features. Differences in zonal wind speed with depth at constant latitude and time imply vertical wind shear. Assuming the average variations in the zonal wind profiles result from wind shear over 3–5 scale heights, we predict vertical wind shears between −1.0 and −2.2 m/(s km) at the equator (increasing with height).The standard thermal wind equation and meridional thermal profile for Neptune given by Voyager/IRIS spectra predict wind shear of the wrong sign relative to the observations. We consider two effects that reconcile this inconsistency. First, we calculate the meridional temperature gradients at pressures outside the Voyager/IRIS narrow sensitivity window required to match our predicted wind shears. Second, we generalize to a thermal wind equation that considers global methane variations and re-derive the temperature structure needed to match the observed wind shear. If methane is uniformly distributed or weakly varying, the equator must be 2–15 K cooler than the mid latitudes below 1 bar. If methane is strongly varying, the equator can be 2–3 K warmer than the mid latitudes below 1 bar, qualitatively consistent with observed temperature contrasts. These findings may imply a stacked-celled circulation pattern in Neptune’s troposphere and lower stratosphere.