Neptune's NDS-2018: The Dark Vortex That Would Not Die

Tuesday, 15 December 2020
Michael H. Wong1, Amy A Simon2, Lawrence Anthony Sromovsky3, Agustin Sanchez-Lavega4, Jon Legarreta4, Patrick M Fry5, Joshua Tollefson1, Ricardo Hueso6, Patrick GJ Irwin7, Imke De Pater8 and Glenn Orton9, (1)University of California Berkeley, Berkeley, CA, United States, (2)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (3)University of Wisconsin Madison, Madison, WI, United States, (4)University of the Basque Country, Donostia, Spain, (5)University of Wisconsin Madison, Space Science and Engineering Center, Madison, WI, United States, (6)University of the Basque Country, Fisica Aplicada I, Donostia, Spain, (7)University of Oxford, Oxford, United Kingdom, (8)Univ California Berkeley, Berkeley, CA, United States, (9)Jet Propulsion Laboratory, Pasadena, CA, United States
The newest dark vortex on Neptune, NDS-2018, was discovered in Hubble Space Telescope (HST) annual imaging data from the OPAL program (Simon et al. 2019, GRL, DOI 10.1029/2019GL081961). OPAL observations in the following year, 28-29 August 2019, showed that the dark spot had drifted southward toward the equator, from planetographic latitude 23 deg N in 2018, to 18 deg N in 2019. The exciting implication was that the equatorward drift could have led to vortex disruption as early as February 2020, as described in models by LeBeau and Dowling (1998, Icarus, DOI 10.1006/icar.1998.5918).

Follow-up HST observations instead found that the equatorward motion of the vortex had arrested, or even reversed, with the most recent latitude being 18.3 deg N on 23 June 2020. Under the standard zonal wind profile, which was measured by tracking CH4-ice clouds in Voyager data (Sromovsky et al. 1993, Icarus, DOI 10.1006/icar.1993.1114), simulations predict an equatorward drift about 10 times faster than observed in 2018-2019 (Hadland et al. 2020, MNRAS, DOI 10.1093/mnras/staa1799). We will present numerical simulations using the EPIC code, which indicate that the equatorward migration is slower if the vortex intensity (rotation speed) is reduced, or if there is higher meridional shear in the zonal winds, compared to the Sromovsky et al. (1993) winds.