A seasonally recurrent annular cyclone in Mars northern latitudes and observations of a companion vortex

Abstract : We study a seasonally recurrent cyclone and related cloud phenomena observed on Mars at Ls ~ 120°, latitude ~ 60°N and longitude 90°W from images obtained with cameras in different spacecraft between 1995 and 2018. A remarkable double cyclone formed in 2012 and we present a detailed study of its dynamics using images from Mars Express and Mars Reconnaissance Orbiter obtained between June 6 and July 9. A double cyclone was also observed in 2006 and 2008. In other Martian Years the primary cyclone showed an annular cloud morphology with a large water ice cloud observed eastward of it. The cyclones have a size of ~ 600‐800 km with a cloud‐free core of a radius ~ 100‐300 km. Tangential velocities measured from cloud tracking in 2012 images are ~ 5‐20 ms^‐1 at 10 km altitude and DC moved eastwards with a velocity of 4 ms^{‐ 1} during its lifetime of one month. The vortices grow in the morning hours, but with the increasing insolation as the sol progresses, a part of the clouds evaporate, the winds weaken and the vortices lose coherence. This phenomenon forms under high temperature gradients in a region with a large north‐south topographic slope and has been recurrent each Martian year between 1995 and 2018. We argue the interest of studying its changing properties each Martian year in order to explore their possible relationship to the state of the Martian atmosphere at Ls ~ 120°.