A Seasonally Recurrent Annular Cyclone In Mars Northern Latitudes And Observations Of A Companion Vortex

We study a seasonally recurrent cyclone and related cloud phenomena observed on Mars at L-s similar to 120 degrees, latitude similar to 60 degrees N, and longitude 90 degrees 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 6 June and 9 July. 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 similar to 600-800km with a cloud-free core of a radius similar to 100-300km. Tangential velocities measured from cloud tracking in 2012 images are similar to 5-20m/s(-1) at 10-km altitude and double cyclone moved eastward with a velocity of 4m/s(-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 L-s similar to 120 degrees.Plain Language Summary We study a remarkable annular vortex observed some years to be formed by two coupled cyclones that grow every Martian northern summer at the same longitude and latitude of the planet. Each vortex has a size of 700km and its center is a cloud-free area with a radius of about 200km. The vortex is formed by water ice clouds at about 10-km altitude where the winds blow with speeds of 5 to 20m/s. The clouds grow in the morning hours and sublimate as the vortex weakens with increasing insolation. Dynamical models show that the vortex forms in a region with a large north-south terrain slope that combines with north-south temperature gradient. The study mainly uses images taken in 2012, but a survey shows that the vortex recurrently forms every Martian year between 1995 and 2018. This vortex system can be used as a proxy, studying its variability, to characterize the state of the atmosphere at the time of its formation.