A super-rotating equatorial jet during the 2018 Martian Global Dust Storm

Abstract

By assimilating temperature and dust data from the Mars Climate Sounder (MCS) and Atmospheric Chemistry Suite (ACS) instruments into a Mars Global Circulation Model (MGCM), we demonstrate that super-rotation in the Martian atmosphere doubled for the majority of the global dust storm period in Mars year (MY) 34, as compared to the same period in MY 33. During this period, the tropical band was dominated by a strong super-rotating jet that extended to 60 km. Our findings illustrate how dust events can significantly alter Martian tropical wind profiles away from climatology, and they underscore the need to better understand and constrain the dynamical processes that drive these phenomena.

Background

Super-rotation is a dynamical phenomenon in which the total axial angular momentum of a planetary atmosphere exceeds its pure solid-body component [1]. Super-rotation is commonly observed on slowly rotating bodies such as Venus and Titan, and often entails the presence of a westerly jet in the equatorial region [2].

Modelling studies indicate that super-rotation is also present in the Martian atmosphere [3]. The strength of super-rotation on Mars is modulated by the amount of dust in the atmosphere: this is due to dust-driven enhancement of the diurnal thermal tides, which then induce westerly acceleration in the tropics. This effect is most substantial during global dust storms, when large quantities of dust are lifted into the atmosphere.

Methods

The MGCM used at the Open University (OU) is the UK version of the Laboratoire de Météorologie Dynamique (LMD) MGCM, using a spectral dynamical core and a semi-Lagrangian transport scheme [4,5], and with physical parameterizations developed in collaboration between the LMD, the OU, the University of Oxford, and the Instituto Astrofísica de Andalucía [6]. The data assimilation scheme used is a version of the Analysis Correction scheme [7], modified for application to the Martian atmosphere [4].

We assimilated two Martian years for comparison: MY 34, which featured a global dust storm between L_s 180°-240°; and MY 33, a year that did not feature any major dust events. The observational datasets used in the assimilation were retrieved temperature profiles and dust column products from MCS, a limb sounder aboard the Mars Reconnaissance Orbiter (MRO) [8], and  retrieved temperature profiles from the ACS spectrometers aboard the ExoMars Trace Gas Orbiter [9] (available for the second half of MY 34).

Results

We calculated the variation of the global super-rotation index S against solar longitude for the two Mars years. The index S is defined as the ratio between the atmospheric and solid-body components of the total axial angular momentum of the atmosphere, minus one [1,3]. Positive values of S indicate super-rotation. We found that the atmosphere of Mars was in a state of global super-rotation for much of the year. The super-rotation had a semi-annual structure, with peaks at equinoxes and troughs at solstices. Most strikingly, we found a prominent peak in S during the dust storm period of MY 34, indicating strong super-rotation with a maximum value that was twice as large as the reference MY 33 value in the same period.

We also calculated a local super-rotation index, s, to investigate the spatial distribution of super-rotation. s is a three-dimensional field in time, latitude and height, and is defined as the ratio of the specific zonal mean axial angular momentum of an air parcel at that location to the specific axial angular momentum of an air parcel at rest at the equator, minus one [3]. Calculations of s during the onset phase of the dust storm (L_S 180°-210°) show that the tropical latitude band was dominated by a strong and deep super-rotating jet during this period. The jet height extended to around 60 km in altitude, with wind speeds of 30-50 m/s between altitudes of 10-30 km.

Future work will focus on understanding and constraining the dynamical processes that drive the enhancement of super-rotation during global dust storms.

Acknowledgements

KR, SRL, JAH and MRP acknowledge the support of the UK Space Agency under the following grants: ST/R001405/1 (KR, SRL, MRP), ST/S00145X/1 (JAH, SRL, MRP), and ST/P001262/1 (SRL, MRP). SRL and MRP also acknowledge the support of the UK Science and Technology Facilities Council (STFC) under grant ST/P000657/1. PMS acknowledges the support of the STFC under grant ST/N50421X/1, and the Open University in the form of a PhD studentship.

References

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