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Mars’ Polar Vortex

A surprising radial distribution of potential vorticity (PV) of Martian atmosphere represents a minimum PV value at the winter pole, which is surrounded by an annular circumpolar vortex with high PV.

The potential vorticity (PV) is the absolute circulation of an air parcel in a fluid motion that is enclosed between two isentropic surfaces. PV plays a major role in operational weather diagnosis, weather forecasts, and research.

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Credit: Masoud Rostami

According to the instability rules, the PV structure of Mars, which looks like a hollow PV tower, is unstable and should not exist for a long time. This profile is in contrast to what is expected from the nonlinear evolution of an unstable cyclonic vortex that satisfies Rayleigh-Kuo instability criterion. A high centered PV on the pole would be a result of a typical evolution of instability without external forcing. The North polar vortex in the Earth’s stratosphere is an example of such monotonic increase of PV towards the pole.

It should be stressed that the Martian polar vortex is annular only on average [1]. In order to understand this apparent paradox,  the simplest atmospheric model which is obtained by vertical averaging of the adiabatic ”primitive” equations of the atmosphere [2] is employed in the recent research by Rostami et al (2018) [3]. The rotating shallow water (RSW) model is improved by adding radiative cooling and convective fluxes due to the phase transitions of CO2, which is the major constituent of Mars’ atmosphere.

The main objective of the recently-published article by Rostami et al. (2018) [2] is to understand the dynamical influence of radiative relaxation and COdeposition as two major diabatic phenomena acting during Mars’ winter upon Mars’ polar vortex. As there are no direct measurements in the Martian atmosphere, all reanalysis datasets have been extracted by using Martian general circulation models. Reanalysis dataset presents patches of high PV at lower altitudes, on the daily time scales, during winter solstice; nevertheless, maps of 30 sol mean PV represent a near-continuous elliptical ring of high PV. The South pole exhibits a similar characteristic with smaller intensity.

A new simple parametrization of COdeposition of Mars’ atmosphere is proposed in the framework of the RSW model. The deposition mechanism includes nuclei as an active tracer in the heterogeneous atmospheric environment. The intersection of the regions with high pressure and high concentrated nuclei causes a deposition of CO2, which is a direct transition from gas to the solid phase, and consequently, a latent heat release that is accompanied by ascending the potential vorticity.

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While non-linear adiabatic saturation of the instability tends to reorganize the vortex, the diabatic effects prevent this and, thus, provide an explanation of the vortex’s form and longevity. Furthermore, the patchiness of the PV map is explained by the role of both spatially inhomogeneous nucleation of COand radiative relaxation at the same time. Minimal parameterizations in an idealized numerical model enable us to understand the causalities among available phenomena, e.g., in the aforementioned study. COdeposition substantially contributes to the formation of blobs of high PV encircling the pole.

These findings are described in the article entitled On the role of spatially inhomogeneous diabatic effects upon the evolution of Mars’ annular polar vortex, recently published in the journal IcarusThis work was conducted by Masoud Rostami from the University of Cologne and Sorbonne Universités (Université Pierre et Marie Curie, UPMC) and Ecole Normale SupérieureVladimir Zeitlin from Sorbonne Universités (Université Pierre et Marie Curie, UPMC) and Ecole Normale Supérieure, and Luca Montabone from the Space Science Institute

References:

  1. L. Montabone, K. Marsh, S.R. Lewis, P.L. Read, M.D. Smith, J. Holmes, A. Spiga, D. Lowe, A. Pamment The Mars analysis correction data assimilation (MACDA) dataset v1.0 Geosci. Data J., 1 (2) (2014), pp. 129-139.
  2. Rostami, M. and Zeitlin, V. (2018), Improved moist‐convective rotating shallow water model and its application to instabilities of hurricane‐like vortices. Q J R Meteorol Soc. Accepted Author Manuscript. . doi:10.1002/qj.3292
  3. Masoud Rostami, Vladimir Zeitlin, Luca Montabone, On the role of spatially inhomogeneous diabatic effects upon the evolution of Mars’ annular polar vortex, Icarus, Volume 314, 2018, Pages 376-388, https://linkinghub.elsevier.com/retrieve/pii/S0019103517306073.

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