Martian polar vortices: Comparison of reanalyses

Abstract

The structure and evolution of the Martian polar vortices is examined using two recently available reanalysis systems: version 1.0 of the Mars Analysis Correction Data Assimilation (MACDA) and a preliminary version of the Ensemble Mars Atmosphere Reanalysis System (EMARS). There is quantitative agreement between the reanalyses in the lower atmosphere, where Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data are assimilated, but there are differences at higher altitudes reflecting differences in the free‐running general circulation model simulations used in the two reanalyses. The reanalyses show similar potential vorticity (PV) structure of the vortices: There is near‐uniform small PV equatorward of the core of the westerly jet, steep meridional PV gradients on the polar side of the jet core, and a maximum of PV located off of the pole. In maps of 30 sol mean PV, there is a near‐continuous elliptical ring of high PV with roughly constant shape and longitudinal orientation from fall to spring. However, the shape and orientation of the vortex varies on daily time scales, and there is not a continuous ring of PV but rather a series of smaller scale coherent regions of high PV. The PV structure of the Martian polar vortices is, as has been reported before, very different from that of Earth's stratospheric polar vortices, but there are similarities with Earth's tropospheric vortices which also occur at the edge of the Hadley Cell, and have near‐uniform small PV equatorward of the jet, and a large increase of PV poleward of the jet due to increased stratification.

What causes Mars' annular polar vortices?

“…This is also generally true for other 30 sol averaged periods, but this behavior is less distinct when viewed instantaneously. As in Martian reanalyses [ Waugh et al ., ], there is a wide variation in the shape and orientation of the PV annulus, both on hourly and daily time scales, in the standard MarsWRF simulation (e.g., Figure ). There is also a wide variation in the latent heating, including times when there is latent heating (and hence temperatures at or just below the CO 2 saturation temperature) covering nearly all the low PV region (e.g., Figure a), times when the region of latent heating is offset from the region of low PV (e.g., Figures b and c) and times when there is no, or very minimal, latent heating (e.g., Figure d).…”

“…While the model reproduces the main features in the Martian reanalysis there are some differences, with a weaker polar minimum and the region of low PV extending to higher altitudes in the simulation. As in the reanalyses [ Mitchell et al ., ; Waugh et al ., ], the simulated annular vortices form in autumn and persist until spring (Figure a), and annular vortices occur in both hemispheres during winter, with a weaker and less disturbed vortex in the SH (Figure a).…”

“…An annular polar vortex is also observed in the southern hemisphere during winter [ Mitchell et al ., ; Waugh et al ., ]. Similar PV structure to that shown in Figure is also found in the Ensemble Mars Atmosphere Reanalysis System [ Waugh et al ., ].…”

“…Previous studies have shown that annular vortices can be produced in free‐running Martian general circulation model simulations [ Mitchell et al ., ; Waugh et al ., ]. This is also the case for our MarsWRF simulations.…”

“…In contrast, on Mars, there is near‐uniform PV equatorward of the core of the westerly jet, a large increase in PV just within the jet, and then a decrease in polar regions, resulting in an annulus of high PV. These annuli are seen in observations [ Banfield et al ., ], reanalyses [ Mitchell et al ., ; Waugh et al ., ], and free‐running general circulation model simulations of Mars [ Barnes and Haberle , ; Mitchell et al ., ; Waugh et al ., ]. This annular PV structure likely has implications on the mixing of gases and aerosols between polar and middle latitudes and on the vertical propagation of waves, and it is important to identify the cause of the annulus.…”