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The Tropical Cloud Oscillation (TCO) in the Martian atmosphere is a shift of clouds in the northern spring and summer tropical cloud belt between the eastern and western hemispheres on an intra‐seasonal timescale of about 10–40 sols. The TCO is a significant intraseasonal variation and may strongly affect the Martian general circulation, water cycle, and dust cycle. We examine TCOs using multiple data sets with a focus on the clouds observed in Mars Daily Global Maps during Mars Year (MY) 29–35. One or more TCO cycles are observed in each MY and the phenomenon is most prominent during Ls = 135°–185°. Space‐time spectral analysis shows a variety of waves which appear to follow the theoretical dispersion relationships of equatorial waves, such as Kelvin waves, Rossby waves, and Mixed Rossby Gravity waves. The TCO appears to be controlled by zonal wavenumber one traveling waves with Kelvin and Rossby wave characteristics and exhibits a fine‐scale latitudinal structure that requires modeling with sufficient resolution. Issues with current data assimilation products for use in studies of Martian equatorial waves due to this fine‐scale structure are discussed. Plain Language Summary: During martian northern spring and summer, a relatively thick water ice cloud belt is observed in the tropics (approximately 25°S–25°N), which modifies large‐scale circulation, global water transport and the atmospheric dust cycle. We report on a Tropical Cloud Oscillation (TCO) that is a periodic shift in the thickness of this cloud belt between the eastern and western hemispheres on timescales (about 10–40 Mars days) longer than that of weather but shorter than that of seasons. Since the TCO involves changes in clouds and implies changes in circulation, it may also modulate the dust and water cycles. We examine TCOs using image data for Mars Year 29–35 (late 2007 through early 2021). One or more TCO cycles are observed in each Mars Year and the phenomenon is most prominent in late northern summer. A wide variety of atmospheric waves are found during TCOs; however, the most important are equatorial waves traveling eastward and westward with one peak and one trough around the planet. These waves have narrow latitudinal length scales and thus require high resolution numerical models to simulate and properly represent in data assimilation products. Key Points: The Martian tropical cloud belt exhibits eastern—western zonal oscillations that are most prominent in late northern summerTropical Cloud Oscillations are controlled by large‐scale low frequency equatorial Kelvin and Rossby wavesEquatorial wave dynamics may have an important effect on the Martian water and dust cycles [ABSTRACT FROM AUTHOR]

The kinetic temperature of the Martian seasonal caps is controlled by the partial pressure of atmospheric CO2 at the surface. When carbon dioxide condenses, typically near the poles, light non‐condensable species (Ar, N2, CO, etc.) accumulate in the atmosphere, resulting in a decrease of the CO2 partial pressure and depressing the local frost point temperature. The buoyant air should mix laterally and vertically within the polar vortices. Observations show that the Martian seasonal caps' kinetic temperatures are ∼0–4 K below the expected CO2 frost point, depending on latitude and season, indicating atmospheric CO2 gas depletion at the surface/atmosphere interface. In the North and South, we find relatively similar non‐condensable peak enhancement factors (e.g., EFNC ∼ 6–8, up to ∼8.7 in the North) at most latitudes, confirming the efficient meridional mixing within the polar vortices, despite steep surface condensation gradients. In the South, this surface enhancement is similar to column‐integrated values derived from Gamma Ray Spectrometer data, indicating efficient vertical mixing. But in the North, the surface depletion is much larger than in the entire column, suggesting poor vertical mixing. Reduced infrared emission of the seasonal caps stemming from CO2 depletion is not a major energy balance factor. This work illustrates how the atmosphere's composition at the surface can be significantly different from column‐integrated values. Plain Language Summary: The condensation and sublimation of the Martian seasonal CO2 caps remove and then reinject massive amounts of carbon dioxide in the atmosphere at different seasons, thus changing the relative composition of the air in the polar regions, as non‐condensable species remain in gaseous form. Consequently, the CO2 partial pressure and therefore the frost point temperature vary over the course of a year in a measurable manner. Using Mars Climate Sounder surface observations, we leverage this connection between ice temperature and air composition to constrain the properties of the surface/atmosphere interface in the polar regions where few observations are generally available. We find that in the South, the air composition at the surface is similar to the average composition of the atmospheric column, whereas in the North, more CO2‐depleted gas remains close to the ground without efficiently spreading upward. These findings have multiple implications: on the energy budget of the caps as the ice is generally slightly colder than usually modeled, on the modeling of the polar vortices and their dynamics, and on the uniqueness of near‐surface atmospheric conditions and composition. Key Points: Mars Climate Sounder observations of the seasonal caps are used to derive the kinetic temperature of the ice and atmospheric CO2 pressureThe temperatures of the seasonal caps are consistent with non‐condensable gases enrichment by factors 6–8The composition of the northern near‐surface atmosphere differs significantly from the column‐integrated average, in contrast with the South [ABSTRACT FROM AUTHOR]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.07.020 Byline: Ai Inada (a), Mark I. Richardson (a), Timothy H. McConnochie (b), Melissa J. Strausberg (c), Huiqun Wang (d), James F. Bell (e) Keywords: Mars; atmosphere; Mars; polar caps Abstract: High-resolution observations of atmospheric phenomena by the Mars Odyssey Thermal Emission Imaging System (THEMIS) during its first mapping year are presented. An atmospheric campaign was implemented on the basis of previous spacecraft imaging. This campaign, however, proved of limited success. This appears to be due to the late local time of the Odyssey orbit (the locations of activity at 4-6 p.m. appear to be different from those at 2 p.m.). Ironically, images targeting the surface were more useful for study of the atmosphere than those images specifically targeting atmospheric features. While many previously recognized features were found, novel THEMIS observations included persistent clouds in the southern polar layered deposits, dust or condensate plumes on the northern polar layered deposits, dust plumes as constituent parts of local dust storms, and mesospheric clouds. The former two features tend to be aligned parallel and normal to polar troughs, respectively, suggesting a wind system directed normal to troughs and radially outward from the center of the polar deposits. This is consistent with katabatic drainage of air off the polar deposits, analogous to flow off Antarctica. The observation of dust lifting plumes at unprecedented resolution associated with local dust storms not only demonstrates the importance of mean wind stresses (as opposed to dust devils) in initiation of dust storms, but is also seen to be morphologically identical to dust lifting in terrestrial dust storms. As Odyssey moves to earlier local times, we suggest that the atmospheric campaign from the first mapping year be repeated. Author Affiliation: (a) California Institute of Technology, M.S. 150-21, 1200 E. California Blvd., Pasadena, CA 91125, USA (b) NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (c) University of California, Los Angeles, 7229 Math Science Bld., Los Angeles, CA 90095-1565, USA (d) Harvard-Smithsonian Center for Astrophysics, MS-50, 60 Garden Street, Cambridge, MA 02138, USA (e) Cornell University, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA Article History: Received 28 February 2007; Revised 29 June 2007

Author(s): Mark I. Richardson (corresponding author) [1]; R. John Wilson [2] Large seasonal and hemispheric asymmetries in the martian climate system are generally ascribed to variations in solar heating associated [...]