General Circulation

Mean Meridional Circulation (Hadley Cell)

Fundamental to theories of global atmospheric circulations of rapidly rotating, shallow, and differentially heated terrestrial planetary atmospheres like Earth and Mars are the longitudinally averaged (i.e., zonally symmetric) mean meridional “overturning” circulations (MMCs). In the tropics and subtropics, this is the so-called thermally direct Hadley Cell. In the middle and high latitudes, it is the thermally indirect Ferrel Cell. Finally, in high latitudes, it is the thermally direct Polar Cell. The MMCs and their statistical temporal and spatial constructions are relevant when discussing decomposition of the atmospheric global circulation, and when formulating diagnostics of atmospheric energy, angular momentum and trace species’ budgets. The MMCs are necessarily two-dimensional (2D), that is, they are constructed (i.e., statistically-based) in a latitude-pressure (or height) plane.

Stationary Eddies

Due to continental, large-scale east-west variations in surface properties of terrestrial-like worlds (e.g., geographic landmasses, oceans and large seas, and/or vast variations in surface thermal properties, etc), longitudinally varying global circulation patterns become embedded within a zonally symmetric (2D) atmospheric decomposition perspective. Many of these variations arise due to physically based, large-scale atmospheric waves and disturbances induced by geophysical fluid dynamics.  There exist non-traveling (i.e. quasi-stationary or “standing”) large-scale waves such as forced Rossby modes that arise due to topography and surface heating variations associated with thermal inertia and albedo patterns coupled with differential rotation in latitude inherent to a terrestrial-like planetary world.

Transient Eddies

Large-scale, west-to-east traveling waves associated with equator-to-pole (warm to cold) temperature gradients and large-scale wind “shear”, (changes in wind velocity horizontally and vertically) exist in the form of barotropic and baroclinic shear instabilities. Such traveling Rossby waves encompass extratropical “weather systems” on both large and planetary scales (i.e., at the Rossby deformation length scale, LD = NH/ѡ where N is the buoyancy frequency; H is the scale height; and, ѡ is the planetary rotation rate. For Mars and Earth, LD = O(2000 km)). 

Since Mars is smaller than Earth, there are fewer weather systems about a midlatitude circle. As on Earth, the weather systems are impressions of synoptic-period (i.e., day-to-day) traveling eddies or disturbances in the middle latitudes (i.e., “high” and “low” pressure systems or “anticyclones” and “cyclones”). Traveling Rossby waves present with long-to-medium periods (weeks to days) are not forced by daily solar heating variations from a rising and setting sun as are the global thermal tidal modes. Such large-scale waves arise due to shear instabilities within the geophysical fluid envelope. Traveling synoptic weather systems and their accompanying cyclogenesis/frontogenesis dynamical processes are very effective at mixing heat, momentum, and tracer quantities between the middle and high latitudes. In addition, these large-scale waves can interact significantly with tropical/subtropical circulation components.

Thermal tides

Thermal tides are the atmospheric response to diurnally varying thermal forcing caused by aerosol heating within the atmosphere and radiative and convective heat transfer from the surface. The thermal tides are planetary-scale gravity waves with periods that are harmonics of the solar day. As particularly prominent features of the Mars atmosphere, temperature and wind fields have a strong dependence on local solar time. 

Thermal tides include westward-propagating (migrating, sun-synchronous) waves driven in response to solar heating as well as non-migrating waves forced by zonal variations in the thermotidal forcing. Zonal modulation of forcing can arise from longitudinal variations of the boundary (topography and surface thermal inertia) and radiatively active aerosols (dust and water ice clouds). Tides strongly influence the near-surface climate. Additionally, they are easily amplified by dust heating, therefore providing an important feedback mechanism for dust lifting that suggests tides likely play a critical role in the lifting and transport of dust. Non-migrating tides appear as diurnally varying upslope/downslope circulations within the near-surface boundary layer. Like their migrating counterparts, they are also able to propagate vertically to the lower thermosphere, transporting momentum from their source regions in the lower atmosphere to the upper atmosphere.

Mass Condensation Flow

Mars is unique in that it has a wind system associated with the flow of mass to and from the polar caps. This mass flow arises from the seasonal condensation and sublimation of CO₂, the main constituent of the atmosphere, in the polar regions of each hemisphere. These flows, termed “condensation flows,” are directly responsible for the large seasonal changes in surface pressure measured by the Viking Landers and Curiosity Rover. Modeling studies suggest that the condensation flows can significantly enhance surface winds near the edge of the growing or retreating cap. For diagnostic purposes, the condensation flow at a given time is defined as the zonal average of the vertically integrated meridional mass flux.