Annular modes of climate variability
Annular modes of climate variability capture considerable percentages of the variance in the zonal-mean winds and eddy strength on Earth, but they were thought to not be particularly important in other atmospheres. Battalio and Lora (2021) showed that annular modes are even more important on Mars and potentially Titan than they are for Earth. A mode of climate variability is simply a particular set of changes that happen to the climate due to the intrinsic nature of the climate system, like ENSO (El Niño or La Niña). What makes a mode of variability annular, unlike ENSO, is that the changes occur along a set of latitudes more evenly. The jet stream and eddy activity vary in this way on Earth, Mars, and Titan.
Mars’s mode in the jet stream acts similarly to Earth’s, though explains more variance, but Mars’s annular mode in the eddy kinetic energy (EKE) behaves a bit differently in that it relates to changes in momentum as well as heat. However, this change may be tied to how eddies develop on Mars (see Mars’s transient waves below). Both Mars and Earth’s annular mode in EKE impact important fields: For Earth, the mode impacts clouds and precipitation. For Mars, the mode impacts dust activity. Thus, being able to quantify the annular mode will improve forecasts of dust activity, which will be critical to ensuring the safety of crewed missions to the surface and to the entry, descent, and landing maneuvers of all spacecraft. More importantly, the EKE mode exhibits leading behavior to dust storms, so diagnosing the state of the mode may enable predictions of large dust storms. The mode regresses strongly on dust storms at a 4 sol lag (see Figure below). A Mars Data Analysis Program grant will provide funding to continue this research.
Martian dust storms
The connection between the annular mode and dust storms is made possible by comparison to a long-term dust storm dataset. To better understand the evolution, climatology, and decay of dust storms, Battalio and Wang (2021) presented a multi-year long dataset of all dust storms in the modern imager record (1999–2015) called the Mars Dust Activity Database. We showed that dust storms develop in three primary tracks in the north and one main track in the south (see Figure below), each of which have the potential to develop into the more rare Global Dust Events. The dust storms evolve during specific times of year related to the prevalence of waves in fall and winter in both the northern and Southern Hemispheres.
The MDAD organizes individual dust storms into dust storm sequences, whereby multiple individual dust storms are collected together based on proximity in the same trajectory. An example from Mars Year 31 is shown below in green, with non associated storms in black.
Storm tracks of transient waves on Mars
Large dust storms on Mars develop from the same type of storm systems that instigate blizzards in New England, atmospheric rivers on the West Coast of the US, and severe weather in the Great Plains. Because waves cause the largest dust events, studying their development provides insight into that of dust activity.
A serendipitous feature of the MACDA dataset is that it contains a year (MY 25) that has a global dust storm. The dust lifted during these events dramatically changes the dynamics of the atmosphere. The dust warms the midlevels and cools the surface, which changes the vertical stability profile. Baroclinic instability (middle panel of Fig. 1) is reduced (the red line is not as large in magnitude) so that synoptic-scale systems gain a barotropic component (Fig. 1 bottom). Also, eddies maintain the same absolute strength but decrease in number (see in the top panel of Fig. 1 that the peaks of each of the three lines are about the same height, but the red line [MY 25] has about half the number).
Eddies in the southern hemisphere are weaker due to topography and because there are no in situ observations of the southern hemisphere midlatitudes (Curiosity is just barely in the Southern Hemisphere.), southern hemisphere waves are not nearly as studied as their northern counterparts. While northern hemisphere waves frequently circumnavigate the planet, southern hemisphere waves are focused into two areas. On area is concentrated in the Hellas Basin around 60 E, and the second is just south of the Tharsis Plateau between 180 and 300 N (Fig. 2, top). These waves are baroclinic in nature, evidenced by the collocation of baroclinic energy conversion (Fig. 2 middle) with the eddy kinetic energy (Fig. 2 top). The waves seem to lose energy barotropically (Fig. 2 bottom) just as northern hemisphere waves do.
For more, see Wave energetics of the southern hemisphere of Mars.
Martian Gravity Waves
My research into Martian atmospheric dynamics also extends to some of the smallest scales. Gravity wave activity on Mars can modulate atmospheric escape, impact high cloud formation, and produce dynamical warming in the polar middle atmosphere. Previous climatologies of gravity wave activity in the 10—100 km horizontal wavelength range have been limited to gravity wave activity oriented in the meridional direction. To sample activity in the zonal direction, the analysis of other, complementary datasets is required.
The Thermal Emission Imaging System (THEMIS) Band-10 dataset is sensitive to 10–100 km horizontal wavelengths at 25 km. THEMIS Band-10 over multiple Mars years during the Ls=120—150 season is analyzed to determine the disposition of waves in the northern hemisphere summer. A special pre-processing pipeline, consisting of removal of THEMIS drift and wobble and band-correlated artifacts, enables the measurement of gravity waves. The incredibly long duration of the THEMIS dataset (~8 Mars years) allows for the assessment of interannual variability and direct comparison to other datasets to test the coverage and sensitivity of THEMIS to gravity wave orientation over multiple baselines.