Abstract
Mars Global Surveyor (MGS) orbiter observed a planet-encircling dust
storm (PDS) in Mars year (MY) 25 from Ls=176.2-263.4°. We present an
examination of Mars Orbiter Camera (MOC) dust storms and transient
baroclinic eddies identified from Fast Fourier Synoptic Mapping (FFSM)
of Thermal Emission Spectrometer (TES) temperatures for the first two
phases of the storm: precursor, Ls=176.2- 184.7°, and expansion,
Ls=184.7-193°. FFSM analysis of TES 3.7 hPa thermal data shows the
presence of eastward traveling waves at 60° S with a period of about
three sols. We hypothesize that these waves are transient baroclinic
eddies that contributed to the initiation of precursor storms near
Hellas. Integration of FFSM and MOC MY 24, 25, and 26 data shows
interesting temporal and spatial associations between the evolution of
eddies and storms, including: 1) comparable periodicities of travelling
waves and pulses of storm activity, and 2) concurrent eastward
propagation of both eddies and storms. These results suggest a causal
relationship between baroclinic eddies and local storm initiation. Based
on our analysis of these MGS data, we propose the following working
hypothesis to explain the dynamical processes responsible for PDS
initiation and expansion. Six eastward-traveling transient baroclinic
eddies triggered the MY 25 precursor storms in Hellas during
Ls=176.2–184.6° due to the enhanced dust lifting associated with their
low-level wind and stress fields. This was followed by a seventh eddy
that contributed to expansion on Ls=186.3°. Increased opacity and
temperatures from dust lifting associated with the first three eddies
enhanced thermal tides which supported further storm initiation and
expansion out of Hellas. Constructive interference of eddies and other
circulation components including sublimation flow, anabatic winds
(daytime upslope), and diurnal tides may have contributed to storm onset
in, and expansion out of Hellas.