A CLIMATOLOGY OF THE MARTIAN NORTHERN POLAR VORTEX.
P. M. Streeter, School of Physical Sciences, The Open University, Milton Keynes, U.K. (paul.streeter@open.ac.uk),
S. R. Lewis, J. A. Holmes, K. Rajendran, School of Physical Sciences, The Open University, Milton Keynes, U.K.,
M. R. Patel, School of Physical Sciences, The Open University, Milton Keynes, U.K., Space Science and Technology
Department, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Oxfordshire, U.K..

Understanding the behaviour of Mars’ polar vortices,
and their modification under different dust loading conditions, is important for understanding its contemporary
and ancient climate, including aerosol and chemical cycles. We present an eight martian year climatology of
the northern polar vortex, via the assimilation of orbital
temperature and dust observations into a global climate
model. We show that the general seasonal behaviour
and morphology of the vortex is highly interanually repeatable, with the important exception of dust storm
effects. Previous work has focused on individual regional or global dust storm effects, specifically solstitial
events for the latter; by investigating an eight-year period
covering two global and multiple regional dust storms,
we are able to investigate the potentially crucial impact
of dust storm timing on vortex effects as suggested by
previous work [10]. We find that the seasonal timing
of atmospheric dust loading does indeed have a crucial
role in determining the dynamical effects of a storm
on the northern polar vortex. Storms later in the dusty
perihelion season, such as solstitial global dust storms
and C-type regional dust storms [11], have a significantly greater disruptive effect on both the morphology
(including latitudinal extent) and intensity of the vortex
than those early in the season, such as equinoctial global
dust storms and A-type regional dust storms.

Introduction
Mars’ winter atmosphere is characterized by the presence of a region of low temperatures around the winter
pole, surrounded by a powerful westerly jet; this region is known as the polar vortex [1, 2]. Mars’ polar
vortices are important components of the global atmospheric circulation. The cold temperatures enable the
condensation of CO2 , dramatically altering the global
atmospheric mass budget on a seasonal timescale [3],
while the high westerly winds form an effective barrier to intra-polar transport of suspended aerosol and
chemical species including water vapour [4] while also
controlling the shape of the polar hood cloud belt via
planetary wave activity [5]. In addition, understanding the polar vortices may lead to insights into Mars’
historical climate, recorded in layers of dust and ice at
the permanent polar ice caps [6]. Mars’ polar vortices
are therefore intimately linked to many of the key concerns of martian atmospheric science, from elemental

and tracer cycles through global dynamics to paleoclimate. We present results which show the seasonal trends
in behaviour of Mars’ contemporary northern polar vortex, and the highly seasonally dependent effects of dust
storms on the vortex’s intensity and morphology.
Both north and south polar vortices have a distinctive
annular shape, differentiating them from Earth’s vortices, with this annularity thought to be caused by latent
heat release from CO2 condensation over the seasonal
cap [7, 8, 9]. The northern vortex in particular also
has an elliptical morphology [1], linked to stationary
wave activity forced by the zonal topographic structure
of Mars’ northern mid-latitudes [2, 8, 10].
Mars’ polar vortices appear to have a complicated
relationship with atmospheric dust loading, including
regional and global-scale dust storms. The northern
vortex was found in the MACDA reanalysis to be substantially weakened and shifted off pole by ∼10° latitude by a late season (C-type [11]) regional dust storm
[2]. Atmospheric modelling work using different prescribed dust distributions found that solstitial regional
and Global Dust Storms (GDS) could disrupt the northern polar vortex for up to 10s of sols, via the mechanism
of shifting the descending Hadley cell branch poleward
[12]. An analysis of the OpenMARS reanalysis dataset
found that the northern vortex was in general highly
sensitive to atmospheric dust loading, with the solstitial MY 28 GDS in particular substantially reducing the
intensity of the vortex [13]. On the other hand, work
using a reanalysis of the MY 34 equinoctial GDS found
that the northern polar vortex, while it was altered in
morphology and extent, remained relatively robust despite the extreme atmospheric dust loading [10]. These
results suggest that the seasonal timing of large dust
events could be crucial in determining their effects on
the northern polar vortex.
Large, planet-scale dust events are regular occurrences in Mars’ atmosphere. Regional-scale dust storms
have been observed to occur every martian year during Mars’ dusty (perihelion) season, in a sufficiently
repeatable fashion so as to be classifiable into so-called
A, B, and C-type regional dust storms [11]. B-storms
are confined to the southern polar cap edge and so are
unable to have any significant dynamical effect on the
northern polar vortex. A and C-storms, however, tend
to occur at tropical/mid-latitudes, allowing them to impact the northern vortex dynamically via modification
of the meridional overturning circulation. By defini-

tion A-storms occur early in the dusty season, while
C-storms occur towards its end. A reanalysis of these
events would therefore allow for the investigation of the
effects of dust storm seasonal timing on northern polar
vortex impacts.
More generally, reanalyses using observations from
the Mars Climate Sounder (MCS) now have access to
nearly eight full martian years’ worth of MCS retrievals,
covering a period containing two GDS and many regional dust storms. This provides an opportunity for
study of the climatological, yearly behaviour of Mars’
northern polar vortex; specifically, both its seasonally
repeatable behaviour as well as interannual variability
associated with dust storms. Understanding both the
morphology (including latitudinal extent) and intensity
of the vortex under different dust loading conditions
could have significant implications for the transport of
atmospheric tracers into the polar regions.

riod from when MCS began observing in MY 28 until
the end of MY 35.

Potential vorticity diagnostic
Ertel potential vorticity (PV) is a valuable diagnostic for
studies of the polar vortex. PV is a measure of air circulation based on the relative vorticity and stratification of
the atmosphere; it is also conserved under adiabatic processes, making it especially useful for diagnosing polar
dynamics. The polar vortices themselves can be defined
as regions of high PV around the poles. For this work,
PV is defined on isentropic surfaces (surfaces of constant potential temperature), as the PV of air masses on
isentropic surfaces is conserved and cannot be created
or destroyed [22]. Analysing isentropic PV enables investigation of the morphology of Mars’ polar vortex, its
intensity, and how it is affected by factors such as dust
storms.

Model, assimilation scheme, and retrievals used
We use the Open University’s Mars Global Climate
Model (MGCM), which is the result of a collaboration
between the Laboratoire de Météorologie Dynamique,
The Open University, the University of Oxford, and the
Instituto de Astrofı́sica de Andalucı́a [14]. The MGCM
solves the equations of fluid dynamics, together with
parametrized radiative and other physics, to calculate
the state of Mars’ atmosphere. The MGCM contains a
spectral dynamical core and a semi-Lagrangian tracer
transport scheme [15]. For this study, the MGCM was
run at a spectral resolution of T42, with 50 vertical levels.
The assimilation scheme used is a modified version
of the UK Met Office developed Analysis Correction
scheme [16], adapted for the martian atmosphere [17].
This method is computationally inexpensive, allowing it
to be be run at a similar cadence to the MGCM dynamical core. The scheme’s use of the repeated insertion
method also helps to counteract a common issue with
assimilation schemes for Mars, that of the low thermal
inertia of the atmosphere causing rapid relaxation of the
assimilated atmospheric state. Dust is assimilated as
column dust optical depth (CDOD) from MCS CDOD
products, with MGCM dust tracers being scaled to match
the assimilated CDOD [18]. The vertical distribution of
dust is allowed to evolve freely. Temperature is assimilated in the form of MCS temperature profiles, following
previous work with the same MGCM and assimilation
scheme [19].
The retrievals assimilated into the MGCM were from
MCS [20, 21] aboard the Mars Reconnaissance Orbiter
(MRO) , which has now amassed close to eight full martian years worth of atmospheric observations. For this
study, the assimilated MCS outputs were temperature
profiles and derived CDOD products, covering the pe-

Results and discussion
We present the results of a reanalysis of close to eight full
martian years of MCS temperature profiles and derived
CDOD products into the MGCM, focussing specifically
on the behaviour of the northern polar vortex. We show
the interannually repeatable structure of the northern
polar vortex, from its inception to decay, and how its
morphology (including annularity and latitudinal extent)
varies seasonally. We relate this behaviour to the general
circulation of the martian atmosphere and the growth and
decay of the northern seasonal CO2 cap.
We then demonstrate how the northern polar vortex is affected by tropical and mid-latitude dust storm
activity, specifically A and C-type regional dust storms
and the GDS of MY 28 and 34. We examine both the
zonally averaged PV impacts as well as the longitudinal distribution of PV over the north polar region. A
and C-type regional dust storms in particular provide a
natural experiment regarding the effects of seasonal timing on PV disruption, given roughly similar latitudinal
distributions and CDOD of the storms themselves.
Figure 1 shows northern mid-high latitude PV on the
300 K isentropic surface together with tropical CDOD.
The general pattern of the northern polar vortex is seen
to show a high degree of interannual repeatability, with
the exception of large dust loadings (discussed below).
Roughly speaking, the vortex (on this isentropic surface) first appears between LS =150-180°, and grows in
intensity. However, it only gains its characteristic annular PV structure from approximately LS =210°; this
lasts until approximately LS =330°. In Fig. 1, this is
visible as a peak in PV at around 80° N, with a local
PV minimum over the north pole itself. The peak in PV
intensity occurs between approximately LS =210-330°.

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While these general trends hold for most of the MYs
presented here, there is apparent interannual variability,
linked to either gaps in the MCS data for assimilation
or high tropical atmospheric dust loading. The latter are
discussed below.

causing greater disruption of the vortex (specifically,
compression towards higher latitudes) even when they
have similar CDOD magnitudes (for example, compare
the A-storm at around LS =220-250° and the C-storm at
around LS =320-340° of MY 33).

Conclusions
Mars’ polar vortices are crucial components of its global
atmospheric circulation, and affect the transport of aerosols
and chemical species. Understanding their behaviour,
and how they are modified by the ubiquitous martian
phenomena of dust storms, is therefore of great importance for general studies of the martian atmospheric system. We present results from assimilation of eight martian years’ worth of MCS observations into an MGCM to
explore the general behaviour and climatology of Mars’
northern polar vortex.

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