The solsticial pause on Mars: 2 modelling and investigation of causes
Introduction
In Lewis et al. (2016), hereafter referred to as P1, martian transient waves were diagnosed from a reanalysis of Thermal Emission Spectrometer (TES) data. A particular feature identified in the record was the ‘solsticial pause’: a weakening of waves near the surface around winter solstice of each year analysed, seen in both hemispheres.
Solsticial pauses with a range of depths have been simulated by martian global climate models (MGCMs) (Hourdin et al., 1995, Basu et al., 2006, Wang et al., 2013, Kavulich et al., 2013), and existing modelling literature suggests that the depth of a modelled solsticial pause is enhanced by the presence of one or both of a large solsticial dust loading (representative of a global dust storm) (Hourdin et al., 1995, Kuroda et al., 2007) and radiatively active water ice clouds (Wilson, 2011). The reduction in eddy activity under conditions of high dust loading is understood to be a result of changes to the zonal jet (Kuroda et al., 2007), and several important features of winter hemisphere eddies have been drawn from observational data (Wang, 2007), but a detailed description of the mechanisms behind the solsticial pause, which occurs during years both with and without a global dust storm, is currently lacking.
The midwinter minimum in North Pacific atmospheric storminess on Earth, though different in several ways to the martian solsticial pause (see P1), has been studied in detail in recent years, and some of the explanations put forward for the terrestrial case may be relevant to Mars. These include: an increase in barotropic damping around solstice (Deng and Mak, 2006); eddy dissipation through diabatic effects (Chang, 2001); and localised effects of interaction with topography (Penny et al., 2010, Park et al., 2010).
In this paper we use an MGCM to measure the net effects of several of these factors, some of which may contribute to the formation of the pause, in both hemispheres. First, in Section 2, the model used is described. In Section 3, the set of simulations are introduced, along with summary measures of the extent to which each is able to reproduce solsticial minima similar to those shown in P1. Section 4 analyses the contributions of various mechanisms to the formation of the solsticial pause, from seasonal variation in atmospheric baroclinicity, to other means of reducing eddy growth, and finally to the more fundamental impact of surface topography. In Section 5, the role of topography is discussed further, and our insights into the nature of the pause are used to provide explanation for its interannual variability as presented in P1. Finally, the key results of the paper are summarised in Section 6.
Section snippets
Model description
The model used from this study is an updated, UK version of the LMD/UK Mars Global Climate Model (UKMGCM) described in P1. This version includes a microphysical cloud scheme, which predicts ice particle sizes and growth rates taking into account temperature, humidity and local density of dust nuclei (Montmessin et al., 2004, Madeleine et al., 2012). Water vapour and ice tracers undergo parameterised turbulent diffusion, convective adjustment and gravitational sedimentation. The radiative effect
Model simulations
To test whether or not the model reproduces the reanalysis as presented in P1, and to investigate the conditions necessary for the formation of a solsticial pause, a series of simulations were performed using, in each case, a prescribed dust opacity field. The simulations are named and summarised in Table 1. Two used the ‘MY24’ dust scenario described previously, one including the radiative effects of water ice clouds () and the other neglecting clouds (). A similar pair of runs with
Changes in atmospheric baroclinicity
Having found that the inclusion or removal of water ice clouds strongly affects the ability of the MGCM to simulate the solsticial pause, the model runs were analysed more closely to determine the manner in which the clouds exert their influence. The most obvious mechanism for this is that the clouds alter the structure of the atmosphere so as to reduce baroclinicity at low levels around solstice.
The role of topography
From comparing the zonally averaged topography simulation zonav to those described in earlier sections, it is clear that the zonal asymmetry of the midlatitude topography exerts a very strong influence on transient activity in both hemispheres. In the case of the southern hemisphere, the removal of this asymmetry increases model RMS temperature during autumn and winter by around a factor of two, bringing magnitudes close to those of the northern hemisphere, and implying that, although
Conclusions
The martian solsticial pause, as presented in P1, has been convincingly simulated by the UKMGCM, when run with a realistic dust loading and the inclusion of radiatively active water ice clouds. Both increased dustiness and ice clouds alter the thermal structure of the polar atmosphere, decreasing the zonal wind and its vertical shear near the surface at mid- and high latitudes around winter solstice, causing a decrease in baroclinic growth rates.
Baroclinic eddy activity near the surface in both
Acknowledgments
The authors thank Don Banfield and an anonymous reviewer for their careful and helpful reviews. We acknowledge useful discussions with John Wilson and Luca Montabone. This work was funded by the UK Science and Technology Facilities Council.
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