Investigating the semiannual oscillation on Mars using data assimilation

Tao Ruan,Neil T Lewis,Stephen R Lewis,Luca Montabone,Peter L Read

doi:10.1016/j.icarus.2019.06.012

Tao Ruan, Neil T Lewis + Show 3 more

Open Access

https://doi.org/10.1016/j.icarus.2019.06.012

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Abstract

A Martian semiannual oscillation (SAO), similar to that in the Earthâ€™s tropical stratosphere, is evident in the Mars Analysis Correction Data Assimilation reanalysis dataset (MACDA) version 1.0, not only in the tropics, but also extending to higher latitudes. Unlike on Earth, the Martian SAO is found not always to reverse its zonal wind direction, but only manifests itself as a deceleration of the dominant wind at certain pressure levels and latitudes. Singular System Analysis (SSA) is further applied on the zonal-mean zonal wind in different latitude bands to reveal the characteristics of SAO phenomena at different latitudes. The second pair of principal components (PCs) is usually dominated by a SAO signal, though the SAO signal can be strong enough to manifest itself also in the first pair of PCs. An analysis of terms in the Transformed Eulerian Mean equation (TEM) is applied in the tropics to further elucidate the forcing processes driving the tendency of the zonal-mean zonal wind. The zonal-mean meridional advection is found to correlate strongly with the observed oscillations of zonal-mean zonal wind, and supplies the majority of the westward (retrograde) forcing in the SAO cycle. The forcing due to various non-zonal waves supplies forcing to the zonal-mean zonal wind that is nearly the opposite of the forcing due to meridional advection above ∼3 Pa altitude, but it also partly supports the SAO between 40 Pa and 3 Pa. Some distinctive features occurring during the period of the Mars year (MY) 25 global-scale dust storm (GDS) are also notable in our diagnostic results with substantially stronger values of eastward and westward momentum in the second half of MY 25 and stronger forcing due to vertical advection, transient waves and thermal tides.Mars; Mars, atmosphere; Mars, climate; Atmospheres, dynamics; Atmospheres, structure; Meteorology.

Highlights

In the upper stratosphere and mesosphere of Earth, the semiannual oscillation (SAO) of the mean zonal wind in the tropical stratosphere and mesosphere is commonly observed as a clear feature
Suspended dust particles are widespread in the Martian atmosphere, and they strongly affect its dynamical and thermal structure
The dataset used in this study for diagnosing the Martian SAO is the MGS-TES Mars Analysis Correction Data Assimilation (MACDA) version 1.0 (Montabone et al, 2011, 2014)

Summary

Introduction

In the upper stratosphere and mesosphere of Earth, the semiannual oscillation (SAO) of the mean zonal wind in the tropical stratosphere and mesosphere is commonly observed as a clear feature Various numerical modeling studies have proved capable of reproducing the full seasonal variability of the Martian climate, but MGCMs still struggle to fully represent the realistic interannual variability of the Martian climate system (Newman et al, 2002a,b; Basu et al, 2004, 2006; Mulholland, 2012; Mulholland et al, 2013) In this context, data assimilation offers another approach towards obtaining a four-dimensional representation of atmospheric behavior that has the additional advantage of being consistent with both Martian observations and modeled physical constraints (Lewis and Read, 1995; Montabone et al, 2006; Lewis et al, 2007). A further question that can be answered by this observationallybased approach is whether components of an SAO can be observed outside of the Martian tropics

Data and methodology

Revealing the phenomenology of the observed SAO

Findings

Disussion