Periodicity of Mars's Northern Annular Mode May Help Explain Global Dust Storm Frequency

Annular modes represent some of the largest sources of climate variability for Earth and Mars.  Annular modes are zonally symmetric patterns of variability that represent the jet stream shifting north and south in time and are associated with anomalous occurrences of atmospheric eddies, clouds, and dust.  Mars's Northern Annular Mode (MNAM) develops as anomalies of zonal-mean zonal wind emerging near the subtropics and migrating towards the pole.  New anomalies appear in the subtropics and begin propagating with a period of ~150 sols, as diagnosed from the OpenMARS reanalysis.  The propagation is induced by the interaction of the two leading empirical orthogonal functions that define the MNAM, which can be predicted using a pair of simple prognostic equations similar to what is done for Earth.  The shifting jet stream coincides with bands of anomalous surface wind stress and encourages anomalous dust optical depth.  The MNAM's internally forced periodicity on the surface wind stress combined with the seasonally forced dust cycle may help explain the inter-annual variability of global dust events, as suggested by a Monte Carlo estimate that correctly approximates the observed incidence of global dust events.  Further quantification of the other modes of internal climate variability of Mars may be possible as the record of Mars's present-day climate surpasses a decade of measurement.