MARCI observations of a wavenumber-2 large-scale feature in the north polar hood of Mars: Interpretation with the NASA/Ames Legacy Global Climate Model

Abstract

The Mars Color Imager on the Mars Reconnaissance Orbiter has observed a prominent Fourier wavenumber-2 planetary-scale pattern in the north polar hood clouds that appears in early fall, strengthens as the seasons progress, becomes most pronounced during midwinter, and then disappears by mid-spring as the clouds dissipate. We use the NASA/Ames Legacy Global Climate Model, which is supported by the Mars Climate Modeling Center, to interpret this feature and find that it is caused by a trapped, yet deep, mechanically forced quasi-stationary wavenumber-2. We show that the perturbation temperature and wind fields set up by this forced wave control the boundaries of the polar hood, locking them to fixed topographic ridges and troughs. These perturbation fields also determine the longitudes where the polar hood clouds are thickest, which is generally in the cold sectors set up by the stationary wave. Moisture advection during midwinter, however, changes this pattern, leading to warm regions having the thickest clouds. Observations do not show this pattern, though there is some ambiguity in the data. The source of moisture for the polar hood during midwinter is water released by the retreating south polar seasonal CO2 ice cap, which is carried into the Northern Hemisphere by different components of the global circulation, and in different forms. The model also predicts a seasonal variation in the annular structure of the polar hood for which existing observations are unable to confirm. We discuss the implications of these results and speculate on explanations for the apparent model/observation discrepancies.