What causes seasonal variation of migrating diurnal tide observed by the Mars Climate Sounder?

摘要

We use two Martian years' temperature data from Mars Climate Sounder (MCS) cross-track observations to extract the migrating diurnal tide (MDT) and investigate its spatial and seasonal variation from the perspective of classical tidal theory by Hough mode decomposition. The results suggest that during the equinox the vertically propagating (1, 1) mode is dominant at all altitudes from the near surface to ~0.1 Pa with the magnitude growing with height. The trapped modes (1, -2), (1, -4), and (1, -6) are restricted to the lower altitudes with a similar vertical structure. Both the (1, 1) and (1, -2) modes have clearly semiannual variations. The comparison between Hough modes components of MDT and dust and water ice heating rate indicates that both the dust and water ice contribute to tidal excitation. However, both the annual and semiannual variations of dust heating rate are out of phase with those in MDT, while semiannual variation of the water ice heating rate is in phase, indicating that the water ice may contribute to the semiannual variation of MDT. Using model wind results, we also find that the zonal mean zonal wind and its latitudinal shear at the low latitudes modify the vertical propagation condition of the MDT (1, 1) mode and further affect its seasonal variation. The semiannual variations of equatorial MDT and its corresponding mechanism on Mars are comparable to those on Earth, suggesting that the two planets may have more common characteristics. Plain Language Summary Tides are huge waves in the atmosphere on Mars, like that in the sea on our Earth. Scientists have noticed that tide is very important for the motions of the atmosphere and the change of climate. In this work, we study the tide which rises only once in a day and see how it varies throughout a year of Mars and find out what causes that. An instrument called Mars Climate Sounder (MCS) flies around Mars and observes it, from which we can see this tide and lots of dust and water ice in the atmosphere. We find that both dust and water ice can heat the atmosphere to excite the tide, just like throwing a stone into the water to make the waves. This tide is strong in spring and autumn but weak in summer and winter. Only heating from water ice varies like this and can explain it. Besides, the wind can also have effect on the travelling of tides. We find that changes in wind speed with various places and time can change the conditions for the tide to travel from lower places to higher places. In the spring and autumn, the tide can travel up and become strong, while in the summer and winter it cannot travel up and so becomes weak. The phenomenon of tide and its causes on Mars are like those on Earth making it more interesting to study the similarity of the two planets.