27.3-day and 13.6-day atmospheric tide
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An analysis of time variations in the earth’s length of day (LOD) for 25 years (1973–1998) versus atmospheric circulation changes and lunar phase is presented. It is found that, on the average, there is a 27.3-day and 13.6-day period oscillation in global zonal wind speed, atmospheric geopotential height, and LOD following alternating changes in lunar phase. Every 5–9 days (6.8 days on average), the fields of global atmospheric zonal wind and geopotential height and LOD undergo a sudden change in relation to a change in lunar declination. The observed atmospheric oscillation with this time period may be viewed as a type of atmospheric tide.
Ten atmospheric tidal cases have been analyzed by comparing changes in LOD, global zonal wind speed and atmospheric geopotential height versus change in lunar declination. Taken together these cases reveal prominent 27.3-day and 13.6-day tides. The lunar forcing on the earth’s atmosphere is great and obvious changes occur in global fields of zonal wind speed and atmospheric geopotential height over the equatorial and low latitude areas.
The driving force for the 27.3-day and 13.6-day atmospheric tides is the periodic change in lunar forcing during the moon’s revolution around the earth. When the moon is located on the celestial equator the lunar declination equals zero and the lunar tidal forcing on the atmosphere reaches its maximum, at this time the global zonal wind speed increases and the earth’s rotation rate decreases and LOD increases. Conversely, when the moon reaches its most northern or southern positions the lunar declination is maximized, lunar tidal forcing decreases, global zonal wind speed decreases, earth’s rotation rate increases and LOD decreases.
27.3-day and 13.6-day period atmospheric tides deserve deeper study. Lunar tidal forcing should be considered in models of atmospheric circulation and in short and medium range weather forecasting.
Keywordsatmospheric tide earth’s rotation subseasonal oscillation atmospheric circulation lunar forcing
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