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Annual cycle of sea level height
The annual cycle of sea level height (or seasonal cycle or annual harmonic) describes the variation of sea level that occurs with a period of one year. Historically, analysis of the annual cycle has been limited by locations with tide gauge records, i.e., coastlines and some islands in the deep ocean, and by sparse records in the Southern Hemisphere. Since 1992, satellite-based altimeters have provided near global coverage of sea level variability, allowing for a more thorough understanding of the annual cycle both in the deep ocean and in coastal margins.
==Tide Gauge Era==
Prior to the introduction of satellite-based altimeters, sea level was measured by a network of tide gauges located along continental coastlines and some deep ocean islands. The first analysis of the annual cycle of sea level height on a global scale was done in 1955 by Pattullo ''et al.''〔Pattullo, J., W. Munk, R. Revelle, and E. Strong, 1955: The seasonal oscillation in sea level. ''Journal of Marine Research'', 14, 1, 88-113.〕 An update to this initial study was completed by Tsimplis and Woodworth in 1994〔Tsimplis, M.N., and P.L. Woodworth, 1994: The global distribution of the seasonal sea level cycle calculated from coastal tide gauge data. ''Journal of Geophysical Research'', 99, 16031-16039.〕 at the beginning of the satellite altimetry era. In general, amplitudes of the annual cycle are less than 150mm (6 inches) and are typically larger in the Northern Hemisphere than in the Southern Hemisphere. Annual amplitudes also vary within ocean basins and along stretches of coastline. On the European side of the North Atlantic Ocean annual amplitudes generally increase from south to north, in contrast with the American side where annual amplitudes generally decrease from south to north. Similarly, on the American side of the North Pacific Ocean annual amplitudes generally increase from south to north (with increased amplitudes in the north likely linked to meteorological forcing), while the western side of the Pacific generally sees annual amplitudes decrease with increasing latitude.〔
Determining the exact cause of the annual cycle is difficult as there are several factors that influence the amplitude and timing of the annual cycle along the coast including winds, water temperature and salinity, local bathymetry, coastal geometry, ocean currents, land movements, and river outflows.〔〔Vinogradov, S.V., and R.M. Ponte, 2011: Low-frequency variability in coastal sea level from tide gauges and altimetry. ''Journal of Geophysical Research'', 116, C07006, doi:10.1029/2011JC007034.
〕 The largest amplitudes in the annual cycle are generally noted near areas with large river outflow such as along the Bay of Bengal, the Gulf of Bohai, the St. Lawrence River, and the Rio Uruguay.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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