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Equatorial waves are ocean waves trapped close to the equator, meaning that they decay rapidly away from the equator, but can propagate in the longitudinal and vertical directions.〔Holton, James R., 2004: ''An Introduction to Dynamic Meteorology''. Elsevier Academic Press, Burlington, MA, pp. 394–400.〕 Wave trapping is the result of the Earth's rotation and its spherical shape which combine to cause the magnitude of the Coriolis force to increase rapidly away from the equator. Equatorial waves are present in both the tropical atmosphere and ocean and play an important role in the evolution of many climate phenomena such as El Niño. Many physical processes may excite equatorial waves including, in the case of the atmosphere, diabatic heat release associated with cloud formation, and in the case of the ocean, anomalous changes in the strength or direction of the trade winds.〔 Equatorial waves may be separated into a series of subclasses depending on their fundamental dynamics (which also influences their typical periods and speeds and directions of propagation). At shortest periods are the equatorial gravity waves while the longest periods are associated with the equatorial Rossby waves. In addition to these two extreme subclasses, there are two special subclasses of equatorial waves known as the mixed Rossby-gravity wave (also known as the Yanai wave) and the equatorial Kelvin wave. The latter two share the characteristics that they can have any period and also that they may carry energy only in an eastward (never westward) direction. The remainder of this article discusses the relationship between the period of these waves, their wavelength in the zonal (east-west) direction and their speeds for a simplified ocean. ==Equatorial Rossby and Rossby-gravity waves== Rossby-gravity waves, first observed in the stratosphere by M. Yanai,〔Yanai, M. and T. Maruyama, 1966: Stratospheric wave disturbances propagating over the equatorial pacific. J. Met. Soc. Japan, 44, 291–294. https://www.jstage.jst.go.jp/article/jmsj1965/44/5/44_5_291/_article〕 always carry energy eastward. But, oddly, their 'crests' and 'troughs' may propagate westward if their periods are long enough. The eastward speed of propagation of these waves can be derived for an inviscid slowly moving layer of fluid of uniform depth H.〔Zhang, Dalin, 2008: Personal Communication, “Waves in Rotating, Homogeneous Fluids,” University of Maryland, College Park (not a WP:RS)〕 Because the Coriolis parameter (''ƒ'' = 2Ω sin(θ) where Ω is the angular velocity of the earth, 7.2921 10−5 rad/s, and θ is latitude) vanishes at 0 degrees latitude (equator), the “equatorial beta plane” approximation must be made. This approximation states that “f” is approximately equal to βy, where “y” is the distance from the equator and "β" is the variation of the coriolis parameter with latitude, .〔 With the inclusion of this approximation, the governing equations become (neglecting friction): * the continuity equation (accounting for the effects of horizontal convergence and divergence and written with geopotential height): :: * the U-momentum equation (zonal wind component): :: * the V-momentum equation (meridional wind component): ::.〔 These three equations can be separated and solved using solutions in the form of zonally-propagating waves, which are analogous to exponential solutions with a dependence on ''x'' and ''t'' and the inclusion of structure functions that vary in the ''y''-direction: :.〔 Once the frequency relation is formulated in terms of ω, the angular frequency, the problem can be solved with 3 distinct solutions. These three solutions correspond to the equatorial gravity waves, the equatorially trapped Rossby waves and the mixed Rossby-gravity wave (which has some of the characteristics of the former two).〔 Equatorial gravity waves can be either westward- or eastward-propagating. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Equatorial waves」の詳細全文を読む スポンサード リンク
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