Royal Netherlands Institute for Sea Research, Den Burg, The Netherlands.
PLoS One. 2012;7(3):e32535. doi: 10.1371/journal.pone.0032535. Epub 2012 Mar 5.
A summer bather entering a calm sea from the beach may sense alternating warm and cold water. This can be felt when moving forward into the sea ('vertically homogeneous' and 'horizontally different'), but also when standing still between one's feet and body ('vertically different'). On a calm summer-day, an array of high-precision sensors has measured fast temperature-changes up to 1 °C near a Texel-island (NL) beach. The measurements show that sensed variations are in fact internal waves, fronts and turbulence, supported in part by vertical stable stratification in density (temperature). Such motions are common in the deep ocean, but generally not in shallow seas where turbulent mixing is expected strong enough to homogenize. The internal beach-waves have amplitudes ten-times larger than those of the small surface wind waves. Quantifying their turbulent mixing gives diffusivity estimates of 10(-4)-10(-3) m(2) s(-1), which are larger than found in open-ocean but smaller than wave breaking above deep sloping topography.
一个在海滩上进入平静海面的夏日游泳者可能会感觉到交替的温水和冷水。当人向前进入大海时（“垂直均匀”和“水平不同”），会感觉到这种情况，而当人站在两脚之间和身体之间时（“垂直不同”），也会感觉到这种情况。在一个平静的夏日，一系列高精度传感器已经在特塞尔岛（荷兰）海滩附近测量到高达 1°C 的快速温度变化。测量结果表明，所感觉到的变化实际上是内部波、锋和湍流，部分受到密度（温度）垂直稳定分层的支持。这种运动在深海中很常见，但在浅海中通常不会发生，因为人们预计那里的湍流混合足够强烈，可以使海水均匀化。内部海滩波浪的振幅比小的表面风浪大十倍。对其湍流混合进行定量分析，得出的扩散系数估计值为 10(-4)-10(-3) m(2) s(-1)，这比开阔海域中的值大，但比深斜坡地形上方的波浪破碎值小。
