Mixed layers and surface ducts

The upper ocean is strongly effected by the sun and the wind. In an idealized view, this leads to a warm mixed layer with a nearly constant temperature and salinity. Then, the increase in pressure with depth causes an increase of about 1.7 m/s in sound speed for every 100 m in depth (i.e., a gradient of 0.017/s). This gradient forms an acoustic surface duct, which is a nearly universal feature of the world’s oceans. They can be just tens of meters thick or several hundred meters in the Southern Ocean or the North Atlantic during periods of winter storms. Not surprisingly, the surface duct is very important in terms of sound propagation; However, as we shall describe, the surface duct has some peculiar acoustic properties and can produce vast shadow zones inside the duct. This simplified description is a good beginning; however, “mixed” layers are generally not well-mixed. It is more precise to speak of the upper ocean as a zone where there may be periods of mixing and heating driven by changes in winds and cloud cover. Upper ocean models predict this evolution and often lead to much more complicated sound speed structures. This, in turn, has lead to a wide variety of ways of characterizing the features of the surface duct. This talk will touch on the interesting acoustics of surface ducts and how variations in different (oceanographic) upper ocean models effect the sound propagation.