While it is a well-known fact that dense ocean waters in the high latitudes sink to the bottom carrying dissolved atmospheric carbon with them it is not quite clear even now how and where these waters return to the surface and exhale the dissolved carbon back into the atmosphere. In a commentary piece in the journal Nature, recently, Dr. Raffaele Ferrari of MIT (Massachusetts Institute of Technology) explains some of the research and theories proposed until now.
The first hypothesis was by Walter Munk who suggested in 1966 that breaking internal waves result in mixing of denser waters with lighter waters and this mixing goes on right up to the surface. Direct measurements in the years after Munk’s paper did not detect enough mixing to bring to the surface all of the high latitude waters that sink into the abyss (calculated to require mixing at the rate of 30 x 10 raised to the power of 6 cubic metres per second).
Scientists surmised that there were areas of weak mixing where the sampling was done and the areas of intense mixing were missed. This led to competition among oceanographers to find the missing mixing.
In 1998 the amount of missing mixing was quantified on a global scale by Munk and Carl Wunsch. It was estimated that potential energy had to be supplied at a rate of roughly 0.4 tera watts (1 terawatt is 10 raised to the power of 12 watts) to continuously lift dense bottom waters to the ocean surface. During an internal-wave-breaking event, about 20 per ent of the wave energy is converted into potential energy and lifts fluid, with the rest being dissipated by inconsequential small-scale motions. Internal waves would thus have to be generated at a rate of approximately 2 TW to mix bottom waters back to the surface.
The hunt now began in earnest to find the sources contributing to the 2 TW of energy.
At that time, it was thought that internal waves were mainly generated by variable surface wind at a rate of less than 1 TW. Munk suggested, and later work confirmed, that internal waves are also generated by tidal action at a rate greater than 1 TW. More recently, it was shown that another roughly 0.5 TW is supplied by large-scale currents impinging on the bottom topography.
But just as the 2 TW requirement came tantalisingly close it was found that the mixing was limited only up to a depth of 2000 metres from the surface, not enough to exhale dissolved carbon into the atmosphere.
Then came another study in 1998 in which it was shown that a system of powerful winds which flow around Antarctica, known as the roaring forties causes the southern ocean water to upwell (rise from the bottom to the surface) and may be the answer to the question nagging scientists for so long.
The most recent perception is that mixing brings bottom waters up to about 2000 m and then they flow at that depth all the way to the southern ocean, where the roaring forties lift them to the surface. In this new scenario the potential energy needed from mixing is only half of the earlier estimate.
Mixing is strong where the bottom topography of the ocean is rough and weak where it is smooth. This heterogeneity must be mapped on a global scale to determine the amount of mixing. It has been shown that the waves break and mix preferentially along rough mid ocean ridges and continental slopes. Thus the abyssal waters make their way back to the surface along the slopes of continents and ridges.
