The surface waters of the ocean have a net flow from low to high latitudes with an associated poleward heat flux that is central in establishing the earth’s climate. Some fraction of the high latitude waters are cooled sufficiently (and increased in salinity by the freezing of sea ice) that they sink deep into the sea, perhaps to the sea floor. These basic elements of the so-called thermohaline circulation of the ocean have been known for more than a century. How the deep waters eventually return to the surface and the degree to which they gain heat / lose salt as they return is much less clear.

As with the human circulatory system, where arteries and veins are prominent structures, but the capillaries that link them are microscopic, the processes that govern the evolution of the deep waters on their return to the surface occur at much smaller temporal and spatial scales than that of the general circulation.

A key player in the process of water mass evolution in the deep sea is the internal gravity wave-field. Internal waves are the deep-sea analog of the sea surface waves we are familiar with. Their existence is enabled by the weak vertical density gradient found in the ocean interior. With horizontal scales of meters to hundreds of kilometers, these waves can carry energy and momentum both horizontally and vertically in the sea. They can also break, leading to “internal surf,” deep-ocean turbulence that modifies the temperature and salinity of water parcels.

Searching for deep-ocean turbulence and understanding where and why it occurs has been a major research effort over the past five decades. In the seminar I will present a somewhat personal view of the evolution of this research, emphasizing instrumentation advances and observations. Technical developments range from the creation of very high speed electric winches to Doppler sonars for the remote sensing of ocean currents and to high-resolution probes for measuring turbulent fluctuations. In concert with these developments, numerous observational campaigns have been fielded, both in typical deep-sea conditions and at ocean-mixing “hot spots” that are just now being discovered around the globe. At such special sites, internal waves of tidal frequency go unstable, forming undersea breakers 100 m high. The breakers evolve in ‘slow motion’, taking a half hour to collapse and leading to elevated levels of deep-sea turbulence that persist for three hours or more.

The physical disconnect between the processes that cause dense waters to sink at high latitudes and the processes that mix heat downward at low latitudes to enable the return-circulation is striking. Stabilizing feed-back mechanisms that link these two branches of the thermohaline circulation have yet to be identified, rendering this an exciting frontier of research.