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Abyssal Flows and Mixing

posted Jul 6, 2011, 1:29 PM by mikutas   [ updated Dec 20, 2012, 10:25 AM by Matthew Alford ]
The abyssal ocean was once thought to be stagnant.  Recent work is showing it to be anything but.  This is important since the water that has sunk at the poles must return via tortuous routes to lower latitudes - and then must upwell to complete the loop.  This flow is known as the "conveyor belt" or the "meridional overturning circulation," and is a key element of the ocean's circulation and a vital part of the Earth's climate.  This upwelling appears to occur at least in part due to turbulent mixing - which warms the cold water by mixing with the warmer water above.  This mixing seems to happen at special "hotspots."  This is why internal waves matter (since their breaking often leads to the mixing).  Other processes such as hydraulics are also important.

Observations in the Samoan Passage (Figures 1-2), part of our ongoing work in a critical "choke point" in the conveyor belt.
Figure 1: A breaking lee wave as water spills over a sill 5000 m beneath the surface in the Samoan Passage.  (Top) temperature (colors) and "Thorpe temperatures" (profiles) - think of jagged profiles as showing turbulence as colder water has been swept above warmer water.  (Bottom) velocity (colors) and turbulent dissipation estimated from these overturns (contours).  

Figure 2: Map of the Samoan Passage bathymetry and vectors showing our measurements of the transport of the coldest water (colors refer to two temperature classes).

These flows are also very intermittent in time, as seen by observations from another of our moorings north of Hawaii.  A strong cold flow detected at our profiling mooring at station ALOHA (figure 3) showed a 250-m-high overturn, indicating mixing 10,000 times stronger than usually seen in the open ocean - but only once in a 2-1/2 year record.  This suggests that mixing in the deep flows is intermittent not only in space, but time too.

Figure 3: A cold event at station ALOHA detected in October 2010.  (Upper left) Bathymetry and flow vectors.  Gray indicates depth < 4400 m.  Arrows are mean velocity averaged from 4000 m to the sill depth from the selected time periods (indicated at upper right).  (Bottom) Entire 2.5-year time-series from the combined thermistor chain records.   (Upper right) measured eastward and northward velocity.