Hydrography, Circulation, and Mixing at the Calypso Deep (the Deepest Mediterranean Trough) during 2006–09

The mass and flow fields from June 2006 to May 2009 in the Calypso Deep (bottom depth ~5.2 km) are investigated using eddy-resolving surface-to-bottom hydrography (station grid spacing ~0.2°) and two tall moorings yielding current-meter records at depths from 700 m to near bottom. A salty warm lens (excess core salinity and temperature are ~0.01 and 0.025°C relative to the surrounding water) of Cretan Deep Water with a core at ~3000 m and a horizontal (vertical) scale of ~50 km (1.5 km) is identified in June 2006 to be locked over the trough. The lens coincides with local maxima in dissolved oxygen. In October 2006 the salinity content of the lens and of all deeper layers is increased; the oxygen maxima are shifted to the bottom layers, indicating an episodic intrusion of higher-density ventilated Adriatic water. The circulation changes from anticyclonic at all depths in June 2006 to cyclonic below ~2.5 km in October 2006, whereas after January 2007 it is cyclonic at all instrumented depths. The measured currents are weak (mean speeds < 5 cm s −1 ) and persistent in direction, being mostly along the bottom topography at all current-meter depths. After October 2006, the lens erodes due to salt/heat loss caused predominantly by lateral (intrusive) mixing, which works from the outside toward the lens center. The horizontal diffusivity is on the order of ~10 m 2 s −1 , near the center of the lens, and ~10 2 to 10 3 m 2 s −1 , at its periphery, with an average error ~15 times the diffusivity value. In the deepest part of the trough and in periods of predominance of vertical mixing the vertical diffusivity at 4400 m is ~(4 ± 3) × 10 −3 m 2 s −1 .