Intermediate and deep currents of the Black Sea obtained from autonomous profiling floats

Float trajectories observed at three depths (200, 750 and 1550 m) in the Black Sea during a 1.5-yr period offer new insights on the circulation below the permanent pycnocline. The float observations for the first time provided direct, quantitative evidence for strong currents and a well-organized flow structure at intermediate and deep layers, in contrast to prior ideas of a rather sluggish deep circulation of the Black Sea. The magnitudes of intermediate and deep currents are typically about 5–10% of the surface currents, reaching as much as 5 cm s−1 at 1550 m, adjacent to the steep topographic slope on the periphery. The combination of float and sea-level height altimeter data suggests that a well-defined cyclonic circulation extends from the surface to the bottom, without reversal in its direction. Deep currents are steered by the steep topographic slope, and well correlated with surface currents at seasonal and longer time scales.