Overflow into the deep Caribbean: Effects of plume variability

The deepest connection between the eastern Caribbean and the Atlantic is over the Jungfern-Grappler Sill complex at 1815 m depth. Through these gaps flows the sole source of water for the deep Caribbean, presumably balanced by diffusively driven upwelling elsewhere in the basin. Fourteen-month-long moored observations at the sills in 1991-1992 reveal a mean transport of 0.11+/-0.05 Sv (1 Sv 10(6) m(3) s(-1)) of Atlantic water colder than theta=3.965 degrees C flowing into the Caribbean, one quarter of which comes over the previously unmeasured Grappler Sill. This is about twice the transport seen in previous, shorter experiments. The overflow is highly episodic, with similar to 10 "events" per year, The range of overflow density is comparable to the total vertical stratification of the deep Caribbean. A numerical stream-tube model of the overflow plume is run with 13 different initial conditions, representing the observed range of overflow strength and density. Results indicate that much of the overflow only penetrates to 2200-3000 m depth and was dense enough to descend to the bottom of the Caribbean only similar to 1% of the time. Combined plume model results are used to drive a model of the basin stratification. A steady balance is found to be possible with small diapycnal diffusivity in the basin, between 0.1 and 0.6 cm(2) s(-1). The reason for the small diffusivity is that the stratification is essentially built in place by the different detrainment depths, and so less diffusive transformation is needed.