[1] Surface drifters deployed in the subtropical and subpolar North Atlantic from 1990 to 2002 show almost no connection between the subtropical and subpolar gyres; only one drifter crosses the intergyre boundary even though other data types (e.g., dynamic topography and tracers) suggest a major connection. Two of several possible causes for the lack of intergyre connectivity in this two-dimensional data set are examined: (1) undersampling and short drifter lifetime leading to underestimation of the northward flow, and (2) the southward mean Ekman velocity. Advection of a large number of long-lived synthetic drifters through the observed mean velocity results in a 5% increase in cross-gyre flux compared with that for synthetic drifters with realistic lifetimes. By further advecting synthetic drifters through the observed mean velocity field with and without the Ekman component, estimated from the wind field associated with the actual drifters, it is shown that removal of the Ekman component further increases the intergyre flux by up to 6%. With a turbulent component added to the mean velocity field to simulate the eddy field, there is a further increase in connection by 5%. Thus the Ekman and eddy contributions to the drifter trajectories nearly cancel each other. Consideration of three-dimensional processes (subduction and obduction) is reserved for complete modeling studies.

[1] Subpolar Mode Waters (SPMW) in the eastern North Atlantic subpolar gyre are investigated with hydrographic and Lagrangian data (surface drifters and isopycnal floats). Historical hydrographic data show that SPMWs are surface water masses with nearly uniform properties, confined between the ocean surface and the permanent pycnocline. SPMWs represented by densities 27.3s q , 27.4s q , and 27.5s q are present in the eastern subpolar gyre and are influenced by the topography and the regional circulation. Construction of an absolute surface stream function from surface drifters shows that SPMWs are found along the mean path of each of the several branches of the North Atlantic Current (NAC) and their density increases gradually downstream. The Rockall Trough branch of the NAC carries 27.3s q , 27.4s q , and 27.5s q SPMW toward the IcelandFaroe Front. In the Iceland Basin, the Subarctic Front along the western flank of the Rockall Plateau carries a similar sequence of SPMW. The western side of the Central Iceland Basin branch of the NAC, on the other hand, veers westward and joins the East Reykjanes Ridge Current, feeding the 27.5s q SPMW on the Reykjanes Ridge. The separation among the various NAC branches most likely explains the different properties that characterize the 27.5s q SPMW found on the Reykjanes Ridge and on the IcelandFaroe Ridge. Since the branches of the NAC have a dominant northeastward direction, the newly observed distribution of SPMW combined with the new stream function calculation modify the original hypothesis of McCartney and Talley (1982) of a smooth cyclonic pathway for SPMW advection and density increase around the subpolar gyre.

[1] The processes that lead to the transformation and origin of the eastern North Atlantic Subpolar Mode Waters (SPMW) are investigated from observational data using an extended Walin framework. Air-sea flux data from the National Oceanography Center, Southampton (NOCS), and hydrographic data from the A24 cruise collected during the World Ocean Circulation Experiment (WOCE) are used to estimate the contribution of diapycnal and isopycnal fluxes to the density classes that include SPMW. Surface diapycnal volume flux is the dominant source of waters in the SPMW density. In the North Atlantic subpolar gyre the diapycnal volume flux occurs along the main branches of the North Atlantic Current (NAC) and it has an average transport of 14 ± 6.5 Sv, with a maximum of 21.5 Sv across the 27.35s q isopycnal. The regional distribution of the diapycnal flux on isopycnal surfaces is computed to identify the areas with the largest diapycnal flux. These regions coincide with the location of SPMW based on potential vorticity. The surface diapycnal flux is associated with obduction and subduction through the permanent pycnocline. Therefore, the water involved in the transformation of SPMWs is continuously exchanged with the ocean interior. In addition, we suggest that subduction is not associated with smooth advection from the mixed layer to the ocean interior, but is water mass loss entrainment into the deep overflows of the subpolar gyre. The isopycnal component of the SPMW throughput is estimated from the geostrophic transport across the A24 section from Greenland to Scotland and is 10% to 40% of the diapycnal flux.

[1] For the Mediterranean Sea a large set of historical conductivity-temperature-depth casts has been investigated for the occurrence of double-diffusive vertical mixing due to salt finger convection. All casts were screened in terms of the density ratio R r , providing probability distributions of R r for the upper 1000-dbar range of eight geographical areas. Vertical profiles of the salt finger-driven diffusivities k S and k T of salinity and temperature have been evaluated from the statistics of R r and a presumed mixing law, and analytic expressions are provided, which can be used to parameterize the effects of salt finger mixing in ocean circulation models of the Mediterranean. In most areas the diffusivity profiles exhibit an absolute maximum below the core of the Levantine Intermediate Water. Toward the surface the diffusivities decrease rapidly, while the downward decay is less pronounced. A different behavior is found in the Tyrrhenian and Adriatic Seas, where the diffusivities remain high also at greater depth. INDEX TERMS:4243 Oceanography: General: Marginal and semienclosed seas; 4524 Oceanography: Physical: Fine structure and microstructure; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: Mediterranean, double diffusion, salt fingering, vertical mixing, density ratio Citation: Onken, R., and E. Brambilla, Double diffusion in the Mediterranean Sea: Observation and parameterization of salt finger convection,