Discrete eddies in the northern North Atlantic as observed by looping RAFOS floats

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 52 (2005): 627-650, doi:10.1016/j.dsr2.2004.12.011.RAFOS float trajectories near the 27.5 density level were analyzed to investigate discrete eddies in the northern North Atlantic with the objective of determining their geographical distribution and characteristics. Floats that made two or more consecutive loops in the same direction (loopers) were considered to have been in an eddy. Overall 15% (24 float years) of the float data were in loopers. One hundred and eight loopers were identified in 96 different eddies. Roughly half of the eddies were cyclonic (49%) and half were anticyclonic (51%), although the percentages varied in different regions. A few eddies were quasi-stationary for long times, one for over a year in the Iceland Basin, and many others clearly translated, often in the direction of the general circulation as observed by non-looping floats. Several floats were trapped in eddies in the vicinity of the North Atlantic Current just upstream (west) of the Charlie Gibbs (52ºN) and Faraday (50ºN) Fracture Zones, which seem to be preferred routes for flow crossing the mid-Atlantic ridge. Five floats looped in four anticyclones which translated southwestward away from the eastern boundary near the Goban Spur (47ºN-50ºN). These could have been weak meddies forming from remnants of warm salty Mediterranean Water advected northward along the eastern boundary.Funds for this research were provided by National Science Foundation grants OCE-9531877 to WHOI and OCE-9906775 to URI. This work was also supported by a grant from the WHOI Associates

Optimum fields and bounds on heat transport for nonlinear convection in rapidly rotating fluid layer

By means of the Howard-Busse method of the optimum theory of turbulence we investigate numerically the effect of strong rotation on the upper bound on the convective heat transport in a horizontal fluid layer of infinite Prandtl number Pr. We discuss the case of fields with one wave number for regions of Rayleigh and Taylor numbers R and Ta where no analytical asymptotic bounds on the Nusselt number Nu can be derived by the Howard-Busse method. Nevertheless we observe that when R > 108 and Ta is large enough the wave number of the optimum fields comes close to the analytical asymptotic result α1 = (R/5)1/4. We detect formation of a nonlinear structure similar to the nonlinear vortex discussed by Bassom and Chang [Geophys. Astrophys. Fluid Dyn. 76, 223 (1994)]. In addition we obtain evidence for a reshaping of the horizontal structure of the optimum fields for large values of Rayleigh and Taylor numbers