Isopycnal Lagrangian statistics from the North Atlantic Current RAFOS float observations

One hundred isopycnal floats were tracked on the 27.2 and 27.5 σθ surfaces in the Newfoundland Basin (NFB) from July 1993 to July 1995 to study the current structure and exchanges of waters between the subtropical and subpolar gyres. The float-mapped mean flow consists of weak flows in the NFB and a strong boundary current (the North Atlantic Current (NAC)), which separates from the boundary at the Northwest Corner, becoming a diffusive zonal drift. The NAC meanders are linked to topography and have similar patterns on the two isopycnals despite the fact that the upper layer velocities are twice as fast as the lower layer ones. Perturbation velocity from the mean is used to compute isopycnal turbulent dispersion and diffusivity. This large data set allows us to resolve a narrow mean NAC and results in a Gaussian turbulence. The turbulence approximately follows the classic Taylor dispersion theory. Integral timescales and length scales and turbulent isopycnal diffusivity are of 1.5–2.5 days, 20–30 km, and (1 − 7) × 103 m2 s−1, respectively. The timescale increases with depth and decreases with latitude, the length scale decreases with depth and longitude, and the diffusivity decreases with depth and from NAC to NFB. Compared to previous results from surface drifters and isobaric floats, our isopycnal statistics are more isotropic and agree better with the Taylor dispersion theory because (1) the mean velocity has a better resolution and (2) the isopycnal floats are better Lagrangian followers. The diffusivity scales better with the rms velocity and length scale than with the velocity variance and timescale.

[1]  Philip L. Richardson,et al.  A census of eddies observed in North Atlantic SOFAR float data , 1993 .

[2]  Geoffrey Ingram Taylor,et al.  Diffusion by Continuous Movements , 1922 .

[3]  R. Davis,et al.  Modeling eddy transport of passive tracers , 1987 .

[4]  Russ E. Davis,et al.  Observing the general circulation with floats , 1991 .

[5]  M. Prater,et al.  An Alternative Hypothesis for the Origin of the “Mediterranean” Salt Lens Observed off the Bahamas in the Fall of 1976 , 1999 .

[6]  W. Krauss,et al.  Mean circulation and eddy kinetic energy in the eastern North Atlantic , 1984 .

[7]  Edward J. Kearns,et al.  Why are the meanders of the North Atlantic Current stable and stationary? , 2000 .

[8]  Philip L. Richardson,et al.  Eddy kinetic energy in the North Atlantic from surface drifters , 1983 .

[9]  S. Bauer.  Eddy-mean flow decomposition and eddy-diffusivity estimates in the tropical Pacific Ocean , 1998 .

[10]  S. Levitus,et al.  Temporal variability of the Subarctic Front near the Charlie‐Gibbs Fracture Zone , 1996 .

[11]  Edward J. Kearns,et al.  Isopycnal RAFOS floats as roving hydrographers in the North Atlantic Current Region , 1997 .

[12]  A. Provenzale,et al.  Velocity Probability Density Functions for Oceanic Floats , 2000 .

[13]  M. Ollitrault,et al.  SOFAR Floats Reveal Midlatitude Intermediate North Atlantic General Circulation. Part I: A Lagrangian Descriptive View , 2002 .

[14]  C. Iselin.  A study of the circulation of the western North Atlantic , 1936 .

[15]  M. Swenson,et al.  Statistical analysis of the surface circulation of the California Current , 1996 .

[16]  S. Riser,et al.  Quasi-Lagrangian structure and variability of the subtropical western North Atlantic circulation , 1983 .

[17]  S. Dutkiewicz,et al.  Pathways of cross-frontal exchange in the North Atlantic Current , 2001 .

[18]  C. Böning.  Characteristics of particle dispersion in the North Atlantic: an alternative interpretation of SOFAR float results , 1988 .

[19]  S. Riser,et al.  The Western North Atlantic - A Lagrangian Viewpoint , 1983 .

[20]  A. Verdière.  Lagrangian eddy statistics from surface drifters in the eastern North Atlantic , 1983 .

[21]  Edward J. Kearns,et al.  Historical position of the North Atlantic Current , 1998 .

[22]  W. Krauss.  The North Atlantic Current , 1986 .

[23]  A. Warn-Varnas,et al.  Near-surface circulation of the Nordic seas as measured , 1996 .

[24]  W. Brechner Owens,et al.  A statistical description of the mean circulation and eddy variability in the northwestern Atlantic using SOFAR floats , 1991 .

[25]  Michael A. Spall,et al.  Advection and eddy mixing in the Mediterranean salt tongue , 1993 .

[26]  Edward R. Levine,et al.  The isopycnal swallow float—A simple device for tracking water parcels in the ocean , 1985 .

[27]  W. Krauss,et al.  Lagrangian properties of eddy fields in the northern North Atlantic as deduced from satellite-tracked buoys , 1987 .

[28]  Huai-min Zhang.  Application of an inverse model in the community modeling effort results , 1995 .

[29]  J. Price,et al.  Lateral mixing and the North Atlantic Tracer Release Experiment: Observations and numerical simulations of Lagrangian particles and a passive tracer , 1998 .

[30]  T. Rossby.  The North Atlantic Current and surrounding waters: At the crossroads , 1996 .

[31]  W. Owens.  A Synoptic and Statistical Description of the Gulf Stream and Subtropical Gyre Using SOFAR Floats , 1984 .

[32]  H. Rossby,et al.  Pathways of the North Atlantic Current from surface drifters and subsurface floats , 2001 .

[33]  H. W. Hill,et al.  Current System South and East of the Grand Banks of Newfoundland , 1980 .

[34]  Amy S. Bower,et al.  Evidence of Cross-Frontal Exchange Processes in the Gulf Stream Based on Isopycnal RAFOS Float Data , 1989 .

[35]  C. Mann.  The termination of the Gulf Stream and the beginning of the North Atlantic Current , 1967 .

[36]  R. Davis,et al.  Mid-depth recirculation observed in the interior Labrador and Irminger seas by direct velocity measurements , 2000, Nature.