Gulf Stream

Near-surface velocity and PV structure of the Gulf Stream
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Velocity Structure >> Potential Vorticity Structure >> Eddy Exchange Processes >> Discussion and Conclusions

Velocity Structure
Eulerian and Stream coordinate mean fields

Eulerian Mean

Mean velocity field at 52 m depth (1992 to 1999).
The large vectors and variance ellipses indicate the position of the Gulf Stream.
The mean flow vectors north of the Gulf Stream suggest a weak outflow, whereas to the south little exchange appears to be present.
The suprisingly sharp decrease in variance away from the mean path of the current suggests that the observed variance results mostly from the current itself and is not part of the high background eddy fields.

Figure 1. The Eulerian mean field and variance ellipses at 52 m depth from >300 sections across the NW Atlantic. The maximum velocity vector is 500 km from New Jersey. The direction of mean flow is 65°. The solid dot indicates the location of the H4 mooring in SYNOP (37° 40'N 68° 35'W). The scale to the right indicates 1 m/s and 1 J/kg. Bathymetry is indicated in 1000 m increments.

A more organized pattern of flow emerges when the data are plotted in a stream coordinate system.

A sharp velocity maximum flanked by rapidly decreasing velocities to either side. The mean peak value is 2.05 m/s with a standard deviation of only 0.25 m/s.
The standard deviations are relatively invariant across the stream.

Figure 2. The stream-averaged velocity field of the Gulf Stream at 52 m depth using the same sections as in Figure 1 (above). The dots indicate all velocity components parallel to the maximum velocity vector. The heavy line indicates an exponential fit to these excluding data within 10 km of the maximum (the scale-widths are indicated). The error bars indicate standard deviations in 20 km increments. The light line exponentials are forced to asymptote to zero.

The mean velocity estimates are used to estimate an exponential fit to either side of the velocity maximum.

Force the velocity to zero at large |y| (light line in Figure 2). Allow the fit to asymptote to the mean flow of the recirculating gyres for large |y| (heavy line in Figure 2).

u(y) = 2.61exp(y/20) - 0.56exp(y/5) (y<0)
u(y) = 2.75exp(-y/34) - 0.7exp(-y/8) (y>0)
u(y) = 2.61exp(y/25) - 0.33exp(y/3)-0.23 (y<0)
u(y) = 2.75exp(-y/37) - 0.62exp(-y/3)-0.08 (y >0)

This set better represents the total velocity field, but includes the substantially barotropic component of -0.23 and -0.08 m/s in the recirculation gyres north and south of the Gulf Stream (Hogg, 1992).

Meandering versus mesoscale eddies Velocity and deformation scales Thermal structure

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Eulerian Mean
Mean velocity field at 52 m depth (1992 to 1999). The large vectors and variance ellipses indicate the position of the Gulf Stream. The mean flow vectors north of the Gulf Stream suggest a weak outflow, whereas to the south little exchange appears to be present. The suprisingly sharp decrease in variance away from the mean path of the current suggests that the observed variance results mostly from the current itself and is not part of the high background eddy fields.
	Figure 1. The Eulerian mean field and variance ellipses at 52 m depth from >300 sections across the NW Atlantic. The maximum velocity vector is 500 km from New Jersey. The direction of mean flow is 65°. The solid dot indicates the location of the H4 mooring in SYNOP (37° 40'N 68° 35'W). The scale to the right indicates 1 m/s and 1 J/kg. Bathymetry is indicated in 1000 m increments.
A more organized pattern of flow emerges when the data are plotted in a stream coordinate system.
A sharp velocity maximum flanked by rapidly decreasing velocities to either side. The mean peak value is 2.05 m/s with a standard deviation of only 0.25 m/s. The standard deviations are relatively invariant across the stream.
	Figure 2. The stream-averaged velocity field of the Gulf Stream at 52 m depth using the same sections as in Figure 1 (above). The dots indicate all velocity components parallel to the maximum velocity vector. The heavy line indicates an exponential fit to these excluding data within 10 km of the maximum (the scale-widths are indicated). The error bars indicate standard deviations in 20 km increments. The light line exponentials are forced to asymptote to zero.

The mean velocity estimates are used to estimate an exponential fit to either side of the velocity maximum.
Force the velocity to zero at large \|y\| (light line in Figure 2).	Allow the fit to asymptote to the mean flow of the recirculating gyres for large \|y\| (heavy line in Figure 2).
u(y) = 2.61exp(y/20) - 0.56exp(y/5) (y<0) u(y) = 2.75exp(-y/34) - 0.7exp(-y/8) (y>0)	u(y) = 2.61exp(y/25) - 0.33exp(y/3)-0.23 (y<0) u(y) = 2.75exp(-y/37) - 0.62exp(-y/3)-0.08 (y >0)
	This set better represents the total velocity field, but includes the substantially barotropic component of -0.23 and -0.08 m/s in the recirculation gyres north and south of the Gulf Stream (Hogg, 1992).