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East Pacific equatorial front

Figure 2.21 Trajectories of Gulf Stream rings. Cold rings south of the stream: unbroken lines represent inferred ring time series; dashed and dotted lines indicate estimated trajectories of rings based on incomplete data. Warm rings north of the stream: trajectories are confined between the continental slope and the Gulf Stream. (Lai and Richardson, 1977; J. Phys. Oceanogr., 7, 670-83; copyright by the American Meteorological Society.)

Figure 2.21 Trajectories of Gulf Stream rings. Cold rings south of the stream: unbroken lines represent inferred ring time series; dashed and dotted lines indicate estimated trajectories of rings based on incomplete data. Warm rings north of the stream: trajectories are confined between the continental slope and the Gulf Stream. (Lai and Richardson, 1977; J. Phys. Oceanogr., 7, 670-83; copyright by the American Meteorological Society.)

separate water-mass entities contained within their own enclosed circulation and surrounded by water having different characteristics (Kerr, 1977).

Cold-core Gulf Stream rings form on the south side of the Gulf Stream. These rings are typically 100-300 km in diameter and rotate cyclonically (i.e. counterclockwise in the northern hemisphere) at speeds up to 3 knots at the surface (The Ring Group, 1981). These rings generally drift in a southwestward direction in the Sargasso Sea and can persist for up to a year before losing their identity or being reabsorbed into the Gulf Stream (Figure 2.21). It has been estimated that five to eight cold-core rings form each year (Lai and Richardson, 1977). These rings have cold cores of slope water encircled by warm Sargasso Sea waters, as illustrated in the temperature section of Figure 2.22. Therefore, these rings represent strong thermal anomalies.

Warm-core Gulf Stream rings form on the north side of the Gulf Stream, trapping pockets of warm Sargasso Sea water in the cold slope water (Figure 2.21). The resulting circulation is anticyclonic (i.e. clockwise in

XBT Locations

XBT Locations

Figure 2.22 Temperature (°C) section through a cyclonic (cold-core) Gulf Stream ring as observed during December 1975 near 36° N, 58° W. This cross-section, which is oriented southwest to northeast, shows that the central core of the ring contains cold, low-saline slope water with isotherms rising about 500 m above the normal depth in the surrounding Sargasso Sea water. The overall size of the ring is approximately 200 km. (Lai and Richardson, 1977; J. Phys. Oceanogr., 7, 670-83; copyright by the American Meteorological Society.)

Distance (km)

Figure 2.22 Temperature (°C) section through a cyclonic (cold-core) Gulf Stream ring as observed during December 1975 near 36° N, 58° W. This cross-section, which is oriented southwest to northeast, shows that the central core of the ring contains cold, low-saline slope water with isotherms rising about 500 m above the normal depth in the surrounding Sargasso Sea water. The overall size of the ring is approximately 200 km. (Lai and Richardson, 1977; J. Phys. Oceanogr., 7, 670-83; copyright by the American Meteorological Society.)

the northern hemisphere). An average of 22 warm-core rings are formed annually between 75° and 44° W (Auer, 1987).

Another example of rings is provided by Goni and Johns (2001), who conducted a census of anticyclonic rings shed by the North Brazil Current in the western tropical Atlantic off the northeast coast of South America near 8° N latitude. The North Brazil Current is separate from the Guiana Current, which is noted on the ocean frontal chart of Figure 2.20. These rings were tracked over the period of October 1992-December 1998 using sea height anomaly data derived from TOPEX/Poseidon satellite radar altimeters. On an average, five rings were formed each year with estimated translation speeds of 14 km day-1. One in six of these rings penetrated into the Caribbean Sea through the southern Lesser Antilles while the rest followed a northern trajectory past Barbados. Available data suggest that the vertical structures of these rings can vary widely. Some rings may have intense subsurface structure but weak surface signatures, which would make them difficult to detect from satellites.

The acoustic impacts of ocean fronts and eddies will be further explored in Chapter 5 (Section 5.6.7).

2.5.2.2 Internal waves

Internal waves are sub-surface waves that propagate along interfaces separating fluid layers of different densities. They can also exist within fluid layers where vertical density gradients are present. These waves can be generated by a number of mechanisms including surface waves, wind forcing, submarine earthquakes, submarine landslides, air pressure changes and current shears, among others.

In the open ocean, internal waves appear to take the form of progressive waves. In partially closed water bodies, standing internal waves are generally found. Internal waves are commonly observed over continental shelves. Such waves are probably generated by the scattering of the barotropic tide into baroclinic modes at the edge of the shelf, and then propagate shoreward on the shelf where they are absorbed and reflected as they break on the sloping bottom. Evidence suggests that such internal waves may have lifetimes of several days on the shelf. Over the shelf, internal waves may be manifested as solitary wave packets (or solitons).

The distribution of internal wave amplitudes as a function of depth is influenced by the vertical density distribution. Specifically, as the density boundary weakens the amplitudes become larger. Internal waves will normally have amplitudes several times greater than surface waves. Crest-to-trough wave heights for internal waves can be on the order of 10 m. Wavelengths can range from a few hundred meters to many kilometers. At the long-wavelength end of the internal wave spectrum are internal tides.

In theory, free internal waves can only exist in the frequency range bounded at the lower limit by the inertial frequency (which is a function of latitude) and at the upper limit by the buoyancy frequency (which is a function of depth). The inertial frequency (&>i) is equivalent to the Corio-lis parameter (f). Specifically, &>i = f = (2n sinL)/12radh , where L is the latitude (in degrees). The associated period (T) is 12h/sinL. At latitude 30° N, the inertial period is 24 h. The buoyancy frequency (N), also referred to as the Brunt-Vaisala frequency, is related to water density (p), depth (z), gravitational acceleration (g) and sound speed (c) by:

where water depth (z) is measured in the negative (downward) direction (Apel, 1987: 169-70). The buoyancy frequency measures the stability of the

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