Dynamic topography
4.3.5 MAPPING THE GULF STREAM USING WATER CHARACTERISTICS
Figure 4.29 shows the paths taken by a number of Sofar floats in the region of the Gulf Stream and, for comparison, the mean current flow as it would have been measured by means of moored current meters (although, in fact, such measurements were not made).
For completeness, we should mention the use of coloured d y e - in a sense, the ultimate Lagrangian method. This approach is generally used to study relatively small-scale turbulence, and to investigate rates of dispersion, i.e. the extent to which a patch of initially coherent water becomes widely spread, or dispersed, as a result of mixing and turbulence (cf. Figure 4.24(a)).
All the methods mentioned above provide information about horizontal velocity but nothing at all about the vertical component of current velocity. Vertical flow velocities are generally very much smaller than horizontal velocities but in certain unusual circumstances have been measured using floats with fins so arranged that the float rotates at an angular velocity proportional to the vertical current velocity.
An instrument which measures both vertical and horizontal velocities is the Acoustic Doppler Current Profiler (ADCP). ADCPs are now widely used, and can be deployed in various ways (e.g. on moorings, cf. Figure 4.27(b)).
The way in which they exploit sound waves is explained in Section 4.3.7.
123
Figure 4.30 Three interpretations of temperature data collected in August 1953.
The tracks along which measurements were made are shown as red lines.
Interpretation (a) shows a single, simple stream, while (b) shows a double stream (one part stronger than the other) with some branching. The third interpretation (c) shows the Gulf Stream as a series of disconnected fragments.
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125
Figure 4.31 Distributions of (a) sea-surface temperature and (b) phytoplankton pigments, off the eastern coast of the United States and Canada (Cape Cod and Long Island may be seen two-thirds of the way up the image). The data were collected on 14 June 1979, by the Coastal Zone Color Scanner on the Nimbus-7 satellite. The image in (a) is based on measurement of infrared radiation: the warmest water (shown red) is about 25 ~ and the coldest (shown dark blue) is about 6 ~ The brown colour is the land and the white streaks are cloud (which often limits the usefulness of such images). The image in (b)is the same as that shown on the front cover. The highest concentrations of phytoplankton pigment are shown in brown; intermediate concentrations in red, yellow and green; and lowest levels in blue (concentrations have been deduced from the relative absorption and reflection of red and green light by organic pigments).
Figure 4.32 (a) The evolution of Gulf Stream eddies or 'rings', as deduced from infrared satellite images made in February-March 1977.
The warm Sargasso Sea water is shown as light pink, the cool continental shelf water as blue and the Gulf Stream as darker pink.
Incidentally, the Gulf Stream meanders that tend to develop into tings are sometimes referred to as 'baroclinic instabilities', because they are perturbations of a flow with strong density gradients (Figure 4.32(b)) and hence velocity gradients, vertical as well as horizontal. Their kinetic energy is believed to be derived from the potential energy of the mean flow, i.e. from the 'relaxation' of sloping isopycnals in the Gulf Stream (cf. Section 3.5).
Other types of eddies develop as a result of large lateral variations in velocity, or lateral current shear (cf. Figure 4.6), in which case the original disturbances are referred to as barotropic instabilities.
Figure 4.31(a) and (b) show vividly the role that eddies play in transferring water properties across frontal boundaries. Together, the two images show how the formation of a warm-core eddy results in warm, relatively unproductive Sargasso Sea water being transferred across the Gulf Stream into the cool, productive (because nutrient-rich) waters over the continental margin. Similarly, cold-core eddies will carry cool productive coastal water into the Sargasso Sea. Eddy generation may also be important in transferring water characteristics b e t w e e n o c e a n s . Eddies similar to Gulf Stream rings
127
Figure 4.32 (b) Temperature section along the black line in (a)(iv), showing that the eddies extend to significant depths.
form from the Agulhas Current 'loop' off the tip of South Africa (see Figure 3.1), and are believed to be an important agent in the transfer of water between the Indian and Atlantic Oceans.
The temperature section in Figure 4.32(b) shows that, like the western boundary current from which they form, Gulf Stream rings extend to considerable depths. Cold-core eddies may extend to the sea-floor at a depth of 4000-5000 m, while warm-core eddies impinge on the continental slope and rise when, after forming, they drift erratically towards the south-west.
Gulf Stream rings tend to move westwards and/or equatorwards (as do similar eddies elsewhere in the ocean) rather than polewards and/or eastwards.
Their survival times seem to depend to a large extent on the path they take:
warm-core eddies often last until they are entrained back into the large-scale north-easterly flow of the Gulf Stream, and may have lifetimes of anything from a few months to a year; cold-core eddies, which can more easily escape being caught up in the Gulf Stream again, generally survive somewhat longer.
Gulf Stream eddies are not only deep, they also extend over large areas.
A newly formed cold-core eddy typically has a diameter of 150-300 kin;
a warm-core eddy has a diameter of about 100-200 kin. Furthermore, at any one time as much as 15c~ of the area of the Sargasso Sea may be occupied by cold-core eddies, and as much as 40% of the continental shelf water by warm-core eddies. They have a significant influence on the North Atlantic as a whole, continually exchanging energy, heat, water, nutrients and
organisms with their surroundings. Locally, they also greatly affect exchanges of heat and water between the ocean and the overlying atmosphere.
Cold-core eddies are always cyclonic (anticlockwise in the Northern Hemisphere) and warm-core eddies are always anticyclonic - this is true of all mesoscale eddies, not just Gulf Stream rings. You have already
encountered this idea in Question 4.11: the 'highs" on Figure 4.25 correspond to eddies with warm central regions and the lows to eddies with cold central regions.
All Gulf Stream eddies, whether warm-core or cold-core, contain a ring of Gulf Stream water. Rotational velocities are highest in this r i n g - as much as
1.5-2.0 m s -] - and decrease both towards the centre of the eddy and towards the outer 'rim'. Such information was initially obtained largely through direct current measurements. Satellite images like those in Figure 4.31 are extremely effective in portraying horizontal variations in water properties, and they suggest flow patterns whose complexity could not have been fully appreciated through traditional oceanographic techniques, but they cannot
Figure 4.33 Variation in the height of the sea-surface along a satellite track (see inset map) in the western North Atlantic. The measurements were made by the Seasat radar altimeter from 17 September to 8 October 1978 The black line represents the local height of the marine geoid, and the distances in centimetres are departures from this level.
129 A remote-sensing technique that can provide information about current velocity is satellite altimena', which you have already encountered in Section 3.3.4.
The Frontispiece shows the dynamic topography of the sea-surface - i.e. the sea-surface height m i n u s the geoid (Figure 3 . 2 2 ) - for one pass of the T O P E X - P o s e i d o n satellite. Such instantaneous pictures of the sea-surface may be used to deduce the velocities of surface currents at that time, if geostrophic equilibrium is assumed. Figure 4.33 shows how the shape of the sea-surface along a south-east-north-west satellite track in the region of the Gulf Stream changed over the course of 21 days. The Gulf Stream itself shows up clearly, as does a cold-core ring, which was moving away to the side of the satellite track during the period in question.
Satellite altimetry is very exciting to physical oceanographers as it enables them to see the dynamic topography of the sea-surface (Section 3.3.4).
Also, comparison of directly measured current velocities with values calculated from the observed sea-surface slopes enables the depths at which geostrophic current velocities become zero (i.e. the depths at which isobaric surfaces become horizontal) to be accurately determined (Section 3.3.3).
This almost concludes our survey of recent measurements and observations of the Gulf Stream - an example of an intense western boundary current. It is likely that as more becomes known about the other western boundary currents, they will be found to share many of the characteristics of the Gulf Stream. Before moving on to look at the equatorward-flowing eastern limbs of the subtropical gyres - the eastern boundary currents - we will briefly mention some methods of current determination that have not been discussed so far, and look at some of the results of a project to model the circulation in the North Atlantic.