Mapping The Gulf Stream Using Water Characteristics

As discussed in Section 4.1.1. seafarers have long been aware of the high temperatures associated with the Gulf Stream. They have also noted that the edge of the Gulf Stream is often marked by accumulations of Sargassum (a floating seaweed, endemic to the Sargasso Sea), and that the waters of the Stream are a clear blue, contrasting strongly with the relatively murky waters between the Stream and the coast.

Since Franklin's time, there have been various surveys of the temperature distribution in the region of the Gulf Stream. These have greatly added to knowledge of the flow: for example, it was through measurement of surface-water temperatures that the north-easterly extension of the Gulf Stream towards Britain and Scandinavia was discovered by Captain Strickland in 1802.

Nevertheless, measurements made directly from ships are time-consuming and expensive, and so are relatively few in number and widely spaced. That this can lead to difficulties in interpreting the results is graphically illustrated by Figure 4.30(a)-(c). These maps show plots made from temperature data collected in 1953 in the vicinity of the Gulf Stream, and the important point to note is that all three have been drawn using the same data, collected while the ship moved along the tracks indicated by the red lines. Of course, obtaining measurements at sea has become easier and quicker since the 1950s, but the inherent problem of large gaps in the data (in both space and time) still remains.

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.

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.

To add to such difficulties of interpretation^ scientists using observations made from ships have to cope with the problem that measurements are made over periods of perhaps weeks and it is therefore impossible to obtain a 'snapshot' of the flow at any one time. This is not the case with satellite measurements, which enable us to see complex spatial variations of surface waters over a wide area, effectively instantaneously (i.e they provide synoptic information), Nevertheless, satellite orbits are such that a given area of ocean is 'viewed' relatively infrequently (e.g. every ten days for TOPEX-Poseulon).

Figure 4.31(a) and (b) show the distributions of sea-Surface temperature and phytoplankton oft'the eastern coast of North America, as measured by the satellite-borne Coastal Zone Color Scanner (C7.CS).

QUESTION 4.13 In Figure 4.31(a). the blue end of the colour range represents (he coldest water and the orange-red the warmest water; the green region corresponds to cool water over the continental shelf and slope. Bearing this in mind, and referring to Figure 3.1 and/or Figure 4.20(a) if necessary, can you identify: (i) the water in the Labrador Current; and (ii) the Sargasso Sea water and water flowing in the Gulf Stream?

In interpreting images like Figure 4.31. it is important to remember that the boundaries between areas of different colours are. in a sense, arbitrary, because colours have been assigned to particular ranges qtf temperature or chlorophyll content, often in order to bring out certain features. Changing either the ranges or the colours themselves could significantly alter the appearance of an image. More fundamentally, the patterns seen in images like Figure 4.31 are a result of the flow pattern, not the aciua) flow pattern itself, and other factors are always at work. In particular, chlorophyll distributions (Figure 4.3 Kb)) are significantly affected by nutrient supply, rates of growth of the phytoplankton species present, etc.

4.3.6 GULF STREAM RINGS'

Perhaps the most dramatic aspect of Figure 4.31 is the complexity of the (implied) flow pattern, which no conventional method of current measurement could ever have revealed. Various types of eddies can he seen, but the most striking is the circular feature between the Gulf Stream and Cape Cod to the north. This is an example of a "Gulf Stream ring' - an eddy that formed from a meander which broke off to form an independent circulatory system

Figure 4.32(a) (overleaf) shows the evolution o! Gulf Stream rings on both the landward and Sargasso Sea side of the Stream, over about a month. Gulf Stream eddies are often described as "cold-core" or "warm-core".

Given the way in which Gull Stream rings form (Figure 4.32(ai>. would you expect the continental-margin side of the Gulf Stream to be characterized by warm-core or cold-core eddies? Is this borne out by Figure 4.31 la)?

As eddies form rather in the manner of river meanders cutting off ox-bow lakes, enclosing ami pinching off volumes of water so that they end up on the opposite side of the Stream, eddies on the continental-margin side of the Gulf Stream must be warm-core eddies. This is borne out by Figure 4.31(a), in which the central region of the eddy is yellow, corresponding to warm Gulf Stream and Sargasso Sea water. (Note that the rotatory flow in the eddy extends over an area considerably larger than thai shown in yellow.I

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-7satellite. The image in (a) is based on measurement of infrared radiation: the warmest water (shown red) is about 25 °C and the coldest (shown dark blue) is about 6 °C. 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 4.31 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-7satellite. The image in (a) is based on measurement of infrared radiation: the warmest water (shown red) is about 25 °C and the coldest (shown dark blue) is about 6 °C. 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.

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 rings 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 between oceans. Eddies similar to Gulf Stream rings

Figure 4.32 (b) Temperature section along the black line in (a)(iv), showing that the eddies extend to significant depths.

iqi ^stance ■«mi 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 km; a warm-core eddy has a diameter of about 100-200 km. Furthermore, at any one time as much as 159r of the area of the Sargasso Sea may be occupied by cold-core eddies, and as much as 409c 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 w ater between the ocean and the overlying atmosphere.

Reluming lor a moment to the (¡nil Stream rings in Figure 4 32. \\ hat can sou sav about the directions ol rntulion <it cold-core ami warm-core eddies '

Stream

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.

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 ring - as much as 1.5-2.0 m s_l - 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 provide information about current \elocity

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.

A remote-sensing technique that can provide information about current velocity is satellite altimetiy. 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 minus the geoid (Figure 3.22) - for one pass of the TOPEX-Poseidon 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.

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