Insights From Mode

Up until the 1970s, it was generally believed that the deep ocean was a steady, unchanging environment with large bodies of water moving slowly and coherently. There was very little evidence to the contrary, because available oceanographic techniques meant that current, temperature and salinity data came from discrete measurements (or a relatively short series of measurements), widely separated in space and (generally) time. As a result, it was impossible to get any meaningful information about how these properties might vary over relatively short distances or over relatively short time periods (cf. Section 3.5). Then during 1971-73. an international expedition was mounted to study intensively an area of ocean several hundred kilometres across, in the western North Atlantic to the east of the Gulf Stream and to the south of Bermuda, at -28° N, 70° W (cf. Figure 4.20(b)). The aim of the expedition was to reveal relatively small-scale variability within the ocean by combining results obtained from fixed current meters, freely drifting floats, and dynamic topography calculated from temperature and salinity data. This project, which in its design was a turning point in oceanographic experimentation, was named the Mid-Ocean Dynamics Experiment (MODE).*

The freely drifting floats used during MODE were of the same type as those which provided the first direct evidence for counter-currents within the Gulf Stream (Section 4.3.2). The floats consisted of aluminium tubes, about 6 m long (the prototypes were constructed out of lengths of scaffolding!), each containing a battery, an acoustic beacon, and a precisely determined amount of ballast. At the surface, such a float is negatively buoyant and so initially it sinks; however, as it is less compressible than seawater its density increases more slowly with increasing pressure than does the density of the seawater. Eventually, at a density level that can be predetermined by adjusting the ballast, the float has the same density as the surrounding seawater. It is then neutrally buoyant and stops sinking; thereafter, it drifts with the water that surrounds it. Neutrally buoyant floats are also known as Swallow floats, after John Swallow, the British oceanographer who invented them.

The original Swallow floats were designed to sink to about 1000 m, but in the 1970s they began to be deployed in the sound channel, the level of minimum sound velocity where sound waves become trapped by refraction, which varies between about 1000 and 2000 m depth. Large numbers of these floats could be continuously tracked over ranges of 1000 km or more, from acoustic receiving stations on the shore or the sea-bed. Sofar (SOund Fixing And hanging) floats like these were used in MODE, along with the conventional neutrally buoyant floats.

* Around that time. Soviet oceanographers were also investigating intermediate-scale variability in the ocean, and in 1970 they carried out Polygon-70 in the Atlantic North Equatorial Current. Then, during 1974—75 (at the height of the Cold War), the Americans and Soviets collaborated in PolyMODE which investigated mesoscale activity over much of the western part of the North Atlantic subtropical gyre. These experiments had results consistent with those of MODE, but unfortunately we do not have room to discuss them here.

Figure 4.24 (a) A compilation of the tracks of all the Sofar floats launched in the MODE area (in the vicinity of the red dot), as recorded from August 1972 to June 1976. Note how the floats became dispersed from the original area, so that after a number of years they had become scattered over the western part of the North Atlantic subtropical gyre.

Figure 4.24 (a) A compilation of the tracks of all the Sofar floats launched in the MODE area (in the vicinity of the red dot), as recorded from August 1972 to June 1976. Note how the floats became dispersed from the original area, so that after a number of years they had become scattered over the western part of the North Atlantic subtropical gyre.

The most exciting result to come out of analysis of the MODE records was the ubiquity of mesoscale eddies, which were found to dominate flow with periods greater than the tidal and inertial periods (Section 3.5.2). This was despite that fact that - as has since become clear - the MODE site is relatively 'quiet' eddy-wise, when compared with areas closer to intense frontal currents (Figure 3.34).

Figure 4.24(a) is a compilation of the tracks recorded during MODE. Most of the floats seem to have been carried in eddy currents at some time or another, and at about 30° N. a number of floats were trapped in an eddy (or eddies) for a considerable time. The track of one of these floats is shown in Figure 4.24(b) (overleaf), which shows that before being caught in a fast southward flowing current, the float spent most of the monitoring period circulating within a cyclonic eddy which was moving due west.

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Figure 4.24 (b) The path of one of the Sofar floats tracked during the MODE experiment. The float was drifting in the sound channel at 2000 m. Dots show daily positions, so the track shows the path taken by the float over 51 months. The float first circulates at speeds of about 0.5 m s~1 within a westward-moving eddy. It is then caught in a fast southward-flowing current.

Figure 4.25 illustrates geostrophic flow patterns at a depth of 150 m in the MODE area, on the basis of density determined from T and S data.

day 105

day 135

day 165

day 105

day 135

day 165

Figure 4.25 The mesoscaie structure in tit® region of 69= 40'W. 23 - N (marked by (he cross in the centre of each map], as revealed by the dynamtc topography at 150 m depth, calculated from rand S dat?. collected during MODE. The numbers on the contours are a measure ot the total flow between the contour in question and the zero contour The 'highs' (H) correspond to waimer water and the 'lows' (L) to cooler water The three pictures show the situation at intervals of 30 days.

100 km

Figure 4.25 The mesoscaie structure in tit® region of 69= 40'W. 23 - N (marked by (he cross in the centre of each map], as revealed by the dynamtc topography at 150 m depth, calculated from rand S dat?. collected during MODE. The numbers on the contours are a measure ot the total flow between the contour in question and the zero contour The 'highs' (H) correspond to waimer water and the 'lows' (L) to cooler water The three pictures show the situation at intervals of 30 days.

100 km

Maps similar to Figuie 4.35 were drawn for different depths in ihe water column and tt was found that although the fastest speeds occurred near the surface I being about 0.15 in s 1 at 150m depth) the eddies persisted down to at least 1500 m. It w as also found that the 'axes of rotation' of the eddies were not all vertical.

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