The Observed Thermohaline Circulation

The time-mean abyssal flow in the ocean is so weak that it cannot be measured directly. However abyssal circulation, and the convective processes forcing it, leaves its signature in the distribution of water properties, from which much can be inferred.

11.2.1. Inferences from interior tracer distributions

Water masses modified by deep convection are tagged with T and S values characteristic of their formation region, together with other tracers, such as tritium from the atomic weapon tests of the 1960s and chlorofluorocarbons (CFCs) from industrial and household use. Tracers can be tracked far from their formation region, revealing interior pathways through the ocean.

Zonal-average sections of T and S across the Atlantic Ocean are shown in Fig. 11.13; see also the hydrographic section along 25° W in the Atlantic shown in Fig. 9.9. We see three distinct layers of deep and abyssal ocean water, fed from different sources. Sliding down from the surface in the southern ocean to depths of 1 km is Antarctic Intermediate Water (AAIW), with low salinity (34.4 psu) and, near the surface, slightly lower temperature than water immediately above and below. This water appears to originate from about 55° S and is associated with regions of deep mixed layers in the circumpolar ocean seen in Fig. 9.10. At a

Circumpolar Deep Water

FIGURE 11.10. The Woods Hole ship KNORR cuts through harsh Labrador Sea conditions during the winter Labrador Sea Deep Convection Experiment (Feb—Mar, 1997) taking observations shown in Fig. 11.11. Waves such as those shown at the top caused continual ice build-up on the ship, as can be seen at the bottom. Courtesy of Bob Pickart, WHOI.

FIGURE 11.10. The Woods Hole ship KNORR cuts through harsh Labrador Sea conditions during the winter Labrador Sea Deep Convection Experiment (Feb—Mar, 1997) taking observations shown in Fig. 11.11. Waves such as those shown at the top caused continual ice build-up on the ship, as can be seen at the bottom. Courtesy of Bob Pickart, WHOI.

FIGURE 11.11. Top: Sections of potential density, a (contoured), and stratification, da/dz (colored), across the Labrador Sea in October, 1996, before the onset of convection, and in March, 1997, after and during wintertime convection. Purple indicates regions of very weak stratification. Bottom: A horizontal map of mixed layer depth observed in Feb-Mar, 1997, showing convection reaching to depths in excess of 1 km. The position of the sections shown at the top is marked by the dotted line. Courtesy of Robert Pickart, WHOI.

FIGURE 11.11. Top: Sections of potential density, a (contoured), and stratification, da/dz (colored), across the Labrador Sea in October, 1996, before the onset of convection, and in March, 1997, after and during wintertime convection. Purple indicates regions of very weak stratification. Bottom: A horizontal map of mixed layer depth observed in Feb-Mar, 1997, showing convection reaching to depths in excess of 1 km. The position of the sections shown at the top is marked by the dotted line. Courtesy of Robert Pickart, WHOI.

depth of 2 km or so—indeed, filling most of the Atlantic basin—is North Atlantic Deep Water (NADW), with high salinity (34.9 psu) originating in high northern latitudes, but identifiable as far south as 40° S and beyond. At the very bottom of the ocean is the Antarctic Bottom Water (AABW), less saline but colder (and denser) than NADW. Together, these give us a picture of a multilayered pattern of localized sinking and horizontal spreading of the dense water, which were represented schematically by the arrows in Fig. 11.1.

Another useful tracer of the circulation is dissolved oxygen. Surface waters are near saturation in oxygen content (in fact, they are slightly super-saturated). As the water leaves the surface (the source of oxygen), its

Preconditioning Convection and Mixing Sinking and Spreading

FIGURE 11.12. Schematic diagram of the three phases of open-ocean deep convection: (a) preconditioning, (b) deep convection and mixing, and (c) sinking and spreading. Buoyancy flux through the sea surface is represented by curly arrows, and the underlying stratification/outcrops are shown by continuous lines. The volume of fluid mixed by convection is shaded. From Marshall and Schott (1999).

Preconditioning Convection and Mixing Sinking and Spreading

FIGURE 11.12. Schematic diagram of the three phases of open-ocean deep convection: (a) preconditioning, (b) deep convection and mixing, and (c) sinking and spreading. Buoyancy flux through the sea surface is represented by curly arrows, and the underlying stratification/outcrops are shown by continuous lines. The volume of fluid mixed by convection is shaded. From Marshall and Schott (1999).

Zonal-Average T (°C) And S (psu) across The Atlantic

90s aos so s er xx bon wn

Latitude

FIGURE 11.13. Zonal average (0° —> 60° W) temperature (top) and salinity (bottom) distributions across the Atlantic Ocean. Antarctic Intermediate Water (AAIW), Antarctic Bottom Water (AABW), and North Atlantic Deep Water (NADW) are marked. Compare this zonal-average section with the hydrographic section along 25° W shown in Fig. 9.9.

90s aos so s er xx bon wn

Latitude

FIGURE 11.13. Zonal average (0° —> 60° W) temperature (top) and salinity (bottom) distributions across the Atlantic Ocean. Antarctic Intermediate Water (AAIW), Antarctic Bottom Water (AABW), and North Atlantic Deep Water (NADW) are marked. Compare this zonal-average section with the hydrographic section along 25° W shown in Fig. 9.9.

oxygen content is slowly used up by biological activity. Hence, oxygen content gives us a sort of clock by which we can get a feel for the "age" of the water (i.e., the time since it left the surface); the lower the content, the "older" the water. Oxygen content (expressed as a percentage of saturation) for the Atlantic and Pacific Oceans is shown in Figs. 11.14, top and bottom, respectively.

In the Atlantic, water in the deep ocean shows a progressive aging from north to south, implying the dominant source is in the far north. However, the water is generally "young" (oxygen saturation > 60%) everywhere except at depths shallower than 1 km in low latitudes, where "old" water is (we infer) slowly upwelling from below (cf. the isopycnals in Fig. 9.7). That

Zonal Average Oxygen (ml/1) across Atlantic and Pacific

□___I_I_1_I_I_I_I_I___1_I_1_I_I_I___1_I_1___,_1_1_I_!_I_,_1_I_____□

Latitude

□___I_I_1_I_I_I_I_I___1_I_1_I_I_I___1_I_1___,_1_1_I_!_I_,_1_I_____□

Latitude

1 b_i_,_i_,_i_i_i_i_i_i___i_i_i_i_i_i_i_i_i_i___i_i_i_i_i_,_i_i_i_i_d so s 60 s 90 s 0' 30'n 60 n 90 n

Latitude

FIGURE 11.14. Zonally averaged oxygen saturation (in mll-1) in the Atlantic (0°—60° W) and Pacific (150-190° W) oceans.

1 b_i_,_i_,_i_i_i_i_i_i___i_i_i_i_i_i_i_i_i_i___i_i_i_i_i_,_i_i_i_i_d so s 60 s 90 s 0' 30'n 60 n 90 n

Latitude

FIGURE 11.14. Zonally averaged oxygen saturation (in mll-1) in the Atlantic (0°—60° W) and Pacific (150-190° W) oceans.

water is relatively young near the Antarctic coast, around 40°-50°S and, especially, in high northern latitudes, is evidence that surface waters are being mixed down in these regions of the Atlantic. In contrast, the Pacific Ocean cross-section, Fig. 11.14 (bottom), shows young water only near the Antarctic. Deep water in high northern latitudes has very low oxygen content, from which we infer that there is no sinking of surface waters in the North Pacific, except in the Arctic basin.

11.2.2. Time scales and intensity of thermohaline circulation

Water whose properties are set (ocean-ographers use the term ''formed'') at the source regions marked in Fig. 11.9 must spread out before slowly upwelling back to the surface to complete the circuit of mass flow. Estimates of the strength of the major source, NADW, are about 14 Sv (see Section 11.4). Using this source rate we can make several estimates of parameters indicative of the strength of the circulation. The area of the Atlantic Ocean is about 1014m2. The depth of the ocean ventilated by the surface sources is perhaps 3 km (see Fig. 11.13). So one estimate of the time scale of the overturning circulation

:s .__ocean volume = 1014m2 x 3 x 103m 700 y is volume flux _ 1.4 x 107m3s-1 -/uuy-

The net horizontal flow velocity in the deep ocean must be about v = ^phmfuh =

3x11403Xm°X5mx31s061m - 10-3 m s-1.Ifcompensating upwelling occupies almost all of the ocean basin, the upwelling velocity must be about w = voluifflux = i-4^m85-1 - 4my-1(!!), area of ocean 1014 m2

ten times smaller than typical Ekman pumping rates driven by the wind (cf. Fig. 10.11).

Thus the interior abyssal circulation is very, very weak, so weak that it is all but impossible to observe directly. Indeed progress in deducing the likely pattern of large-scale abyssal circulation has stemmed as much from the application of theory as direct observation, as we now go on to discuss.

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