Other figures. Observations from Challenger expedition in 1873 (Worthington, 1976). Geographical relation of formation and properties of Eighteen Degree Water to the Gulf Stream (Talley and Raymer, 1982). Iselin (1939) central water T/S relation. Salinity maximum water (Worthington, 1976). Salinity and tritium penetration on isopycnals (Sarmiento). 11-12C pycnostad penetration into subtropical gyre (McCartney, 1982).

North Atlantic Deep Water (NADW) formation (not covered in this lecture, but included for completeness)

(For a much more complete Talley review text about NADW formation, see Physica D reference listed in study questions.)

There are five sources of NADW, classified in a bit of an oversimplification as: Antarctic Intermediate Water, Antarctic Bottom Water, Mediterranean Overflow Water, Labrador Sea Water and Nordic Sea Overflow Water. Two of these are the original southern sources, and the other three are the North Atlantic sources.

The deep thermohaline circulation of the Atlantic involves flow of waters from the southern hemisphere into the North Atlantic, modification and convection of these waters in the North Atlantic and its adjacent seas, and outflow in a thick deep layer. This deep layer affects the world ocean, where it can be tracked through its high salinity signature since the North Atlantic is the most saline of all the oceans. The deep layer flowing out of the North Atlantic is called North Atlantic Deep Water and is notable for its vertical salinity maximum, vertical oxygen maximum (actually two) and vertical nutrient minima.

Inflow to the North Atlantic occurs in two layers - a relatively warm one and a cold one, sandwiching the outflowing NADW. The upper layer is composed of thermocline and Antarctic Intermediate Water (evidenced by a salinity minimum in the vertical at about 800 m depth, resulting from its source at the sea surface near the tip of South America where surface salinity is low). The AAIW can be tracked as a salinity minimum up to the subtropical gyre in the North Atlantic. Because the North Atlantic is a very well-ventilated ocean, from top to bottom, it has lower nutrients throughout its depths compared with other oceans. AAIW comes from the South Atlantic where nutrients are higher, and it so happens that it can be tracked through the Gulf Stream and on up into the subpolar North Atlantic through its silica, which is slightly higher than the North Atlantic silica.

The other southern source for NADW is at the ocean bottom - the Antarctic Bottom Water. This is actually deep water (not bottom water) from the South Atlantic sector of the Antarctic. (The true bottom waters, formed in the Weddell Sea and Ross Sea, do not escape very far northward, mainly because of topography that confines them. The Antarctic Bottom Water in the South and North Atlantic is essentially the same as what we call Circumpolar Deep Water in the Pacific and Indian Oceans.) AABW is apparent as a cold, lower salinity bottom layer and extends northward in the North Atlantic up to the Gulf Stream latitude. It upwells into the southward-flowing NADW layer above it (and hence does not reach the sea surface).

In the North Atlantic, the components of NADW are formed at three sites, all involving intermediate depth convection: inside the Mediterranean Sea, in the Labrador Sea and in the Greenland Sea.

The Mediterranean Sea is connected to the North Atlantic through the narrow Strait of Gibraltar. Flow is into the Mediterranean at the sea surface in the Strait. Within the Mediterranean there is large evaporation and cooling and production of dense water. This flows out into the North Atlantic at the bottom of the Strait. The resulting Mediterranean Water (or Mediterranean Outflow or Overflow Water) in the North Atlantic is found at mid-depth and is marked by its salinity maximum both in the vertical and in the horizontal along isopycnals. The MOW forms the upper part of the North Atlantic Deep Water. (In the tropical Atlantic, Wust referred to the salinity maximum core of the NADW as Upper North Atlantic Deep Water - UNADW clearly originates as MOW.) MOW properties at Gibraltar are 13C, 38.45 psu and 29.07 sigma theta. This is actually denser than the bottom water of the North Atlantic. As the plume of MOW exits Gibraltar, it moves to the north (boundary to the right, in the Kelvin wave sense) and down the slope. Vigorous entrainment as it moves down the slope reduces its high salinity and density. It finally equilibrates after mixing at a depth of about 1000 m. (Figures from Ochoa and Bray, Zenk, Reid (1994) showing salinity distribution at about 1000 m, outflow characteristics.)

Salinity at 25W showing salinity maximum of MOW (30-40N at 1000m), salinity minimum of LSW (40-60N at 1500-2000m). Also - salinity minimum of AAIW (south of 20N at 500-1000m) and overall salinity maximum of NADW (south of 20N and 1500-3000m).
Salinity at 24N showing salinity maximum of MOW in the east at 1000m.

The two northern sources of NADW are formed from surface waters that flow northward in the Gulf Stream and North Atlantic Current and then eastward and northward in the subpolar region. These surface waters cool and freshen along this path towards the ultimate intermediate-depth convection regions. Cooling of the subpolar surface waters creates very thick mixed layers in the subpolar gyre. These thick layers are called Subpolar Mode Water, in analogy with the Subtropical Mode Water of the subtropical gyres. SPMW temperature range from about 14C near the North Atlantic Current, to 8C where SPMW enters the Norwegian Sea, to 4C where SPMW enters the Labrador Sea. (Figure from McCartney and Talley, 1982.)

The two northern parts of NADW are Labrador Sea Water and Nordic Sea Overflow Water. (The latter is also referred to by its three separate components resulting from overflows at three separate sites into the North Atlantic, and also sometimes as Greenland-Iceland-Norwegian Sea overflow water or GIN-Sea overflow.)

Labrador Sea Water is formed in the western Labrador Sea through convection to about 1500-2000 meters in late winter. This forms a relatively homogeneous water mass within the Labrador Sea. Much attention has been focused in recent years on both the formation of LSW and on its changing properties (temperature changing from 3.5C to about 2.9C on decadal time scales). Labrador Sea Water spreads out into the North Atlantic, filling both the subpolar gyre and entering the subtropical gyre. Within the subpolar gyre, it is marked by a salinity minimum in the vertical. Within both the subpolar and subtropical gyres it is marked by an oxygen maximum in the vertical. Within both gyres it is also marked by a thickness maximum resulting from its convective source. (Instead of thickness, the quantity that is often mapped is called potential vorticity, which we have not discussed in this class. Potential vorticity is inversely proportional to the thickness of the layer and proportional to the Coriolis parameter; PV is conserved following flow, essentially like angular momentum, whereas thickness is not conserved.) (Figures from McCartney and Talley, 1982; Lazier, 1993; Talley and McCartney, 1982; Lavender et al., 2000.)

LSW moves southward along the western boundary of the North Atlantic as the upper part of the Deep Western Boundary Current, above the denser water that originates in the Nordic Seas. The oxygen maximum of the LSW persists to the tropical Atlantic where Wust referred to it as "Middle North Atlantic Deep Water". (Figure from Wust.)

Potential Temperature at 47N,
Salinity at 47N,
Potential vorticity at 47N
Salinity at 47N as a function of density rather than depth.

The densest part of the NADW is formed through convection in the Greenland Sea offshore of the East Greenland Current and ice edge. This portion of NADW has many different names, but we will call it Nordic Sea Overflow Water. Flow into the Nordic Seas is in the Subpolar Mode Water layer, east of Iceland. The Greenland Sea convection is usually to intermediate depths, and is colder and denser than convection in the Labrador Sea, hence producing the denser part of the NADW. (Experiments in recent years, involving acoustic tomography to track convection through the winter, have examined the Greenland Sea convection. Much attention has been given to climatic variations in convection in the Greenland Sea.) (Figures from Morawitz et al., 1996.)

Outflow of the dense NSOW occurs in three locations along the Greenland-Faroe ridge: through Denmark Strait (between Greenland and Iceland), across the Iceland-Faroe ridge (between Iceland and the Faroe Islands), and through the Faroe-Shetland channel (between the Faroe and Shetland Islands). All three locations are relatively shallow sills. The dense water flowing southward over the sills plunges downward in plumes, entraining (mixing with) surrounding water in the process. This modifies the properties of the NSOW as it enters the North Atlantic. (Figures from Dickson and Brown.)

The deep part of the NADW thus produced as NSOW fills the western North Atlantic through the lower part of the Deep Western Boundary Current. The DWBC crosses under the Gulf Stream. The presence of the DWBC is not obvious in isopycnal slopes crossing the Gulf Stream. Now that we have mapped many different properties, the DWBC is apparent as a core layer of higher oxygen. The DWBC was predicted in a theory by Stommel and Arons, and then its presence was detected through the earliest use of deep tracked floats (Swallow and Worthington). (Figures from Jenkins, Reid, and Swallow and Worthington.)

The DWBC continues to the South Atlantic. One feature of the DWBC is that is exhibits recirculations to the north on its offshore side. These spread the DWBC waters into the ocean interior. (Cartoon from McCartney.)

Transports for the various components of the NADW overturn have been computed. The net transport involved in the overturn is 15 to 20 Sv. (Figures from Schmitz, and from Schmitz and McCartney)

Some online figures.

North Atlantic meridional section at 64W (crossing the Gulf Stream)

North Atlantic meridional section at 35W

North Atlantic zonal section at 47N

North Atlantic zonal section at 24N

South Atlantic circulation.

At the southern end of South America, the Antarctic Circumpolar Current (ACC) loops far to the north as a western boundary current called the Malvinas or Falkland Current. It then loops back southward nearly immediately offshore (called the "Falkland loop") and rejoins the ACC.

Salinity sections show the saline evaporation region of the subtropics at the sea surface. North-south property sections show the southward progress of the saline, high oxygen, low silica North Atlantic Deep Water between about 1500 and 3500 m. Above the NADW is the low salinity Antarctic Intermediate Water, whose origin is discussed below in the southern ocean paragraphs. It is flowing northward on average. Below the NADW is the Antarctic Bottom Water (also known as Lower Circumpolar Deep Water), whose origin is also discussed below, and which is also flowing northward on average.

Isopycnal plots show that the low salinity AAIW extends northward into the tropics along the western side of the South Atlantic. The saline NADW moves southward into the South Atlantic also along the western side. On the deepest isopycnals, water is only present west of the Mid-Atlantic Ridge - hence the Antarctic Bottom Water of the 25W vertical section is confined west of the ridge. The densest waters (the true bottom waters of the Weddell Sea) are confined to the far south, essentially south of the Antarctic Circumpolar Current.

Indian Ocean subtropical circulation.

The eastern boundary current of the Indian subtropics, the Leeuwin Current, is an anomaly compared with other oceans - it flows poleward. It is connected to waters that flow westward through the Indonesian archipelago from the Pacific Ocean, although most of the water that comes through the Indonesian passages flows directly westward along about 10S towards the western boundary. This can be seen as lower salinity in the section shown below. The Leeuwin Current and the northern Indian Ocean circulation will be discussed in the eastern boundary current and monsoon lecture (topic 7).

Abyssal flow in the Indian Ocean is northward from the Antarctic since no bottom water is formed in the Indian Ocean. A deep water is formed in the Red Sea, similar to Mediterranean Water, but affecting a deeper layer since there is no denser northern source of water. The bottom topography of the Indian Ocean is very complicated, leading to a fractured abyssal circulation with deep western boundary currents along each of the ridges.

North-south sections of properties show the high salinity evaporation region of the subtropical gyre (with its equatorward subsurface salinity maximum, the Subtropical Underwater). Below the saline surface layer is the salinity minimum of the Antarctic Intermediate Water (AAIW). In the south Indian Ocean, the salinity maximum below the AAIW extends northward from the Antarctic Circumpolar Current. Much of the high salinity of this layer comes from its source in the North Atlantic (remnant of North Atlantic Deep Water). At the bottom is the fresher Antarctic Bottom Water or Lower Circumpolar Deep Water. In the north Indian Ocean, the deep-reaching saline water originates in the Red Sea, which is an area of high evaporation. The near-surface high salinity water is from the Arabian Sea as well as the Red Sea and Persian Gulf. On the section that I am showing, which is in the central Indian Ocean, there is a very thin layer of low salinity water at the sea surface in the north, which originates in the Bay of Bengal and results from major river runoff into this area, including the Ganges and many other large rivers.

There is a subsurface oxygen maximum that is somewhat shallower than the Antarctic Intermediate Water. This can be traced back to the sea surface south of Australia where winter mixed layers are very deep. This water mass is called Southeast Indian Subantarctic Mode Water, and is a special, voluminous type of the more general water mass described below.

South Pacific subtropical circulation.

Indonesian throughflow. Upper ocean water leaves the North Pacific in the tropics and flows westward through the Indonesian archipelago into the Indian Ocean. Thus relatively fresh, warm water from the Pacific is carried into to Indian Ocean. This flow of about 10 Sv is important to global heat and freshwater budgets, and is included in all concepts of the global overturning circulation.

The abyssal flow of the South Pacific consists of northward flow from the Antarctic, entering mainly in the west as shown by deep silica distributions, and flowing northward through Samoan Passage to the tropics and then up along the western side of the North Pacific.

Salinity sections show the evaporation region at the sea surface in the subtropical gyre with an equatorward-extending salinity maximum just below the sea surface, the Subtropical Underwater. There is a Subtropical Mode Water associated with the northern side of the East Australia Current. Below the surface high salinity layer is a salinity minimum layer, the Antarctic Intermediate Water. In the tropics it is somewhat difficult to distinguish in this vertical section from the North Pacific Intermediate Water that extends southward, but consideration of basin-wide properties allows the boundaries to be drawn (see below). The abyssal Pacific is higher salinity, with the highest salinity in the south in the Antarctic Circumpolar Current. The reduction of salinity towards the north indicates that there must be diapycnal mixing of fresh water downwards since there is no source of deep water in the North Pacific.

Oxygen sections show high oxygen in the AAIW layer and in the abyssal layer, with low oxygen extending southward from the North Pacific. This low oxygen (high silica) layer is the Pacific Deep Water, returning southward after being formed from upwelled abyssal water (and possibly downwelled intermediate water).

Southern Ocean circulation and water masses.

Subpolar (cyclonic) circulations are found south of the ACC, most particularly in the Weddell and Ross Seas, both of which have western boundary currents that can support a wind-driven cyclonic gyre.

Subantarctic Mode Water (SAMW) is the name given to the very thick mixed layers north of the Subantarctic Front. These mixed layers are thickest in the eastern Indian and across the Pacific sectors of the ACC. Their temperatures decline from 14C in the western South Atlantic (just north of the SAF which is also just east of the Falkland Loop) to 4C in the eastern South Pacific, just west of Chile where the ACC enters Drake Passage. Part of the SAMW of the southeast Indian Ocean circulates northward into the Indian Ocean subtropical gyre and is the source of the oxygen maximum layer throughout the Indian Ocean. Part of the SAMW of the southeast Pacific circulates northward into the Pacific Ocean subtropical gyre and is the source of the low salinity Antarctic Intermediate Water there. Part of the latter (SE Pacific) SAMW flows eastward through Drake Passage and is the source of the low salinity AAIW for the combined Atlantic and Indian subtropical regions.

Proxy for winter mixed layer depth - depth of the 95% oxygen saturation (Figure) (From Talley, 1999 with update in Hanawa and Talley, 2001.)

NADW (deep water of the circumpolar current) upwells south of the ACC and is the source water for dense water formation around the Antarctic continent. The dense water formation occurs mainly as a result of ice formation (brine rejection) on the shelves. The primary sites for this are in the Weddell and Ross Seas, although there are also shelf water sources in the region south of Australia. The cold, dense shelf water flows down the shelves, mixing, and then out towards the east. Its pathways are strongly affected by bottom topography.

Global circulation, water masses and ocean heat transport.

The wind-driven circulations of each subtropical region include an poleward western boundary current and anticyclonic circulation. Just offshore and equatorward of the separated western boundary currents is found the relatively thick layer that we have called Subtropical Mode Water, shown in red in the figure, with schematic cyclonic streamlines to indicate the subtropical gyre. These are around 16 to 19C in each ocean. The pink regions in the figure indicate a lower density subtropical mode water of the poleward/eastern portions of the subtropical gyres. This was not discussed in the lectures, and is mentioned only here for completeness. These eastern STMWs are subducted into the subtropical circulations. Not shown in the figure is the location of the Subtropical Underwaters of each subtropical ocean (should be added to the figure).

The other mode waters discussed in class and shown in the figure are the Subpolar Mode Water of the North Atlantic's subpolar gyre (thick mixed layers evident in the previous winter mixed layer map) and the Subantarctic Mode Water (the thickest part shown as the brown patch in the mode water map).

Location of warm, upper ocean mode waters (Figure), including the Subtropical Mode Waters (around 18C) of each subtropical gyre (red). Lower density subtropical type mode waters of the eastern region of each subtropical gyre (not included in class discussion) (pink). Subpolar Mode Water of the North Atlantic, Subantarctic Mode Water north of the Subantarctic Front (only the region of deepest mixed layers is shown), North Pacific Central Mode Water (not discussed in class - Suga et al., 1997) (brown). Schematics of the gyre circulations are indicated as well, but are clearly just cartoons. (From Talley, 1999 with update in Hanawa and Talley, 2001.)

The intermediate waters of the world ocean are usually traced by their salinity extrema. The two salinity maximum intermediate waters are formed in marginal seas in the high evaporation zone of the subtropics. These are the Mediterranean and Red Sea Waters, the former entering the North Atlantic at mid-depth and the latter filling the northern Indian Ocean (neither shown in the figure). The three low salinity intermediate water masses are formed at high subtropical or subpolar latitudes from surface water that has relatively low salinity. The three intermediate waters have different formation processes (in the regions marked by X in the figure). North Pacific Intermediate Water is formed from shelf water in the Okhotsk Sea, densified by brine rejection, mixed with North Pacific water in the Kuril Straits and including the low salinity surface waters of the Oyashio. It is a salinity minimum only in the North Pacific's subtropical gyre, but ventilates the entire subpolar region as well, where it does not have a salinity extremum. Labrador Sea Water forms through open ocean convection in the western Labrador Sea, from relatively fresh surface water. Since it sinks to mid-depth, below the inflowing more saline surface waters of the subpolar gyre, it appears as a salinity minimum throughout the subpolar gyre. Its influence is traced southward along the western boundary into the subtropical gyre and on into the tropical Atlantic through its relatively large thickness (resulting from its convective source) and through high oxygen. Antarctic Intermediate Water is a high latitude subtropical water mass, formed as the densest of the thick Subantarctic Mode Waters where the Antarctic Circumpolar Current is farthest south, just west of Chile. The SAMWs are located just north of the ACC and hence are in the subtropical gyres of the southern hemisphere. AAIW is subducted northward into the South Pacific's subtropical gyre. Part of the AAIW flows eastward through Drake Passage and enters the combined South Atlantic and Indian subtropical gyre through mixing in the Falkland (Malvinas) Loop.

Location of the three major salinity minimum intermediate waters (Figure). The X's mark the primary formation regions for each to the water masses. North Atlantic - Labrador Sea Water (extent of the potential vorticity minimum). North Pacific - North Pacific Intermediate Water (extent of salinity minimum in subtropical gyre, and indication that the ventilated water also completely covers the subpolar gyre, although NPIW is not a salinity minimum there since the surface waters are even fresher). Southern hemisphere - Antarctic Intermediate Water (extent of salinity minimum; extension of high silica northward in the subpolar North Atlantic is not indicated). Hatching indicates regions where mixing is critical to formation and modification. Clearly mixing occurs elsewhere as well, but these special areas set the properties for much larger regions. (From Talley, 1999.)

The influence of Antarctic Bottom Waters is illustrated with the location of a deep isopycnal (next figure), which is the lowest density water of Antarctic origin that is clearly separated from the Nordic Seas Overflow Waters (in the western North Atlantic). All higher densities south of the northern North Atlantic of course have significant Antarctic influence. At somewhat lower densities, one must more carefully differentiate between the Nordic Seas and Antarctic influences.

Location of isopycnal sigma_4 = 45.92 (Figure). (From Talley, 1999.)

Returning to one of the global overturning diagrams from Schmitz, we now look more carefully at the Antarctic sources as well as the North Atlantic sources of deepest waters. We divide the oceans again into four layers - thermocline, intermediate waters, deep waters and bottom waters.

The net meridional directions of water mass transports in each ocean are demonstrated with figures from papers showing net transport as a function of either depth of density. In the North Pacific (Roemmich and McCallister, 1989) we see the shallow overturning subtropical gyre in the upper ocean (northward Ekman transport balanced by southward gyre transport), and inflow at the bottom, returning southward at a level just above the bottom. In the South Pacific (Wunsch et al., 1983) the subtropical gyres are not clearly demonstrated. The AAIW (indicated as Circumpolar Intermediate Water) is shown moving northward. Northward inflow occurs at the bottom, and the thick Pacific Deep Water moves back to the south. In the North Atlantic (Roemmich and Wunsch, 1985), we again don't see the subtropical gyre well, but see northward flow of upper ocean waters, northward flow of Antarctic Bottom Water, and southward flow of North Atlantic Deep Water. In the South Atlantic (Rintoul, 1991), we see northward flow of AAIW and Upper Circumpolar Water, northward flow at the bottom (Antarctic Bottom Water, or Lower Circumpolar Water), and southward flow of the North Atlantic Deep Water. In the south Indian Ocean (Toole and Warren, 1993), we see northward flow of Circumpolar Water and North Atlantic Deep Water, and southward flow of thermocline water.

The total heat transports associated with these mass transports are shown on a map of net surface heat flux Superimposed on the figure are arrows and magnitudes (PW) of meridional ocean heat transport from Roemmich and Wunsch (1985, N. Atlantic), Rintoul (1991 - S. Atlantic), Robbins and Toole (1997- Indian), Roemmich and McCallister (1991 - N. Pacific), Wunsch et al. (1983 - S. Pacific). We note the northward direction of heat transport in the South Atlantic, in contrast with poleward heat transport in all other basins, due to location of northern hemisphere overturning (North Atlantic only, not North Pacific).

Net surface heat flux (W/m^2) from Hsiung, 1985 with net heat transports superimposed based on quoted references, from Talley, 1999.

Selected figures

Atlantic Ocean meridional section at about 25W

The data were described in previous lectures.

Pacific Ocean meridional section at about 150W

along a meridional section at approximately 150W.

Data were collected in 1984 (north of Hawaii), and in 1991 between 38S and Hawaii, and in 1992 south of 38S. The 1991 and 1992 portions were collected as part of WOCE. A reference for the 1984 portion is:

Talley, L.D., T.M. Joyce and R. A. deSzoeke, 1991. Transpacific sections at 47N and 152W: distribution of properties. Deep-Sea Res., 38 (Suppl), S63-S82.

Indian Ocean meridional sections at about 80E and 95E

The data were collected in 1994-1995 as part of WOCE.