SIO 210 Talley Topic 5: North Atlantic circulation and water masses. Thermohaline forcing.

Lynne Talley, 2000
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This lecture on the North Atlantic is divided into two major parts. The first concerns the wind-driven circulation, and is developed in parallel to the North Pacific lecture of topic 4. The principal purpose is to show the commonality of the elements of the wind-driven circulation, which then can also be extended to the three southern hemisphere subtropical gyres. The second portion concerns the thermohaline circulation, focused on the production of North Atlantic Deep Water, including the intermediate and deep inflows from the southern hemisphere, the transport of these waters through the North Atlantic, their modification therein, the three sites of convection that create the NADW, and the outflow of NADW back to the south.

This topic emphasizes the water masses of the North Atlantic, most recognized by at least one property extremum pointing to a particular formation mechanism. (The exception is the Central Water which has no property extrema but which is formed through a well-defined mechanism.)

Wind driven circulation, in parallel with the North Pacific topic

The prevailing winds north of the tropics consist of trades (easterlies) and the mid-latitude westerlies. These create Ekman convergence in the mid-latitude North Atlantic, driving the subtropical gyre. The northern side of the westerlies creates Ekman divergence at the higher latitudes of the North Atlantic, driving the subpolar circulation. The subtropical gyre's western boundary current is the Gulf Stream system. The subtropical gyre actually has two western boundary currents - the Gulf Stream, which is off the east coast of the U.S., and the North Atlantic Current, which is off the east coast of Newfoundland/Flemish Cap. The North Atlantic Current makes a major bend offshore at the southern entrance to the Labrador Sea (called the "northwest corner") and then extends eastward into the mid-Atlantic and then turns northward into the subpolar region as the Subarctic Front. The subtropical gyre also has an eastern boundary current called the Canary Current. The southern side of the subtropical gyre shrinks poleward (northward) and westward with increasing depth, that is, towards the Gulf Stream Extension. The wind-driven subtropical gyre is largely gone by about 2000 meters depth.

The subpolar circulation is not quite the closed cyclonic circulation of the North Pacific, because the northeastern boundary (between Greenland and the United Kingdom) is partially open, unlike the Gulf of Alaska. Part of the subpolar flow proceeds on north into the Norwegian Sea - this component is essentially part of the thermohaline circulation, discussed below. Part of the subpolar circulation makes its way westward past Iceland (although not at the sea surface) and follows the deep boundaries to Greenland, around Greenland into the Labrador Sea and then southward out of the Labrador Sea. Western boundary currents are found along the eastern side of Greenland ("East Greenland Current") and along the Labrador coast ("Labrador Current"). The strong boundary currents around the rim of the subpolar gyre penetrate to great depth without much change in velocity, unlike the subtropical currents. The Subarctic Front extends northeastward from the North Atlantic Current, and passes east of Iceland into the Norwegian Sea.

Figures. Cartoon from Tomczak and Godfrey. Reid (1994) adjusted steric height at the sea surface, 500 dbar, 800 dbar, 3000 dbar, 4000 dbar. Worthington (1976) "two gyres" (Gulf Stream and North Atlantic Current as two western boundary currents for the subtropical gyre) with transports. Wind stress curl pattern showing relation of wind to the Gulf Stream and North Atlantic Current (McCartney, 1982).

Gulf Stream

The Gulf Stream system has three named components - the Florida Current where it passes between Florida and the Bahamas, the Gulf Stream where it flows along the coast of the U.S. and the Gulf Stream Extension after it separates from the coast. The warm part of the Gulf Stream comes from the Gulf of Mexico, in which the circulation is first confined through the Yucatan Channel (between the Yucatan Peninsula and Cuba) and then forms a dramatic loop (the "Loop Current") in the Gulf of Mexico before exiting into the Florida Current. A marked westward flow exists south of the Gulf Stream ("Gulf Stream recirculation"). A westward flow usually carrying subpolar types of water is found north of the Gulf Stream ("Slope Water Current"). Floats deployed at several depths and current meter arrays across the Gulf Stream Extension show that the eastward flow penetrates to the ocean bottom. Near the ocean bottom there are narrow westward counterflows on either side of the Gulf Stream Extension and so therefore the full surface structure of the Gulf Stream Extension (strong eastward flow flanked by westward countercurrents) extends from top to bottom. This picture is much more striking than for the Kuroshio, whose structure is apparently similar to the ocean bottom, but upon which a weak westward deep flow appears to be superimposed.

The Gulf Stream is an energetic, unstable flow. It meanders vigorously, with the meanders occasionally cutting off to form rings. Anticyclonic warm core rings are found north of the Gulf Stream and cyclonic cold core rings are found south of the Gulf Stream. The rings migrate westward and occasionally remerge with the Gulf Stream.

There isn't time in this single lecture to discuss the North Atlantic Current, but the connection of some of the transport in the Gulf Stream Extension (which is well separated from the coast) to the North Atlantic Current is required by mass considerations for the thermohaline circulation. Inshore of the North Atlantic Current there is southward flow (between Newfoundland and Flemish Cap) and north of the separated Gulf Stream there is westward flow along the continental slope (called the "Slope Water Current").

Figures in overheads. Gulf Stream and Kuroshio anatomies from Kawai (1972). Davis (1991) and Owens (1992) floats at surface, 700 m and 2000 m. Schmitz (1980) current meters at 55W. Showing penetration of Gulf Stream to bottom, meandering, deep recirculations (westward flow on either side of the eastward Gulf Stream).

SST in the western N. Atlantic. Shown as a bright red band, the Gulf Stream is about 27C (~80F) in this sea surface temperature image of the Western North Atlantic during the first week of June 1984. This image is based on data from NOAA-7 Advanced Very High Resolution Radiometer (AVHRR) infrared observations, analyzed at the U. Miami Rosenstiel School's Remote Sensing Laboratory. Warmer hues denote warmer temperatures.
Credit: O. Brown, R. Evans and M. Carle, University of Miami Rosenstiel School of Marine and Atmospheric Science, Miami, Florida.

The Franklin-Folger map of the Gulf Stream, printed in 1769-1770. This early map of the Gulf Stream location was produced by B. Franklin for the mail service from England, based on information from whaling captain Timothy Folger. This map was rediscovered by P. Richardson (1980), and is remarkably accurate. This image is from R. Peterson et al. (1996), article in Progress in Oceanography.

Ventilation of the subtropical gyre

Subtropical Mode Water ("Eighteen Degree Water"). The isopycnal bowl immediately south of the Gulf Stream created by the Gulf Stream itself and its westward recirculation south of the Gulf Stream is a site of winter convection. Convection is favored here for two reasons - the isopycnal bowl has reduced vertical stratification compared with other regions simply because of the bowl, and heat losses to the atmosphere in this region are very large because of the conjunction of the warm Gulf Stream waters and cold, dry air blowing off the North American continent. The convected water mass is called "Eighteen Degree Water" because of its dominant temperature. It is the Subtropical Mode Water that is associated with the Gulf Stream. (Recall that there is an STMW for each of the subtropical gyres' western boundary currents.) The Eighteen Degree Water is spread by the circulation into the whole of the western subtropical gyre, even though the only location where it outcrops is close to the Gulf Stream. It is recognized far from its source as a vertical maximum in thickness between isopycnals, which results from the initial convection.

Potential temperature at 64W (crossing the Gulf Stream, showing 18 degree water).
Potential Density (0 dbar),
Potential Density (4000 dbar)
Oxygen (ml/l)

Central Water and Subtropical Underwater. As in the North Pacific, subduction in the Ekman convergence region moves water from the sea surface southward into subsurface layers of the subtropical gyre. Central Water is the general name for the whole of the thermocline. As mentioned in topic 4, Iselin (1939) proposed that the subtropical thermocline properties are set by this subduction process, which was further elaborated by Stommel and then received its name and formal theory from Luyten, Pedlosky and Stommel (1983). Also as mentioned before, vertical mixing between warm surface water and cold deep water is considered the other explanation for the thermocline; both subduction (along isopycnals) and mixing (across isopycnals) likely are operative.

The surface waters of the central subtropical gyre are very salty as a result of evaporation under the atmospheric high pressure region. As this salty water subducts southward beneath water that is not quite as saline, it forms a salinity maximum in the vertical. This salinity maximum is typical of every subtropical gyre (we did not describe it for the North Pacific), and is sometimes called Subtropical Underwater.

Subduction throughout the subtropical gyre is evidenced in salinity (and tritium) along isopycnals that outcrop in the subtropical gyre (Sarmiento figures). Somewhat thick near-surface layers in the northeastern subtropical gyre also subduct southward (temperatures around 11-12C) (cartoon from McCartney, 1982 including wind stress curl pattern).

Salinity along 25W showing subducted high salinity

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

(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.

A large number of vertical CTD sections can be accessed using the vertical section atlas for the Atlantic.

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

Potential Temperature,
Potential Density (0 dbar),
Potential Density (4000 dbar)
Oxygen (ml/l)

North Atlantic meridional section at 35W

Potential Temperature,
Potential Density (0 dbar),
Potential Density (4000 dbar)

North Atlantic zonal section at 47N

Potential Temperature,
Potential vorticity
Salinity as a function of density rather than depth

North Atlantic zonal section at 24N

Potential Temperature,