Circulation Driving Mechanisms

A striking feature of the temperature distribution in the oceans Fig. 1 displays a section of temperature along the meridional extent of the Atlantic is the strong vertical contrast in temperatures at low and mid-latitudes, with warm upper-ocean waters floating on top of cold deep and abyssal waters. The vertical layering of waters of different temperatures (densities) is referred to as stratification. It was already recognised as early as 1798 by Count Rumford that in the absence of any deep-ocean heat sinks at low latitudes those cold waters had to originate from high latitudes propagating equatorward at depth.6 Today, it is well established that the observed temperature distribution is a consequence of the AMOC, that moves roughly 19 Sv of warm, saline waters northward throughout the Atlantic and the same amount of cold water back south at depth ([16,17]; Fig. 2). Carried northward within the Gulf Stream/North Atlantic Current system the near surface waters release heat to the atmosphere and thus become gradually denser. The waters eventually reach the Nordic Seas and the Labrador Sea. Here, deep-reaching wintertime convection (i.e. vertical mixing throughout the upper 2000 m of water column) can occur [4,18,19], when the vertical stratification has eroded after periods of excessive heat loss (Fig. 2). The bulk of the newly formed deep waters that

6 Longworth and Bryden [15] give an exciting account of the history of the recovery of the Atlan tic Meridional Overturning Circulation.

FIGURE 1 Section of potential temperature along the meridional extent of the Atlantic. For tem peratures less than 5 °C and greater than 5 °C, the black contours have a spacing of 0.2 and 1 °C, respectively. The red, indian red, salmon, cyan, light blue and dark blue areas denote temperatures above 16 °C, from 10 to 16 °C, from 4 to 10 °C, from 3 to 4 °C, from 1 to 3 °C and below 1 °C, respectively. Lowered temperature measurements acquired during three research expeditions aboard RV Ronald H. Brown in 2003 (section A16N, PI: Bullister [PMEL]) and in 2005 (section A16S; PIs: Wanninkhof [NOAA]/Doney [WHOI]) and aboard RV James Clark Ross in 1995 (section A23; PIs: Heywood/King [NOCS]) were joined together to compile this figure. Data source: Clivar and Carbon hydrographic data office (http://whpo.ucsd.edu/atlantic.htm). Adapted from a figure of Lynne D. Talley (http://sam.ucsd.edu/vertical sections/Atlantic.html#a16a23).

FIGURE 1 Section of potential temperature along the meridional extent of the Atlantic. For tem peratures less than 5 °C and greater than 5 °C, the black contours have a spacing of 0.2 and 1 °C, respectively. The red, indian red, salmon, cyan, light blue and dark blue areas denote temperatures above 16 °C, from 10 to 16 °C, from 4 to 10 °C, from 3 to 4 °C, from 1 to 3 °C and below 1 °C, respectively. Lowered temperature measurements acquired during three research expeditions aboard RV Ronald H. Brown in 2003 (section A16N, PI: Bullister [PMEL]) and in 2005 (section A16S; PIs: Wanninkhof [NOAA]/Doney [WHOI]) and aboard RV James Clark Ross in 1995 (section A23; PIs: Heywood/King [NOCS]) were joined together to compile this figure. Data source: Clivar and Carbon hydrographic data office (http://whpo.ucsd.edu/atlantic.htm). Adapted from a figure of Lynne D. Talley (http://sam.ucsd.edu/vertical sections/Atlantic.html#a16a23).

are subject to overflow and entrainment processes constitute the North Atlantic Deep Water (NADW). The NADW is subsequently exported southward, partly confined to the deep western boundary current (DWBC) along the Americas below roughly 1000 m. The intensity of the strongly localised, buoyancy-loss induced formation of NADW at high latitudes (Fig. 2) 'pushing' surface waters downwards has long been thought to control the strength of the AMOC.

To close the circulation, the dense NADW needs to return to the upper ocean eventually. This is assumed to be accomplished mainly by two processes. The first process relates to winds and tides that represent the major sources of mechanical energy input into the ocean [20]. Ultimately, this energy input is balanced by dissipation into small scale motions, a process by which turbulent mixing occurs. Dissipation and mixing are ubiquitous in the open ocean; however, they seem most active in the vicinity of rough bathymetry such as exhibited by mid-oceanic ridges [21]. As a consequence deeper (denser) waters from below are mixed with overlying warmer (less

^ Surface flow

G Wind-driven upwelling

L

Labrador Sea

^ Deep flow

Mixing-driven upwelling

N

Nordic Seas

_ Bottom flow

Salinity > 36 %»

W

Weddell Sea

m Deep Water Formation

■ Salinity < 34 %„

R

Ross Sea

FIGURE 2 Strongly simplified sketch of the global overturning circulation system. In the Atlan tic, warm and saline waters flow northwards all the way from the Southern Ocean into the Lab rador and Nordic Seas. By contrast, there is no deep water formation in the North Pacific and its surface waters are fresher. Deep waters formed in the Southern Ocean are denser and thus spread in deeper levels than those from the North Atlantic. Note the strongly localised deep water formation areas in comparison with the wide spread zones of mixing driven upwelling. Wind driven upwelling occurs along the Antarctic Circumpolar Current (ACC). This figure has been published by Kuhlbrodt et al. [17].

FIGURE 2 Strongly simplified sketch of the global overturning circulation system. In the Atlan tic, warm and saline waters flow northwards all the way from the Southern Ocean into the Lab rador and Nordic Seas. By contrast, there is no deep water formation in the North Pacific and its surface waters are fresher. Deep waters formed in the Southern Ocean are denser and thus spread in deeper levels than those from the North Atlantic. Note the strongly localised deep water formation areas in comparison with the wide spread zones of mixing driven upwelling. Wind driven upwelling occurs along the Antarctic Circumpolar Current (ACC). This figure has been published by Kuhlbrodt et al. [17].

dense) waters, thus making deep waters gradually lighter. This allows them to rise and to return to the upper ocean. The fact that as a direct consequence of vertical mixing even at the deep ocean below 1000 m exhibits a notable stable stratification (i.e. water becoming denser with depth, as shown in Fig. 1) has been used to argue that dissipation induced vertical mixing 'pulling' deep water upwards might ultimately have a stronger control on the vigour of the AMOC than the downwards 'pushing' at high latitudes. To move waters vertically across surfaces of constant density, vertical mixing is required and this cannot be generated by high latitude buoyancy forcing [22].

The second potentially powerful mechanism to 'pull' deep water back to the upper ocean to close the overturning circulation can be motivated by a careful inspection of Fig. 1. The NADW flows southward away from the regions of its formation and eventually partly reaches the Southern Ocean. While north of 40° S the deep surfaces of constant temperature show only a weak upwards slope towards the south, the situation changes dramatically south of 40°S. This is a direct result of 70% of the global wind energy input into the ocean taking place in this area. Due to the Ekman balance the strong westerly winds over the Southern Ocean push large amounts of near-surface waters northward, which are then replaced by waters being sucked upwards from the deep ocean. The manifestation of this process is the drastic increase in the upwards tilt of the deep temperature surfaces towards the south (Fig. 1). In this scenario, the transition from cold to warm waters (mixing across density surfaces) occurs near the sea surface in the Southern Ocean as suggested from model findings by Toggweiler and Samuels [23]. Whether the Southern Ocean's control on the vigour of the AMOC is stronger than that deep-ocean mixing mechanism is subject to current debate [17], as clear observational evidence is still lacking.

Besides the AMOC, a second major pattern of meridional overturning exists. This involves formation of deep waters by means of convection around Antarctica. The Antarctic Bottom Water (AABW) spreads northward and represents the coldest and therefore deepest water mass in the Atlantic, Pacific and Indian Oceans (roughly represented by the dark blue shaded part of the temperature field in Fig. 1).

In the Atlantic, the waters gradually mix into the lower parts of the overlying NADW, and eventually return southward. Even though the volume of NADW flowing southward and AABW moving northward are comparable in size [24] or possibly larger for the southern cell [25], the contribution of the AABW related meridional overturning cell to meridional heat transport is negligible, as the vertical temperature contrast between its upper and low branches is very small [26].

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