Meridional overturning circulation

In the open ocean, the major advective water motion is associated with the surfece-water geostrophic currents and meridional overturning circulation. These flow paths are shown in Figures 4.4b and 4.6. Advection is much fester than molecular diffusion and turbulence. This enables water masses to retain their original temperatures and salinities as they are advected away from their sites of formation. Slow turbulent mixing with adjacent water masses eventually alters this temperatme and salinity signal beyond... 

As noted in Chapter 4, the term meridional overturning circulation, abbreviated MOC, is used to collectively describe all the water movements associated with the creation, lateral flows, and upwelling of the subsurfece water masses. 

North Atlantic to 500 m in the North Pacific. This reflects an increasing addition of CO2 to deep waters as meridional overturning circulation moves them from the Atlantic to the Indian and then to the Pacific Ocean. Thus, as a water mass ages, it becomes more corrosive to calcium carbonate. Since aragonite is more soluble than calcite, its saturation horizon lies at shallower depths, rising from 3000 m in the North Atlantic to 200 m in the North Pacific. 

These data suggest that biogeochemical domains undergo a systems switch on time scales required for return of exported nutrients to the sea surfece. In oligotrophic waters, this return requires decades to centuries as the strong density stratification at these sites forces the return to proceed through the meridional overturning circulation. 

The initial decrease in ocean uptake rate will eventually be reversed once enough time has passed (millennia) for meridional overturning circulation to recycle the subsurface waters. This will serve to inject CO2 below the mixed layer. Once the surface waters have been sufficiently acidified and are transported into the deep sea, they will eventually start dissolving sedimentary calcium carbonate. The resupply of alkalinity via this route will provide sufficient enhancement to ocean carbon uptake capacity to... 

Other important feedbacks in the hydrological cycle include a possible shutdown or slowdown in meridional overturning circulation due to reduced formation of NADW. 

This is a matter of considerable controversy given the current lack of consensus on the importance of winds and tidally driven internal waves in driving meridional overturning circulation. Nevertheless, evidence for changes in circulation have been construed from a freshening of low-latitude surfece seawater in the Atlantic Ocean and a slowdown in NADW formation between 1998 and 2004. 

Cyclical phenomena, such as ENSO events, the North Atlantic Oscillation, the North Pacific Oscillation, and a phenomenon in the Southern Ocean called the Southern Annular Mode, have important impacts on the solubility pump. This was illustrated for the ENSO events in Figure 25.5b. These oscillations are all sensitive to global climate change. In the case of the Southern Ocean, the impact of climate change on circulation is complicated by the zonation associated with the polar and subpolar frontal boundaries (Figure 10.10). As a result, some parts of the Southern Ocean are expected to respond differently than others to changes in increases and meridional overturning circulation rates. 

Changes in climate and meridional overturning circulation are likely to affect plankton species distributions. As discussed in Chapter 25.5.3, such changes have the potential to alter the biological carbon pumps. The associated feedbacks are also likely to be affected due to synergistic interactions with other pollution problems including nutrient loading. 

Slowing of meridional overturning circulation in the North Atlantio from 1998 to 2004. Gulf Stream was deflected southward. Deep southerly flows deolined by 50%. 

Ecosystems changes due to weakening of the meridional overturning circulation ... 

Injection of compressed CO2 into the deep ocean has already been tested. The goal of this approach is to emplace the CO2 into waters with low temperatures, ensuring the formation of relatively immobile gas hydrates. This strategy has the potential to sequester thousands of gigatonnes of carbon, but likely environmental impacts include (1) a change in the pH in the seawater near the emplaced gas hydrates, (2) benthic kills, (3) other ecosystem impacts, and (4) release back to the atmosphere as an eventual consequence of meridional overturning circulation. 

Horizontal segregation The horizontal gradient in biogenic materials, such as nutrients and O2, that is established by the interacUon between the biogeochetnical cycling of pardculate organic matter and meridional overturning circulation. 

Meridional overturning circulation (MOC) Deep water circulation driven by thermohaline processes, tides and winds. 

---