Why is the Atlantic So Salty.

Lecture Figures (PowerPoint).

Comparing the Atlantic and Pacific Oceans

The surface waters of the Atlantic Ocean are a lot saltier than the Pacific surface water (Fig. 1), Why?

The water vapor transport (about 0.3 Sv, equal to about two Amazon Rivers) carried by the trade winds across the Isthmus of Central America (Fig. 2) may explain the salty North Atlantic. The freshwater evaporating from the North Atlantic subtropics feeds excess rainfall in the Pacific Ocean's western tropical regions. The Atlantic gets salty, the Pacific gets fresher. However the freshwater export across Central America can't explain the salty South Atlantic. That may be explained by the removal of water vapor from the South Atlantic subtropics to the Antarctic Circumpolar Current (Fig. 3) by a mean southwest-directed wind. The South Atlantic gets saltier and the excess freshwater is carried into the Indian and ultimately to the Pacific Ocean by the Antarctic Circumpolar Current. Both routes of water vapor, from the North Atlantic and from the South Atlantic, make for a salty Atlantic. But that's not all, there is also an influx of Indian Ocean water around the southern rim of Africa by the Agulhas Current (Fig. 4). This is referred to as the Agulhas Leakage, and this is where the story becomes interesting.

Naturally as water vapor is removed from the Atlantic Ocean by the winds and the Atlantic gets saltier there must be some compensating action that brings freshwater back to the Atlantic, otherwise the Atlantic will continuously become saltier, the Pacific fresher. Compensation is accomplished by interocean exchange, the fresher Pacific waters make their way to the salty Atlantic. Much of the interocean exchange is part of a sluggish global scale circulation associated with North Atlantic Deep Water.

Cooling of the salty surface water of the northern North Atlantic leads to the formation of North Atlantic Ocean Deep Water. The production rate is thought to be about 15 Sv. NADW flows southward between 1000 and 3500 m where upon reaching the Antarctic Circumpolar Current it upwells and advected eastward around Antarctica, into the Indian and Pacific Oceans. Slowly the NADW water enters into the upper kilometer of the ocean, altered in temperature and salinity. This surface layer water winds it way back to the Atlantic Ocean (Fig. 5, 6). Two important links in this global scale circulation occur just south of Africa- the Agulhas leakage and within the Indonesian seas, the Indonesian Throughflow.

Agulhas Leakage

On rounding the southern rim of Africa, the Agulhas Current abruptly turns anticlockwise, to flow back to the Indian Ocean as the Agulhas return current (Fig. 7). The special nature of the Agulhas retroflection stems from the unique regional geography and wind patterns. The southern coast of Africa is about 5° of latitude closer to the Equator than the westerly wind maximum, the latitude where western boundary currents in the subtropics are expected to separate from the continental margin to turn into their ocean's interior. The Agulhas 'runs out' of western continental margin before the wind allows for such separation. The resulting retroflection is shaped by the sea-floor morphology and by regional to large-scale winds; its form is expected to alter with changes of the maximum westerlies, or with the strength of the Agulhas Current. The warm surface water trapped within the Agulhas retroflection transfers heat to the atmosphere, the largest such exchange in the Southern Hemisphere. The effects are considerable, for instance in influencing rainfall patterns as far away as Australia or deep-ocean 'overturning' in the northernmost Atlantic.

The significance of the Agulhas retroflection does not end with its momentary loop into the southeast corner of the South Atlantic, for there are its outputs and inputs to consider. A major output, for instance, is the considerable •leakage' of water from the Indian Ocean into the upper kilometer of the Atlantic Ocean. The magnitude of this phenomenon has been the subject of conflicting results and much debate over the past 15 years, as have the wider implications. These include the effects of Agulhas leakage on the heat flow towards the Equator within the South Atlantic, and on the formation of NADW.

The various and widespread inputs to the Agulhas system are shown in Fig. 8. Streams of water are derived from far-distant parts of the Pacific Ocean, and may be part of a NADW-induced global balance. Red Sea intermediate water is part of the picture, as is eastward-flowing water in the South Atlantic, both of which contribute to the mix in the Indian Ocean. All of the return pathways must pass through or become entangled with the Agulhas retroflection, which may act as a 'valve' regulating the buoyancy of water in the upper kilometer of the South Atlantic Ocean and may in turn regulate NADW overturning. There are indications in models and paleo-climate data that the Agulhas valve does indeed play a central role in governing the formation rate of NADW and perhaps of the glacial ®interglacial swings.

The big ocean is the Pacific Ocean, and it provides important streams feeding the Agulhas, notably the Pacific to Indian Ocean flow through the Indonesian seas. Low salinity Pacific waters streak across the tropical Indian Ocean (Fig. 9).

Indonesian Seas Throughflow [ITF]

Pacific to Indian transfer of upper layer water within the Indonesian Seas strongly influences the Pacific and Indian heat FW budgets (Fig. 10). The Indonesian seas offer a complex array of pathways connecting the tropical Pacific and Indian Oceans (Fig. 11). The ITF is mostly composed of North Pacific water (Fig. 12, 13). There have been attempts to directly measure the ITF, such as the 1996-1998 current meter array set in Makassar Strait, considered as the main ITF pathway (Fig. 14, 15). The ITF amounts to about 10 SV (Fig. 16), but the magnitude depends on season and on the phase of El Niño (to be discussed in the ENSO lecture). Might severing the ITF or Agulhas leakage shut down or drastically reduce the NADW, and all of the climate phenomena linked to NADW (Fig. 17)?

Text by Arnold Gordon, 2004.