![]() Today this is occurring through the production and export of North Atlantic Deep Water.At times in the past, rapid melting of ice sheets surrounding the North Atlantic was great enough to alter surface salinities, likely reducing the density of deep water formed, and slowing the export of deep water from the North Atlantic. If the Atlantic salinity is in balance, then it must be exporting the excess salt (enough to compensate for the lost fresh water) through ocean circulation processes. ![]() In the absence of other processes, this would raise the salinity of the Atlantic by about 1 salinity unit each 1000 years. (1990a) noted that higher Atlantic salinities are the result of a net transfer of water vapor from the Atlantic to the Pacific over the Isthmus of Panama, equivalent to approximately 0.35 Sverdrup (10 6 m 3 per second). Interestingly, Warren also noted that the North Atlantic had much greater river runoff than the North Pacific, so its higher surface salinities must be the result of greater evaporation in the Atlantic basin.īroecker et al. (2003) reevaluated the Warren (1983) results and fundamentally confirmed his thesis, noting that atmospheric moisture transport from the Asian monsoon was also an important source of fresh water to the North Pacific not originally considered by Warren. Warren (1983) noted that the salinity of the North Pacific was low because of relatively low evaporation, little exchange with salty tropical waters, and an influx of fresh water from precipitation and river runoff. Wintertime cooling occurs in both the North Atlantic and North Pacific, but since the surface waters of the North Atlantic are much closer in salinity to the mean of the ocean's deep water, they achieve a density high enough to sink to great water depths. Salty water, when cooled, achieves a higher density and is thus able to sink to greater depth in the water column. Warren (1983) first noted that the difference in salinity between the North Pacific and the North Atlantic (Figure 1) was the principal reason deep water formation occurs today only in the North Atlantic. The residence time of deepwater in the western Atlantic is approximately 100 years (Broecker 1979), meaning that the average water parcel spends about a century in the deep Atlantic. The presence of these three water masses in the Atlantic Ocean is evident in cross-sections of many water properties, including salinity, phosphate concentration and carbon isotope ratios (Figure 2). Antarctic Intermediate Water (AAIW) flows northward above NADW. AABW is confined to water depths below 4000 meters in the tropical and North Atlantic. Antarctic Bottom Water (AABW), which is formed close to Antarctica, is denser than NADW, and flows northward in the Atlantic below NADW. This North Atlantic Deep Water (NADW), as it is called, flows from the Atlantic to the Southern Ocean where much of it upwells - or rises to the surface - around Antarctica, and some of it circulates Antarctica before entering the rest of the world's deep oceans. Once dense enough, these waters sink and flow southward between ~ 10m. Warm salty tropical surface waters flowing northward in the western Atlantic cool in transit to and within the high-latitude North Atlantic, releasing heat to the overlying atmosphere and increasing seawater density. The Atlantic Ocean is the only ocean basin that features the transformation of surface-to-deepwater near both poles. Reconstructions of past ocean circulation using the geochemistry of microfossils preserved in marine sediments provide critical information to test these models. Instead, scientists use computer climate models to predict how the Earth's climate will change. Unfortunately the observations are too limited to provide insight into how the deep oceans and climate interact on the longer time scales of ocean circulation and also how this interaction might change in response to rising greenhouse gases. Subsequent measurements revealed that properties of deep water in key regions vary from decade to decade, and that these changes are linked to oscillations in surface climate (Dickson et al. The first field program to systematically measure physical and chemical properties of all the world's deep oceans took place from 1973–1978. However, knowledge of the deep oceans, their properties, and their climatic significance has been acquired relatively recently. ![]() Early seafaring nations recognized the practical and economic benefits of mapping surface currents and winds in great detail. ![]()
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