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Freshwater inflow and bottom water formation explained
« on: August 14, 2013, 04:04:54 PM »
Is the following statement correct?

Even more ominously, a wedge of cold water at the surface spreading out from the poles would push hotter, saltier water toward the ocean bottom. Fresh water is less dense than salty water, so the fresh water pulses from glaciers and melting ice bergs will act as a wedge, driving the denser, warmer, saltier water toward the bottom The net effect of such changes would be a shallower and weaker ocean circulation system as more warm water is averted toward the ocean bottom near the equator and then spreads northward and as warmer surface waters toward the poles and temperature regions are driven toward the sea-bed.

If above statement is correct, how far (if) does this influence bottom water formation in the Arctic Circle, especially in regions like the ESAS?

Is there a better explanation about the impact of increased freshwater runoff into the Arctic through the major rivers? (Links to science papers are welcome)


Deep Arctic Ocean warming during the last glacial cycle
In the Arctic Ocean, the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier Atlantic Layer by a halocline. Ongoing sea ice loss and warming in the Arctic Ocean1, 2, 3, 4, 5, 6, 7 have demonstrated the instability of the halocline, with implications for further sea ice loss. The stability of the halocline through past climate variations8, 9, 10 is unclear. Here we estimate intermediate water temperatures over the past 50,000 years from the Mg/Ca and Sr/Ca values of ostracods from 31 Arctic sediment cores. From about 50 to 11 kyr ago, the central Arctic Basin from 1,000 to 2,500 m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1–2 °C warmer than modern Arctic Intermediate Water, with temperatures peaking during or just before millennial-scale Heinrich cold events and the Younger Dryas cold interval. We use numerical modelling to show that the intermediate depth warming could result from the expected decrease in the flux of fresh water to the Arctic Ocean during glacial conditions, which would cause the halocline to deepen and push the warm Atlantic Layer into intermediate depths. Although not modelled, the reduced formation of cold, deep waters due to the exposure of the Arctic continental shelf could also contribute to the intermediate depth warming.

An international team of scientists, including Martin Jakobsson from the Department of Geological Sciences and Johan Nilsson from the Meteorological Department at Stockholm University, has published a new study in Nature Geoscience entitled "Deep Arctic Ocean warming during the last glacial cycle”. The researchers have reconstructed the temperature history of the intermediate and deep Arctic Ocean during the past 50,000 years, using novel geochemical techniques on microfossils in sediment cores from across the central Arctic Ocean. Remarkably, the results show that in the last ice age, from about 50,000 to 11,000 years ago, the central Arctic Basin between 1,000 and 2,500 m water depth was occupied by water that was generally 1–2 °C warmer than in the modern Arctic. This extraordinary finding, indicating that the glacial Arctic Ocean operated in a different dynamical regime, challenges the view of a general glacial cooling of the ocean.

The Arctic region has received considerable attention due to its sensitivity to the changing climate. Of particular concern is the rapid decline of the summer sea ice extent, which this year even may approach another record low since satellite observations begun 1979. The sea ice in the Arctic forms at the top of the ‘halocline’, a 200–300 m thick layer of low salinity seawater capping the Arctic Ocean. The low salinity of the halocline layer is reflects the high freshwater to the Arctic Ocean. The halocline is also very cold, close to freezing point of seawater near -2°C, protecting the sea ice from the deeper laying warmer and more saline Atlantic Water Layer that flows into the Arctic Ocean from the North Atlantic.

The new study published as a Letter in Nature Geoscience shows that the warm intermediate Atlantic Layer was displaced far downward in the glacial Arctic Ocean, resulting in a substantial warming at depths between 1000 and 2500 m. Based on a conceptual oceanographic model, the researchers propose a mechanism for the subsurface warming of the glacial Arctic Ocean: A reduced influx of freshwater to the Arctic Ocean acted to deepen the halocline and push the warm Atlantic Layer downward. Based on their results, the researchers conclude that the Arctic Ocean has a previously unrecognized high sensitivity to changes of the freshwater input over multiple timescales, which is manifested in large temperature excursions of the intermediate water layers.
« Last Edit: August 15, 2013, 05:46:09 PM by prokaryotes »