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The effect of Arctic precipitation changes on the Atlantic meridional overturning circulation

Sleen, N.R. van der (2015) The effect of Arctic precipitation changes on the Atlantic meridional overturning circulation.

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Abstract:In this study the effect of an increase in precipitation on the Atlantic meridional overturning circulation (AMOC) is studied. The processes involved that lead to a weakening of the AMOC, like the change in sea surface salinity, mixed layer depth and sinking are described and explained. This research is done because the IPCC expects an increase in precipitation of more than 50 % in some parts of the Arctic region with the RCP 8.5 scenario. The expected result of an increase in precipitation is that a relatively fresh surface layer will form on the ocean. If this is the case in the convection regions, the fresh surface layer will prevent convection to occur and therefore indirectly affect the AMOC. This study continues on the research performed by Bintanja & Selten (2014). They have performed model runs in which the precipitation above the Arctic Ocean is changed. The change in precipitation in the model runs range from a -50 % decrease to a +300 % increase of precipitation above the Arctic Ocean north of 70 °N. The model runs are performed with the coupled atmosphere (IFS), ocean (NEMO) and sea ice model (LIM) EC-Earth. The used version is 2.3 with a 1 degree resolution. In addition to the model runs performed by Bintanja & Selten also new model runs are performed to further study the effect of an increase in precipitation above 80 degrees north. The mixed layer depths and the AMOC are more sensitive to an increase in precipitation above the Arctic Ocean compared to the sea surface salinities. This is because the sea surface salinities have a much smaller natural variability. The sea surface salinities decrease with 6.8 %, the maximum mixed layer depth with 76 % and the strength of the AMOC decreases 43 % in the model run in which the precipitation is increased with 400 % in the region above 70 degrees north. Density profiles near the convection regions show that there will indeed form a layer of relatively fresh surface water on top of the ocean. This makes that the convection in these regions is much less. For the Nordic Seas this means that the deeper layers will be less dense, which results in less overflow at the Greenland Scotland ridge. In the Labrador Sea the fresh layer results in small mixed layer depths. This affects the amount of sinking according to the theory of Spall & Pickart (2001) which says that the amount of net sinking is dependent of the mixed layer depth. It turns out that 10.93 Sv sinks in the North Atlantic Ocean, this makes up a large part of the AMOC which has a strength of 12.91 Sv in the control run. The most important sinking regions in this study are the region west of Great Britain and the overflows at the Greenland Scotland ridge. The sinking region west of Great Britain is not in line with observations, this is an important result and in the further development of EC-Earth it is important to find out what causes the sinking in this region in the model. If the precipitation increases, the sinking as a result of the overflow at the GS ridge and the sinking west of Great Britain are decreasing the most. Therefore most of the decrease in AMOC strength because of an increase in precipitation in the Arctic can be attributed to these regions. If the precipitation is only increased in the region above 80 degrees north instead of 70 degrees north, the system responds differently. The average value of the mixed layer depth in the Nordic Seas is less affected, also the sinking in the North Atlantic Ocean decreases less. The cause of this is that the precipitation is not directly increased above the convection region in the North Atlantic Ocean. For the strength of the AMOC it does not matter whether the precipitation is increased above 70 degrees north of 80 degrees north.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/68772
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