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Simulating Earth's Southern Ocean and Its Climate

Reference
Weijer, W., Sloyan, B.M., Maltrud, M.E., Jeffery, N., Hecht, M.W., Hartin, C.A., van Sebille, E., Wainer, I. and Landrum, L. 2012. The Southern Ocean and its climate in CCSM4. Journal of Climate 25: 2652-2675.
Writing as background for their study, Weijer et al. (2012) state that "the Southern Ocean is a region of extremes: it is exposed to the most severe winds on the Earth (Wunsch, 1998), the largest ice shelves (Scambos et al., 2007), and the most extensive seasonal sea ice cover (Thomas and Dieckmann, 2003)." And they indicate that various interactions among the atmosphere, ocean, and cryosphere in this region "greatly influence the dynamics of the entire climate system through the formation of water masses and the sequestration of heat, freshwater, carbon, and other properties (Rintoul et al., 2001)." Against this backdrop, Weijer et al. "explored several key aspects of the Southern Ocean and its climate in the new Community Climate System Model, version 4 (CCSM4)," including "the surface climatology and inter-annual variability, simulation of key climate water masses (Antarctic Bottom Water [AABW], Subantarctic Mode Water [SAMW], and Antarctic Intermediate Water [AAIW]), the transport and structure of the Antarctic Circumpolar Current [ACC], and inter-basin exchange via the Agulhas and Tasman leakages and at the Brazil-Malvinas Confluence [BMC]."

In describing their findings, the nine researchers state that "the CCSM4 has varying degrees of accuracy in the simulation of the climate of the Southern Ocean when compared with observations," some of which we list as follows: (1) "the seasonally ice-covered regions are mildly colder (ΔSST > -2°C) than observations," (2) "sea ice extent is significantly larger than observed," (3) "north of the seasonal ice edge, there is a strong (-4°C < ΔSST < -1°C) cold bias in the entire Pacific sector south of 50°S and in the western Australian-Antarctic Basin," (4) "positive biases (1° < ΔSST < 4°C) are found in the Indian and Atlantic sectors of the Southern Ocean," (5) "significant differences are found in the Indian and Pacific sectors north of the ACC, with the CCSM4 model being too cold (< -2°C) and fresh (<-0.3 psu)," (6) "AABW adjacent to the Antarctic continent is too dense," (7) "North Atlantic Deep Water is too salty (>0.2 psu)," (8) "in the Indian and Pacific sectors of the Southern Ocean, north of 50°S and below 3000 meters, the too-salty AABW penetrates northward, resulting in a denser-than-observed abyssal ocean in CCSM4," (9) "the model underestimates the depth of the deep winter mixed layers in the Indian and eastern Pacific sectors of the Southern Ocean north of the ACC," (10) "in the southern Tasman Sea and along the eastern Indian Ocean boundary ... the model mixed layer depth is deeper than observed by more than 400 meters," (11) "in all sectors of the Southern Ocean, Model CFC-11 concentrations in the lower thermocline and intermediate waters are lower than observed," (12) "model CFC-11 concentrations in the deep ocean (below 2000 meters) are lower than observed in the basins adjacent to the Antarctic continent," (13) "model surface CFC-11 concentrations are higher than observed," (14) "the production of overflow waters in the Ross Sea is too low by about a factor of 2 relative to the limited observations," (15) "the depth at which the product water settles was also shown to be too shallow by about a factor of 2," (16) "the subtropical gyre of the South Atlantic is too strong by almost a factor of 2, associated with a strong bias in the wind stress," (17) the mean position of the BMC is too far south in the CCSM4," and (18) "the model variability in the position of the BMC is significantly less than observations."

In light of their several findings, Weijer et al. conclude that as the CCSM4 currently stands, it "may underestimate the sequestration of heat, carbon, and other properties to the interior ocean," with the result that its parameterizations may "lead to significant biases in the representation of the Southern Ocean and its climate," which is not a characteristic one would hope to see in the models.

Additional References
Rintoul, S.R. and Sokolov, S. 2001. Baroclinic transport variability of the Antarctic Circumpolar Current south of Australia (WOCE repeat section SR3). Journal of Geophysical Research 106: 2815-2832.

Scambos, T.A., Haran, T.M., Fahnestock, M.A., Painter, T.H. and Bohlander, J. 2007. MODIS-based Mosaic of Antarctica (MOA) data sets: Continent-wide surface morphology and snow grain size. Remote Sensing of Environment 111: 242-257.

Thomas, D.N. and Dieckmann, G. 2003. Sea Ice: An Introduction to Its Physics, Chemistry, Biology, and Geology. Wiley-Blackwell, Hoboken, New Jersey, USA.

Wunsch, C. 1998. The work done by the wind on the oceanic general circulation. Journal of Physical Oceanography 28: 2332-2340.

Archived 12 September 2012