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Antarctic Sea Ice Trends of the Southern Ocean
Fan, T., Deser, C. and Schneider, D.P. 2014. Recent Antarctic sea ice trends in the context of Southern Ocean surface climate variations since 1950. Geophysical Research Letters 41: 2419-2426.

Neukom et al. (2014) have recently written that "earth's climate system is driven by a complex interplay of internal chaotic dynamics and natural and anthropogenic external forcing," and they note that "recent instrumental data have shown a remarkable degree of asynchronicity between Northern Hemisphere and Southern Hemisphere temperature fluctuations, thereby questioning the relative importance of internal versus external [i.e., anthropogenic] drivers of past as well as future climate variability," citing the work of Thompson et al. (2010), Deser et al. (2012) and Friedmann et al. (2013). And almost simultaneously, Fan et al. have produced additional data that are germane to this issue.

What was done
Fan et al. first examined "the relationships among Antarctic sea ice concentration, Southern Ocean sea surface temperature (SST), surface air temperature (SAT), sea level pressure (SLP), and surface zonal wind (U) trends over the period 1979-2011, using un-interpolated gridded surface marine data sets, land station archives, atmospheric reanalysis and satellite products," after which they extended their analysis of Southern Ocean climate trends "back to earlier decades (1950-1978) to illustrate the low-frequency behavior of surface climate trends over the Southern Ocean."

What was learned
"For the Southern Ocean as a whole," in the words of the three U.S. researchers, "sea surface temperature has decreased by approximately 0.6°C in December-February (0.4°C in the annual mean) while Antarctic sea ice cover has increased by approximately 9% in December-February (12% in the annual mean) during 1979-2011." In addition, they found that "all data sets (SST, SAT, SLP and U) show a consistent reversal in sign of the Southern Ocean surface climate trends between 1979-2011 and 1950-1978." And they also say that "the meridional extent of the Southern Ocean surface climate trends shows a consistent northward expansion of approximately 10° of latitude during 1950-1978 compared to 1979-2011 in all data sets," thereby providing "a broader context for the recent increase in Antarctic sea ice."

What it means
As for the cause of what they documented, Fan et al. say it could have been "low-frequency variability generated by tropical dynamics," citing Okumura et al. (2012) and Schneider and Noone (2012). Be that as it may, it is clear that the strong inter-hemispheric coupling that Neukom et al. (2014) found to be characteristic of the CMIP5 models is not what is found in real-world data, which suggests, in the words of Neukom et al., that "models overestimate the strength of externally-forced [i.e., anthropogenic] relative to internal climate system variability, therefore implying more limited predictability not only on regional (Dreser et al., 2012; Braconnot et al., 2012) but also hemispheric scales," which conclusion is now further confirmed by the work of Fan et al. (2014).

Braconnot, P., Harrison, S.P., Kageyama, M., Bartlein, P.J., Masson-Delmotte, V., Abe-Ouchi, A., Otto-Bliesner, B. and Zhao, Y. 2012. Evaluation of climate models using palaeoclimatic data. Nature Climate Change 2: 417-424.

Deser, C., Knutti, R., Solomon, S. and Phillips, A.S. 2012. Communication of the role of natural variability in future North American climate. Nature Climate Change 2: 775-779.

Friedmann, A., Hwang, Y., Chiang, J. and Frierson, D. 2013. Inter-hemispheric temperature asymmetry over the 20th century and in future projections. Journal of Climate 26: 5419-5433.

Neukom, R., Gergis, J., Karoly, D.J., Wanner, H., Curran, M., Elbert, J., Gonzalez-Rouco, F., Linsley, B.K., Moy, A.D., Mundo, I., Raible, C.C., Steig, E.J., van Ommen, T., Vance, T., Villalba, R., Zinke, J. and Frank, D. 2014. Inter-hemispheric temperature variability over the past millennium. Nature Climate Change 4: 362-367.

Okumura, Y., Schneider, D.P. and Deser, C. 2012. Decadal-intertidal climate variability over Antarctica and linkages to the tropics: Analysis of ice core, instrumental, and tropical proxy data. Journal of Climate 25: 7421-7441.

Schneider, D.P. and Noone, D.C. 2012. Is a bipolar seesaw consistent with observed Antarctic climate variability and trends? Geophysical Research Letters 39: 10.1029/2011GL050806.

Thompson, D.W.J., Wallace, J.M., Kennedy, J.J. and Jones, P.D. 2010. An abrupt drop in Northern Hemisphere sea surface temperature around 1970. Nature 467: 444-447.

Reviewed 6 August 2014