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Analyses based on SST anomalies will also have an uncertainty associated with the climatological reference fields used to calculate the anomalies. Sub-surface analyses have been shown to be particularly sensitive to choice of base period [Lyman et al., 2010], due in a large part to the relative sparseness of the data sets. Although the problem is likely to be less severe for the better-observed SST record, there are still regions the Southern Ocean and Arctic Ocean where observations are few. Yasunaka and Hanawa [2011] found that differences between long-term-average SSTs from different analyses were typically less than 0.5 K, but that they exceeded 1 K in places. The largest differences were at high latitudes and in regions with strong SST gradients. There are also likely to be pervasive systematic errors in the climatological averages [Kennedy et al., 2011c].
Other structural differences arise from the way that SSTs are extended to the edge of the sea ice. SSTs can be estimated from measurements of sea-ice concentration [Rayner et al., 2003; Smith et al., 2008; Hirahara et al., 2013, Huang et al. 2015, 2017]. Although their global impact is likely to be small, the uncertainties in these relationships and estimates need also to be factored into the uncertainty of SSTs in these regions. At the moment, the uncertainty associated with historical sea-ice concentrations is poorly understood. Huang et al. [2017] compared the effects of using two sea ice data sets, Rayner et al. [2003] and Titchner and Rayner [2014].
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