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Satellite altimeter observations provide a further data source for wave height variability since the mid-1980s. Altimetry is of particular value in the southern hemisphere, and in some poorly sampled regions of the northern hemisphere, where analysis of SWH trends remains a challenge due to limited in situ data and temporal inhomogeneity in the data used for reanalysis products. In the Southern Ocean, altimeter- derived SWH and model output both show regions with increasing wave height although these regions cover narrower areas in the altimeter analysis than in the models and have smaller trends (Hemer et al., 2010). Young et al. (2011a) compiled global maps of mean and extreme (90th and 99th percentile) surface wind speed and SWH trends for 1985–2008 using altimeter measurements. As the length of the data set is short, it is not possible to determine whether their results reflect long-term SWH and wind speed trends, or are part of a multi-decadal oscillation. For mean SWH, their analysis shows positive linear trends of up to 10 to 15 cm per decade in some parts of the Southern Ocean (with the strongest changes between 80°E and 160°W) that may reflect the increase in strength of the wind stress since the early 1980s (see Section 3.4.4). Young et al. (2011a) note, however, that globally the level of statistical significance is generally low in the mean and 90th percentile SWH trends but increases for the 99th percentile. Small negative mean SWH trends are found in many NH ocean regions and these are of opposite sign to, and thus inconsistent with, trends in wind speed — the latter being primarily positive. Nevertheless, for the 99th SWH percentile, strong positive trends up to 50 to 60 cm per decade were identified in the Southern Ocean, North Atlantic and North Pacific and these are consistent in sign with the extreme wind speed trends. Subsequent analysis has shown that the Young et al. (2011a) wind speed trends tend to be biased high when compared with microwave radiometer data (Wentz and Ricciardulli, 2011; Young et al., 2011b).

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