220.127.116.11 Regional Sea Level Change
Two regions are discussed here to give examples of local variability in sea level: the northeast Atlantic and small Pacific Islands.
Interannual variability in northeast Atlantic sea level records exhibits a clear relationship to the air pressure and wind changes associated with the NAO, with the magnitude and sign of the response depending primarily upon latitude (Andersson, 2002; Wakelin et al., 2003; Woolf et al., 2003). The signal of the NAO can also be observed to some extent in ocean temperature records, suggesting a possible, smaller NAO influence on regional mean sea level via steric (density) changes (Tsimplis et al., 2006). In the Russian Arctic Ocean, sea level time series for recent decades also have pronounced decadal variability that correlates with the NAO index. In this region, wind stress and atmospheric pressure loading contribute nearly half of the observed sea level rise of 1.85 mm yr–1 (Proshutinsky et al., 2004).
Small Pacific Islands are the subject of much concern in view of their vulnerability to sea level rise. The Pacific Ocean region is the centre of the strongest interannual variability of the climate system, the coupled ocean-atmosphere ENSO mode. There are only a few Pacific Island sea level records extending back to before 1950. Mitchell et al. (2001) calculated rates of relative sea level rise for the stations in the Pacific region. Using their results (from their Table 1) and focusing on only the island stations with more than 50 years of data (only 4 locations), the average rate of sea level rise (relative to the Earth’s crust) is 1.6 mm yr–1. For island stations with record lengths greater than 25 years (22 locations), the average rate of relative sea level rise is 0.7 mm yr–1. However, these data sets contain a large range of rates of relative sea level change, presumably as a result of poorly quantified vertical land motions.
An example of the large interannual variability in sea level is Kwajalein (8°44’N, 167°44’E) (Marshall Archipelago). As shown in Figure 5.18, the local tide gauge data, the sea level reconstructions of Church et al. (2004) and Church and White (2006) and the shorter satellite altimeter record all agree and indicate that interannual variations associated with ENSO events are greater than 0.2 m. The Kwajalein data also suggest increased variability in sea level after the mid-1970s, consistent with the trend towards more frequent, persistent and intense ENSO events since the mid-1970s (Folland et al., 2001). For the Kwajalein record, the rate of sea level rise, after correction for GIA land motions and isostatic response to atmospheric pressure changes, is 1.9 ± 0.7 mm yr–1. However, the uncertainties in rates of sea level change increase rapidly with decreasing record length and can be several mm yr–1 for decade-long records (depending on the magnitude of the interannual variability). Sea level change on the atolls of Tuvalu (western Pacific) has been the subject of intense interest as a result of their low-lying nature and increasing incidence of flooding. There are two records available at Funafuti, Tuvalu; the first record commences in 1977 and the second (with rigorous datum control) in 1993. After allowing for subsidence affecting the first record, Church et al. (2006) estimate sea level rise at Tuvalu to be 2.0 ± 1.7 mm yr–1, in agreement with the reconstructed rate of sea level rise.
Figure 5.18. Monthly mean sea level curve for 1950 to 2000 at Kwajalein (8°44’N, 167°44’E). The observed sea level (from tide gauge measurements) is in blue, the reconstructed sea level in red and the satellite altimetry record in green. Annual and semi-annual signals have been removed from each time series and the tide gauge data have been smoothed. The figure was drawn using techniques in Church et al. (2004) and Church and White (2006).