Contemporary sea level in the Chagos Archipelago, central Indian Ocean

Long term mean sea level in the Chagos Archipelago has been relatively stable over the ~ 20 year length of the available instrumental records. Tide-gauge data from Diego Garcia (1988–2000, and 2003–2011) show no statistically significant long-term rise, whilst the rates of rise obtained from the satellite altimeter record for 1993–2011 span the range of 0.16–4.56 mm yr− 1 in the surrounding sea areas (70–74°E and 4–9°S) and are also consistent with a zero rate except in the far south of the region. The dominant feature is one of considerable inter-annual variability in mean sea level of up to ~ 10 cm, such that the very weak seasonal pattern of highest and lowest sea level in February and May respectively, is absent or reversed in some years. The Indian Ocean Dipole appears to exert an important influence on mean sea level in the area, with positive and negative dipole mode indices preceding periods of elevated or lowered sea levels respectively. The Chagos also lie outside the Indian Ocean cyclone belt and experience relatively low wind speeds, and there is no evidence of changes in the wind or wave environment in the past 20 years. Although in an area of seismic activity, there is no record of island subsidence, indeed on Diego Garcia minor crustal uplift of 0.63 ± 0.28 SE mm yr− 1 has occurred between 1996 and 2009. Collectively, these results suggest that this has been a relatively stable physical environment, and that these low-lying coral islands should continue to be able to support human habitation, as they have done for much of the last 200 years. Nonetheless, future sea-level rise and its effect on the Chagos remains an important issue for further studies such as those of the Intergovernmental Panel on Climate Change.

► Long term sea level in the Chagos Archipelago has been stable in the last 20 years. ► The dominant feature is of large inter-annual variability in sea level. ► The Indian Ocean Dipole appears to exert an influence on sea level in this region.