Tides & Sea Level - 2021 Guide

Summary

Tidal form at Diego Garcia

1. The tidal pattern is semidiurnal with a range of 155 cm at springs and 42 cm at neaps.

2. There is significant variation in mean sea level (MSL) between months. Highest monthly MSL occurs in November each year with a lower peak in February and the lowest between May and August. The amplitude of the annual variation in monthly MSL is approximately 14 cm.

Diego Garcia sea level

1. Between 1996 and July 2021 (26 years) there has been no detectable vertical land movement on Diego Garcia.

2. Based on a 15-year tide-gauge record (Sep 2006-Jun 2021) mean sea-level rise (MSL) is estimated to be 5.09 mm yr^-1 (95% CI[1] -1.38 to 11.56).

3. From satellite altimetry data, MSL rise is estimated to be 4.12 mm yr^-1 (95% CI -2.14 to 10.39) between 1 Jan 1993 - 3 Jun 2020 (27 years). For the period corresponding to the tide gauge record the rate is 3.36 mm yr^-1 (95% CI ‑11.8 to 18.6).

4. Although the altimetry and tide-gauge rates appear to differ, the estimated trends and 95% confidence intervals overlap. It is not possible therefore to conclude that rates differ, nor would they be expected to do so given that there has been no detectable land movement at Diego Garcia.

5. The shortness of and variability in both the satellite altimetry and the tide gauge records mean that it is not yet possible to give a robust estimate of long-term trend.

6. There is evidence that the top 1% sea level quantile (extreme spring tides) is increasing faster than the rate of mean sea-level rise. It is possible that this is temporary, following high sea levels during the recent extreme pIOD[2] in Sep-Nov 2019.

7. Extreme high tides similar to those in March 2012 that caused flooding at the Donkey Gate on the western arm of the atoll have been present in 130 values of the hourly record since 2004. Most (62) occurred during the recent 2019 pIOD. There was also a higher frequency (17) of extreme high tides in 2020.

8. A review of the tide gauge data from Diego Garcia conducted in 2019 by the UHSLC[3] concluded that there were errors in the vertical referencing between 2003 and 2019. Major changes have since been made to the published data which means that computations of rates of sea-level rise based on the earlier data are now erroneous.

Peros Banhos

1. From satellite altimetry data, MSL rise is estimated to be 2.96 mm yr^-1 (95% CI -1.85 to 7.78) between 1 Jan 1993 - 3 Jun 2020 (27 years).

2. Rates of MSL rise decline with latitude towards the north of the Chagos sea area.

Other estimates of sea-level rise

1. Global MSL as measured by satellite altimetry rose by 3.3 ± 0.4 SE[4] mm yr^-1 over the period 1993-2020[5] (Annex A). Until a robust estimate exists for the Chagos area, this rate is likely to be the most realistic assessment.

2. A study in 2014 of 20^th Century regional sea-level change suggests that Chagos sea level rose by about 6 mm yr^-1 during the first half of that century (1901-1949) decreasing to 2 mm yr^-1 in the latter half (1950-1999).

3. The estimation of trends computed over very limited timespans where sea level fluctuates widely, are not robust and are particularly susceptible to extreme events if these coincide with the start and end points of the period of analysis. The sea level records for the Chagos fall into this category.

4. The latest IPCC assessment of sea level concludes that it is very likely to virtually certain that regional mean relative sea-level rise will continue throughout the 21st century and that extreme sea-level events that occurred once per century in the recent past are projected to occur at least annually at more than half of all tide gauge locations by 2100 (high confidence). Relative to 1995-2014, the likely global mean sea-level rise by 2100 ranges from 0.28-0.55 m under the very low GHG[6] emissions scenario to 0.98-1.88 m under the very high scenario. By 2050, sea level is expected to rise an additional 10–25 cm whether or not greenhouse gas emissions are reduced.

Regional Climatic factors and their contribution to sea level at Diego Garcia

1. Statistical models identify regional climatic factors which affect sea level. These include both the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO). Both exert a significant influence (10% and 17% respectively), with positive IODs and El Niño contributing to periods of elevated sea level and vice versa.

2. The 2019 positive IOD coincided with the highest observed sea levels in the Chagos. At Diego Garcia there were reports of flooding in November 2019. The highest daily MSL anomaly during this period at Diego Garcia was 42.5 cm and at Peros Banhos 36.0 cm.

3. The frequency of extreme positive IOD events (such as occurred in 1997 and 2019) has been forecast to increase under the highest GHG scenario (RCP 8.5) from one event every 17.3 years during the 20^th century to one event every 6.3 years during the 21^st century. Research findings concerning changes to El Niño occurrence under global warming remain equivocal.

4. Positive IOD extreme events and El Niños are likely to be a more important factor contributing to sea water inundation of low-lying parts of the Chagos islands in the near future compared with the effect of current rates of sea-level rise.

Richard Dunne

9 September 2021

[1] Confidence Interval. A CI that spans 0 as here indicates that the sea level could equally be falling.

[2] Positive Indian Ocean Dipole

[3] University of Hawaii Sea Level Center – the tide gauge owner/operator

[4] Standard Error

[5] https://sealevel.colorado.edu/data/2020rel1-global-mean-sea-level-seasonal-signals-retained

[6] Green House Gas

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