Programa de evaluación

Several analyses of the sea-level data for Tarawa Atoll since 1974 have been undertaken. The first analysis was performed by Howorth in 1985 in order to relate the elevations of the beaches measured from the beach profiles to MSL. As discussed earlier in Section 2.3.7, MSL at this time was based on the Cox datum. Using the UH tide gauge data from 1974 to 1976, MSL was calculated to be 1.082 m above Cox datum which is expressed as 1.189 m above UH tide gauge zero (Howorth, 1985) (Table 2.2) (Figure 2.17).

Table 2.2. MSL values from Tarawa Atoll tidal records and their characteristics

MSL Heights (m) Heights (m) above

SEAFRAME datum Period Source

MSL 1 1.189 above UH 1.608 1974 - 1976 Howorth, 1985

MSL 2 1.615 above UH 2.034 1980 - 1999 Solomon, 1997

MSL 3 or MSL 1.637 above SEAFRAME 1.637 1974 - 2008 Ramsay et al. 2010

The second analysis of the sea-level data for Tarawa Atoll since 1974 covering 23 years shows an average rise of +0.078 m (Solomon, 1997) (Tables 2.2 and 2.3). However, the calculated rate of sea-level rise over that period seems abnormally high at a rate of 33 mm/yr, which Solomon suggests maybe attributed to uncertainty surrounding the vertical datum as it moved several times. Based on the data, Solomon proposes that the sea-level pattern shows a rise, but the rate is uncertain. Contributing towards this uncertainty is the 0.419 m difference between the tide datums, which was later discovered in 2011. Another aspect of sea level calculated

38 from this analysis is the MSL, which is reported to be +1.615 m above UH datum in 1980. In Table 2.2 and Figure 2.17, this is referred to as MSL 2.

Table 2.3. Calculated rates of sea-level rise around Tarawa Atoll from tide records and their characteristics

Rate Tide datum Dates (period) Source

33 mm/yr UH 23 yrs ( 1974 to 1997) Solomon, 1997

2.9 mm/yr SEAFRAME 17 yrs (1993 to 2010) Bureau of Meteorology, 2010

1.7 mm/yr UH 9 yrs (1988 to 1997) Becker et al. 2012

2.2 mm/yr UH 22 yrs (1988 to 2009) Becker et al. 2012

1.8 mm/yr SEAFRAME 35 yrs (1974 to 2008) Ramsay et al. 2010

A third, and more recent, analysis calculated the average sea-level rise for Tarawa Atoll to be 2.2 mm/yr (Becker et al., 2012). This analysis involved two datasets: 1) a 9 year tide record (1998–1997) when the UH tide gauge was reinstalled in Betio but excluding the SEAFRAME data, and 2) satellite altimetry data covering 13 years (1997-2009). The average sea-level rise calculated from the tide gauge record shows a rise of 1.7 mm/yr. Analysis of the longest sea-level records for Tarawa Atoll shows MSL of 1.637 (Table 2.2, Figure 2.17) and a linear rise of 1.8 mm/yr over the past 35 years since 1974 (Ramsay et al., 2010). The two rates are comparable even though one is derived from much shorter records. The difference between the rates of sea level rise presented in Table 2.3 is associated with the large effect of ENSO and the duration of the data records. In the case of the research by Becker et al. (2012), the inclusion of the satellite altimetry data may have contributed to the large increase in sea level, as measurements were obtained from a wider grid. The other common factor of the studies by Solomon, Becker and Bureau of Meteorology, is that they all fall short of the required 25 years to remove any noise associated with ENSO and the Interdecadal Pacific oscillation (IPO) (Ramsay et al., 2010). The enormous difference in rates of sea-level rise observed by Solomon (33 mm/yr) and Ramsay (1.8 mm/yr) may be associated with differences in tide gauge datums and non- adjustment of the historical data to SEAFRAME datum making the results obtained from Solomon unreliable. The average rate provided by Ramsay incorporates these factors and is reported relative to SEAFRAME datum.

The rate of sea-level rise of 1.8 mm/yr obtained from Ramsay et al.(2010) is comparable to other global rates (Church, 2006), including that of the western Pacific

39 (Becker et al., 2012) but higher than the regional rate of +1.4 mm/yr (Church et al., 2006). The reason for this is that the local average sea level is derived from site- specific data located close to the equator where the influences of ENSO are greater, whereas the regional average is obtained from a larger area spread across the Indo- Pacific region (Ramsay et al., 2010). The study by Ramsay shows that with longer trends, the pattern of sea-level rise, the variability, and the factors that control it may be estimated.

Sea level in Tarawa Atoll and the Gilbert Group is influenced by several other variables including astronomical tides and storm surges (Figure 2.19). These factors have been discussed by Ramsay et al. (2010). The astronomical tide in Tarawa Atoll is semi-diurnal i.e. that there are two high tides and two low tides each day. These tides result from the gravitational effects of the moon and the sun on the earth and can be classified as Mean High Water Perigean Spring (MHWPS), Mean High Water Spring (MHWS) and Mean High Water Neap (MHWN). The effect of astronomical tides especially MHWPS which is also known as King tides can be devastating on low-lying areas along South Tarawa, particularly so if they coincide with strong wind conditions. This scenario occurred in February, 2005 and several assets on South Tarawa such as causeways and Betio Hospital were threatened by a king tide and strong winds (Donner, 2012). Finally, sea level is affected by storm surges caused by low barometric pressure and wind set-up.

(0.43 m), and c) storm surges (0.32 m) (Figure 2.18).

SEAFRAME DATUM 0

Figure 2.19. Magnitude of influences on the mean sea level (MSL) (modified from Meteorology and CSIRO, 2011, p 48).

40 As Tarawa Atoll lies outside the cyclone belt, sea level is rarely affected by intense storms and cyclones. Storm surge is the difference between observed still water level at a tide gauge, (not including setup and runup) and predicted (tidal) water level (Ramsay et al., 2010). Storm surges of less than 0.15 m primarily affects the ocean shorelines for a short length of time (maximum 3 days) (Ramsay et al., 2010). But compared to the effects of wave setup caused by waves breaking on the seaward edge of the reef flat raising the water levels up to 1 m this is very minimal.

Mean sea level varies constantly. In order to capture that variability Ramsay et al. (2010) considered it is best reported as Mean Level Of the Sea (MLOS). They define MLOS, as the average level of the sea over a certain period of interest; for example the MLOS in Ramsay et al. study is the average level of the sea from 1974 to 2007. The tide gauge data over the past 33 years show that the MLOS is 1.637 m (1974 – 2007). The MLOS reaches a maximum value of more than 1.72 m and a minimum of about 1.45 m. For the 33 years from 1974 to 2007, the heights of the astronomical tides are as follows, MHWPS is 2.641m, MHWS is 2.528 m and MHWN is 1.930 m (Table 2.4). The total sea level range in relation to SEAFRAME datum is 3.0 m. The factors that vary sea level in descending order of contribution are: a) astronomical tides (with a maximum range of 2.25 m); b) long-term fluctuations in MLOS due to factors such as ENSO and c) storm surges (0.32 m) (Figure 2.20). Storm tide is the temporary increase in water level which develops offshore beyond the wave breaker. The storm tides are a combined effect of MLOS, astronomical tides and storm surge height (Ramsay et al., 2010).

Table 2.4. Tide elevations in relation to MLOS based on the 33 year tide data relative to SEAFRAME datum (source: Ramsay et al. 2010).

MHWPS (m) above MLOS relative to SEAFRAME

MHWS (m) above MLOS relative to SEAFRAME

MHWN (m) above MLOS relative to SEAFRAME

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