PACIFIC COUNTRY REPORT

PACIFIC COUNTRY REPORT

SEA LEVEL & CLIMATE: THEIR PRESENT STATE

TUVALU

June 2004

Executive Summary

● A SEAFRAME gauge was installed in Funafuti, Tuvalu, in March 1993. It records sea
level, air and water temperature, atmospheric pressure, wind speed and direction. It is
one of an array designed to monitor changes in sea level and climate in the Pacific.

●  This report summarises the findings to date, and places them in a regional and
historical context.

● The sea level trend to date is +5.9 mm/year (as compared to a global average of 1-2
mm/year) but the magnitude of the trend continues to vary widely from month to month
as the data set grows. Accounting for the geodetic survey results and inverted
barometric pressure effect, the trend is +5.4 mm/year. A nearby gauge, with a longer
record but less precision and datum control, shows a trend of +0.9 mm/year.

●  Variations in monthly mean sea level are dominated by seasonal cycles and were
affected by the 1997/1998 El Niño.

● Variations in monthly mean air and water temperatures are likewise dominated by
seasonal cycles and were affected by the 1997/1998 El Niño.

● The seasonal cycle shows a peak early in the year, a time when Funafuti frequently
experiences flooding. The seasonal cycle is due to a combination of atmospheric
factors, but is sometimes exacerbated by a local tidal effect which is due to the
geometry of the atoll lagoon.

● Since installation, at least two cyclones have passed through Tuvalu, but only one,
Tropical Cyclone Gavin, was registered as extreme low pressure at Funafuti.

●  A tsunami caused by the Peru earthquake of June 2001 was registered by the
SEAFRAME, but only weakly (a few centimetres).

June 2004

Contents

Page

Executive

Summary

Introduction

Regional

Overview

2.1. Regional Climate and Oceanography

2.2. Historical Sea Level Trends and their Confidence Intervals

2.3. Short-Term Sea Level Trends from SEAFRAME stations

2.3.1.

Geodetic

Levelling

Summary

2.3.2. Inverted barometric pressure effect

2.3.3. Combined net rate of relative sea level trends

3. Project Findings to Date – Tuvalu

3.1.

Extreme

events

3.1.1.

Tropical

Cyclones

3.1.2.

Tsunamis

3.2. Short Term Sea Level Trend

3.3. Historical Sea Level Trend Assessment

3.4. Predicted highest astronomical tide

3.5. Monthly mean air temperature, water temperature,

and

atmospheric

pressure

3.6. Geodetic Levelling Results for Tuvalu

Appendix

A.1. Definition of Datum and other Geodetic Levels at Funafuti

June 2004

1. Introduction

As part of the AusAID-sponsored South Pacific Sea Level and Climate Monitoring
Project (“Pacific Project”) for the FORUM region, in response to concerns raised by its
member countries over the potential impacts of an enhanced Greenhouse Effect on
climate and sea levels in the South Pacific region, a SEAFRAME (Sea Level Fine
Resolution Acoustic Measuring Equipment) gauge was installed in Funafuti, Tuvalu, in
March 1993. The gauge has been returning high resolution, good scientific quality data
since installation.

SEAFRAME gauges not only measure sea level by two independent means, but also a
number of “ancillary” variables - air and water temperatures, wind speed, wind direction
and atmospheric pressure. There is an associated programme of levelling to “first
order”, to determine vertical movement of the sea level sensors due to local land
movement. Continuous Global Positioning System (CGPS) measurements are now also
being made to determine the vertical movement of the land with respect to the
International Terrestrial Reference Frame.

When change in sea level is measured with a tide gauge over a number of years one
cannot be sure whether the sea is rising or the land is sinking. Tide gauges measure
relative sea level change, i.e., the change in sea level relative to the tide gauge, which
is connected to the land. To local people, the relative sea level change is of paramount
importance. Vertical movement of the land can have a number of causes, e.g. island
uplift, compaction of sediment or withdrawal of ground water. From the standpoint of
global change it is imperative to establish absolute sea level change, i.e. sea level
referenced to the centre of the Earth which is to say in the terrestrial reference frame. In
order to accomplish this the vertical land movement and in particular the rate at which
the land moves must be measured separately. This is the reason for the addition of
CGPS near the tide gauges.

June 2004

2. Regional Overview

2.1. Regional Climate and Oceanography

Variations in sea level and atmosphere are inextricably linked. For example, to
understand why the sea level at Tuvalu undergoes a much larger annual fluctuation
than at Samoa, we must study the seasonal shifts of the trade winds. On the other
hand, the climate of the Pacific Island region is entirely ocean-dependent. When the
warm waters of the western equatorial Pacific flow east during El Niño, the rainfall, in a
sense, goes with them, leaving the islands in the west in drought.

Compared to higher latitudes, air temperatures in the tropics vary little throughout the
year. Of the SEAFRAME sites, the most extreme changes are naturally experienced by
those furthest from the equator – the Cook Islands (at 21

°S) recorded the lowest

temperature, 13.1

°C, in August 1998. The Cook Islands regularly fall to 16°C while

Tonga (also at 21

°S) regularly falls to 18°C in winter (July/August).

Table 1. Range in air temperatures observed at SEAFRAME stations

SEAFRAME location

Minimum recorded
air temperature (

°
°
°
°C)

Maximum recorded
air temperature (

°
°
°
°C)

Cook Islands

13.1 32.0

Tonga

16.0 31.4

Fiji (Lautoka)

16.6 33.4

Vanuatu

16.5 33.3

Samoa

18.7 32.3

Tuvalu 22.8 32.6
Kiribati 22.4 32.9
Nauru 22.4 32.5
Solomon Islands

20.1 34.5

Papua New Guinea

21.5 31.8

Marshall Islands

20.0

31.9

FSM 23.0

31.3

The most striking oceanic and climate fluctuations in the equatorial region are not the
seasonal, but interannual changes associated with El Niño. These affect virtually every
aspect of the system, including sea level, winds, precipitation, and air and water
temperature. Referring to Figure 1, we see that at most SEAFRAME sites, the lowest
recorded sea levels appear during the 1997/1998 El Niño. The most dramatic effects
were observed at the Marshall Islands, PNG, Nauru, Tuvalu and Kiribati, and along a
band extending southeastward from PNG to Samoa. The latter band corresponds to a
zone meteorologists call the “South Pacific Convergence Zone” or SPCZ (sometimes
called the “Sub-Tropical Convergence Zone”, or STCZ). In Figure 1, we see the effect of
the 1997/1998 El Niño on all SEAFRAME stations.

June 2004

Figure 1. Sea level anomalies* at SEAFRAME sites

Marshall Islands

Federated States of Micronesia

Papua New Guinea

Solomon Islands

Kiribati

Nauru

Tuvalu

Samoa

Vanuatu

Fiji

Tonga

Cook Islands

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

SEA LEVEL ANOMALIES THROUGH JUNE 2004 (m)

0.0

-0.2

+0.2

* Sea level “anomalies” have had tides, seasonal cycles and trend removed from the
sea level observations.

June 2004

Most Pacific Islanders are very aware that the sea level is controlled by many factors,
some periodic (like the tides), some brief but violent (like cyclones), and some
prolonged (like El Niño), because of the direct effect the changes have upon their lives.
The effects vary widely across the region. Along the Melanesian archipelago, from
Manus Island to Vanuatu, tides are predominantly diurnal, or once daily, while
elsewhere the tide tends to have two highs and two lows each day. Cyclones, which are
fueled by heat stored in the upper ocean, tend to occur in the hottest month. They do
not occur within 5

° of the equator due to the weakness of the “Coriolis Force”, a rather

subtle effect of the earth’s rotation. El Niño’s impact on sea level is mostly felt along the
SPCZ, because of changes in the strength and position of the Trade Winds, which have
a direct bearing on sea level, and along the equator, due to related changes in ocean
currents. Outside these regions, sea levels are influenced by El Niño, but to a far lesser
degree.

Figure 2. Mean Surface Water Temperature

Note the warm temperatures in the SPCZ and just north of the equator.

The convergence of the Trade Winds along the SPCZ has the effect of deepening the
warm upper layer of the ocean, which affects the seasonal sea level. Tuvalu, which is in
the heart of the SPCZ, normally experiences higher-than-average sea levels early each
year when this effect is at its peak. At Samoa, the convergence is weaker, and the
seasonal variation of sea level is far less, despite the fact that the water temperature
recorded by the gauge varies in a similar fashion. The interaction of wind, solar heating
of the oceanic upper layer, and sea level, is quite complex and frequently leads to
unexpected consequences.

June 2004

The Streamlines of Mean Surface Wind (Figure 3) shows how the region is dominated
by easterly trade winds. In the Southern Hemisphere the Trades blow to the northwest
and in the Northern Hemisphere they blow to the southwest. The streamlines converge,
or crowd together, along the SPCZ.

Figure 3. Streamlines of Mean Surface Wind

Much of the Melanesian subregion is also influenced by the Southeast Asian Monsoon.
The strength and timing varies considerably, but at Manus Island (PNG), for example,
the NW monsoon season (winds from the northwest) runs from November to March,
while the SE monsoon brings wind (also known as the Southeast Trade Winds) from
May to October. Unlike many monsoon-dominated areas, the rainfall at Manus Island is
distributed evenly throughout the year (in normal years).

2.2. Historical Sea Level Trends and their Confidence Intervals

With the great diversity in climatic environments, vertical land movement and ocean
variability, one might expect that the relative sea level trends measured at different
stations over different time periods may also vary. This is indeed the case and is
demonstrated by Table 2, which contains the relative sea level trends from all the
‘historical’ regional stations. The term ‘historical’ in this case refers to tide gauges that
were installed prior to the South Pacific Sea Level and Climate Monitoring Project. In
general, these historical gauges were designed to monitor the sea level variability
caused by El Niño and shorter-term oceanic fluctuations rather than long-term sea level
change, for which a high level of precision and datum control is required.

June 2004

Table 2. Historical Sea Level Data and their Relative Sea Level Trends

Location Country Years

data

Trend
(mm/year)

Standard
Deviation
mm/year

Pago Pago

U S Trust

49.7

+1.43

1.5

Rarotonga Cook

22.2 +3.80 3.7

Penrhyn Cook

Is 21.6 +0.89 3.4

Pohnpei

F S of Micronesia

26.9

+0.42

3.7

Kapingamarangi

F S of Micronesia

19.9

-1.04

4.7

Truk

F S of Micronesia

27.6

+1.79

3.3

Guam

U S Trust

50.1

+0.37

1.9

Yap

F S of Micronesia

30.9

-0.20

3.6

Suva Fiji 24.8

+3.99

3.0

Christmas

Rep of Kiribati

40.3

-0.68

2.2

Kanton

Rep of Kiribati

45.0

+0.26

1.5

Fanning

Rep of Kiribati

16.8

+2.17

5.1

Tarawa

Rep of Kiribati

23.6

-2.24

3.6

Majuro

Rep of Marshall Is 30.8

+2.79

2.6

Enewetok

Rep of Marshall Is 24.5

+1.18

3.3

Kwajalein

Rep of Marshall Is 54.4

+1.13

1.3

Nauru

Rep of Nauru

24.2

-2.03

4.2

Malakal

Rep of Palau

30.1

+0.64

4.0

Honiara Solomon

24.5 -2.21 4.8

Funafuti Tuvalu 21.6 +0.92 5.1

Mean trend: 0.67 mm/year (all data) Mean trend of data > 25 years: 0.8 mm/year

Data from University of Hawaii as at June 2002

Figure 1 illustrates that sea level can undergo significant short-term fluctuation, the
occurrence of which can affect any estimate of the underlying long-term trend. The
expected width of the 95% confidence interval (±1.96 times the standard error) of a
linear trend as a function of data length based on the relationship for all National
Oceanographic and Atmospheric Administration (NOAA) gauges with a data record of
at least 25 years are shown in Figure 4. A confidence interval or precision of 1 mm/year
should be obtainable at most stations with 50-60 years of data on average, providing
there is no acceleration in sea level change, vertical motion of the tide gauge, or abrupt
shifts in trend due to tectonic events.

June 2004

Figure 4. 95% Confidence Intervals for Linear Mean Sea Level
trends (mm/year) plotted as a function of the year range of data.
Based on NOAA tide gauges with at least 25 years of record

2.3. Short-Term Sea Level Trends from SEAFRAME stations

The importance of precise measurements and vertical datum control for long-term sea
level monitoring is integral to the South Pacific Sea Level and Climate Monitoring
Project. Nevertheless the data collection program to date has been operating for a
relatively short term, and so the trends are still prone to the effects of shorter-term
ocean variability (such as El Niño and decadal oscillations). The nature of this effect is
shown in Figure 5, which depicts the evolution of the short-term sea level trends, at
SEAFRAME stations, from one year after installation to the present. Please note that
the trendlines have not yet stabilised.

1. Zervas, C. (2001) Sea Level Variations of the United States 1854-1999. NOAA, USA.

June 2004

Figure 5. Short Term Sea Level Trends (mm/year)

0.0

-25

+25

Fiji

Vanuatu

Tonga

Cook Islands

Samoa

Marshall Islands

Kiribati

Tuvalu

Nauru

Solomon Islands

Papua New Guinea

Federated States of Micronesia

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

SEA LEVEL TRENDS THROUGH JUNE 2004 (mm/year)

June 2004

2.3.1 Geodetic Levelling Summary

Precise datum control is an essential component of the Pacific Project. Surveys are
carried out every eighteen months. Table 3 shows the distances K

(km) between the

SEAFRAME Sensor benchmark and the Tide Gauge Benchmark (TGBM) and also
gives the “maximum allowable vertical movement misclosure” between the two. The
misclosure, an indicator of the precision to which the survey must be performed, forms
part of Project design specifications. Additional coastal benchmarks are also used to
ensure the vertical stability of the TGBM. These latter, which are known as the “coastal
array”, are not presented here.

Table 3 also shows the rate of vertical movement of the gauge relative to the TGBM
(determined by fitting a straight line to the survey results) that is contributing to the
observed sea level trend. For example, a substantial subsidence of the tide gauge at
Samoa is occurring at a rate of -1.2 mm/year. Subsidence is also occurring at Marshall
Islands and Solomon Islands. The Cook Islands tide gauge is rising at 0.9 mm/year with
respect to the tide gauge benchmark. The rates of vertical tide gauge movement listed
in Table 3 should be added as a correction to the sea level trend estimates and is
accounted for in section 2.3.3. Combined net rate of relative sea level trends.
Additional levelling details for individual countries are provided in section 3.6. Geodetic
Levelling Results.

Location

(km)

(mm)

Number of
Surveys

Vertical
movement
(mm/year)

Cook Is

0.491

1.4 7 +0.9

FSM 0.115 0.7

N/A

Fiji 0.522

1.4 7 +0.2

Kiribati 0.835

1.8 8 +0.1

Marshall Is

0.327

1.1 8 -0.5

Nauru 0.120 0.7

+0.0

PNG 0.474

1.4 6 -0.2

Samoa 0.519

1.4 7 -1.2

Solomon Is 0.394

1.3 4 -0.4

Tonga 0.456

1.4 7 -0.0

Tuvalu 0.592

1.5 7 -0.0

Vanuatu

1.557 2.5

+0.3

Table 3. Distance (km) between SEAFRAME and TGBM, misclosure, and
the rate of movement of the SEAFRAME relative to the TGBM.

During Phase 3 of the Project, Continuous GPS stations are being installed on islands
in association with the SEAFRAME gauges. Their purpose is to close the final link in
establishing vertical datum control – that is, to determine whether the island as a whole
(or in some cases the coastal array) is moving vertically. To date, the length of the
CGPS data time series is inadequate to make definitive conclusions.

June 2004

2.3.2. Inverted barometric pressure effect

Another parameter that influences the estimates of relative sea level rise is atmospheric
pressure. Known as the inverted barometer effect, if a 1 hPa fall in barometric pressure
is sustained over a day or more, a 1 cm rise is produced in the local sea level (within
the area beneath the low pressure system). Therefore, if there are trends in the
barometric pressure recorded at the tide gauge sites, there will be a contribution to the
observed relative sea level trends. The contribution will be a 10 mm/year increase
(decrease) in relative sea levels for a 1 hPa/year decrease (increase) in barometric
pressure.

Table 4 contains the estimates of the contribution to relative sea level trends by the
inverted barometric pressure effect in mm/year at all SEAFRAME sites over the period
of the project.

Location

Length of data
(months)

Barometric Pressure
Contribution to Sea Level
Trend (mm/yr)

Cook Is

135 +0.07

FSM 30

-0.34*

Fiji

140 +1.25

Kiribati

133 +0.58

Marshall Is

128 +0.31

Nauru

130 +0.55

PNG

102 +1.56

Samoa

135 +0.27

Solomon Is

115 -0.12

Tonga

136 +0.81

Tuvalu

133 +0.47

Vanuatu

126 +1.63

Table 4. Recent short-term barometric pressure trends
expressed as equivalent sea level rise in mm/year based upon
SEAFRAME data to June 2004. *The trend at FSM is from a
comparatively short series and therefore varies considerably.

Table 4 shows that the contribution of pressure to the observed rates of sea level rise
are substantial when compared to the global average rates of between one and two
millimetres per year. In this region for the past decade the contribution is mostly
positive, that is, the relative sea level trends are overestimated without this effect being
taken into consideration.

June 2004

2.3.3. Combined net rate of relative sea level trends

The effects of the vertical movement of the platform and the inverse barometer effect
are removed from the estimated relative rates of sea level change and presented in
Table 5. There is a high degree of spatial coherency in these rates with the smallest
rates of +2.1 mm/year being in the southeast grading up to +7.5 mm/year in the
northwest. There is an anomalous value of +10.8 mm/year at Tonga. This may be due
to a vertical motion of the whole island, but since the CGPS station has only recently
been installed (in February 2002) the estimates are still too noisy to be reliable.

Location Length

data
(months)

Sea Level
Trend
(mm/yr)

Barometric
Pressure
Contribution
(mm/yr)

Vertical Tide
Gauge
Movement
Contribution
(mm/yr)

Net Sea Level
Trend
(mm/yr)

Cook Is

135 +1.3 +0.07 -0.9

+2.1

FSM 30 +24.3* -0.34* N/A

+24.6*

Fiji

140 +4.1

+1.25 -0.2

+3.1

Kiribati

133 +5.5 +0.58 -0.1 +5.0

Marshall Is

128 +5.8 +0.31 +0.5 +5.0

Nauru

130 +7.5 +0.55 +0.0

+7.0

PNG

102 +8.7 +1.56 +0.2 +6.9

Samoa

135 +5.1 +0.27 +1.2 +3.6

Solomon Is 115 +6.3 -0.12 +0.4 +6.0

Tonga

136 +11.6** +0.81 +0.0 +10.8**

Tuvalu

133 +5.9 +0.47 +0.0

+5.4

Vanuatu

126 +6.1

+1.63 -0.3

+4.8

Table 5. The net relative sea level trend estimates after the inverted
barometric pressure effect and vertical movements in the observing platform
are taken into account.
*FSM is a comparatively short series and therefore varies considerably.
**The relative sea level trend at Tonga appears to be affected by vertical
movement of the island as a whole.

This overview was intended to provide an introduction to the Pacific Islands regional
climate, in particular those aspects that are related to sea level. This is an area of active
research, and many elements, such as interdecadal oscillations, are only beginning to
be appreciated.

June 2004

3. Project findings to date – Tuvalu

3.1. Extreme Events

3.1.1. Tropical Cyclones

Tropical Cyclone Gavin originated close to the Southwest of Funafuti on the 3

March

1997 (see Figure 6). The storm surge (the non-tidal part of the recorded sea level)
generated by Gavin reached a peak of 0.3 metres on the 5

of March but since this was

at a time of Neap tides, did not cause as much damage as it might have at Spring tides.
However, Gavin did cause considerable erosion through wave action reaching into the
lagoon.

Figure 6. Track of Tropical Cyclone Gavin

June 2004

3.1.2. Tsunamis

A tsunami can be defined as "A wave usually generated by seismic activity. Also called
seismic sea wave, or, incorrectly, a tidal wave. Barely discernible in the open ocean,
their amplitude may increase to over ten metres in the shallow coastal regions.
Tsunamis are most common in the Pacific Ocean."

Two tsunamis have been recorded at Tuvalu since the SEAFRAME was installed,
although both do not seem to have caused any damage at Tuvalu.

(1) Vanuatu event

At 1321 UTC on the 26

of November, 1999 a large undersea earthquake with Richter

magnitude 7.1 was triggered off Vanuatu, causing considerable damage there. The
resultant tsunami travelled on to be recorded throughout the region.

At Tuvalu a 6 cm tsunami was recorded arriving at 1550 hours UTC on the 26

2 hours, 29 minutes after the event (Figure 7).

Figure 7. Funafuti sea level in response to Vanuatu Earthquake

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

Height (m)

26.50

26.75

27.00

27.25

27.50

November (UTC)

(2) Peru event

At 2033 UTC on 23

of June, 2001 a large undersea earthquake registering Richter 8.4

occurred off the coast of Peru, causing considerable damage there. The resultant
tsunami travelled across the Pacific with several locations recording tsunami amplitudes
of 30 cm.

At Tuvalu there was a recorded tsunami of 6 cm peak-to-trough arriving at 1430 hours
UTC on the following day, 18 hours after the earthquake (Figure 8). A computer model
of the tsunami (Figures 9 and 10) confirmed these conclusions.

June 2004

3.2. Short-term sea level trend

A fundamental goal of the Project is to establish the rate of sea level change. It has
been recognised since the beginning that this would require several decades of
continuous, high quality data. However, in response to increasing requests from the
region for information regarding the trends as they gradually emerge from the
background “noise”, combined with concern that less experienced users might attempt
to fit a trend line to the data without properly accounting for processes such as
seasonality that can bias the result, the preliminary findings are now being provided.
These are given in the form of plots (see Figure 5. Short Term Sea Level Trends) which
show how the trend develops as more data becomes available. We caution against
drawing conclusions prematurely.

As at June 2004, based on the short-term sea level trend analyses performed by the
National Tidal Centre of the eleven years of Tuvalu data, a rate of +5.9 mm per year
has been observed. Accounting for the inverted barometric pressure effect and vertical
movements in the observing platform, the sea level trend is +5.4 mm per year. By
comparison, the Intergovernmental Panel on Climate Change (IPCC) in its Third
Assessment Report (IPCC TAR, 2001) estimates that global average long-term sea
level rise over the last hundred years was of the order of 1 to 2 mm/yr.

Figure 5 shows how the trend estimate has varied over time, and because the data set
is still relatively short, varies considerably from month to month. In the early years, the
trend appeared to indicate an enormous rate of sea level rise. Later, due to the
1997/1998 El Niño when sea level fell 35 cm below average, the trend actually went
negative, and remained so for the next three years. Over most of the past four years,
the sea level appears to have been falling. Only in August 2001 did the trend return to
positive values. It is still far too early to deduce a long-term trend (or even whether it will
be positive or negative) from this data.

Figure 11

June 2004

The sea level data recorded since installation is summarised in Figure 12. The middle
curve (green) represents the monthly mean sea level. The upper and lower curves show
the highest and lowest values recorded each month. The two most notable features of
the monthly averages are the annual peaks, which appear every year around March
except in 1998, and a large drop in sea level recorded during the 1997/1998 El Niño.
Tuvaluans are accustomed to the annual peaks, which bring well-documented flooding
throughout the low-lying atoll nation. In the past decade or so, as our understanding of
El Niño has improved, they also have come to expect lower sea levels during such
events.

Although sea levels in the Tuvalu region normally fall in response to El Niño, the
decrease that occurred during 1997/1998 El Niño can be considered extraordinary. Sea
levels were lowered by 35 cm in March and April of 1998. By November 1998, sea level
had completely recovered. Following the El Niño, the sea level resumed its normal
seasonal cycle.

Figure 12

Monthly sea level at Funafuti

SEAFRAME gauge

0.4

0.6

0.8

1.2

1.4

1.6

1.8

2.2

2.4

2.6

2.8

3.2

3.4

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

Year

Sea L

evel (

res)

Minimum

Mean

Maximum

The maximum recorded sea level for March 1997, during Tropical Cyclone Gavin, was
3.332 m, whereas the SEAFRAME record maximum is 3.348 m at 05:06 UTC on the 9

March 2001, 1.6 cm higher.

The latter maximum was not caused by a tropical cyclone, but was the result of a
combination of a high monthly anomaly with the equal highest spring tide for the year at
3.20 m, occurring at 17:00 local time on the 9

March 2001.

June 2004

3.3. Historical Sea Level Trend Assessment

A longer sea level record is available at Tuvalu, from the Funafuti tide gauge operated
by the University of Hawaii (UH) from 1977 until the end of 1999 - about 22 years of
data (see Figure 13). The UH data exhibits a sea level rise of +0.9 mm/year over the
period 1977 to the end of 1999. The UH gauge was designed to monitor the variability
caused by El Niño and shorter-term oceanic fluctuations, for which the high level of
precision and datum control demanded by the determination of sea level trend were not
required. Hence, even with 22 years of data, the trend can not be established without
sizeable uncertainties.

Figure 13

Monthly sea level at Funafuti

University of Hawaii data

-0.5

0.5

1.5

2.5

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

2002

2004

Year

Sea Level (m

etr

es)

Minimum

Mean

Maximum

Levelling surveys undertaken since 1993 by NTC of the UH gauge have shown that it
appears to have been sinking relative to the NTC benchmarks by an average of about 1
mm per year since 1994. This highlights the need for regular surveys. The sea levels
discussed in this report are all taken relative to local benchmarks. If a tide gauge is
slowly moving vertically relative to the nearby coast, such a vertical movement can and
must be accounted for by local survey. All SEAFRAME stations are re-surveyed every
18 months or less. These surveys have shown that the bench mark network associated
with the SEAFRAME gauge at Funafuti has exhibited excellent stability. As at the most
recent survey (May 2003) there was, to within the measurement tolerances, no net
movement of the SEAFRAME gauge relative to the primary tide gauge benchmark
since installation, and at every one of the seven surveys, the vertical changes have
been within Project specifications.

June 2004

3.4. Predicted highest astronomical tide

The component of sea level that is predictable due to the influence of the Sun and the
Moon and some seasonal effects allow us to calculate the highest predictable level
each year. It is primarily due to the ellipticity of the orbit of the Earth around the Sun,
and that of the Moon around the Earth resulting in a point at which the Earth is closest
to the Sun, combined with a spring tide in the usual 28 day orbit of the Moon around the
Earth. Figure 14 shows that the highest predicted level (3.24 m) over the period 1990 to
2016 will be reached at 17:26 Local Time on the 28

February 2006.

Figure 14

Predicted highest tide each year for Funafuti

2.8

2.9

3.1

3.2

3.3

3.4

1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

2012

2014

2016

2018

Year

Sea level (metr

es)

There is an apparent fluctuation in these annual predicted highest tides with a period of
about 4 ½ years. This unusual periodicity has been observed elsewhere in semi-
enclosed embayments (like Funafuti lagoon), and has been ascribed to interactions
between the major tidal components.

The location of the gauge within the atoll lagoon leads to unique characteristics showing
up in the data, such as the small 4 ½ year tidal oscillation and the effect of solar heating
of the lagoon waters. It also shelters the gauge from ocean wave swell, particularly from
the east. Swell is caused by surface winds. It is an important source of error in many
tide gauges, especially the older conventional gauges with stilling wells.

June 2004

3.5. Monthly means of air temperature, water temperature, and atmospheric
pressure

The data summarised in Figures 15-17 follows the same format as the monthly sea
level plot: the middle curve (green) represents the monthly mean, and the upper and
lower curves show the highest and lowest values recorded each month.

The air temperature at the Funafuti SEAFRAME gauge shows a very slight downward
trend in the monthly means. The figure also shows that since around the middle of
2000, air temperature maxima have been relatively low. The highest air temperature
recorded over the period is 31.6°C at 0200 UTC on the 24

April 1994 with the lowest

being 22.8°C at 2200 UTC on the 14

January 1999.

Figure 15

Monthly Air Temperature at Funafuti

SEAFRAME Gauge

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

Year

Air Temperature (

Minimum

Mean

Maximum

Since the start of recording, the mean water temperatures have initially declined to
reach a low in 1998, the El Niño year, then rebounded to record highs in recent months.
The SEAFRAME record maximum is 32.7°C at 0600 UTC on the 26

November 2001

and the minimum is 27.6ºC at 1600 UTC on the 26

July 1993. Also notable are the

recent highs and the typical annual high in November each year. At the time of El
Niños also note that the seasonal cycle of sea level and water temperature are
interrupted.

June 2004

Figure 16

Monthly water temperature at Funafuti

SEAFRAME gauge

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

Year

t
e

r T

erat

r
e (

Minimum

Mean

Maximum

The monthly atmospheric pressure at Funafuti shows a decline over the years after the
El Niño of 1998. The highest pressure recorded was 1016.4 hPa at 21:00 UTC on July
2

1998, while the lowest was 995.4 hPa at 15:00 UTC on 5

March 1997 coinciding

with the passage of Tropical Cyclone Gavin.

Figure 17

Monthly atmospheric pressure at Funafuti

SEAFRAME gauge

960

970

980

990

1000

1010

1020

1030

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

Year

e
ric Pressu

re (

Pa)

Minimum

Mean

Maximum

June 2004

3.6. Geodetic Levelling Results for Tuvalu

While the SEAFRAME gauge exhibits a high degree of datum stability, it is essential
that the datum stability be checked periodically by precise levelling to an array of deep-
seated benchmarks located close to the tide gauge. For example, the SEAFRAME is
normally supported by a wharf. Wharf pilings are often subject to gradual vertical
adjustment, which in turn can raise or lower the SEAFRAME.

Precise levelling is carried out on a regular 18-monthly cycle between the SEAFRAME
Sensor Benchmark and an array of at least six deep benchmarks. The nearest stable
benchmark is designated the “Tide Gauge Benchmark (TGBM)”, and the others are
considered the “coastal array”.

Figure 18 summarises the most important survey information being the movement of
the SEAFRAME Sensor benchmark relative to the TGBM. The graph does not include
the results for the other benchmarks on the coastal array. In this graph, the first survey
was performed in “year zero”. Each subsequent survey is plotted relative to the first.
Thus, the second survey at Tuvalu found that the SEAFRAME Sensor benchmark had
risen relative to the TGBM by 0.5 mm, however the Sensor level has had 0.0 mm/year
rate of change overall. Thus at Tuvalu, no adjustment to the sea level trend is required
for survey results.

Figure 18. Movement of SEAFRAME Sensor

relative to the Tide Gauge Bench Mark

Years

i
l
lim

t
r
e

-10

-5

Tuvalu -0.0 mm/yr

Levelling of SEAFRAME Sensor
benchmark. Seated next to SEAFRAME:
Andrick Lal, SOPAC. Standing: John
Ovenden, NTC. Photo credit: Steve
Turner, NTC.

June 2004

Appendix

A.1. Definition of Datum and other Geodetic Levels at Funafuti

Newcomers to the study of sea level are confronted by bewildering references to “Chart
Datum”, “Tide Staff Zero”, and other specialised terms. Frequently asked questions are,
“how do NTC sea levels relate to the depths on the marine chart?” and “how do the UH
sea levels relate to NTC’s?”.

Regular surveys to a set of coastal benchmarks are essential. If a SEAFRAME gauge
or the wharf to which it is fixed were to be damaged and needed replacement, the
survey history would enable the data record to be “spliced across” the gap, thereby
preserving the entire invaluable record from start to finish.

Figure 19

June 2004

The word “datum” in relation to tide gauges and nautical charts means a reference
level. Similarly, when you measure the height of a child, your datum is the floor on
which the child stands.

“Sea levels” in the NTC data are normally reported relative to “Chart Datum” (CD), thus
enabling users to relate the NTC data directly to depth soundings shown on marine
charts – if the NTC sea level is +1.5 metres, an additional 1.5 metres of water may be
added to the chart depths.

Mean Sea Level (MSL) in Figure 19 is the average recorded level at the gauge over the
two year period 1993/1994 (as indicated). The 1993/1994 MSL at Tuvalu was 1.985
metres above CD.

Lowest Astronomical Tide, or “LAT”, is based purely on tidal predictions over a 19 year
period. In this case, LAT is 0.7859 metres, meaning that if the sea level were controlled
by tides alone, the sea level reported by NTC would drop to this level just once in 19
years.

UH “tide staff zero” is also placed on the figure. It is 0.7933 metres, which explains why
the NTC sea levels in the figure appear to be about 0.8 metres higher than the UH sea
levels.

Document Outline