THE WAVE CLIMATE OF THE KINGDOM OF

Stephen Barstow and Ola Haug OCEANOR*

June 1994 SOPAC Technical Report201

*OCEANOR - Oceanographic Company of Norway AS Pir-Senteret N-7005 Trondheim Norway

Prepared for; South Pacific Applied Geoscience Commission (SOPAC) The Wave Climate of the Kingdom of Tonga i

TABLE OF CONTENTS

1 Introduction...... 1

2 Some Basics...... 2

3 Oceanic Winds...... 3 3.1 General Description...... 3 3.2 Representativity of the measurement period...... 5 3.3 Winds in the source region for swell...... 5

4 Ocean Waves...... 9 4.1 Buoy Measurements...... 9 4.2 Ocean Wave Statistics...... 11

5 Special Events...... 18 5.1 ...... 18 5.2 Other Cyclones...... 18 5.3 Swell Events...... 19 5.4 September 1990 Swell Event...... 21 5.5 High Swells in June 1991...... 21

6 Conclusions...... 24

7 Recommendations...... 26

8 References...... 27

APPENDIX A Time series plots of wave and wind parameters; 1987-92...... 28 APPENDIX B Time series plots from the Directional Waverider; June to August 1992...... 41 APPENDIX C Comparison of wind and wave data from the UK Met . Office Global Wave Model with measurements from the Directional Waverider in open water to the south of ...... 43

28400/REPORT/93085/SFB/akm/11 May 1994 The Wave Climate of the Kingdom of Tonga 1

1 Introduction

The South Pacific Applied Geoscience Commission (SOPAC) acting through its Secretariat in Suva, , embarked on a wave data collection program in 1987 with the aim to map the ocean wave climate off the shores of several South Pacific island nations. The principal application of the wave data was seen to be the mapping of the wave energy resource of the islands needed to study the feasibility of developing wave power as a future energy source.

In the present report, data from the wave measurement programme recorded by a Waverider buoy, moored off the southern coast of Tongatapu, are combined with satellite altimetric data, island wind measurements and data from the UK Met. Office global wave model in building up a picture of the climatology of ocean waves in the Tonga group. Wave data from special events such as tropical cyclones and energetic swells are highlighted. Influence of the El-Nino Southern Oscillation phenomenon on the wave climate is also discussed.

Companion reports describing the ocean wave climate of the , Western Samoa, Fiji, and are also available. Further, a report has been produced discussing region-wide differences in the wave climate, entitled The Wave Climate of the South West Pacific (Barstow and Haug, 1994) also giving further details on the various data sources used, in particular, the GEOSAT satellite altimeter data. It is recommended that readers should acquaint themselves with this report first before reading the present one.

This report was produced by the SOPAC Secretariat with assistance from the Oceanographic Company of Norway AS (OCEANOR) and the Norwegian Hydrotechnical Laboratory (NHL) who were contracted as consultants. The work was financed by the Norwegian Government through the Norwegian Agency for International Development, NORAD.

28400/REPORT/93085/SFB/akm/ 3 May 1994 The Wave Climate of the Kingdom of Tonga 2

2 Some Basics

The ocean waves we are concerned with here are those waves generated by wind as opposed to tsunamis (or tidal waves) which are generated by subterranean seismic activity or landslide and the tides caused by the gravitational attraction of the moon and sun on the earth. In the South Pacific ocean wind waves are always present, it never being perfectly calm, and the energy involved is obvious to anyone at the coast as wave after wave dissipates on the coast. Ocean waves have a typical range of wave periods from 2 secs. (short wind waves) to 25 secs. (long swell).

The generation of ocean waves in response to wind depends both on the wind speed, the fetch or distance over which the wind blows and the duration of the wind in a given direction. A shift in wind direction leads to new waves growing in the new direction. There is, however, a limit to the growth of wind waves for a given wind speed. If the wind blows long enough over a long enough fetch the waves reach the so-called fully developed state.

Storm waves once created are known to attenuate very slowly and can travel many days across the ocean before dissipating on distant shores. Ocean waves are known as swell away from their generation area. The classic studies of ocean swell propagation were carried out in the Pacific during the early 1960s (e.g., Snodgrass et al, 1966) confirming the prediction of the linear wave theory that predicts that ocean waves travel across the ocean with a speed which increases with increasing wave period or wavelength. In a storm area waves of a range of wave periods are generated. When this spectrum of waves leaves the storm area, the longer waves travel faster, so that an observer at a distant point will detect the longer period swell waves first.

In reality, at a given location on the shore of a Pacific island, waves may be present arising from several different wind systems which may include local trade winds, storms in the southern ocean or northern Pacific, and, occasionally, from tropical cyclones. The exposure of the actual location is also very important, so that a location on the northern coast of an island will only experience swell from the northern hemisphere due to the island sheltering the location from southerly swell. It is, therefore, important to have information on the variability of oceanic winds on different time scales in both the local area and the source areas for swell in order to understand the variability of the wave climate. This is discussed in the next section.

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3 Oceanic Winds

3.1 General Description

A good overview of the wind climate of the Tonga group is to be found in the New Zealand Meteorological Service's publication the Climate and Weather of Tonga (Thompson, 1986a).

Another useful reference is Harrison and Luther (1 990) who present climatological monthly mean winds from 33 island stations in the Pacific. The data were quality controlled by comparing neighbouring stations and long term averages from ship observations near to the islands. Generally, agreement is very good with differences generally no more than about 1 m/s in speed and 10 degs. in direction. We are grateful to Harrison for providing us with his long term climatological means. These statistics are reproduced for the one island station in the Tonga group which has been analysed (Nukualofa on Tongatapu) in Fig. 1.

Time series of wind speed and direction were obtained in digitised form from the New Zealand's National Institute of Water and Atmospheric Research Ltd. (NIWA) for Fua'amotu airport on Tongatapu (38 m a.s.l.) for the period November 1979 to February 1993.

In the Tonga group, taking the year as a whole, easterly winds are most common north of 19'S whereas south easterlies predominate south of that latitude. In the south of the Tonga group there is little variation in the wind speed throughout the year although there is a tendency for higher winds during November to April due to cyclone activity. The wind direction tends to be more south easterly than easterly during the winter months. In the north, however, east and south easterlies are most frequent during winter (May-October). At that time of year wind speeds are also strongest. During November to April the northern Tonga islands experience more frequent northerlies and northwesterlies.

At any time of year, the influence of higher latitude weather affects the winds at Tonga. The wind time series often exhibit rotating wind direction in response to the movement of these weather systems to the south. This can be seen in the record from Fua'amotu shown in Appendix A. Harrison and Luther (1990) analysed the variability of winds on different scales and find greater variability on time scales less than 30 days in subtropical areas such as Tonga compared to more tropical locations.

The record at Fua'amotu also shows a diurnal periodicity in the wind speed in response to the differential heating of land and sea.

High winds occur in Tonga mostly as a result of tropical cyclones. In some years,, no tropical cyclones are experienced, in others 2 or 3 may occur. Years with several tropical cyclones tend to occur during the negative phase of the Southern Oscillation (characterised by a negatiave Southern Oscillation Index).

The Southern Oscillation Index (SOI) describes the well known pressure oscillation between the western part of the South Pacific, represented by Tahiti and the eastern part (Darwin in Australia), and is intimately related to the El-Nino phenomenon. When air pressure is relatively

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 4

high at Darwin, the SOl is negative. A time series of the SOl is shown from 1980 to the end of 1992 in Fig. 2. The major 1983 event is clearly seen as well as more moderate events in 1987 and a rare two year event in 1991-1992. These negative SOl episodes occur about every 4-5 years on average.

Island name NUKUALOFA N Latitude -21.13 E Longitude -175.18 Monthly climatology and standard deviation in MIS (~enerated from daily average observations from 02 Jul to 31 Oct

Month U Stddev V Stddev Speed Stddev Jan ;;'3.22 '2.92 0.29 2.73 4.80 1.85 Feb -2.99 2.70 -0.41 2.81 4.60 1.77 Mar -2.35 2.98 0..08 3.02 4.42 1.99 Apr -2.36 2.62 0.75 2.47 4.01 1.74 May -2.61 3.01 0.98 2.34 4.33 1.90 Jun -2..45 2.85 0.85 2.66 4.29 1.86 Jul -2.11 2.99 0.99 2.82 4.34 1.86 -2.20 3.34 0.74 Aug<.'. 2.85 4.50 2.11 oj:! p -2..85 2.99 1.04 2.62 4.61 1.94 O(::t -3.29 2.89 0.89 ~.55 4.76 1.96 Nov -3.51 2.93 0.84 2.50. 4.89 2.00 Dec -3.27 2.88 0.51 2.64 4.77 1.84

Fig. 1 Climatological statistics for winds measured at Nukualofa in the Tonga group (courtesy of Ed Harrison, Pacific Marine Environmental Laboratory in Seattle). U is the zonal (E-W) component velocity and V the meridional (N-S) component velocity.

Fig. 2 Time series of the Southern Oscillation Index from 1980 to 1992

Apart from an increased likelihood of cyclones, the winds do not seem to be significantly affected by the EI-Nifio Southern Oscillation phenomenon although in the north there may be increased northerlies associated with negative SOl periods, as is the case in Western Samoa.

28400/R EPORT/93085/SF B/akm/3 May 1994 1949197: The Wave Climate of the Kingdom of Tonga 5

3.2 Representativity of the measurement period

It is important to describe the temporal representativity of the wave measurement period (May 1987 -August 1992) with respect to the long term climatology. In other words, can the period for which wave measurements exist be considered to be representative for long term conditions as far as local winds (and hence wind seas) are concerned? This has been assessed by computing monthly mean wind statistics for Fua'amotu for 1986-92 and comparing with the long term conditions. In Fig. 3 are shown time series of monthly averaged wind speed and zonal and meridional component velocities (as defined in Fig. 1). The measured values at Fua'amotu are shown by the solid line and the long term climatology is given by the dashed line.

Although the overall wind speed does not significantly deviate from the long term climatology, a more southerly meridional component (positive) occurred during the 1987 El-Nino than normal. The same feature was also found for Rarotonga in the Cook Islands (see The Wave Climate of the Cook Islands report in this series). For the Cook Islands it is known that negative SOI usually results in more persistent south easterlies. This also seems to have been the case in the southern Tonga group during the 1987 El-Nino. In 1988, when the SOl was positive the zonal wind speed was higher than normal. The usual minimum in the easterlies during winter 1988 did not occur.

Next, a prolonged anomaly with more persistent than normal northerlies occurred during most of 1989, with a large anomaly very early in the year, particularly in February. February was anomalous due to the monsoonal trough being a strong feature on the weather charts for much of the month. Harry developed over the Vanuatu group during this time. Tonga lay on the eastern edge of the trough with northerly winds for much of the time.

Finally, for several months both in early 1990 and early 1992 more persistent than normal easterlies occurred.

In conclusion, although mean wind speeds vary little from month to month and year to year there have been several periods during the period for which wave measurements have been made, lasting several months, with anomalously persistent winds from certain directional sectors.

3.3 Winds in the source region for swell

The region of higher latitudes to the south and southwest is the most important source area for ocean swell observed in the Tonga group. An attempt has been made to quantify the variability of ocean swell on the shores of Tonga by looking at long term wind measurements in the source area. Unfortunately, there are very few meteorological stations in this area. The two stations most representative of open ocean conditions are those on Chatham Island at 44'S, 176.6'W, which is about 600 km to the south east of Wellington in New Zealand, and Campbell Island at 52.6'S, 169.2'E, about 650 km to the south of Invercargill on the southern tip of South Island, New Zealand. Time series of wind speed and direction were obtained from NIWA in New Zealand for the period April 1964 to February 1993 for Chatham and July 1941 to February 1993 for Campbell Island.

It was decided to compute a southerly monthly gale index defined by the number of days in each month when wind speed exceeded 10 m/s from the 170-230' sector at Chatham and 15 m/s from 200-260" at Campbell. Such winds could potentially generate waves later appearing as swell off the Kingdom of Tonga.

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 6

The seasonal variation of this gale index at the two island stations is shown in Fig. 4 both for the long term (solid line) and for the months with wave measurements at Tongatapu

Strong southerlies appear to occur at different times of the year at the two meteorological stations. At Campbell Island the gale index peaks in autumn and spring with lower frequencies in mid-winter and summer. At Chatham Island, May and June are the months with most frequent south westerly gales. In December and January, south westerly gales occur about half as often.

During the wave measurement period the gale index at Chatham Island was not significantly different from normal in all months apart from August in which month gales were significantly less common than normal. For Campbell gales were about twice as frequent in May. Otherwise conditions were fairly normal.

It has also been investigated whether there is any relationship between the gale index and the Southern Oscillation Index (section 3.1). This analysis based on 32 years of data (1961-1993) revealed almost no correlation between the two indices overall. The time series plots in Fig. 5 show time series of the two indices from 1961 to 1993. This shows that some El-Nino years (SOI-ve) do coincide with a greater frequency of southerly gales. The 1987 El-Nino is a good example with more than average gale index for 6 months at Campbell Island. On the other hand, in the exceptional 1983 El-Nino more frequent southerly gales did not occur.

28400/REPORT/93085/SFB/akm/3May 1994 The Wave Climate of the Kingdom of Tonga 7

a) m"

'--"S '"0 a.> a.> 0.- m '"0 ~ .~

b) "cn --e ""d Q) Q) 0.. cn

""d I:::: .~ ~ I:::: 0 N

c) co "..r/) ~ '-'".E "d Q) Q) C\1 0- r/) "d 0 ~ .~ C\1 .-'0 ~ Q) s

Fig. 3 Comparison of mean monthly a) wind speed, b) zonal and c) meridional velocity components from the Fua 'amotu Airport wind measurements (solid line) against the long term climatology (dashed line).

28400/REPORT/93085/SFB/akm/19 May 1994 Year The Wave Climate of the Kingdom of Tonga 8

CAMPBELL IS. a I I I I I I I j I I I I I I I I , I , I , I .,-.. "rn "'-C/] 8 8 LQ 0 co 1\ 1\ rn >- >-C/] aj ro "0 -0 "--' to '-" >< >< Q) L Q) "0 -0 .s /l oS

Q) ~ Q) ~ 'id d 0 ~ ~ aj N ro ~Q) ~Q)

0 1'1 I I I I I I I I I I I I I I .1 2 3 7 B 9 10 11 12 Month

Fig. 4 Southerly gale index from Campbell and Chatham Island long term wind data (solid line) and for the duration of the wave measurements (dashed line). For Campbell the index is defined as the mean number of days in each month for which wind speed exceeds 15 mls from 200-260 °; for Chatham 10 mls and 170- 230°.

Fig. 5 Time series of the southerly gale index (solid line) and the Southern Oscillation Index for a) Chatham Island and b) Campbell Island; 1961-1993.

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 9

4 OceanWaves

4.1 Buoy Measurements

Wave measurements were carried out off the southern coast of Tongatapu (Fig. 4) in the southern Tonga group between May 1987 and July 1992 with a Waverider buoy. The buoy was initially moored in position 1 (Fig. 6) in a water depth of 154 m, subsequently in December 1989 being moved to position 2 further from the coast and in deeper water (309 m). Full details of the measurements and the data analysis together with comprehensive statistics can be found in the data reports (e.g., Barstow and Olsen, 1993). 3 months of directional wave data were also collected using a Directional Waverider buoy in a deep water location to the south of Tongatapu (21'50.3' S, 175'24.9' W) far enough away from land to be considered representative of open ocean conditions. This buoy measures the wave direction at both wind sea and swell wave periods in addition to the parameters measured by the Waverider buoy. The only wave directions for which the buoy cannot be considered to represent open ocean conditions is northerly due to island sheltering during northerly swell events. Northerly waves were, however, rare during the measurement period. The Directional Waverider data are not presented elsewhere.

Time series of selected wave parameters for the duration of the measurement period are to be found in Appendix A. Wind speed and direction measurements from Fua'amotu Airport on Tongatapu are also plotted. A brief definition of the various wave parameters is given below:

S(f) Wave spectrum (m's). Based on a 17 min. registration of the wave elevation relative to mean water level. The wave spectrum is computed using the Fast Fourier Transform over the frequency range f = 0.025 to 0.5 Hz (wave period, T = 1/f).

Hm0: Significant wave height (m). This is numerically close to the classical definition of significant wave height which is the height of the 1/3 highest waves during the 17 min. measurement period. Hm0 is computed from the wave spectrum as follows:

Hm0 = 4(/ s(f) df)

Tp: Peak wave period (s). This is the wave period at which the wave spectrum attains its maximum value.

Tm-10: Energy period (s). There are many definitions of wave period. Tm-10 tends to be somewhat higher than Tm02. It is computed as follows:

28400/REPORT/93085/SFB/akm/10 May 1994 The Wave Climate of the Kingdom of Tonga 10

Tm-10 = m _1/mo

where

Tm-10 is used in the computation of wave power.

TmO2 Mean wave period (s).

JT: Wave power (kW/m)

Jr = 0.49 HmO2 Tm-10

Hmax: Maximum wave height (m). Maximum wave height is the height of the highest single wave during a wave record. It is typically 1.4 -1.6 times the significant wave height for 17 min wave records.

r.1oIfw ,

() .. "". ?\.....J" ~ ~ ...g.,..'"" f\1 0 --~ :~~ ~ Q ~

OIr'6'4 - ,'4 ,o(j

-2

Fig. 6 Wave measurement locations south of Tongatapu.

28400/REPORT/93085/SFB/akm/10 May 1994

.-._.---""1'l.11 The Wave Climate of the Kingdom of Tonga 11

4.2 Ocean Wave Statistics

The monthly variation of various wave parameters measured by the Waverider at Tongatapu are shown in Fig. 7. Note that the number of records (Fig. 7.d) are not equally distributed through the year. However, each month has at least data from 3 years available and in some cases up to 5 years.

Unlike winds, the waves on the south coast of Tongatapu are seasonal, reaching a maximum between May and July of about 2 m and decreasing to 1.5 m during the summer months. Note that the months with highest average wave heights were also the months with most frequent southerly gales at Campbell (May) and Chatham Island (July). In particular, gales at Campbell Island were twice as frequent than normal during the months with measurements at Tongatapu. Thus, the peak of 2 m during May is probably due to unusually high swells and, therefore, not representatiave of long term conditions at Tongatapu. The measurement location is poorly exposed to wind sea energy out of the eastern sector due to the sheltering of the island Eua (see Fig. 12). The direction of the wind sea has to be south of southeasterly before a significant proportion of wind sea energy will reach the measurement site.

The influence of sheltering on the Waverider data is illustrated in Fig. 6 which shows simultaneous time series of significant wave height from the Directional Waverider in the undisturbed location 3 (Fig. 6) and the Waverider just off the coast of Tongatapu (location 2). Together with the wave height are shown the wave direction at the spectral peak period measured by the Directional Waverider and the wind direction at Fua'amotu. It is apparent from these time series that when the wave direction at the outer location is from the south and southwest then the difference in wave height between the two buoys is much less than when east to south easterly waves dominate the wave spectrum, in which case the wave energy dissipates by breaking on the east coast of Eua before reaching the Waverider location.

Analysis of the GEOSAT altimeter data for the South Pacific (see Barstow and Haug, 1994 for more details on these satellite measurements of average wave and wind fields) reveals that the Southern Tonga group has significant wave heights averaging about 2.2 m in November to March increasing to about 2.4 m in May - September.

The satellite data represent the offshore, i.e., relatively undisturbed, wave conditions in the region off Tongatapu. These wave heights are significantly higher than the Waverider measurements which range, as we have seen, from 1.5 to 2.0 m over the year. The difference between satellite and buoy is least during the winter as it is at that time of year that most southerly swell occurs off the shores of Tonga. In addition, southerly wind driven seas are more frequent in winter. Sheltering by Eua is therefore less important at that time of year.

A comparison of monthly mean buoy and satellite measured significant wave heights is shown in Fig.9. If one compares mean wave heights from the satellite and the Waverider in the same months (compare crosses with triangles) one can see that the difference can be up to 1 m but other months have significantly lower differences. This is again explained by the poor exposure of the Waverider to easterly wind seas. The month with the largest deviation (October 1987) was also the month with the highest easterly wind component of the months for which data exists for both Waverider and satellite (see zonal wind speed time series, Fig. 3). On the other hand the two months with the smallest difference in wave heights were also the two months with the lowest zonal wind speed (or easterlies), suggesting also anomalous westerly weather during these months, to which the Waverider is better exposed.

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 12

I I I I I I I I I I I t (1') a) -s 0 j S ::r::

C\2 /' / /

I I I I I I I I I I I -I J F M A M J J AS 0 N 0

,--. b) rn a

8 E-.

a L{) c) s a ;3:: 'o:t' '-"'~ -+-' a ~ C'j

0 N

a .-4

0

I I I I I I I I I I I I I d) ~ ~ c§' 0 ~ () Q) "'~ ~ r{;" '"'" ~ """ 0 r{;" / 0 v z i c§' 'I;

<;::) , ' I I I I I I I I I I I t J FHA M J J A SON 0 Fig. 7 Annual variation of a) Significant wave height, HmO, b) Energy Period, Tm-10, c) Wave Power, Jr and d) Number of wave records for the Waverider measurements at Tongatapu

28400/REPORT/93085/SFB/akm/3 May 1994 /~-."""~.-.I The Wave Climate of the Kingdom of Tonga 13

Comparing GEOSAT wave heights during the relatively strong El-Nit70 in 1987 with the following year when the SOI was positive shows no significant difference in the two years. Further to the series of low pressure systems further north than normal to the south of the Cooks with accompanying strong south westerly winds, particularly during winter 1987. These south westerlies do not seem to have affected Tonga to the same degree in the same period.

The subsequent El-Nino episode occurred during 1991-92. During March and April 1992 wave heights measured by the Waverider were relatively high for the time of year (Fig. 9). March 1992 was the coldest March on record in New Zealand due to southerly winds for most of the month. It is likely that the high wave heights observed at Tongatapu at this time were a result of the abnormal weather further south manifesting itself as higher swell than normal, although abnormal southerly gales were not registered at Chatham and Campbell Islands. Inspection of weather charts shows that the weather picture was dominated by high pressure over the Tasman Sea and low pressure in the southern ocean to the south east of New Zealand for much of the period March-April 1992 with strongest winds to the east and south east of New Zealand. The Waverider on the south coast of Kadavu (Fiji) also recorded high wave heights for the time of year during the same period.

The variation of the wave period, Tm-10, over the year (Fig. 7.b) shows that wave periods are somewhat higher during the winter at about 10 secs. decreasing to a little under 9 secs. in the summer when a greater proportion of short period wind seas and less long period swells are experienced in the more frequent southerly winds at that time of year.

The distribution of the peak wave period, Tp, for the whole measurement period is shown in Fig, 8. The vast majority of records have wave spectra with peak period greater than 10 secs., confirming the swell dominance on the wave climate on the southern shore of Tongatapu.

The frequency distribution for wave direction associated with waves at the spectral peak period is shown for the 3 month measurement period in the offshore directional Waverider location in Fig. 11. Separate wind sea and swell directions are shown in Appendix C. It comes as no surprise that wind seas are predominantly easterly whilst swells come from southerly directions. It is interesting to note that the data shows a minimum in the swell direction distribution corresponding to the direction of New Zealand. Long swells arrive only from directions corresponding to the Tasman Sea and the sea area to the east of New Zealand as would be expected. A comparison of wave directions from the Directional Waverider and from the UK Met. Office Global numerical wave model is also discussed in Appendix C.

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 14

a)

b)

c)

Fig. 8 a) Simultaneous measurements at Tongatapu (- ---) and the directional Waverider measurement site (-) of significant wave height (m); b) wave direction at the spectral peak from the directional Waverider and c) wind direction from Fua 'amotu airport

28400/REPORT/93085/SFB/akm/11 May 1994 The Wave Climate of the Kingdom of Tonga 15

'---'s 0 S ::r:: ~ ro ~OJ

19817 1988 1989 1990 1991 1992

Year

Fig. 9 Comparison of monthly mean significant wave height from the Waverider buoy south of Tongatapu (crosses) compared with data from the GEOSAT Satellite altimeter (triangles) for the sea area around the Southern Tonga group.

1.O I I I I I I I I I I :>.. N ~() Q) a ;:j N ~ Q) ~ 1.O ~ ~ ~ a ~

lO

0 I ~I 1 I 1 \/"-.1 I I 0 5 10 15 20 25

Fig. 10 Frequency distribution of the peak wave period, Tp, for the Waverider at Rarotonga.

28400/REPORT/93085/SFB/akm/11 May 1994 The Wave Climate of the Kingdom of Tonga 16

TONGA

0

Fig. 11 Frequency distribution of the wave direction at the spectral peak period, ThTp, from the 3 month Directional Waverider deployment.

The wave power on the southern coast of Tongatapu averages in most months greater than 15 kW/m, reaching 20 kW/m during March to July, and falling below 15 kW/m during November to January. It is probable, however, that the measured average of 20 kW/m for March is not representative for long term conditions due to the unusual conditions in March 1992 discussed above and the high wave conditions at the end of March 1990 due to Tropical Cyclone activity.

Next, we look at the variation of significant wave height along the GEOSAT satellite's tracks in the neighbourhood of the Tonga group. Fig. 12 shows colour coded average wave heights along the tracks for the whole period 1987-89 (the satellite follows an exact repeat orbit returning to the same track every 17 days). Away from the islands the significant wave height is fairly invariant but, particularly to the west of the islands, somewhat lower wave heights are found than to the east. This is due to the islands sheltering to a certain extent both wind sea and swell energy. In amongst the islands wave heights are considerably lower, less than 1 m in places.

The northern Tonga group has wave height and power levels similar to that for the southern group, although the wind sea will have closer affinity to that found around Western Samoa (see the companion report The Wave Climate of Western Samoa) due to the stronger seasonality in the wind direction in the north of the group.

28400/REPORT/93085/SFB/akm/3 May 1994

The Wave Climate of the Kingdom of Tonga 18

5 Special Events

In this section, wave data are presented for a number of special events and we start with a review of wave data from tropical cyclones in the Tonga group.

5.1 Cyclone Sina

During the measurement period at Tongatapu only two tropical cyclones produced high seas. During 23rd - 24th March 1990 Tropical Cyclone Rae passed on a south eastward track to the east of Tongatapu. Significant wave height peaked at 6 m, with a strong swell component, as winds locally backed from northerly to south easterly, peaking at 15 m/s from the south.

The second cyclone was Cyclone Sina which passed the southern Tonga group on 29th November 1990. Time series of wave parameters and wind data from Fua'amotu Airport are shown in Fig. 13. Significant wave height at the Waverider increased late on the 28th in response to the increasing south easterly winds. The wave height peaked at 6.7 m soon after the wind speed maximum early on the 29th. Wind speeds weakened rapidly in the southerlies which developed behind the storm.

Wave spectra leading up to the storm peak are shown in Figs. 14 and 15. Late on the 28th a swell peak at about 0.08 Hz begins to grow. This swell probably originates from strong south westerlies off the east coast of New Zealand during the previous days. By 0030 UTC the swell peak had increased to dominate the wave spectrum. At the same time one can see a growing local wind sea peak at about 0.15 Hz as the cyclone moves closer. By the time of the storm peak the two peaks in the wave spectrum had merged to form a one-peaked spectrum. Thus the peak wave conditions comprised not only local storm waves but also a fairly strong swell.

5.2 Other Cyclones

Only two tropical cyclones marginally affected the wave climate during the lifetime of the GEOSAT mission from November 1986 to September 1989 for which satellite wave height measurements are available. The first was which, although at some distance from Tonga, produced 5-6 m seas near to Tongatapu on 4th March 1988. This storm is discussed in some more detail in the companion report on the Wave Climate of Vanuatu.

The second storm was which caused considerable damage in Fiji in late December 1986. In the early stages of the storm it skirted the northern Tonga group with Niuafoou lying on the edge of the area of gales. The satellite recorded wave heights exceeding 5 m in the northern Tonga group during this time. A full presentation of satellite data for this storm is given in the Wave Climate of Fiji report in this series.

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 19

5.3 Swell Events

There are historical records for the South Pacific of unusually energetic swells causing damage. Matthews (1971) demonstrates that damage caused on the northern shores of South Pacific Islands in December 1969 was due to an unusually intensive storm in the North Pacific, the swell travelling over 7000 km before dissipating its energy over a wide area of the South Pacific from Tuvalu to Tahiti. No records of damage were, however, reported from Tonga during this event. It is probable that the Samoa islands sheltered Tonga from this swell to a certain extent. to the east, on the other hand, reported very high swells. Damaging southerly swells are also known from the Southern Cook Islands. Similar swells will occasionally occur in the Tonga group but the more rugged coastline there will serve to protect property from wave action.

a)

b)

c)

d)

Fig. 13 Time series of a) significant wave height, HmO; b) peak wave period, Tp; c) Wind speed and d) Wind Direction from Tongatapu during the passage of Cyclone Sina on 29th November 1990.

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 20

0

-0 --u. 0 UJ J, 1\

i 0. i 0 i I i I ". I.i V,," ..'." 1..':- ~ " .\ , .,: '.\""';"." '\...,., (:) I I L. I ,.J.- I r/ " 1'I I..' , (..' " j I: " j I:! ': .' ~ " " ,~"". "'---~;2~l ~~ 0.0 0.2 0.3 0.'\Frequency(Hz)

Fig. 14 Wave spectra, S(f) [mZs] for 28th November at 1500 (-),1800 ( ), 2000 (- ---) and 0300 UTC on 29th November (-. ) measured by the Tongatapu Waverider. The swell peak is indicated by an arrow.

0 ~ 0 u.-- 0 i, (/) f "\ -1-/ f\ ! .. ' \ \ '" " ':,;, \ 0 -- ~" 0 .~"'" I \ - \ .\ '.,., ~ .", , ~ ., , ~., " I , " I " , 0 .. ,.[! : II " I i: I i ; : I i i: , \ ; : , \ ~t. , 0,\ , ,I - 0- 0:0 0.1 0.2 0.3 0.'+ FrequencyCHz) Fig. 15 Wave spectra, S(i) [m2s] for 29th November at 0030 (-), 0200 ( ), 0330 (- ---) and 0630 UTC on 29th November (-. ) measured by the Tongatapu Waverider. The swell peak is indicated by an arrow.

28400/REPORT/93085/SFB/akm/3 May 1994 o.'\':" Wave Climate of the Kingdom of Longa 21

5.4 September 1990 Swell Event

During 8th -9th September an extremely long swell can be detected in the wave records from all stations then operational in the South Pacific. In particular. the peak wave period peaks at 20 secs. at all stations (Fig. 16). Plots of simultaneous wave spectra from the three wave stations (Rarotonga, Tonga and Western Samoa) are shown in Fig. 17. One can see that the swell signal is first picked up by the buoy off Tonga (swell peak is indicated by an arrow on the first figure at about 0.05 Hz or 20 secs.). Towards the end of the sequence on the 8th the swell is also detected at Western Samoa and Rarotonga. A powerful low pressure system was located off New Zealand at about this time. This fits in well with the direction of propagation of the swell which may be inferred from the arrival times of the swell at the three locations.

5.5 High Swells in June 1991

The highest wave heights recorded in 1991 at Tongatapu were not a result of cyclone activity but rather a powerful swell originating from a low centred over North Island, New Zealand on 23rd June. Time series plots for Tongatapu are shown in Appendix A. This swell event on the 23rd at Tongatapu resulted in significant wave heights exceeding 5 m and, somewhat later, 4 m was also recorded also at Western Samoa. Wave spectra near to the peak of the swell events at Tongatapu and Western Samoa are presented in Fig. 18.

g, I I I I I I I I I I I I I I I II -~ ~. ~-I ~ h r b ~.. a: 1"\ ---"'J V '--V'-~"--"""\_-~ r-"' 'Q. 0.. -:~ I I I" I I I I I I I I I I I ; ~ I i 1 10 13 \6. DATE -'" f ; .-- a.'" .,., ..r"JV'--'-.f"""J~,V\¥,-""_j'(AJ\.,.M_N\J/-"-")W~N",,","",, 9 '" 0 I i 1 10 \3 16 DATE ...I I I I I I j ~j ~ ~- ;§. I 0-~ ""r..l""\_~-\I-\,,-J\.J 'V'-"'" .n1 0- .I I I I I I I I I I II I I I~ I i 1 to 13 16 DATE

Fig. 16 Time series of peak wave period, Tp, for a) Rarotonga, b) Tonga and c) Western Samoa for 1st -16th September 1990.

28400/REPORT/93085/SFB/akm/11 May 1994

"'""-'V-v'4fI!Je The Wave Climate of the Kingdom of Tonga 22

Fig. 17 Simultaneous wave spectra from Western Samoa ( ~, Tonga ( ) and Rarotonga (- ---) for the following times (from left to right) 900907, 1800; 900908, 0000; 900908, 0600; 900908, 1130; 900908, 1500; 900908, 1800 UTC.

28400/REPORT/93085/SFB/akm/11 May 1994 The Wave Climate of the Kingdom of Tonga 23

a)

- -.

b)

Fig. 18 Wave spectra, S(f) in m2/Hz, recorded by the Waverider at a) Tongatapu and b) Western Samoa during the 23rd June 1991 swell event.

28400/REPORT/93085/SFB/akm/11 May 1994 The Wave Climate of the Kingdom of Tonga 24

6 Conclusions

The purpose of SOPAC's wave data collection programme has been to map the wave energy resource of several South Pacific island nations including the Kingdom of Tonga,

The data confirms that the Tonga group are endowed with a relatively high resource of the same order of magnitude as an area on the south coast of Java where a wave power development project is at a fairly advanced stage.

Although wave power levels are on average not as high as the higher northern latitudes, e.g., European Atlantic coastlines, the resource is considerably more steady through the year.

As the dominant contributions to the wave power in the Tonga group are derived from the easterly trade winds and southerly swells, the highest coastal wave power levels are to be found on the south and east facing coasts. This leads to a large difference in annual wave power levels between the offshore undisturbed region (estimated to be about 25-30 kW/m), and the buoy location off the south coast of Tongatapu (17.5 kW/m) which is relatively sheltered as far as wind sea energy is concerned. Although sheltered from this wind sea energy, the southern shore of Tongatapu is nevertheless probably the most energy rich coastline in the group.

The wave power resource of the other principal groups of islands is most probably of a similar magnitude as at Tongatapu particularly on the south and east facing coasts of the Ha'apai and Vava'u groups.

Although the resource is relatively stable throughout the year in the Tonga group, albeit with somewhat higher winter wave power levels compared to summer, there is considerable variability in wave power levels on other time scales from minutes to years. The tendency for high single waves to appear in groups leads to wave energy varying strongly on the time scale of minutes. There is also variability, on the time scale of days and weeks, in the swell component associated with the passage of storms further south and for wind seas due to trade wind variability. On the longest time scale, the El-Nino Southern Oscillation phenomenon may potentially have a significant impact on coastal wave energy levels due to anomalies in trade wind directions and variability in swell in some years.

Significant wave heights exceeding 10 m can occur in the Tonga group in association with tropical cyclones which occur in this region sometimes 2-3 times per year. On two occasions significant wave height measured by the buoy on the coast of Tongatapu exceeded 6 m due to close passage of Tropical Cyclones Sina and Rae in 1990. The GEOSAT satellite altimeter also recorded significant wave heights exceeding 5 m in the Tonga group as a result of Cyclone Raja in 1986 and Cyclone Bola in 1988.

Very high swells can also lead to high sea states, exceeding 5 m on occasion, as on 23rd June 1991.

28400/REPORT/93085/SFB/akm/11 May1994 The Wave Climate of the Kingdom of Tonga 25

The foundations have been lain in this project for estimates to be made of the wave energy resource at any location in the Tonga group. The data will also be useful with respect to coastal defence, harbour design and improved wave forecasting. Various points need to be stressed to the potential user of this data. First, there are large gradients in the wave conditions in the neighbourhood of the islands and as such the wave measurements should be considered to be site specific. The length of the record is also short compared to the dominant El-Nino Southern Oscillation climate cycle and the data cannot necessarily be considered to be representative for the long term. Finally, wave direction measurements have confirmed that wave directions provided by the UK Met. Office global wave model are not entirely satisfactory due to systematic underestimation of swell heights. Information on wave direction is crucial for the estimation of the coastal wave climate at other locations than the southern Coast of Tongatapu.

In the final section we give recommendations as to how the wave climate can be estimated away from the measurement site and what future work would be beneficial. All data collected in this project has been installed at the SOPAC Secretariat in Suva, Fiji.

28400/REPORT/93085/SFB/akm/11 May 1994 The Wave Climate of the Kingdom of Tonga 26

7 Recommendations

In the course of this project, in addition to the wave measurements and wave climatology work, a number of training workshops have been held for nations of several countries including Tonga. At the present time, in-country knowledge of ocean waves, wave power technology and Waverider buoy operation and maintenance exists. Some wave measuring equipment is also available in the region. In order to maximise the benefits from this training activity it is imperative to continue to the next natural stage of carrying out site specific assessments for wave power. In fact, the Fiji Ministry of Energy has already set up its own wave power programme to look at the wave power potential at 3 or 4 sites in the Fijian Archipelago where energy requirements are presently covered by fossil fuels. It is recommended that the Tonga follows a similar procedure. This is described in the following.

Short term measurements (3-6 months) could be made using a Directional Waverider at the offshore measurement location, simultaneously with Waverider measurements as close to each of the chosen sites as possible. This data can then be used to construct a long term wave climate at the site in question calibrated global wave model data, a coastal wave model and available satellite data.

It is also recommended that SOPAC produces a regionwide brochure describing the results of the wave climate program with emphasis on the wave energy resource, the state-of-the-art of wave power technology internationally, and the potential in the South Pacific. This document would then be invaluable both as an information source within the countries themselves and to attract foreign companies involved in wave power to the region.

It has been pointed out earlier in this report that the length of the measurement series is relatively short compared to the El-Nino/Southern Oscillation cycle, which may have quite a large effect on the inter-annual variability of the wave climate. It is therefore considered to be worthwhile to extend the data base compiled at SOPAC with the ongoing satellite measurements from the ERS-1 and Topex/Poseidon satellites which followed GEOSAT which is widely used in this report. These data will also be useful direcly to SOPAC in their coastal science projects. Cyclone data are, in particular, often captured by the satellites.

A final point is that there is much research activity in large international programs such as TOGA in tropical waters of the Pacific. Future programs should consider adding wave measurements to the instrument arrays. Wave measurements give integrated information on the winds over a wide expanse of ocean and are thus an interesting climate monitoring parameter.

28400/REPORT/93085/SF/akm/11 May 1994 The Wave Climate of the Kingdom of Tonga 27

8 References

Barstow, S.F. and Haug, O., 1994: The Wave Climate of the South West Pacific, SOAPC Technical Report 206.

Barstow, S.F. and Olsen, E., 1993: Wave Data Collection, tongatapu Kingdom of Tonga, May 1987 - December 1991. SOPAC Tech. Rep. No. 154.

Harrison, D.E. and Luther, D.S., 1990: Surface Winds from Tropical Pacific Islands - Climatological statistics. J. Climate 3(2), 251-271.

Matthews, L.S., 1971: Heavy Swell Observed in the South Pacific in December 1969. New Zealand Meteorological Service, Technical Note 196, 11 pp.

Olsen, E. andSelanger, K., 1993: Wave Data Collection, Tongatapu, Kingdom ofTonga, May 1987 - July 1992. SOPAC Tech. Rep. No. 185.

Olsen, E., Barstow, S.F. and Selanger, K, 1991: Wave Data Collection, Tongatapu, Kingdom of Tonga, May 1987 - December 1990, SOPAC Joint Contribution No. 85.

Snodgrass, F.E., et al., 1966: Propagation of swell across the Pacific. Phil. Trans. Roy. Soc. A259, 431-97.

Thompson, C.S., 1986: The climate and weather of Tonga, N.Z. Met. Serv. Misc. Publ. 188 (2), 69pp.

28400/REPORT/93085/SFB/akm/11May 1994 The Wave Climate of the Kingdom of Tonga 28

APPENDIX A

Time series plots of wave and wind parameters; 1987-92

Hm0: Significant wave height (m) Wind Speed: Measurements from Fua'amotu Airport (m/s) 38 m.a.s.l. Tp: Spectral peak wave period (s) Tm-100: Energy (wave) period (s) Tm02: Mean wave period (s) Wind Direction: Measurements from Fua'amotu Airport (m/s), 38 m.a.s.l.

28400/REPORT/93085/SFB/akm/11 May 1994

INSTRUMENTlQQ,aQlQ~ SOPAC WAVE CLIMATOLOGY STUDY LOCATIOO STATION WATERDEPTH INSTRUMENTHEIGHT OBSERVATIONPERIOD Fua'amotu 154 m Om 1987.07.01-1987.12.31- q~ OCEANOGRAPHIC PROJECT FIGURE COMPANYOF NORWAY 28400

lrKCN13'3'/Opp

The Wave Climate of the Kingdom of Tonga 41

APPENDIX B

Time series plots from the Directional Waverider; June to August 1992

Hm0, Significant wave height (m) Wind speed (m/s) from Fua'amotu Airport Tp, Peak Wave Period (s) ThTp, Wave Direction at the Spectral Peak (deg, from which) ThHf, High Frequency Wave Direction (deg. from which); in the 2.5 sec. period band. Wind Direction (deg. from which) from Fua'amotu Airport

28400/REPORT/93085/SFB/akm/11 May 1994

The Wave Climate of the Kingdom of Tonga 43

APPENDIX C

Comparison of wind and wave data from the UK Met. Office Global Wave Model with measurements from the Directional Waverider in open water to the south of Tongatapu.

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 44

DISCUSSION

It became evident as the measurement program with the Waverider buoys progressed that in order to make estimates of the wave climate elsewhere, even on the same island, would require information on both the undisturbed offshore wave heights and, most importantly, the wave direction. This is due to the fact that the offshore wave conditions at any one time will consist of crossing waves from up to several different sources, such as easterly trade wind seas, northerly swells, and southerly swells. Not all of these wave trains will be "seen" on a given stretch of coastline. The presence of waves from a given source on the coast depends mainly on the relative orientation of the coast and the deep water wave direction. In areas with extensive shallow water areas at the coast, wave refraction may also play a role.

It was, therefore, decided to make some short term directional measurements offshore in order to validate the only potentially reliable information on wave direction in the South Pacific provided by the UK Met. Office's Global numerical wave model. Time series of various wave parameters are available at a 6-hourly time step at points on a grid with a resolution of 0.833 deg in latitude and 1.25 deg. in longitude. Data from the nearest grid point to Tonga were obtained (22.04'S, 175.62'W) covering the directional Waverider measurement period from June to August 1992.

A time series comparison between the Directional Waverider and the wave model is shown for the 3 month period in Fig. C.1. The model (dashed line) significantly underestimates the significant wave height. Splitting the wave spectrum into wind sea and swell parts and comparing significant wave height for the two parts separately (lower two plots in Fig. C.1) shows that whilst wind sea wave heights compare very well, the swell height is on average under half of the actual measured height.

Figs. C.2 and C.3 display directional wave roses for wind sea and swell separately. The wind sea directions from the model are generally quite good whereas swell direction is less well correlated with the measurements. However, it should be pointed out that the method for separating swell and wind sea directions is not identical in the model and buoy.

The overall wave direction, ThTp, is compared in Fig. C.4. The swell underestimation discussed above also results in fewer situations with southerly wave directions in the model data.

The difference n the distribution around north does not necessarily reflect poor performance in the model as northerly directions are not possible for the buoy location due to the Tonga group sheltering any waves coming from that direction. The model does not include the South Pacific islands as they are generally smaller than the grid resolution. Scatter diagrams of wind sea and swell significant wave height with regression statistics are given in Figs.C.5 and C.6.

28400/REPORT/93085/SFB/akm/3 May 1994 Fig. C.1 Time series for June-August 1992 from the Directional Waverider (solid line) and the UK Met. Office Wave Model (dashed line) for a) Significant Wave Height, b) Wind Sea Wave Height and c) Swell Wave height.

28400/REPORT/93085/SFB/akm/3 May 1994 The Wave Climate of the Kingdom of Tonga 46

WAVERIDe~

IWAVERIDER -22.0

{}):O GRIDPOINT

185.0 Fig. C.2 Swell direction roses for buoy (uppermost) and UK Met. Office wave model.

r-'--:-"/ WAVER'D~ I

~

DIRECTIONAL WAVERIDER~

-22.0

185.0 Fig. C.3 Wind sea direction for buoy (uppermost) and UK Met. Office wave model.

28400/REPORT/93085/SFB/akm/3 May 1994 5./11RECTIONAL The Wave Climate of the Kingdom of Tonga 48

~ 5 E --' Q) :3: U) :I: 4

~

0

HmOs m

STATISTICS N = 37-f3

Mean x = t .88 Mean y = 0.70 Mean lid-x} =IV. t .18 Sl.de I y-x} = 0.52

Regresslon type: y = a + bx tnc. OX.) a = 0.08 b = 0.33 Correlallon coefflclenl = 0.39 Sl.dev. around regr. = 0.30

Fig. C.5 Comparison of simultaneous swell wave heights from the model (HswelJ against the Directional Waverider (HmOs).

28400/REPORT/93085/SFB/akm/24 May 1994 ~ ~ ::> E

-0 C-' ~ :I:

3 I'~;': '_/, === I :E*+/ + +~ + + + +1++"+ + -t 1-.,

+++11- + ++ + -+++ I I '+11++-+-+- ++ " + -II- + -T' .:-'::.':~H-+ :;r:1+++ +++- / ~+ ,,1-+ + ++ + +* + +I-HI- + +

++ + +

~ +++ +

+// + ~ + 0 ,I,, I I I I I --~ t 0 t 2 4 5 HmO",(m

STATISTICS N = "3,4 Mean x = i. i8 Mean y = i .28 Mean (y-x 0.10 0.413 St .dev. (ItJ-X

y = a + bx (prtnc Regresslon type: ax. a = 0.23 b = 0.90 CorreLallon coerflclenl = 0.85 Sl.dev. around regr. = 0.28

Fig. C.6 Comparison of simultaneous wind sea wave heights from the model (Hwi~ against the Directional Waverider (HmOw).

28400/REPORT/93085/SFB/akm/24 May 1994 TJ~~~,/~