sign in sign up
<<
Home , Niuatoputapu, Tonga

p u Niuafo'ou o Late r G

’u a v a V

South

Tofua Island

p

u o Kotu Group r G i a p ’a a Nomuka Group H

p

u

o NUKU’ALOFA r G

u p a t a ‘Eua Island g n o T

Current and future climate of

> Tonga Meteorological Service > Australian Bureau of Meteorology > Commonwealth Scientific and Industrial Research Organisation (CSIRO) Tonga’s current climate

Tonga has two distinct – a warm wet from to April are also affected by sub-tropical and a cooler dry season from to October (Figure 1). Almost two thirds of high pressure systems that bring the annual rainfall comes during the wet season. cooler air from the south.

Tonga’s climate varies considerably Tonga’s rainfall is affected by the South Temperatures in Tonga change from from year to year due to the El Niño- Pacific Convergence Zone. This band of season to season and are strongly Southern Oscillation. This is a natural heavy rainfall is caused by air rising over tied to changes in the surrounding climate pattern that occurs across warm water where winds converge, ocean temperature (Figure 1). The the tropical Pacific Ocean and affects resulting in thunderstorm activity. It larger differences occur in the south, weather around the . There are extends across the South Pacific Ocean with about 5ºC difference between two extreme phases of the El Niño- from the Solomon to east of the warmest month (February) and Southern Oscillation: El Niño and La the and is most intense coolest month (July) at Nuku’alofa. Niña. There is also a neutral phase. during Tonga’s wet season (Figure 2). Temperatures in the winter months In Nuku’alofa and Lupepau’u, El Niño events tend to bring cooler dry seasons Maximum temperature Average temperature and drier wet seasons than normal, Minimum temperature surface temperature while La Niña events usually bring wetter than normal conditions. 35 300 Nuku’alofa, Tonga, 175.18ºW, 21.13ºS 250 30 200 25 150 Temperature (ºC) Monthly rainfall (mm) 100 20 0

Beach at Kala’au, . 05 15 Jan Apr Jul Oct Figure 1: Seasonal rainfall and temperature at Nuku’alofa.

2 N H o N 20 o Federated States of o n e I n t e r t r o p i c a l C o n v e r g e n c e Z 10 o W a 0 r m p Tra de W inds o o l S o New Guinea u M o t h Tu valu n S s o P o o n a c i f i c C o n v e r g e n c e

S 10

Z o o To nga Cook Islands n e S 20 H o

0 500 1,000 1,500 2,000 30 Kilometres o E E E E E E E W W W W o o o o o o o o o o o 0 180 11 12 0 13 0 14 0 15 0 16 0 17 0 17 0 16 0 15 0 14 0 Figure 2: The average positions of the major climate features in November to April. The arrows show near surface winds, the blue shading represents the bands of rainfall convergence zones, the dashed oval shows the West Pacific Warm Pool and H represents typical positions of moving high pressure systems. Tropical 6 Tropical cyclones 11-yr moving average 5 cyclones

4 Tropical cyclones affect Tonga between November and April. 3 In the 41-year period between 2 1969 and 2010, 71 tropical cyclones passed within 400 km 1 of Nuku’alofa, an average No. of tropical cyclones 0 of one to two cyclones per season (Figure 3). The number of cyclones varies widely from 1969/701971/72 1973/74 1975/76 1977/78 1979/80 1981/82 1983/84 1985/86 1987/88 1989/90 1991/92 1993/94 1995/96 1997/98 1999/00 2001/02 2003/04 2005/06 2007/08 2009/10 year to year, with none in some Figure 3: Number of tropical cyclones passing within 400 km seasons but up to five in others. of Nuku’alofa. Eleven-year moving average in purple. Over the period 1969–2010 cyclones occured more frequently in El Niño years.

3 Tonga’s changing climate

Temperatures Nuku’alofa’s annual Sea level has risen have increased rainfall has decreased As ocean water warms it expands causing the sea level to rise. The Annual and seasonal maximum and Data since 1950 show a clear melting of glaciers and ice sheets minimum temperatures have increased decreasing trend in the annual and wet also contributes to sea-level rise. in Nuku’alofa since 1950 (Figure 4). season rainfall at Nuku’alofa (Figure Maximum temperatures have increased 5) but no clear trend in dry season Instruments mounted on satellites and at a rate of 0.10°C per decade. These rainfall. There are no clear rainfall tide gauges are used to measure sea temperature increases are consistent trends at Lupepau’u. Over this period, level. Satellite data indicate the sea level with the global pattern of warming. there has been substantial variation in has risen near Tonga by about 6 mm rainfall from year to year at both sites. per year since 1993. This is larger than the global average of 2.8–3.6 mm per 26 year. This higher rate of rise may be El Niño La Niña partly related to natural fluctuations that 25.5 take place year to year or decade to 25 decade caused by phenomena such as 24.5 the El Niño-Southern Oscillation. This variation in sea level can be seen in

emperature (ºC) 24 Figure 7 which includes the tide gauge 23.5 record and satellite data since 1993. 23 Average T 22.5 Ocean acidification has been increasing 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Year About one quarter of the carbon dioxide Figure 4: Annual average temperature for Nuku’alofa. Light blue bars emitted from human activities each year indicate El Niño years, dark blue bars indicate La Niña years and the is absorbed by the oceans. As the extra grey bars indicate neutral years. carbon dioxide reacts with sea water it causes the ocean to become slightly more acidic. This impacts the growth 3000 El Niño La Niña of corals and organisms that construct 2500 their skeletons from carbonate minerals. These species are critical to the balance 2000 of tropical reef ecosystems. Data 1500 show that since the 18th century the

Rainfall (mm) level of ocean acidification has been 1000 slowly increasing in Tonga’s waters. 500 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Year Figure 5: Annual rainfall for Nuku’alofa. Light blue bars indicate El Niño years, dark blue bars indicate La Niña years and the grey bars indicate neutral years.

Taking weather observations, Tonga Meteorological Service.

4 Tonga’s future climate

Climate impacts almost all aspects of life in Tonga. Understanding the possible future climate of Tonga is important so people and the government can plan for changes.

How do scientists develop climate projections?

Global climate models are the best tools Intergovernmental Panel on Climate for understanding future climate change. Change (IPCC) developed a series of Climate models are mathematical plausible scenarios based on a set of representations of the climate system assumptions about future population that require very powerful computers. changes, economic development and They are based on the laws of physics technological advances. For example, 2090 and include information about the the A1B (or medium) emissions scenario 800 atmosphere, ocean, land and ice. envisages global population peaking mid-century and declining thereafter, 2055 700 There are many different global climate 600 very rapid economic growth, and 2030 tion (ppm) models and they all represent the a

rapid introduction of new and more t r climate slightly differently. Scientists from 1990 500 efficient technologies. Greenhouse e n the Pacific Climate Change Science 400

gas and aerosol emissions scenarios on c Program (PCCSP) have evaluated 24 300 C

are used in climate modelling to 2

models from around the world and O provide projections that represent found that 18 best represent the climate C a range of possible futures. of the western tropical Pacific . Figure 6: Carbon dioxide (CO2) These 18 models have been used to The climate projections for Tonga concentrations (parts per million, ppm) develop climate projections for Tonga. are based on three IPCC emissions associated with three IPCC emissions scenarios: low (B1), medium (A1B) scenarios: low emissions (B1 – blue), The future climate will be determined and high (A2), for time periods medium emissions (A1B – green) and by a combination of natural and human around 2030, 2055 and 2090 high emissions (A2 – purple). The factors. As we do not know what the (Figure 6). Since individual models PCCSP has analysed climate model future holds, we need to consider a give different results, the projections results for periods centred on 1990, range of possible future conditions, are presented as a range of values. 2030, 2055 and 2090 (shaded). or scenarios, in climate models. The Tonga Meteorological Service Meteorological Tonga crops. Inundation event.

5 Tonga’s future climate

This is a summary of climate projections for Tonga. For further information refer to Volume 2 of Climate Change in the Pacific: Scientific Assessment and New Research, and the web-based climate projections tool – Pacific Climate Futures (available at www.pacificclimatefutures.net).

Temperatures will Changing rainfall Less frequent continue to increase patterns but more intense Projections for all emissions scenarios There is uncertainty around rainfall tropical cyclones indicate that the annual average projections for Tonga as model results On a global scale, the projections air temperature and sea surface are not consistent. However, projections indicate there is likely to be a decrease temperature will increase in the generally suggest a decrease in dry in the number of tropical cyclones by future in Tonga (Table 1). By 2030, season rainfall and an increase in wet the end of the 21st century. But there under a high emissions scenario, this season rainfall over the course of the is likely to be an increase in the average increase in temperature is projected 21st century. Wet season increases maximum wind speed of cyclones by to be in the range of 0.3–1.1°C. are consistent with the expected between 2% and 11% and an increase intensification of the South Pacific in rainfall intensity of about 20% within Convergence Zone. Drought projections More very hot days 100 km of the cyclone centre. are inconsistent across Tonga. Increases in average temperatures In the Tonga region, projections tend will also result in a rise in the number More extreme to show a decrease in the frequency of hot days and warm nights and of tropical cyclones by the late a decline in cooler weather. rainfall days 21st century and an increase in the Table 1: Projected annual average air Model projections show extreme rainfall proportion of the more intense storms. temperature changes for Tonga days are likely to occur more often. for three emissions scenarios and three time periods. Values represent 90% of the range of the models and changes are relative to the average of the period 1980-1999.

2030 2055 2090 (°C) (°C) (°C) Low 0.2–1.0 0.5–1.5 0.8–2.0 emissions scenario Medium 0.2–1.2 0.7–1.9 1.3–2.9 emissions scenario High 0.3–1.1 1.0–1.8 1.9–3.3 emissions scenario Courtesy of NASA Gathering seafood at low tide caused substantial in Nuku’alofa. damage in Tonga in January 2011.

6 Sea level will Table 2: Sea-level rise projections for Ocean acidification Tonga for three emissions scenarios continue to rise and three time periods. Values will continue represent 90% of the range of the Sea level is expected to continue to Under all three emissions scenarios models and changes are relative to rise in Tonga (Table 2 and Figure 7). By (low, medium and high) the acidity the average of the period 1980-1999. 2030, under a high emissions scenario, level of sea waters in the Tonga region this rise in sea level is projected to be 2030 2055 2090 will continue to increase over the 21st in the range of 3-17 cm. The sea-level (cm) (cm) (cm) century, with the greatest change rise combined with natural year-to-year under the high emissions scenario. Low 5–16 10–27 16–47 changes will increase the impact of The impact of increased acidification emissions storm surges and coastal flooding. As scenario on the health of reef ecosystems is there is still much to learn, particularly likely to be compounded by other how large ice sheets such as Antarctica Medium 4–16 10–31 20–59 stressors including coral bleaching, emissions and Greenland contribute to sea-level storm damage and fishing pressure. scenario rise, scientists warn larger rises than currently predicted could be possible. High 3–17 9 –31 21–62 emissions scenario

90 Figure 7: Observed and projected Reconstruction relative sea-level change near Tonga. 80 Satellite altimeter The observed sea-level records are Tide gauge indicated in dark blue (relative tide- 70 Projections gauge observations) and light blue 60 (the satellite record since 1993). Reconstructed estimates of sea level 50 near Tonga (since 1950) are shown in purple. The projections for the 40 A1B (medium) emissions scenario 30 (representing 90% of the range of models) are shown by the shaded 20 green region from 1990 to 2100. The dashed lines are an estimate of 10 90% of the range of natural year- Sea level relative to 1990 (cm) 0 to-year variability in sea level. −10

−20

−30 1950 2000 2050 2100 Year

7 Changes in Tonga’s climate

> Temperatures have > Annual and wet season > By the end of this > Sea level near > Ocean acidification warmed and will rainfall at Nuku’alofa century projections Tonga has risen has been increasing in continue to warm has decreased since suggest decreasing and will continue Tonga’s waters. It will with more very hot 1950. Rainfall patterns numbers of tropical to rise throughout continue to increase days in the future. are projected to change cyclones but a this century. and threaten coral over this century with possible shift reef ecosystems. more extreme rainfall towards more days expected. intense categories.

The content of this brochure is the result of a collaborative effort between the Tonga Contact the Tonga Meteorological Service and the Pacific Climate Change Science Program – a component Meteorological Service: of the Australian Government’s International Climate Change Adaptation Initiative. This web: www.met.gov.to information and research conducted by the Pacific Climate Change Science Program email: [email protected] builds on the findings of the 2007 IPCC Fourth Assessment Report. For more detailed information on the climate of Tonga and the Pacific see: Climate Change in the Pacific: phone: +676 35 355 Scientific Assessment and New Research. Volume 1: Regional Overview. Volume 2: fax: +676 35 123 Country Reports. Available from November 2011. © Pacific Climate Change Science www.pacificclimatechangescience.org Program partners 2011.