Current and Future Climate of Tonga
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p u Niuatoputapu Niuafo'ou o Late Island r G ’u a v a V South Pacific Ocean 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 > Tonga Meteorological Service > Australian Bureau of Meteorology > Commonwealth Scientific and Industrial Research Organisation (CSIRO) Tonga’s current climate Tonga has two distinct seasons – a warm wet season from November to April are also affected by sub-tropical and a cooler dry season from May 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 world. There are extends across the South Pacific Ocean with about 5ºC difference between two extreme phases of the El Niño- from the Solomon Islands to east of the warmest month (February) and Southern Oscillation: El Niño and La the Cook Islands 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 Sea 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, Tongatapu. 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 Micronesia Palau Marshall Islands 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 Kiribati 10 o W a Nauru 0 r m p Tra de W inds o o l S o Papua New Guinea u M o t h Tu valu n Solomon Islands S s o P o East Timor o n a c i f i c C o n v e Samoa r g Vanuatu Fiji e n Niue 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 0 0 0 0 0 0 0 0 0 0 180 11 12 13 14 15 16 17 17 16 15 14 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 T Figure 7 which includes the tide gauge 23.5 record and satellite data since 1993. 23 Average 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 r rapid introduction of new and more t climate slightly differently. Scientists from 1990 500 n efficient technologies. Greenhouse e the Pacific Climate Change Science 400 c gas and aerosol emissions scenarios on Program (PCCSP) have evaluated 24 C 300 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 region. 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 Taro 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).