Samoa - Climate

Samoa is at risk from the effects of climate change. Cyclone Evan in 2012 damaged a number of ports in Upolu and Savai’i, particularly at the old wharf in Apia. During 1969–2010, 52 tropical cyclones passed within 400 kilometers of the capital city of Apia. Samoa’s coastal zone is also vulnerable to ongoing sea level rise—sea level has risen by about 4 millimeters per year since 1993, and is projected to rise by 5–15 centimeters (cm) by 2030, 10–29 cm by 2055, and by 21–59 cm by 2090. Projections indicate that tropical cyclones will intensify. A 2010 World Bank study estimated the present economic value of damage through 2050 resulting from climate change in the absence of additional adaptive measures to be $104 million to $212 million over the same period in Samoa.

The climate is tropical, with a rainy season from November to April.

Temperatures in Samoa are generally consistent throughout the year, with only very small seasonal differences. Average temperatures are coolest in July, when the cool, dry south-east trade winds are strongest. The warmest month is March. The country has two distinct seasons – a wet season from November to April and a dry season from May to October. On average 75% of Samoa’s total annual rainfall occurs in the wet season.

Samoa’s rainfall is greatly influenced by the position and strength of the South Pacific Convergence Zone. This band of heavy rainfall is caused by air rising over warm water where winds converge, resulting in thunderstorm activity. It extends across the South Pacific Ocean from the Solomon Islands to the Cook Islands and lies between Samoa and Fiji during the wet season.

Samoa’s mountains have a significant effect on rainfall distribution. Wetter areas are located in the south-east and relatively sheltered, drier areas in the north-west.

Samoa’s climate varies considerably from year to year due to the El Niño Southern Oscillation. This is a natural climate pattern that occurs across the tropical Pacific Ocean and affects weather around the world. There are two extreme phases of the El Niño Southern Oscillation: El Niño and La Niña. There is also a neutral phase. In Samoa, El Niño events tend to bring wet seasons that are drier than normal, while La Niña events usually bring wetter and cooler than normal conditions.

Droughts and flooding associated with the El Niño Southern Oscillation have impacted the socio-economic livelihoods of the Samoan people on many occasions in the past. Flooding associated with tropical cyclones and strong La Niña events has caused widespread damage in Samoa in the past, particularly in Apia. In early 2008 and 2011, for example, transportation infrastructure and water supplies were severely damaged. Drought impacts are most notable in the north-west regions of the main islands and at times are associated with forest fires. In Asau, there were major forest fires during the dry seasons of 1982-83, 1997-98, 2001-02 and 2002-03.

Projections for all emissions scenarios indicate that the annual average air temperature and sea surface temperature will increase in the future in Samoa. By 2030, under a high emissions scenario, this increase in temperature is projected to be in the range of 0.4–1.0°C. Increases in average temperatures will also result in a rise in the number of hot days and warm nights, and a decline in cooler weather.

There is uncertainty around rainfall projections for Samoa as model results are not consistent. However, projections generally suggest a decrease in dry season rainfall and an increase in wet season rainfall over the course of the 21st century. Increased wet season rainfall is expected due to the projected intensification of the South Pacific Convergence Zone. Drought projections are inconsistent for Samoa.

On a global scale, the projections indicate there is likely to be a decrease in the number of tropical cyclones by the end of the 21st century. But there is likely to be an increase in the average maximum wind speed of cyclones by between 2% and 11% and an increase in rainfall intensity of about 20% within 100 km of the cyclone centre. In the Samoa region, projections tend to show a decrease in the frequency of tropical cyclones by the late 21st century and an increase in the proportion of the more intense storms.