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American Samoa - Climate

Global warming also causes changes to rain patterns. For American Samoa, future average annual rainfall is predicted to stay about the same, but extreme rain events are predicted to increase. Periods of drought are predicted to decrease slightly, or stay about the same.

The islands experience Southeast trade winds that result in frequent rains and a pleasant, warm tropical climate. The year round temperature ranges from 70 to 90 degrees, depending on the warmth of the surrounding ocean. The humidity averages about 80 percent during most of the year. The average rainfall at Pago Pago International Airport is 130 inches per year, while Pago Pago Harbor, only four and one-half miles away, averages 200 inches per year.

Low islands, have fewer choices and resources to reduce the impacts of climate change than do communities on high islands or continents. Low atolls lack the higher elevations that can provide much more security with respect to avoiding flooding, getting fresh water, growing food, and building roads. If the impacts of climate change continue to increase, the low islands in American Samoa, and their way of life, will become increasingly threatened.

American Samoa is in the South Pacific Ocean, between the Equator and the Tropic of Capricorn. A tropical climate prevails with warm or hot temperatures year-round and high humidity. Rain showers are frequent and can last only for a few minutes or all-day. The average annual rainfall is 125-inches in the dryer areas to as much as 300-inches in the highest mountains. Tropical storms are more prevalent during the long, wet summer season (October -May) and a slightly cooler and drier season (June-September).

One of the most important climate patterns is that during the year there is a wetter season and a drier season. The hot and humid rainy season extends from October through May. The drier season is usually between June and September when the average monthly rainfall is much less. Most Pacific Islands near the equator also have wet and dry seasons.The rainy (or monsoon) and typhoon seasons in the South Pacific extend from November to April.

The amount of rain changes a lot from year to year. Some years had 160 inches (406 centimeters) or more of rain, while other years had 80 inches (203 centimeters) or less of rain. This kind of change in rainfall from year to year is also a natural feature of the climate in many Pacific islands that are near the equator.

Over American Samoa, the winds from the equator and from the southern hemisphere come together and cause a band of rain called the South Pacific Convergence Zone or SPCZ. This very cloudy and rainy area can be seen in satellite photos as a band of thunderstorm clouds extending to the southeast from the equator. This long band of rainy area over the South Pacific Ocean does not just stay in one place. The wetter and warmer season occurs from October through May when the area of rain is located over American Samoa. The drier season occurs from June through September when the area of rain moves further north, toward the equator and away from American Samoa.

The east to west trade winds play a large role in the climate of the equatorial Pacific Ocean. These winds can change during a climate pattern that is called the El Niño Southern Oscillation (ENSO). When trade winds are weaker than usual, it is an El Niño year. When the trade winds are stronger than normal, it is a La Niña year. When the winds are normal it is called a neutral year.

American Samoa has both high islands (Tutuila, Ofu, and others) and low islands (Rose Atoll and Swains Island), and the high islands get more rain than the low islands. On the big, high island of Tutuila, the average annual rainfall ranges from about 120 inches (304 centimeters) along the coasts to as much as 300 inches (762 centimeters) in the high mountains. In the small islands and the flat atolls, about 100 inches (254 centimeters) of rain fall each year.

The fact that American Samoa’s high mountains get more rain than its coasts provides the clue why high islands get more rain than low islands. The air is much colder near the top of a high mountain compared with the bottom of that mountain. When warm, humid air blowing in from the ocean hits a mountain, that air is forced upward into the colder mountain areas. When warm humid air becomes colder, its water vapor condenses from the gas state into the liquid state, and forms water droplets. These water droplets become clouds that rain on the island.

In a La Niña year, strong winds blow across the ocean surface into the region of American Samoa. This raises the level of the ocean and can cause coastal erosion (land loss due to wave action) and damaging king tides (the highest tides of the year). La Niña years also tend to be rainy in American Samoa. In an El Niño year, trade winds are weaker than normal (or absent). An El Niño year is usually drier and there is a greater chance of drought (an extended period of little rain). In addition, warm ocean water moves away from American Samoa toward the eastern Pacific Ocean, and causes sea levels to decrease.

Natural disasters have been occurring more frequently and with greater intensity, threatening the stability of fuel sources, and the reliability and affordability of electricity generation. Rapid changes in cost or availability of energy can have serious effects on an entire community. There are a number of power generation options potentially available to American Samoa, such as diesel, heavy fuel oil, liquefied natural gas, modular nuclear, and renewable energy technologies including solar, wind, biomass, waste-to-energy, and geothermal. Each of these technologies has its own operational characteristics, initial and operational costs, implementation time horizon, and near- and long-term environmental impacts. Most power generation choices require a large investment that can impact a community for many years.

In the Samoa region of the western South Pacific, coral bleaching is often related to the warm phase of the El Niño-Southern Oscillation (ENSO), a climate phenomenon characterized by large-scale changes in sea surface temperature and sea level pressure across the tropical Pacific. The warm El Niño phase of ENSO brings higher water temperature and lower sea levels over Samoan reefs, which can expose them to air and increased sunlight and may result in bleaching. The strong El Niño conditions of 2015 resulted in major bleaching of some reef areas.

The Samoan language has its own word for an El Niño bleaching event in which sea levels are low enough to expose corals to air: taisama refers to the stench of dying reefs that accompanies these events. The word may be uttered more often as climate change makes coral bleaching events increasingly common.

Severe rainstorms can cause flooding and landslides, resulting in significant loss of life and extensive damage to infrastructure, and hampering the provision of essential services. Disruptions to air services and to water and power supplies may also occur. Keep informed of regional weather forecasts, avoid disaster areas and follow the advice of local authorities. During a typhoon or monsoon, hotel guests may be required to leave accommodations near the shore and move to safety centres inland. Travel to and from outer islands may be disrupted for some days.

As an island Territory, American Samoa is directly and immediately impacted by global climate change. The implications of increased global temperatures put American Samoa's survival at risk. A majority of the population resides along the shoreline; any rise in ocean level will threaten homes and displace residences. Any loss of coastal land will disturb the delicate balance of the Territory’s ability to provide resources such as food and local building materials. Most of the local water supply comes from underground freshwater aquifers.

Sea level has been increasing in American Samoa at about 0.1 inches per year. This may not seem like a very rapid increase, but its effects can quickly add up year after year. Sea level rise causes beach erosion, flooding during high tide, and increased flooding due to storm surge and tsunamis. Sea level is expected to continue rising, perhaps by more than 3 feet (1 meter) by the year 2100.

This increase in sea level means that anything that makes the ocean waves reach farther inland (such as a king tide or a tropical cyclone) will cause more flooding than when the sea level was lower. For example, a deadly tsunami struck American Samoa in 2009, and because of longterm sea level rise over the past century, it caused more damage than it would have if sea level had not been rising.

Higher sea level also affects the availability of food and water. When the ocean floods the land, the soil becomes salty, which damages the natural plants and trees, and also makes it much harder to grow food. The higher sea level can also reduce the amount and quality of the underground fresh water. Any rise in ocean level will likely compromise the freshwater aquifers via seawater intrusion.

Climate change endangers the survival of American Samoa's coral reefs, the rain forest of the ocean. American Samoa depends on our coral reefs for subsistence fishing and protection from hurricanes. Any loss of coral reefs is detrimental to the capacity to endure severe weather events and to feed families. Therefore, American Samoa must take an active stance in combating climate change at the Territorial level. American Samoa must do all we can locally to help affect a global strategy against the threat of climate change.