Seasons in Samoa
Samoa's weather may seem warm, wet and humid all year, but we do have two seasons. Not exactly summer and winter, because Tutuila is a small dot in a vast tropical ocean, so the ocean moderates our weather year-round. Our seasons are sometimes referred to as tropical wet and dry periods, although you may wonder, when is it ever dry here?
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| Graph after R. Whittaker. 1998. Island biogeography: ecology, evolution, and conservation. Oxford University Press (NY) 285p. |
October through May is our 8-month hot and wet summer. Rainfall at the Tafuna airport weather station is about 11-14 inches per month and air temperature (averaged over a 24-hour period) is about 82-83o F (for the period 1990-2002). Our 'cold' spell occurs from June through September, when averaged air temperatures plummet to 81o F and rainfall is reduced by half.
Actual rainfall can vary much more than this, as we all know. Minimum and maximum records for rainfall over a one-month period are 0.3 and 32.7 inches. The amount of rainfall we receive depends largely on where we are standing. The harbor area is very wet -- it receives 200 inches of rainfall per year. Some mountain areas may get up to 300 inches (25 feet) per year. The Tafuna plains are much drier and receive a mere 125 inches per year.
If it seems like it rains every day here, it almost does. In most years, the airport weather station detects at least a trace of rain 300 days of the year. But a noticeable amount of rain (at least one tenth of an inch) occurs on about 50% of the days of the year. But remember, the airport (where these measurements are taken) is the driest part of our island. Relative humidity at the airport fluctuates in the 80s (daily range 73-90%).
Air temperatures in recent years typically fluctuate from nighttime lows around 77o F to afternoon highs in the upper 80s. We now average 60 days per year at 90o or above. Record highs and lows here are 96o and 62oF.
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if it feels like our climate it is getting warmer, it is, but the picture
is a little complicated. We have two long-term records of air temperature
in American Samoa, the NOAA weather stations at the Tafuna airport and
at Tula. Temperature trends at these two sites are quite different,
probably due to local conditions where the temperatures are actually
measured. Temperatures measured at the airport weather station have
risen steadily over the past 15 years (see graph), but no such increase
has been recorded at the NOAA station in Tula. The airport station measures
air temperatures at a height of about 5.5 feet above the ground, and
it is located on the Tafuna Plains which is rapidly being built up with
buildings and roads, so it is probably detecting a near-ground increase
in temperature that is felt by us. Whether other parts of Tutuila are
warming in a similar fashion is not known. In contrast, the Tula station
measures air temperatures at about 50 feet above the ground, so it is
less affected by near-ground conditions. It shows no increase in temperature,
as would be expected at our remote oceanic location.
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 Hurricane Val in 1991 |
As summer progresses, the temperature of the ocean's surface waters also increases by 2o F. Warmer ocean temperatures, in turn, help provide the energy to start hurricanes, thus the chance of a hurricane is greatest between November and March.
Some plants and animals in Samoa schedule their activities, such as flowering or spawning, according to the seasons. These biological events tend to occur over a much longer period in the tropics compared to colder regions of the world, because the distinction between our two seasons is not great and growing conditions here are generally good year-round. Nevertheless, seasonal patterns of flowering and fruiting mean times of abundance or scarcity for some of our native wildlife, particularly the fruit bats (pe'a) and Pacific pigeon (lupe).
Some animals schedule their seasonal migrations to Samoa to avoid winters elsewhere in the world. We see more golden plovers (tuli) and and tourists during the October to March periodwhen they flee winter in the northern hemisphere, while humpback whales (tofola) are most common here in September-October when they escape winter in the southern hemisphere.
P. Craig, NPS