Samoa Guide, 2. Seasons and cyclones

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 about 200 inches of rainfall per year. Lata Mountain on Ta’u receives over 300 inches (25 feet) per year. The Tafuna plains are much drier and receive a mere 125 inches per year.

But 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 20 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 nearground 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 is on the upwind side of the island and it measures air temperatures at 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.

It is curious that there are no cyclones along the equator or around South America. This occurs because cyclones require two general conditions to develop: (1) warm ocean waters at least 800 F (26.50 C) to fuel the cyclone, and (2) enough Coriolis force (caused by the earth’s rotation) to provide stability and spin to a developing storm. Coriolis force is weakest along the equator, so developing storms there remain unstable and do not develop into cyclones. South America has few cyclones due to cool water temperatures (upwelling along the west coast) or other climatic conditions (on the east coast) that are not conducive for cyclone development.

Cyclones are a regular but erratic feature of the environment in our part of the world, so it is likely that native species inhabiting American Samoa are able to cope with cyclone impacts and recover, given enough time and assuming that their recovery is not jeopardized by human-related conditions such as habitat loss due to human population increases, competition by invasive alien species, pollution, or changing environmental conditions due to global climate change.

Returning to the topic of seasons, 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 our islands to avoid winters elsewhere in the world. We see more golden plovers (tuli) and tourists during the October to March period when they flee winter in the northern hemisphere, while humpback whales (tofola) visit us in August-October when they escape winter in the southern hemisphere.

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 82-83 degrees F (for the period 1990-2002). Our cold spell occurs from June through September when averaged air temperatures plummet to 81 degrees F and rainfall is reduced by half.

A world map of cyclone tracks (next page) helps put some perspective on American Samoa’s susceptibility to tropical cyclones. This map shows the tracks of all cyclones greater than 74 mph during the 10-year period 1994-2003. It shows that cyclones are generated primarily in two bands that lie north and south of the earth’s equator at latitudes of about 10-300. American Samoa lies near the edge of the cyclone band in the southern hemisphere where cyclone activity begins to fade.

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 77 degrees F to afternoon highs in the upper 80s. We now average 60 days per year at 900 or above. Record highs and lows here are 96 degrees and 62 degrees F.

As summer progresses, the temperature of the ocean’s surface waters also increases by about 30 F. Warmer ocean temperatures, in turn, help provide
the energy to start tropical cyclones, thus the chance of a cyclone is greatest between November and April. Tropical cyclones have hit American Samoa at
intervals of 1-13 years during the past 24 years: 1981 Esau, 1987 Tusi, 1990 Ofa, 1991 Val, 2004 Heta and 2005 Olaf. Their severity varied -- Tusi was particularly damaging to Manu’a, the back-toback cyclones Ofa and Val hit Tutuila hard, Heta’s overall impact was moderate, Olaf slammed Manu’a hard.

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?