Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
2.0 PHYSICAL ENVIRONMENT AND HABITATS
The islands of the Mariana archipelago lie between latitude 13 degrees (°) N and 20°N and are
approximately 5,800 km west of Hawai’i, 2,250 km south of Japan, and 7,600 km north of Sydney,
Australia (DoN 1998; DoN 2003a, 2003b). The archipelago extends roughly 800 km from Guam in the
south to the uninhabited island of Farallon de Pajaros in the north (DoN 1998) and is divided into three
relatively parallel arcs. The outer frontal arc is composed of the more southerly limestone islands while
the inner, or active arc, extends to the north to form the only active volcanic islands in Micronesia
(Eldredge 1983).
The Marianas MRA study area and vicinity (study area) extend from the high tide shore line along the
islands of Guam, Tinian, and FDM to over 11,000 m of water depth in the Marianas Trench. The study
area includes several dominant physiographic features including the Marianas Trench, seamounts, and
active submarine volcanoes.
2.1 CLIMATE AND WEATHER
The tropical climate of the study area is influenced by easterly trade winds and can be described as warm
and humid throughout the year, although rainfall and wind exhibit distinct seasonal patterns (Eldredge
1983). Average temperatures range from 29° to 32° Celsius (C) during the day and drop to 21° to 24°C
overnight (Eldredge 1983; DoD 1999). The months of January through March are generally the coolest
months of the year, with May and June being the warmest. Throughout the year, relative humidity ranges
from 65 to 75% in the afternoon and increases at night to 85 to 100%. Annual rainfall ranges from 1,775
to 2,285 millimeters [mm] per year with the more southerly (more tropical) islands in the study area
receiving higher levels of rainfall than the more northern islands. The study area experiences two distinct
seasons, a dry and a rainy season, separated by brief periods of transitional weather (Eldredge 1983;
DoD 1999). Climatic anomalies influencing the study area include El Niño/Southern Oscillation (ENSO),
La Niña, and the Pacific Decadal Oscillation (PDO) (Trenberth 1997; Giese and Carton 1999; Sugimoto et
al. 2001; Mantua 2002; NOAA 2005a, 2005b).
During normal conditions, trade winds blowing west across the tropical Pacific pile up warm water in the
west Pacific (~0.5 m sea surface height difference between Indonesia and Ecuador) (Conlan and Service
2000; NOAA 2005a, 2005b). The equatorward flow of the eastern boundary Peru Current along the South
America coastline and the easterly trade winds cause the offshore transport of cool surface water (Ekman
layer) (Pickard and Emery 1982; NOAA 2005b) visualized as a large “cold tongue” extending westward
across the Equatorial Pacific. The removed surface water is replaced by upwelled cold and nutrient-rich
water which favors increased primary productivity and higher trophic levels (including fisheries). Under
these normal conditions, rainfall is scare in the eastern Pacific and is concentrated over the warmest
water in the west Pacific.
2.1.1 Seasons
The dry season (December to June) is characterized by strong and consistent tradewinds blowing from
the east to northeast at 24 to 40 km per hour (kph) (Eldredge 1983; DoD 1999; Paulay 2003). Winds are
heaviest during the late morning and afternoon and are lightest during the night. On average less than
20% of the study area’s rain falls during the dry season and thunderstorms are rare (Eldredge 1983; DoD
1999).
During the rainy season (July through November), the study area experiences heavy winds and rains,
with squalls and gales becoming more common (Eldredge 1983; DoD 1999). Rain falls during more than
75% of the days. More than 60% of the annual rainfall is received in the study area during the rainy
season.
Tropical cyclones commonly traverse the study area from August to November with the peak typhoon
season extending from July through October (Elsner and Liu 2003). Typhoons are tropical cyclones with
maximum sustained surface wind speeds greater or equal to 33 meters per second (m/sec) and less than
2-1