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

AUGUST 2005 FINAL REPORT
of the water column. Variations in temperature and salinity will cause changes in water density which in
turn drives the thermohaline circulation capable of moving water masses at all levels of the water column
(Pickard and Emery 1982).
The general oceanic circulation surrounding the study area and the Mariana Islands is little known as few
studies have investigated the major current pattern around the islands (Eldredge 1983). Due to the lack of
observational data in the study area, only broad, more generalized patterns can be identified. The
following is a discussion of circulation patterns that influence the study area including sea surface
circulation, deepwater circulation, and the North Pacific Subtropical Gyre (NPSG).
2.4.1.1 Surface Currents
Surface currents in the study area are heavily influenced by the North Pacific Equatorial Current (NPEC)
which flows westward between 8 and 15°N eventually turning to the north to form the Kuroshio current off
of Japan (Pickard and Emery 1982; Wolanski et al. 2003; Figure 2-4a). The North Equatorial Current
(NEC) is driven by the trade winds along the equator (Figure 2-4b). The trade winds force the NEC
through the study area. This results in a net sea surface transport to the west/northwest at an average
speed of 0.1 to 0.2 m/sec (Uda 1970; Wolanski et al. 2003).
However, it also should be noted that the Mariana Islands lie to the southeast of the heaviest tropical
cyclone activity in the Pacific Ocean and current patterns in the study area can be influenced by tropical
cyclones during the rainy season (July through November). As such, the passage of tropical cyclones
(Eldredge 1983), El Niño (Lagerloef et al. 1999), and oceanic cyclonic eddies through the area (Wolanski
et al. 2003) have resulted in a reversal of surface current flow in the study area.
The large mass of the islands within the study area may be capable of producing small eddies (net
eastward coastal flow of several cm s -1 ) on the lee side of the islands capable of returning fish and coral
larvae and eggs to the fringing reefs surrounding most of the islands. While the formation of these eddies
have not been largely investigated throughout the study area, these eddies may provide the explanation
as to why people lost at sea to the west side of the Mariana Islands are not advected to the west by the
NEC as predicted by Coast Guard models (Wolanski et al. 2003).
Many of the islands within the study area are surrounded by fringing coral reefs (Eldredge 1983; Spalding
et al. 2001). There are a number of fine scale currents within the reef and between the reef and shore
(Jones et al. 1974; Eldredge et al. 1977; Marsh et al. 1982). However these fine scale current patterns
are complex and there is a lack of observational data to accurately predict these current patterns
(Eldredge 1983). In Guam, Marsh et al. (1982) found that incoming waves travel shoreward over the reef
flats (Tumon Bay, Pago Bay) and slowly turn to form longshore currents. These currents flow along the
shoreline for distances up to 1,500 m, eventually turn seaward, and then exit through cuts in the reef
margin (Marsh et al. 1982).
2.4.1.2 Deepwater Currents/Water Masses
The colder, mid-depth and bottom waters of the study area do not originate in local waters. Rather, some
of the water travels a great distance prior to reaching the study region, including waters originating in the
North Pacific and the Antarctic Sea (Pickard and Emery 1982). In fact, the water found in the Mariana
Trough and Mariana Trench originates from Lower Circumpolar Water (LCPW) and North Pacific Deep
Water (NPDW) and is influenced by the overlying Antarctic Intermediate Water (AIW) (Kawabe et al.
2003; Siedler et al. 2004).
The NPDW is formed in the northern Pacific as cold water from the North Pacific mixes with high silica
bottom sources (Siedler et al. 2004). The low salinity and high silica content is the signature of the NPDW
water mass. After sinking into the deep subarctic, this water travels from the northeast Pacific with a
general westward propagation south of the Hawaiian Islands. The NPDW extends to the western edge of
the Mariana Trough at a depth to 2,000 to 3,500 m, where net transport of the water mass is southward
(Kawabe et al. 2003; Siedler et al. 2004).
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