Deep-Water Flow in the Mariana and Caroline Basins*

Abstract

Two major water masses dominate the deep layers in the Mariana and Caroline Basins: the Lower Circumpolar Water (LCPW), arriving from the Southern Ocean along the slopes north of the Marshall Islands, and the North Pacific Deep Water (NPDW) reaching the region from the northeastern Pacific Ocean. Hydrographic and moored observations and multibeam echosounding were performed in the East Mariana and the East Caroline Basins to detail watermass distributions and flow paths in the area. The LCPW enters the East Mariana Basin from the east. At about 13ЊN, however, in the southern part of the basin, a part of this water mass arrives in a southward western boundary flow along the Izu-Ogasawara-Mariana Ridge. Both hydrographic observations and moored current measurements lead to the conclusion that this water not only continues westward to the West Mariana Basin as suggested before, but also provides bottom water to the East Caroline Basin. The critical throughflow regions were identified by multibeam echosounding at the Yap Mariana Junction between the East and West Mariana Basins and at the Caroline Ridge between the East Mariana and East Caroline Basins. The throughflow is steady between the East and West Mariana Basins, whereas more variability is found at the Caroline Ridge. At both locations, throughflow fluctuations are correlated with watermass property variations suggesting layerthickness changes. The total transport to the two neighboring basins is only about 1 Sverdrup (1Sv ϵ 10 6 m 3 s Ϫ1 ) but has considerable impact on the watermass structure in these basins. Estimates are given for the diapycnal mixing that is required to balance the inflow into the East Caroline Basin. Farther above in the water column, the high-silica tongue of NPDW extends from the east to the far southwestern corner of the East Mariana Basin, with transports being mostly southward across the basin.

Seasonal variability of water characteristics in the Challenger Deep observed by four cruises

“…At the slope of the Mariana Trench east of Guam, a southwards western boundary current is identified, which branches into westward and an eastward return transport(red arrows in Fig. 1a) 4 . We hypothesize that the highly saline water in the Challenger Deep could have different sources between different seasons, which may result in the seasonal variability of the water mass.…”

“…Conductivity-temperature-depth profiler (CTD) casts were conducted at three stations along the north-south transect of the Challenger deep on 1 December 1992, which identified cyclonic circulation referred to 3000 dbar over the Challenger Deep 3 . With the assumed reference level of 5500 dbar, the water was flowing eastwards between the two pairs of stations in the southern area of the short meridional transect along 145°E, and westwards near the corner point 4 . However, several current meters along 142°35′E in the Mariana Trench showed a mean westwards flow 5 .…”

Distribution and Abundance Estimates for Cetaceans in the Waters off Guam and the Commonwealth of the Northern Mariana Islands

“…NPDW's signature low salinity and high silicate water is formed within the northern Pacific, before translating southwards. Siedler et al (2004) states that these waters are found at a depth of 2,000 -3,500 m, extending to the western edge of the Mariana Trough. LCPW flows northwards from the South Pacific and flows into the Mariana Trough and Trench (Mantyla and Reid, 1978;Kawabe et al, 2003;Siedler at al., 2004).…”

“…Siedler et al (2004) states that these waters are found at a depth of 2,000 -3,500 m, extending to the western edge of the Mariana Trough. LCPW flows northwards from the South Pacific and flows into the Mariana Trough and Trench (Mantyla and Reid, 1978;Kawabe et al, 2003;Siedler at al., 2004). Mantyla and Reid (1978) found that Mariana seafloor ridges halt the progression of LCPW and NPDW into the Mariana Trench and that at depths of 5,585 to 10,933 m in the Trench, the water temperature ranges from 1.5° -2.5°C, have a salinity of 34.7 ppt and 4ml/l of dissolved oxygen.…”

“…However, previous studies such as Kawabe et al (2003); Siedler et al (2004) show that the Mariana Trough and Mariana Trench deepwaters consist of Lower Circumpolar Water (LCPW), also known as Circumpolar Deep Water (CDW) (Pickard and Emery, 1982), and North Pacific Deep Water (NPDW), superimposed by Antarctic Intermediate Water (AIW). NPDW's signature low salinity and high silicate water is formed within the northern Pacific, before translating southwards.…”