Flow of abyssal water into Wake Island Passage: Properties and transports from hydrographic surveys

Water mass characteristics and volume transports of abyssal water flowing northward into Wake Island Passage in the North Pacific Ocean were examined by carrying out high-quality hydrographic surveys in May 2003, October 2004, and December 2005 along with mooring observations from May 2003 to December 2005. Close linear relationships between potential temperature (θ) and salinity, dissolved oxygen, and silicate were seen below θ ≈ 1.1°C (≈4000 m). The relationships above θ 1.1°C were scattered and were separated into relatively salty, oxygen-rich, silicate-poor water to the south, and water with the opposite properties to the north. The results suggested that there was a boundary between water masses at θ ≈ 1.1°C in the deep passage. In addition to the three hydrographic: sections, two hydrographic sections previously surveyed in the deep passage in 1975 and 1999 were reexamined for transport estimates. Geostrophic calculations relative to the θ = 1.1°C surface indicated northward transports of the abyssal water from 0.5 to 2.2 Sv (1 SV = 10⁶ m³ s^-1) below this surface. When 1-year mean estimated velocities at θ = 1.1°C surface were used for reference, mean transport from the five estimates increased from 1.4 to about 4 Sv. The temperature of abyssal water colder than 1.1°C was found to have increased by an average of 0.012°C between 1975 and 2005. This warming is greater than double the standard deviation from the temporal mean temperature profile obtained from mooring observations.