Variations of the deep western boundary current in Wake Island Passage

Two moorings were deployed at 168°30′E (Sta. WPW) and 169°30′E (WPE) on 18°20′N in Wake Island Passage (WIP) to measure the velocity of the eastern branch of the deep western boundary current (DWBC) in the North Pacific. We obtained hourly velocity data from eight current meters located at depths of 3020, 4020, 5020, and 5120 m (near-bottom) at WPW and 2000, 4000, 5000, and 5300 m (near-bottom) at WPE, and analyzed daily mean velocity data for 374 days from 10 February 1999 to 18 February 2000. The velocity of the DWBC and its variations are large near the ocean floor, at depths of 5000 m and more. The DWBC at WPE increases exponentially with depth, with the near-bottom mean velocity being 3.5 cm s −1, whereas the DWBC at WPW is almost uniform at the bottom layer with the mean velocity being 1.9 cm s −1. The direction of the DWBC in WIP is dominant between 15 and 55°T with a mean of approximately 35°T. This orientation is due to the direction of the bottom topography around 5000-m depth. The velocity and its variations decay markedly at 4000 m, indicating the characteristics of the eastern branch current of the DWBC, which is formed primarily by the portion of the DWBC below 4500 m. A peculiar property is that the velocity and its variations of the DWBC are much less in the western part (WPW) than in the eastern part (WPE) of WIP. The presence of several seamounts may decrease the velocity and modify the structure of the DWBC in the western half of WIP. On the other hand, the eastern half of WIP is deep and has no significant seamounts, which may enable the DWBC to flow smoothly there. The volume transport of the DWBC in WIP is estimated to be 3.6±1.3 Sv (10 6 m 3 s −1) northward on average, with a range of −5.3–14.8 Sv, although the uncertainty of the mean value must be larger than 1.3 Sv because of the estimation from just two moorings. The mean volume transport of the eastern branch current of the DWBC is found to be a little more than 3.6 Sv by adding the transport in the passages west of WIP. The velocity and volume transport of the DWBC vary markedly with a period of approximately four months. A pause of the DWBC occurs in relation to the four-month variation. At WPW, the velocity and its variations at 4020 m are similar to those at 3020 m. The variations are sinusoidal with a four-month period, being almost out of phase with those at WPE. These are interpreted to be an eddy that may be propagated by Rossby waves and trapped in WIP. The four-month va