Variability and Linkages of New Guinea Coastal Undercurrent and Lower Equatorial Intermediate Current

Velocity at depths of 700–800 m was measured between September 1998 and October 2002 at 2.5°S, 142°E off the New Guinea coast and at 0°, 138°E to examine the New Guinea Coastal Undercurrent (NGCUC) and the current on the equator carrying Antarctic Intermediate Water (AAIW). Velocity characteristics before November 1999 were markedly different from those after November 1999. The typical state occurred during the second period: the intermediate NGCUC and the Lower Equatorial Intermediate Current (LEIC) varied markedly with an annual cycle in opposite phases. In austral winter, the NGCUC flowed west-northwestward strongly (14 cm s−1, 285°T), especially in May–July during which the LEIC disappeared and eddylike equatorial variations with periods of 20–60 days were significant. In austral summer, the LEIC flowed westward strongly (12 cm s−1, 270°T), especially in October–December, whereas the NGCUC reversed its direction repeatedly to flow east-southeastward in November–February. Thus, the intermediate NGCUC and LEIC are present stably in austral winter and summer, respectively. These variations of the currents must change the pathway of AAIW seasonally. The state during the first period was atypical: the current on the equator flowed eastward strongly (13.0 cm s−1, 81°T), that is, no LEIC was present, and the NGCUC flowed west-northwestward strongly (14.8 cm s−1, 280°T) without changing direction. The atypical state may be related to the 1998–99 La Niña. In addition, power spectral peaks at periods of 14–35 days of meridional velocity at the equator suggest that intermediate tropical instability waves are generated in October–December in the typical state.