Seasonal and Interannual Variability of the Currents off the New Guinea Coast From Mooring Measurements

Seasonal and interannual variability of the New Guinea Coastal Current (NGCC) and New Guinea Coastal Undercurrent (NGCUC) were investigated with 3 years of mooring measurements off the northern coast of New Guinea and outputs from the Ocean General Circulation Model for the Earth Simulator during 1980-2018. Acoustic Doppler Current Profilers mounted on the two moorings captured variations of the currents in the upper 800 m off the New Guinea coast during 2015-2018. NGCC is a seasonally reversing current in the upper 100 m, which flows southeastward in boreal winter with maximum velocity of 63 cm/s near the surface, and flows northwestward in boreal summer with maximum velocity of -55 cm/s at 80 m. NGCUC flows northwestward all year round between 100 and 400 m, and its temporal mean velocity reaches -40 cm/s at 200 m. A seasonally reversing current named New Guinea Coastal Intermediate Current with speed of 10 cm/s is detected below the NGCUC, which is in phase with the NGCC on seasonal time scale. Seasonal variation of the NGCUC is also in phase with that of the NGCC, and it is strong in boreal summer and weak in winter. Such seasonal signal reaches down to the depth of 800 m. Both mooring measurements and model outputs indicate that NGCUC demonstrates significant interannual variations associated with El Nino-Southern Oscillation (ENSO), with its velocity core shoaling during El Nino and deepening during La Nina, but the net transport of NGCUC exhibits no significant relationship with ENSO. Plain Language Summary The currents north of the New Guinea coast transport water masses crossing the equator from the Southern Hemisphere to the Northern Hemisphere, and modulate the thermohaline structure in the equatorial Pacific, playing an important role in the El Nino-Southern Oscillation cycle and the interhemisphere water exchange. To better understand the structure and variability of these currents, we deployed two subsurface moorings and obtained the velocity time series measurements down to 800 m. A seasonally reversing current is detected below the traditionally known upper-ocean currents, and the monsoon wind forcing is noticed to reach the depth of 800 m, much deeper than the previously acknowledged 150 m. The velocity core of the New Guinea Coastal Undercurrent is revealed shoaling and deepening during El Nino and La Nina, respectively, but the transport of this current exhibits no close relationship with ENSO, which is probably due to the transport restriction through Vitiaz Strait. This research improved our understanding of the vertical structure and variability of the currents off the New Guinea coast.