Low-Frequency Currents Observed around the Velasco Reef off Northern Palau

Current were measured in the western equatorial Pacific Ocean in the vicinity of Palau Island near the Velasco Reef. The focus area was near the southern boundary of the westward-flowing North Equatorial Current and just north of the eastward-flowing North Equatorial Counter Current. The major objective was to characterize the flows and their interactions with the reef. Nine moorings containing acoustic Doppler current profilers were deployed from May 2016 to March 2017; 5 moorings in deep water off of the ridge to measure current profiles for the upper 600 m of the water column and 4 moorings near the top of the ridge to measure the full water column. The current profiles generally describe a two-layer system, often with opposing currents. Seasonal current variability was distinct in the lower layer. The currents in the deep water were impacted by a series of westward propagating eddies and intraseasonal Rossby waves propagating at speeds of about 0.15-0.2 m/s triggered by the 2015/2016 El Niño, with long-term averaged flows converging along the ridge. During the El Niño transition period, an intraseasonal oscillation of about 15 to 30 days was set in motion, and both northward and southward transports significantly increased. The reef obstructed the current flows, with different patterns on opposite sides of the reef. While spatial scales of the reef are relatively small, its topographic effect on the current patterns was significant. These observations indicate that currents and Rossby waves likely interact with the topography and generate vorticity off the Velasco Reef. Large upwelling and downwelling events, much larger than in the open ocean, were associated with the vorticity fields. Current variability at the small spatial and temporal scales associated with the reef greatly impact the larger-scale current variability both near the surface and at depth. These processes are difficult to represent by satellite products and are challenging to predict.