Ocean Current Observations Throughout the Water Column in the Clarion-Clipperton Fracture Zone, Tropical North Pacific

Four (4) long-term moorings were deployed in the tropical eastern North Pacific Ocean within the Clarion-Clipperton Facture Zone (CCFZ) to observe physical processes throughout the water column. High-resolution ocean current profiles were obtained within the upper 1000 m of the water column and within the lower 100 m of the near-bottom boundary layer, along with velocity data at discreet depths in the mid-water column. Many of the resulting oceanographic records spanned more than three (3) years. Currents were most energetic near the surface with a mean velocity of ~18–22 cm/sec and mean direction toward the west. The strongest currents observed in the record exceeded 165 cm/sec and occurred during Hurricane Barbara in 2019, but these peak currents were brief, decaying rapidly as the storm passed, leaving only oscillatory currents propagating at near-inertial frequencies as aftereffects. A strong density layer about 75–100 m below the surface, the top of the pycnocline, appeared to act as an effective barrier separating the near-surface layer from deeper layers. CTD casts performed at the mooring location(s) confirmed the strong gradients in the upper water column. Currents at 204 m, below the surface layer, were weaker (mean speed ~12 cm/sec) and flowed eastward much of the time. The velocity variance at this depth was about one-third the velocity variance found nearer the surface. Hurricane storm response at 204 m was much weaker also, further evidence the pycnocline limits mixing between the surface and deeper waters. Currents mid-depth of the water column—from about 1500 m down to 4000 m—were the weakest, with mean speeds of order ~3–4 cm/sec and velocity variance about 1/30th of the variance near the surface. Currents near the seabed were found more energetic, with mean flows of order ~5 cm/sec and peak speeds exceeding 20 cm/sec. Local bathymetry features likely play a role in steering near-bottom flows at each site. Some flow intensification was observed in the near-bottom boundary layer at several sites, where the mean (and peak) near-bottom currents were twice the magnitude of currents measured at the 40 m elevation. This near-bed intensification suggests a velocity shear layer which may enhance vertical mixing. Currents in the upper layer were dominated by tides and near-inertial oscillations, along with significant low-frequency(seasonal) variability. Lunar semi-diurnal tides appeared as the most significant constituent through the water column, especially at the seabed. Near-bottom M2 tides were oriented primarily in the north-south direction with little energy found in the east-west velocity component. Near-inertial oscillations with magnitudes of ~5 cm/sec were ubiquitous through the water column, and especially prominent at the seabed. Low-frequency circulation events were observed often near the seabed—some events featured peak speeds of order ~15–20 cm/sec and varied on ~monthly time scales. These low-frequency events possess properties of large-scale, westward-propagating meso-scale eddies. Comparison of low-frequency currents between moorings showed high correlation, suggesting circulation patterns of length scales greater than 7–9 km. One event estimated a translational speed of ~18 cm/sec, which was consistent with near-bottom eddy speeds described in other studies. Water properties measured in near-bottom boundary layer were extremely stable. Water temperatures varied less than 0.03 °C and salinity values ranged less than 0.005 PSU over the nearly 4-year time series.