Near-bottom currents at Station M in the abyssal Northeast Pacific

Abstract

Deep-sea currents in the Northeast Pacific Ocean are examined using observations at Station M, located on the abyssal plain 220 km offshore of central California. Collected near the seabed at ~4000 m depth from October 2014–October 2018, the current meter observations supplement long-term investigations of deep-sea benthic ecology at this site. To investigate mechanisms for variability at different time scales, the observed currents are separated into low-frequency fluctuations with periods of 1.4–60 days, fluctuations that are coherent with astronomical tidal forcing, and other high-frequency fluctuations. These motions contribute similarly to the overall variability at this location. Low-frequency currents exhibit elevated energy at periods of ~30 days, consistent with previous observations elsewhere in the abyssal North Pacific. However, unlike some other observations in the deep ocean, there is no consistent seasonal cycle in the variability of low-frequency motion. The theoretical prediction of Sverdrup balance between transport and local wind stress curl does not explain the timing or the magnitude of low frequency currents, suggesting that they are driven by remote winds or eddy dynamics. The non-tidal high-frequency fluctuations show evidence of a near-inertial peak and clockwise rotary motion, consistent with theoretical expectations for the internal gravity wave spectrum. Like the low-frequency currents, interannual variability of internal wave energy appears to be stronger than seasonal variability. Observations of the vertical structure 2.5–17.5 m above bottom are used to characterize the interactions between currents and the seabed. Observational estimates of diurnal and semidiurnal tidal ellipses are consistent with results from an analytical model that incorporates the effects of the Earth's rotation and tidal acceleration. A numerical model of the time dependent boundary layer provides estimates of bottom roughness and friction velocity that are consistent with previous observations at this site. This study shows that the strength, time scale and polarization of the near-bottom currents at Station M vary significantly between years, demonstrating the value of multi-year records in studies of the lateral transport of organic matter, sediment and organisms at this abyssal site.