The impact of large atmospheric disturbances on deep benthic communities is not well known quantitatively. Observations are scarce but may reveal specific processes leading to turbulent disturbances. Here, we present high-resolution deep-ocean observations to study potential turbulent mixing by large atmospheric disturbances. We deployed an array of 100-Hz sampling-rate geophysical broadband Ocean Bottom Seismometers (OBSs) on the seafloor.Within the footprint of this array we also deployed an oceanographic 1-Hz sampling-rate vertical temperature sensor string covering the water phase between 7 and 207 m above the seafloor at about 3000 m depth off eastern Taiwan between June 2017 and April 2018. All instruments registered Category 4 cyclone Typhoon Talim's passage northeast of the array one day ahead of the cyclone's closest approach when the cyclone's eye was more than 1,000 km away. For 10 days, a group of near-inertial motions appeared most clearly in temperature.The registration reflects the importance of barotropic response to cyclones and the propagation of inertio-gravity waves in weak density stratification. In addition to internal tides, these waves drove turbulent overturns larger than 200 m that were concurrently registered by OBSs. The turbulent signals were neither due to seismic activity nor to oceansurface wave action. Cyclones can generate not only microseisms and earth hums, as well as turbulence in the water column, producing additional ground motions. Quantified turbulence processes may help constrain models on sediment resuspension and its effect on deep-sea benthic life.
Shear strains, among other ground motions, can be induced by weather‐related processes. As a result, broadband seismic data offer a unique tool for understanding these natural weather events. Here we used continuous seismic, meteorological, and stream data to analyze weather‐related ground motions during typhoons and rainy seasons in Taiwan. In addition to high‐frequency signals, we detected ultralong period seismic signals at the station Mahsi (MASB) during three meteorological events: Typhoon Kalmaegi in 2008, Typhoon Morakot in 2009, and the East Asian rainy season in 2012. Seismic velocity signals with frequencies lower than 0.3 mHz correlate with precipitation and with the time derivative of the water level in a nearby river. We converted the seismic signals to ground tilt and found that the tilt correlates with the time history of river water level, which fluctuates with precipitation. The seismically derived tilt ranges from 10−8 to 10−7 radians, consistent with an analytic circular loading model, indicating water level fluctuations of higher than 1 m in the small river 30 m away from MASB. To quantify precipitation using seismic data, we successfully modeled seismic waveforms using an empirical Green's function method, which gives good fits to both main event seismic waveforms and precipitation time history. This work demonstrates that continuous recordings from broadband seismometers may help determine local fluvial water load and could be useful for decadal rainfall change research.
Turbulent mixing in the deep ocean is not well understood. The breaking of internal waves on sloped seafloor topography can generate deep-sea turbulence. However, it is difficult to measure turbulence comprehensively due to its multi-scale processes, in addition to flow–flow and flow–topography interactions. Dense, high-resolution spatiotemporal coverage of observations may help shed light on turbulence evolution. Here, we present turbulence observations from four broadband ocean bottom seismometers (OBSs) and a 200-m vertical thermistor string (T-string) in a footprint of 1 × 1 km to characterize turbulence induced by internal waves at a depth of 3000 m on a Pacific continental slope. Correlating the OBS-calculated time derivative of kinetic energy and the T-string-calculated turbulent kinetic energy dissipation rate, we propose that the OBS-detected signals were induced by near-seafloor turbulence. Strong disturbances were detected during a typhoon period, suggesting large-scale inertial waves breaking with upslope transport speeds of 0.2–0.5 m s−1. Disturbances were mostly excited on the downslope side of the array where the internal waves from the Pacific Ocean broke initially and the turbulence oscillated between < 1 km small-scale ridges. Such small-scale topography caused varying turbulence-induced signals due to localized waves breaking. Arrayed OBSs can provide complementary observations to characterize deep-sea turbulence.
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