The Arctic Ocean is well‐known for its large number of mesoscale eddies. We observed a lens‐shaped mesoscale eddy by using two roughly perpendicular multichannel reflection seismic sections collected in the Northwind Basin, Arctic Ocean in late September 2011. This is the first seismic imaging of the water column in the Arctic Ocean. The two lines were acquired about seven days apart. The eddy had a horizontal scale of ∼56 km and extended from near‐surface to 300 m. The thickness of the eddy was ∼250 m. The eddy core, characterized by weak reflections, has a radius of ∼13 km. The volume of core water was ∼89 km3. The horizontal current velocity collected by acoustic Doppler current profiler simultaneously with the seismic data shows that the eddy had a rigid core with a rotation period of ∼26 hr, and max azimuthal velocities of 0.59 m s−1. The velocity outside the core decreased in a quadratic inverse curve with the radius. During the observation, the eddy moved westward with an advection velocity of ∼1 cm s−1, and the net transport of the core water was about 0.05 Sv. There were multi‐arm structures at the boundaries of the eddy, forming submesoscale spiral bands. The steep spiral arms (dip angle can be up to ∼4°) may be caused by eddy stirring. Seismic observations provide important insights into our understanding of the vertical structure of ocean eddies, the horizontal and vertical transport of matter, and fluxes of heat that can cause sea ice melting in the Arctic Ocean.

No abstract

No abstract

Abstract. Shoaling internal solitary waves near the Dongsha Atoll in the South China Sea dissipate their energy and enhance diapycnal mixing, which have an important impact on the oceanic environment and primary productivity. The enhanced diapycnal mixing is patchy and instantaneous. Evaluating its spatiotemporal distribution requires comprehensive observation data. Fortunately, seismic oceanography meets the requirements, thanks to its high spatial resolution and large spatial coverage. In this paper, we studied three internal solitary waves in reversing polarity near the Dongsha Atoll and calculated their spatial distribution of diapycnal diffusivity. Our results show that the average diffusivities along three survey lines are 2 orders of magnitude larger than the open-ocean value. The average diffusivity in internal solitary waves with reversing polarity is 3 times that of the non-polarity reversal region. The diapycnal diffusivity is higher at the front of one internal solitary wave and gradually decreases from shallow to deep water in the vertical direction. Our results also indicate that (1) the enhanced diapycnal diffusivity is related to reflection seismic events, (2) convective instability and shear instability may both contribute to the enhanced diapycnal mixing in the polarity-reversing process, and (3) the difference between our results and Richardson-number-dependent turbulence parameterizations is about 2–3 orders of magnitude, but its vertical distribution is almost the same.

Pockmarks consists of depressions on the seafloor and are often considered to originate from fluid escape activities. In this study, we report 125 pockmarks near the Andu Seamount in the southern margin of the South China Sea (SCS) and preliminarily investigate their genesis based on multi-beam bathymetry and multichannel seismic reflection data. Most of them are giant and mega pockmarks and display circular, elliptical, elongated, crescent, comet and irregular shapes on a plan-view. The mapped pockmarks cover a total surface of 291.354 km 2 . Most of them occur in strings or clusters, while some are isolated. The filled volume of pockmark is calculated for the first time in this region, which may have significant implications for sedimentary process. Beneath most of the pockmarks, high-angle faults, high-amplitude reflections, filled-up structures and other fluid escape features are imaged on seismic sections. The structural settings, especially the fault systems, may control the genesis and development of pockmarks in this area. We also speculate that seabed gradients may contribute to generation of crescent pockmarks. According to seismic sections, sediment types and morphological analysis, we finally proposed that the genesis of these pockmarks may mainly be triggered by gas escape activities in the study area. This work may have important implications for the gas exploration in the Nansha Region, southern South China Sea.