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This paper reviews the recent progress in our estimation of ocean dynamic topography and the derived surface geostrophic currents, mainly based on multiple nadir radar altimeter missions. These altimetric observations provide the cornerstone of our ocean circulation observing system from space. The largest signal in sea surface topography is from the mean surface dominated by the marine geoid, and we will discuss recent progress in observing the mean ocean circulation from altimetry, once the geoid and other corrections have been estimated and removed. We then address the recent advances in our observations of the large-scale and mesoscale ocean circulation from space, and the particular challenges and opportunities for new observations in the polar regions. The active research in the ocean barotropic tides and internal tidal circulation is also presented. The paper also addresses how our networks of global multi-satellite and in situ observations are being combined and assimilated to characterize the four-dimensional ocean circulation, for climate research and ocean forecasting systems. For the future of ocean circulation from space, the need for continuity of our current observing system is crucial, and we discuss the exciting enhancement to come with global wide-swath altimetry, the extension into the coastal and high-latitude regions, and proposals for direct total surface current satellites in the 2030 period. [ABSTRACT FROM AUTHOR]

Benthodytes tetrapapillata sp. nov. was collected from a seamount located on the Caroline Ridge at a depth of 2 289 m, during the cruise of R/V Kexue in June 2019. We provided detailed descriptions of external and deposits morphology. The phylogenetic analyses based on cytochrome c oxidase I (COI) and a concatenated dataset of 16S and COI genes showed that the new species belonged to Benthodytes that is not monophyletic. Both details of morphological comparisons and molecular analyses confirmed that Benthodytes tetrapapillata sp. nov. is a new psychropotid species. A state of main morphological characters in valid species of Benthodytes is also provided in this study. [ABSTRACT FROM AUTHOR]

Accurate species identification is a key component of biodiversity research. DNA barcoding is an effective molecular method used for fish species identification. We aimed to study the DNA barcoding of fish in Zhoushan coastal waters, explore the differences and applicability of two gene fragments (12S rRNA and COI) of DNA barcoding in fish species identification, and established a comprehensive fish barcoding reference database. Two hundred and eighty-seven captured fish samples from Zhoushan coastal waters were identified using morphological characteristics and DNA barcoding. A total of 264 12S rRNA sequences (belonging to eight orders, 31 families, 55 genera, and 66 species) and 188 COI sequences (belonging to seven orders, 30 families, 48 genera, and 58 species) were obtained. The lengths of the 12S rRNA sequences ranged from 165 to 178 bp, and the guanine-cytosine (GC) content was 45.37%. The average 12S rRNA interspecific and intraspecific genetic distances (K2P) were 0.10% and 26.66%, respectively. The length of the COI sequence ranged 574–655 bp, and the content of GC was 45.97%. The average 12S rRNA interspecific and intraspecific genetic distances (K2P) were 0.16% and 27.45%, respectively. The minimum interspecific genetic distances of 12S rRNA and COI (1.23% and 1.86%) were both greater than their maximum intraspecific genetic distances (2.42% and 8.66%). Three molecular analyses (NJ tree, ABGD, and GMYC) were performed to accurately identify and delineate species. Clustering errors occurred when the 12S rRNA sequences were delimited using the NJ tree method, and the delimitation results of ABGD and GMYC are consistent with the final species identification results. Our results demonstrate that DNA barcoding based on 12S rRNA and COI can be used as an effective tool for fish species identification, and 12S rRNA has good application prospects in the environmental DNA (eDNA) metabarcoding of marine fish. [ABSTRACT FROM AUTHOR]

Ocean salinity is an important variable that affects the ocean stratification. We compared the salinity and ocean stratification in the tropical Pacific derived from the Argo (Array for Real-time Geostrophic Oceanography data), EN4 (Ensemble 4 analysis), SODA (the Simple Ocean Data Assimilation reanalysis), IAP (Institute of Atmospheric Physics data), and ORAS4 (Ocean Reanalysis System 4) over 2005–2017. Results show that the spatial distribution of climatological mean of sea surface salinity (SSS) in all the products is consistent, and the low salinity region showed large deviation and strong dispersion. The Argo has the smallest RMSE and the highest correlation with the ensemble mean, while the IAP shows a high-salinity deviations relative to other datasets. All the products show high positive correlations between the sea surface density (SSD) and SSS with respect to the deviations of climatological mean from ensemble mean, suggesting that the SSD deviation may be mainly influenced by the SSS deviation. In the aspect of the ocean stratification, the mixed layer depth (MLD) climatological mean in the Argo shows the highest correlation with the ensemble mean, followed by EN4, IAP, ORAS4, and SODA. The Argo and EN4 show thicker barrier layer (BL) relative to the ensemble mean while the SODA displays the largest negative deviation in the tropical western Pacific. Furthermore, the EN4, ORAS4, and IAP underestimate the stability in the upper ocean at the depths of 20–140 m, while Argo overestimates ocean stability. The salinity fronts in the western-central equatorial Pacific from Argo, EN4, and ORAS4 are consistent, while those from SODA and IAP show large deviations with a westward position in amplitude of 0°–6° and 0°–10°, respectively. The SSS trend patterns from all the products are consistent in having ensemble mean with high spatial correlations of 0.95–0.97. [ABSTRACT FROM AUTHOR]

Seamounts affect the surrounding physical oceanography and form unique dynamic processes. The influences of these processes on biological and sedimentary distributions are quite different in seamount areas at different depths. The Y3 seamount is located in the Yap Arc of the tropical Western Pacific Ocean. The water depth of its summit is ∼280 m. Based on field data obtained in December 2014 and other open-access data, the physical oceanography around the Y3 seamount was preliminarily analyzed. The results show that the upper layer (0–150 m) was under the influence of the westward-flowing North Equatorial Current (NEC), while the eastward-flowing North Equatorial Undercurrent (NEUC) controlled the water between 200–800 m. The NEC was strong and steady, but the NEUC was disturbed by the Y3 seamount. The cold dome above the Y3 seamount was not caused by a Taylor cap or tidal rectification but probably by upwelling during the survey time. Tidal currents were squeezed against topography and greatly amplified in the Y3 seamount. The thicknesses of the surface turbulent layers were greatly influenced by the spring-neap tidal cycle. The turbulent diffusivities in the sea surface layer above the Y3 seamount were much larger than those in the open ocean. Calculations showed that the surface wind stress greatly affected the turbulent mixing in the surface layer of the Y3 seamount. The reciprocal action between the amplified tidal currents and topography was the most likely cause of the turbulent mixing near the bottom of the Y3 seamount. This study can provide a scientific basis for further study of biological and depositional characteristics at the Y3 seamount. [ABSTRACT FROM AUTHOR]