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The cruise observation of turbulent mixing in the upwelling region east of Hainan Island in the summer of 2012
Mingming LI, Lingling XIE, Xiaolong ZONG, Shuwen ZHANG, Lei ZHOU, Junyi LI
2018, 37(9): 1-12. doi: 10.1007/s13131-018-1260-y
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Abstract:
The turbulent mixing in the upwelling region east of Hainan Island in the South China Sea is analyzed based on in situ microstructure observations made in July 2012. During the observation, strong upwelling appears in the coastal waters, which are 3°C cooler than the offshore waters and have a salinity 1.0 greater than that of the offshore waters. The magnitude of the dissipation rate of turbulent kinetic energy ε in the upwelling region is O (10–9 W/kg), which is comparable to the general oceanic dissipation. The inferred eddy diffusivity Kρ is O (10–6 m2/s), which is one order of magnitude lower than that in the open ocean. The values are elevated to KρO (10–4 m2/s) near the boundaries. Weak mixing in the upwelling region is consistent with weak instability as a result of moderate shears versus strong stratifications by the joint influence of surface heating and upwelling of cold water. The validity of two fine-scale structure mixing parameterization models are tested by comparison with the observed dissipation rates. The results indicate that the model developed by MacKinnon and Gregg in 2003 provides relatively better estimates with magnitudes close to the observations. Mixing parameterization models need to be further improved in the coastal upwelling region.
The observed tidal and residual currents in the Andaman Sea during the second half of 2016
Yanliang LIU, Jingsong GUO, Huiwu WANG, Yuhuan XUE, Somkiat KHOKIATTIWONG, Weidong YU
2018, 37(9): 13-21. doi: 10.1007/s13131-018-1261-x
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The characteristics of currents and tidal currents in the Andaman Sea (AS) are studied during the second half of 2016 using observed data from a moored acoustic Doppler current profiler (ADCP) deployed at 8.6°N, 95.6°E. During the observation period, the mean flow is 5–10 cm/s and largely southward. The root mean square and kinetic energies of the low and high frequency flows, which are divided by a cutoff period of 5 d, are at the same level, indicating their identical importance to the total current. A power spectrum analysis shows that intraseasonal oscillations, a tidal-related semilunar month signal, a semidiurnal tidal signal and periods of 3–4 d are prominent. The barocliny of an eddy kinetic energy is stronger than the mean kinetic energy, both of which are the strongest on the bottom and the weakest at 70 m depth. Residual currents are largely southward (northward) during the summer (winter) monsoon season. Two striking peaks of the southward flow cause the 80 d period of meridional currents. The first peak is part of a large-scale circulation, which enters the AS through the northern channel and exits through the southern channel, and the second peak is part of a local vortex. The 40 d oscillation of the zonal current is forced by geostrophic variations attributed to local and equatorial remote forcing. The tidal current is dominated by semidiurnal constituents, and among these, M2 and N2 are the top two largest major axes. Moreover, astronomical tidal constituents MM and MSF are also significant. Diurnal constituents are weak and shallow water tides are ignorable. The aims are to introduce the new current data observed in the AS and to provide initial insights for the tidal and residual currents in the Andaman Sea.
Relationship between a function of the northward pressure gradient and the Pacific Equatorial Undercurrent
Xuan CHEN, Chongwei ZHENG, Hui WU, Xiaobao YOU, Xunqiang LI, Huipeng WANG
2018, 37(9): 22-28. doi: 10.1007/s13131-018-1262-9
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The Equatorial Undercurrent (EUC) plays an important role in ocean circulation and global climate change. Near the equator, as the Coriolis parameter goes to 0, equatorial currents cannot be described by geostrophy in which the pressure gradient force term is balanced by the Coriolis force term. Many previous studies focus on the relationships between the EUC and El Niño-Southern Oscillation (ENSO), the thermocline, sea surface topography, the distribution of equatorial wind stress and other atmosphere-ocean factors. However, little attention has been paid to the northward pressure gradient (NGT), which may also be important to the EUC. The pressure can be regarded as a complex nonlinear function of terms including temperature, salinity and density. This study attempts to reveal the connection between a function of the northward pressure gradient (FNP) and the EUC. The connection is derived from primitive equations, by simplifying the equations with using scaling analysis, and shows that the beta effect may be the main reason why the FNP is important to the EUC. The vertical structure of the EUC can be partially described by the FNP. The NGT has an obvious influence on the EUC while the eastward pressure gradient has a relatively smaller effect.
A numerical study on the effect of tidal flat’s slope on tidal dynamics in the Xiangshan Bay, China
Li LI, Taoyan YE, Zhiguo HE, Yuezhang XIA
2018, 37(9): 29-40. doi: 10.1007/s13131-018-1263-8
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The Xiangshan Bay is a semi-enclosed and narrow bay, which is characterized by large scale tidal flats and has been historically utilized through coastal construction and aquaculture engineering. The hydrodynamic model using the Finite Volume Coastal Ocean Model (FVCOM) was constructed to examine the changes of tidal dynamics due to the variation of tidal flat slopes. According to the model results, a decreased slope of a tidal flat would amplify the M2 tidal amplitude and delay the M2 tidal phase in the inner harbor, due to an increased tidal prism, and vice versa. The amplitude of the main shallow-water tide M4 would be amplified/dampened in the entire bay due to the changed bottom friction, if the tidal flat’s slope were reduced/increased at the Tie inlet. The phase was advanced. The change of a tidal flat’s slope at the Tie inlet had greater impacts on tidal amplitude, phase and duration asymmetry, than that at the Xihu inlet. The impact of changes of the tidal flat slope at the Xihu inlet was small and was constrained locally. Changes in the tidal flats’ slopes at the Tie and Xihu inlets changed the tidal duration asymmetry, residual current and tidal energy via modulating tides. The ebb dominance decreased when the tidal flat’s slope at the Tie inlet was changed. Decreased/increased ebb dominance occurred when the tidal flat’s slope was reduced/increased at the Xihu inlet. The residual current and tidal energy density was amplified/dampened and more/less tidal energy was dissipated, with reduced/increased slope at both of the inlets. The findings in this study are instructive to coastal engineering and estuarine management.
A new global gridded sea surface temperature product constructed from infrared and microwave radiometer data using the optimum interpolation method
Weifu SUN, Jin WANG, Jie ZHANG, Yi MA, Junmin MENG, Lei YANG, Junwei MIAO
2018, 37(9): 41-49. doi: 10.1007/s13131-018-1206-4
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A new 0.1° gridded daily sea surface temperature (SST) data product is presented covering the years 2003–2015. It is created by fusing satellite SST data retrievals from four microwave (WindSat, AMSR-E, ASMR2 and HY-2A RM) and two infrared (MODIS and AVHRR) radiometers (RMs) based on the optimum interpolation (OI) method. The effect of including HY-2A RM SST data in the fusion product is studied, and the accuracy of the new SST product is determined by various comparisons with moored and drifting buoy measurements. An evaluation using global tropical moored buoy measurements shows that the root mean square error (RMSE) of the new gridded SST product is generally less than 0.5°C. A comparison with US National Data Buoy Center meteorological and oceanographic moored buoy observations shows that the RMSE of the new product is generally less than 0.8°C. A comparison with measurements from drifting buoys shows an RMSE of 0.52–0.69°C. Furthermore, the consistency of the new gridded SST dataset and the Remote Sensing Systems microwave-infrared SST dataset is evaluated, and the result shows that no significant inconsistency exists between these two products.
An evaluation of sea surface height assimilation using along-track and gridded products based on the Regional Ocean Modeling System (ROMS) and the four-dimensional variational data assimilation
Chaojie ZHOU, Xiaohua DING, Jie ZHANG, Jungang YANG, Qiang MA
2018, 37(9): 50-58. doi: 10.1007/s13131-018-1225-1
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Remote sensing products are significant in the data assimilation of an ocean model. Considering the resolution and space coverage of different remote sensing data, two types of sea surface height (SSH) product are employed in the assimilation, including the gridded products from AVISO and the original along-track observations used in the generation. To explore their impact on the assimilation results, an experiment focus on the South China Sea (SCS) is conducted based on the Regional Ocean Modeling System (ROMS) and the four-dimensional variational data assimilation (4DVAR) technology. The comparison with EN4 data set and Argo profile indicates that, the along-track SSH assimilation result presents to be more accurate than the gridded SSH assimilation, because some noises may have been introduced in the merging process. Moreover, the mesoscale eddy detection capability of the assimilation results is analyzed by a vector geometry–based algorithm. It is verified that, the assimilation of the gridded SSH shows superiority in describing the eddy’s characteristics, since the complete structure of the ocean surface has been reconstructed by the original data merging.
Paleoenvironmental implications of the calcium isotope characteristics in the MD81349 from the Nintyeast Ridge in the Indian Ocean
Feng WANG, Nianqiao FANG, Chaoyong HU, Sarah GOLDSMITH, Yonghang XU, Xuan DING, Aijun WANG, Ping CHEN, Rui BAO
2018, 37(9): 59-66. doi: 10.1007/s13131-018-1264-7
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A δ44Ca curve from shells of the planktonic foraminifera Globigerinoides sacculifer in calcareous biogenic oozes has been extracted from the Nintyeast Ridge in the Indian Ocean since 300 ka. By combining terrigenous inputs (e.g., grain size, magnetic susceptibility, and turbidite frequency) with the oceanic productivity (e.g., biogenic content and Neogloboquadrina dutertrei content), it is found that the curve’s variations are closely related to the historical evolution of the oceanic calcium cycle. The δ44Ca value is in lower tendency and has small oscillation during Marine Isotope Stage (MIS) 6, when the supply of terrigenous detrital is highest. In contrast, during MIS 3, 5 and 7, the δ44Ca values are in higher tendency, and their fluctuations are consistent with the variations of the productivity proxies. These results suggest that the calcium isotopes are mainly influenced by the input of the Himalayan erosion products to the northern Indian Ocean. In addition, the developmental stages of calcareous planktons may have a secondary impact on the fluctuations of the calcium isotope ratio of sea water.
Smectite formation in metalliferous sediments near the East Pacific Rise at 13°N
Kunbo RONG, Zhigang ZENG, Xuebo YIN, Shuai CHEN, Xiaoyuan WANG, Haiyan QI, Yao MA
2018, 37(9): 67-81. doi: 10.1007/s13131-018-1265-6
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A 43 cm long E271 sediment core collected near the East Pacific Rise (EPR) at 13°N were studied to investigate the origin of smectite for understanding better the geochemical behavior of hydrothermal material after deposition. E271 sediments are typical metalliferous sediments. After removal of organic matter, carbonate, biogenic opal, and Fe-Mn oxide by a series of chemical procedures, clay minerals (<2 μm) were investigated by X-ray diffraction, chemical analysis and Si isotope analysis. Due to the influence of seafloor hydrothermal activity and close to continent, the sources of clay minerals are complex. Illite, chlorite and kaolinite are suggested to be transported from either North or Central America by rivers or winds, but smectite is authigenic. It is enriched in iron, and its contents are highest in clay minerals. Data show that smectite is most likely formed by the reaction of hydrothermal Fe-oxyhydroxide with silica and seawater in metalliferous sediments. The Si that participates in this reaction may be derived from siliceous microfossils (diatoms or radiolarians), hydrothermal fluids, or detrital mineral phases. And theirδ30Si values are higher than those of authigenic smectites, which implies that a Si isotope fractionation occurs during the formation because of the selective absorption of light Si isotopes onto Fe-oxyhydroxides. Sm/Fe mass ratios (a proxy for overall REE/Fe ratio) in E271 clay minerals are lower than those in metalliferous sediments, as well as distal hydrothermal plume particles and terrigenous clay minerals. This result suggests that some REE are lost during the smectite formation, perhaps because their large ionic radii of REE scavenged by Fe-oxyhydroxides preclude substitution in either tetrahedral or octahedral lattice sites of this mineral structure, which decreases the value of metalliferous sediments as a potential resource for REE.
A numerical study for boundary layer current and sheet flow transport induced by a skewed asymmetric wave
Xin CHEN, Zichao ZHANG, Fujun WANG
2018, 37(9): 82-89. doi: 10.1007/s13131-018-1267-4
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An analytical model with essential parameters given by a two-phase numerical model is utilized to study the net boundary layer current and sediment transport under skewed asymmetric oscillatory sheet flows. The analytical model is the first instantaneous type model that can consider phase-lag and asymmetric boundary layer development. The two-phase model supplies the essential phase-lead, instantaneous erosion depth and boundary layer development for the analytical model to enhance the understanding of velocity skewness and acceleration skewness in sediment flux and transport rate. The sediment transport difference between onshore and offshore stages caused by velocity skewness or acceleration skewness is shown to illustrate the determination of net sediment transport by the analytical model. In previous studies about sediment transport in skewed asymmetric sheet flows, the generation of net sediment transport is mainly concluded to the phase-lag effect. However, the phase-lag effect is shown important but not enough for the net sediment transport, while the skewed asymmetric boundary layer development generated net boundary layer current and mobile bed effect are key important in the transport process.
Numerical modelling of nonlinear extreme waves in presence of wind
Dezhi NING, Jun DU, Wei BAI, Chongwei ZHANG, Bin TENG
2018, 37(9): 90-98. doi: 10.1007/s13131-018-1268-3
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A numerical wave flume with fully nonlinear free surface boundary conditions is adopted to investigate the temporal characteristics of extreme waves in the presence of wind at various speeds. Incident wave trains are numerically generated by a piston-type wave maker, and the wind-excited pressure is introduced into dynamic boundary conditions using a pressure distribution over steep crests, as defined by Jeffreys’ sheltering mechanism. A boundary value problem is solved by a higher-order boundary element method (HOBEM) and a mixed Eulerian-Lagrangian time marching scheme. The proposed model is validated through comparison with published experimental data from a focused wave group. The influence of wind on extreme wave properties, including maximum extreme wave crest, focal position shift, and spectrum evolution, is also studied. To consider the effects of the wind-driven currents on a wave evolution, the simulations assume a uniform current over varying water depth. The results show that wind causes weak increases in the extreme wave crest, and makes the nonlinear energy transfer non-reversible in the focusing and defocusing processes. The numerical results also provide a comparison to demonstrate the shifts at focal points, considering the combined effects of the winds and the wind-driven currents.
A novel ocean bathymetry technology based on an unmanned surface vehicle
Jiucai JIN, Jie ZHANG, Feng SHAO, Zhichao LYU, Dong WANG
2018, 37(9): 99-106. doi: 10.1007/s13131-018-1269-2
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In ocean bathymetry, the instantaneous depth measured by survey ships or by unmanned surface vehicles (USVs) cannot be directly taken as the chart depth because of the effect of waves and the tide. A novel ocean bathymetry technology is proposed based on the USV, the aim is to evaluate the potential of the USV using a real-time kinematic (RTK) and a single beam echo sounder for ocean bathymetry. First, using the RTK height of the USV with centimeter-level precision, the height of the sea level is obtained by excluding wave information using a low pass filter. Second, the datum distance between the reference ellipsoid and the chart depth is obtained by a novel method using tide tables and the height of the sea level from the USV. Previous work has usually achieved this using long-term tidal observation from traditional investigations. Finally, the chart depth is calculated using the transformation between the instantaneous depth of the USV measurement and the datum of the chart depth. Experiments were performed around the Wuzhizhou Island in Hainan Province using the unmanned surface bathymetry vehicle to validate the proposed technology. The successful results indicate the potential of the bathymetry technology based on the USV.
Recent satellite-derived sea ice volume flux through the Fram Strait: 2011–2015
Haibo BI, Yunhe WANG, Wenfeng ZHANG, Zehua ZHANG, Yu LIANG, Yi ZHANG, Wenmin HU, Min FU, Haijun HUANG
2018, 37(9): 107-115. doi: 10.1007/s13131-018-1270-9
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The Fram Strait (FS) is the primary region of sea ice export from the Arctic Ocean and thus plays an important role in regulating the amount of sea ice and fresh water entering the North Atlantic seas. A 5 a (2011–2015) sea ice thickness record retrieved from CryoSat-2 observations is used to derive a sea ice volume flux via the FS. Over this period, a mean winter accumulative volume flux (WAVF) based on sea ice drift data derived from passive-microwave measurements, which are provided by the National Snow and Ice Data Center (NSIDC) and the Institut Francais de Recherche pour d’Exploitation de la Mer (IFREMER), amounts to 1 029 km3 (NSIDC) and 1 463 km3 (IFREMER), respectively. For this period, a mean monthly volume flux (area flux) difference between the estimates derived from the NSIDC and IFREMER drift data is –62 km3 per month (–18×106 km2 per month). Analysis reveals that this negative bias is mainly attributable to faster IFREMER drift speeds in comparison with slower NSIDC drift data. NSIDC-based sea ice volume flux estimates are compared with the results from the University of Bremen (UB), and the two products agree relatively well with a mean monthly bias of (5.7±45.9) km3 per month for the period from January 2011 to August 2013. IFREMER-based volume flux is also in good agreement with previous results of the 1990s. Compared with P1 (1990/1991–1993/1994) and P2 (2003/2004–2007/2008), the WAVF estimates indicate a decline of more than 600 km3 in P3 (2011/2012–2014/2015). Over the three periods, the variability and the decline in the sea ice volume flux are mainly attributable to sea ice motion changes, and second to sea ice thickness changes, and the least to sea ice concentration variations.
News and Views
A new type of internal solitary waves with a re-appearance period of 23 h observed in the South China Sea
Liang CHEN, Quanan ZHENG, Xuejun XIONG, Yeli YUAN, Huarong XIE
2018, 37(9): 116-118. doi: 10.1007/s13131-018-1252-y
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