Mooring Observations Research Articles

Progress in research of submesoscale processes in the South China Sea

This paper overviews research progress in observation, theoretical analysis and numerical modeling of submesoscale dynamic processes in the South China Sea (SCS) particularly during recent five years. The submesoscale processes are defined according to both spatial and dynamic scales, and divided into four subcategories as submesoscale waves, submesoscale vortexes, submesoscale shelf processes, and submesoscale turbulence. The major new findings are as follows. (1) Systematic mooring observations provide new insights into the solitary waves (ISWs) and the typhoon-forced near-inertial waves (NIWs), of which a new type of ISWs with period of 23 h was observed in the northern SCS (NSCS), and the influences of background vorticity, summer monsoon onset, and deep meridional overturning circulation on the NIWs, as well as nonlinear wave-wave interaction between the NIWs and internal tides, are better understood. On the other hand, satellite altimeter sea surface height data are used to reveal the internal tide radiation patterns and provide solid evidence for that the ISWs in the northeastern SCS originate from the Luzon Strait. (2) Submesoscale offshore jets and associated vortex trains off the Vietnam coast in the western boundary of the SCS were observed from satellite chlorophyll concentration images. Spiral trains with the horizontal scale of 15–30 km and the spacing of 50–80 km were identified.(3) 3-D vertical circulation in the upwelling region east of Hainan Island was theoretically analyzed. The results show that distribution patterns of all the dynamic terms are featured by wave-like structures with horizontal wavelength scale of 20–40 km. (4) Numerical models have been used for the research of submesoscale turbulence. Submesoscale vertical pump of an anticyclonic eddy and the spatiotemporal features of submesoscale processes in the northeastern SCS are well modeled.

The investigation of internal solitary waves over a continental shelf-slope

Internal solitary waves (ISWs) always happen in marginal seas, where stable stratification exists. ISWs may carry large energy when they propagate and affect marine engineering constructions such as marine drilling platforms. Previous studies, including a large number of mooring observations and laboratory experiments, show the speed of ISWs will change when they pass by shelf slopes. Korteweg-de Vries (KdV) theory explain this phenomenon. In the paper, we use a laboratory experiment and a numerical model experiment to verify this theory. In the laboratory experiment, we injected two layers of water of different densities in a tank to simulate marine stratification and make ISWs. We use a CCD camera to record the whole process. The camera can take 16 photos per second. In the numerical experiment, we input the same original conditions as the laboratory one. The results of 18 different original conditions show the dimensionless factor d plays a key role in deciding the amplitudes and shapes of ISWs. The main conclusion also contains that small-amplitude waves match well with KdV theory while mKdV is better for largeamplitude waves. Whether the laboratory experiment or numerical experiment shows results with a high agreement. In future studies, we may use a numerical model with higher resolution to get analysis about phase speed and energy of ISWs.

Response of Near-Inertial Shear to Wind Stress Curl and Sea Level

Near-inertial waves (NIWs) contain a pronounced portion of shear energy in the internal wave field and is of great importance to deep ocean mixing. However, accurate simulation of NIWs remains a challenge. Here we analyzed 3-year long mooring observation of velocity profiles over 80–800 m to study the responses of near-inertial downward shear to varying wind stress curls and sea level anomalies (SLAs). It is demonstrated that moderate (even weak) cyclone makes more contributions to enhanced shear below the pycnocline than very strong cyclone. Because very strong curl can stall the downward propagation of large shear. The large positive and negative SLAs cause the accumulation of large shear in the lower and upper parts of the pycnocline through inducing downwelling and upwelling motions, respectively. Time variation of near-inertial shear was strongly influenced by cases of large curls and interannual variation of SLA, and thus did not follow the seasonal variation of wind stress. Our analyses suggest that matched fields of wind stress curl and SLA, and well representing the ocean response to moderate cyclone are needed in simulating the role of NIWs on mixing.

The Gulf Stream's path and time-averaged velocity structure and transport at 68.5°W and 70.3°W

Full-ocean-depth observations of horizontal velocity, temperature and salinity along 68.5° W chiefly over the period October 2010–May 2014 are analyzed in conjunction with repeated shipboard acoustic Doppler current profiler (SADCP) upper-ocean velocity sections occupied upstream at 70.3° W and regional satellite-altimeter-based sea surface height (SSH) data to construct estimates of the time-averaged Gulf Stream velocity, property structures and transport. A stream-coordinate mean section is created from two moorings near 68.5° W where data are binned relative to distance from the Gulf Stream axis, rotated into along- and across-stream coordinates, and then averaged. Transport in the upper 600 m inferred from the moorings excluding times of large Stream axis curvature and Gulf Stream ring influences is 59.9 Sv (with 95% confidence bounds between 58.6 and 61.6 Sv). This is in good agreement with a mean constructed from the SADCP sections at 70.3° W. Relative to the mean field at 70.3° W, the velocity core of the time-averaged Stream at 68.5° W appears broader with weaker maximum speed, consistent with a companion analysis of the altimetric SSH data. The time-averaged full-ocean-depth transport inferred from the moorings is 102.1 Sv (with 95% confidence bounds between 99.1 and 106.3 Sv), which is stronger than the mean inferred from an ensemble of 10 full-depth lowered acoustic Doppler current profiler (LADCP) sections collected along the moored array. The 2010–2014 time-averaged Gulf Stream inferred from the moored observations is weaker by about 10% than the time-averaged full-ocean-depth transport reported for the late 1980s at the same location using similar procedures, with much of this difference arising from flows at depths greater than 1000 m. Satellite altimetry provides spatial and temporal context for these results and suggests that there are small-scale recirculation cells flanking the separated Gulf Stream west of the New England Seamount Chain. Gulf Stream transport, which includes throughput and recirculating components, appears to be more sensitive to changes in these recirculations at 68.5° W compared to 70.3° W.

Stratification and mixing processes associated with hypoxia in a shallow lake (Lake Kasumigaura, Japan)

Physical processes associated with hypoxia events in a shallow lake, Lake Kasumigaura, Japan, are investigated with long-term mooring observations at the middle of the lake basin. Results show that strong stratification during the summer suppresses vertical mixing resulting in hypoxia in the bottom boundary layer. The dissolved oxygen decreases due to the limited oxygen supply under strongly stratified conditions. The intensity of the stratification is controlled by the ratio between surface wind stress and the surface incoming buoyancy flux. The Monin–Obukhov length scale explains the stratification and shows good agreement with the Froude number and Wedderburn number. The stratification is also enhanced by the heat flux towards the sediments, reaching O (100) W m−2, during the summer season. In addition to field observations, this study conducted numerical simulations to further investigate mixing structure. Results from numerical simulations indicate that the bottom sediment heat flux significantly contributes to mixing and stratification in the lake. This study suggests that mixing processes associated with winds and heat flux at surface and bottom sediments are the key to understanding hypoxia events in Lake Kasumigaura.

Interannual variability of South China Sea winter circulation: response to Luzon Strait transport and El Niño wind

The El Nino wind and Luzon Strait transport are important factors modulating the interannual variability of the southern and northern South China Sea (SCS) winter circulation, respectively. The joint effect of El Nino wind and westward Luzon Strait transport drives a dipolar gyre with an anticyclonic (cyclonic) circulation anomaly in the southern (northern) SCS, which enhances the cross-basin current that connects the SCS western boundary current with the eastern boundary of the SCS around the Mindoro Strait, and then effectively modulates the interannual variability of Mindoro Strait transport. An extreme El Nino and large Luzon Strait intrusion occurred in winter 2015/16, and mooring observations and remote sensing data confirmed the presence of an intense dipolar gyre. This resulted in an extreme eastward cross-basin current and outward SCS Mindoro Strait transport anomaly in 2015/16.

Exploring the deep South China Sea: Retrospects and prospects

Rapid developments of deep-sea researches in China over the past 20 years have promoted the South China Sea (SCS) into the international deep-sea frontiers. The “three deep technologies”, namely scientific drilling, long-term seafloor observation and deep submersible vehicles implemented successively in SCS studies helped to achieve a number of scientific breakthroughs. Over the 20 years, five international ocean drilling expeditions to the SCS recovered nearly 10 km of sediment cores from sites at 3–4 km water depths, and drilling into the magmatic basement at 6 sites shed light on the genesis of the SCS basin. Coupled with other deep-sea short core sediments from the SCS, these records demonstrate evidence that water and carbon cycling in the low latitude regions can directly respond to the orbital forcing, and subsequently nurture a new concept of low-latitude forcing of climate changes, which challenges the classical wisdom of the overwhelming role played by the Arctic ice-sheet in climate changes. The exploration in the continent-ocean transition zone also reveals a number of specific features that characterize the SCS as a marginal basin formed at the subduction zone in the Western Pacific. The features include active magmatism and rapid rupture of lithosphere through the basin formation process, and imply that “the SCS is not a mini-Atlantic” as they can be distinguished as “plate-edge rifting” and “inner-plate rifting” respectively, thus challenging the universality of the Atlantic model for passive margins. Many more discoveries can be assembled from long-term mooring observations and deep diving cruises in the deep SCS, such as the cyclonic nature of the deep-water circulation, deep-water sediment transport by contour currents and turbidites, manganese nodules, extinct hydrothermal vents, and cold-water coral forests. In addition, prominent progress achieved in microbiology and biogeochemistry includes the microbial carbon pump and the coupling of carbon and nitrogen cycles. Clearly, most achievements of the deep-sea explorations in the SCS over the last 20 years have always been of international scale and impact. However, the contributions from Chinese scientists are most prominent, particularly with the research activities undertaken from the major program “Deep Sea Processes and Evolution of the South China Sea (2011–2018)” supported by the National Natural Science Foundation of China.

Winter monsoon promotes the transport of Japanese temperate bass Lateolabrax japonicus eggs and larvae toward the innermost part of Tango Bay, the Sea of Japan

AbstractNorthwesterly cold winds characteristic of the East Asian Winter Monsoon (EAWM) dictate winter climatic conditions over the Japanese Archipelago. Japanese temperate bass Lateolabrax japonicus is a commercially important coastal fish that spawns offshore in winter and uses shallow waters as nursery habitats. To investigate the effects of EAWM on the planktonic period of L. japonicus, eggs, larvae, and juveniles were quantitatively collected in Tango Bay on the Sea of Japan side in winter and spring from 2007 to 2017. Although eggs occurred close to the mouth of the bay, planktonic larvae occurred further inside as they developed. The horizontal distribution of planktonic larvae, combined with water velocity data obtained from mooring observations, indicated that planktonic larvae are transported south‐ to westward through Ekman current and an anticyclonic circulation, which are driven by northwesterly winds. To evaluate survival during the planktonic period in each year class, the abundance of benthic larvae/juveniles was divided by winter total landings of Lateolabrax spp. (proxy of the spawning stock size). This survival index exhibited a positive correlation with the northwesterly component of winter winds, and a negative correlation with winter air temperature (average from December to February, Spearman's correlation, p < .05). There was, however, no significant correlation with winter water temperature or winter freshwater discharge in the bay. We conclude that northwesterly cold winds of EAWM play a critical role in transporting L. japonicus eggs and larvae toward nursery habitats, specifically beaches and estuaries fringing the innermost part of Tango Bay.

Atlantic-Origin Overflow Water in the East Greenland Current

AbstractDense water masses transported southward along the east coast of Greenland in the East Greenland Current (EGC) form the largest contribution to the Denmark Strait Overflow. When exiting Denmark Strait these dense water masses sink to depth and feed the deep circulation in the North Atlantic. Based on one year of mooring observations upstream of Denmark Strait and historical hydrographic profiles between Fram Strait and Denmark Strait, we find that a large part (75%) of the overflow water ( ≥ 27.8 kg m−3) transported by the EGC is of Atlantic origin (potential temperature θ > 0°C). The along-stream changes in temperature of the Atlantic-origin Water are moderate north of 69°N at the northern end of Blosseville basin, but southward from this point the temperature decreases more rapidly. We hypothesize that this enhanced modification is related to the bifurcation of the EGC taking place close to 69°N into the shelfbreak EGC and the separated EGC. This is associated with enhanced eddy activity and strong water mass modification reducing the intermediate temperature and salinity maxima of the Atlantic-origin Water. During periods with a large (small) degree of modification the separated current is strong (weak). Output from a high-resolution numerical model supports our hypothesis and reveals that large eddy activity is associated with an offshore shift of the surface freshwater layer that characterizes the Greenland shelf. The intensity of the eddy activity regulates the density and the hydrographic properties of the Denmark Strait Overflow Water transported by the EGC system.

Evaluation of AMSR-E Thin Ice Thickness Algorithm from a Mooring-Based Observation: How Can the Satellite Observe a Sea Ice Field with Nonuniform Thickness Distribution?

AbstractThe quantification of sea ice production in coastal polynyas is a key issue to understand the global climate system. In this study, we directly compared Advanced Microwave Scanning Radiometer-EOS (AMSR-E) data with the sea ice thickness distribution obtained from a mooring observation during the winter of 2003 off Sakhalin in the Sea of Okhotsk to evaluate the algorithm for estimation of sea ice thickness in coastal polynyas. By using thermal ice thickness as a target physical quantity, we found that the obtained relationship between the polarization ratio (PR) and ice thickness can provide an appropriate AMSR-E algorithm to estimate thin ice thickness, irrespective of the uniform or nonuniform ice thickness field. The relationship between the PR value and thermal ice thickness is likewise consistent with the local PR–thickness relationship that is observed at individual ice floes. This is because both the PR value and thermal ice thickness are more sensitive to thinner ice. Furthermore, we evaluated the method for detection of active frazil in a coastal polynya by comparing with the mooring data, and subsequently modified it to classify the coastal polynya into three thin ice types, namely, active frazil, thin solid ice, and mixed ice (mixture of active frazil and thin solid ice). The improved algorithm successfully represents the thermal ice thickness even for a relatively small-scale polynya off Sakhalin and is expected to be useful for better quantification of sea ice production in the global ocean owing to its high versatility.

Estimating Four-Dimensional Internal Wave Spectrum in the Northern South China Sea

AbstractInternal waves can transfer energy from large-scale to microscale processes; however, the spectra of these waves remain poorly known. A method that combines modal harmonic decomposition and maximum-likelihood method is proposed in this study to estimate four-dimensional internal wave spectrum using limited mooring observations. Using this method, a four-dimensional internal wave spectrum was obtained for the first time based on the mooring measurements collected during the South China Sea (SCS) Internal Wave Experiment in July 2014. The spectrum was then validated by comparing with the spectrum based on Fourier analysis and with the modified Garrett–Munk internal wave spectrum, respectively. The power of the internal wave spectrum decreased obviously with increasing frequency and wavenumber, with a falloff rate of ω−2 beyond tidal frequencies, and with falloff rates of and for horizontal and vertical wavenumbers, respectively. In addition, at a fixed frequency and vertical wavenumber, the propagation direction and phase speed of internal waves can be obtained through the four-dimensional spectrum. In summary, we verified the feasibility of estimating four-dimensional internal wave spectrum using limited mooring observations in this study, and the method we proposed should be applicable to other regions where such mooring observations are available.

Generation of Internal Lee Waves by Lateral Circulation in a Coastal Plain Estuary

AbstractRecent mooring observations at a cross-channel section in Chesapeake Bay showed that internal solitary waves regularly appeared during certain phases of a tidal cycle and propagated from the deep channel to the shallow shoal. It was hypothesized that these waves resulted from the nonlinear steepening of internal lee waves generated by lateral currents over channel-shoal topography. In this study numerical modeling is conducted to investigate the interaction between lateral circulation and cross-channel topography and discern the generation mechanism of the internal lee waves. During ebb tides, lateral bottom Ekman forcing drives a counterclockwise (looking into estuary) lateral circulation, with strong currents advecting stratified water over the western flank of the deep channel and producing large isopycnal displacements. When the lateral flow becomes supercritical with respect to mode-2 internal waves, a mode-2 internal lee wave is generated on the flank of the deep channel and subsequently propagates onto the western shoal. When the bottom lateral flow becomes near-critical or supercritical with respect to mode-1 internal waves, the lee wave evolves into an internal hydraulic jump. On the shallow shoal, the lee waves or jumps evolve into internal bores of elevation.

Abyssal Transport Variations in the Southwest South Atlantic: First Insights From a Long‐Term Observation Array at 34.5°S

AbstractThe zonal structure and time variability of the abyssal flow (below 3,000 dbar) in the South Atlantic western boundary is investigated using a combination of moored observations and simultaneous hydrographic/velocity sections at 34.5°S during 2009–2018. Moored direct velocity measurements near the bottom show strong variability with a peak‐to‐peak range exceeding 80 cm/s and dominant signals at times scales of 1–2 months. Daily time series of the meridional absolute geostrophic volume transport computed from the moorings reveals a highly energetic record with a temporal standard deviation of 8.3 Sv and peak‐to‐peak variations of 49 Sv, suggesting a significant contribution of the abyssal layer flows to the Deep Western Boundary Current time variability. The absolute transport is mostly driven by barotropic changes that are dominated by variations in the bottom pressure ~650 km away from the continental slope.

Dynamic and Statistical Features of Internal Solitary Waves on the Continental Slope in the Northern South China Sea Derived From Mooring Observations

AbstractFor investigation of internal solitary waves (ISWs) in the South China Sea (SCS), most cruise observations are concentrated from Luzon Strait to Dongsha Atoll in the northeastern SCS but few on the continental slope far away from the west of Dongsha Atoll. In this study, we use 1‐year long mooring data to determine dynamic and statistical features of the ISWs on the shelf slope of the northwestern SCS. The analysis results of the mooring data reveal that the ocean internal waves on the shelf slope of the northwestern SCS have physical properties of highly nonlinear waves, which are well described by the solutions of the Korteweg‐de Vries equation. The mean nonlinear phase speeds of mode‐1 and mode‐2 ISWs are 1.38 ± 0.14 and 0.66 ± 0.12 m/s, respectively. The major direction of mode‐1 ISWs is northwestward 305° ± 21°. Strong ISW currents force the major direction of total current velocities to turn 67.5° in the upper layer and 135° in the lower layer. The monthly occurrence frequency distribution of ISWs shows a peak in July with a maximum frequency of 16.2% and a trough in March with a minimum frequency of 3.3%. Mode‐2 ISWs appear most in December, accounting for 50% of total mode‐2 ISWs. The largest mode‐2 ISW on record up today with the depressed amplitude as large as 91 m, and the elevated amplitude of 73 m was observed at mooring station. These new findings and new data are of significance to local internal wave prediction model development.

Observations of radiatively driven convection in a deep lake

AbstractObservations of radiatively driven convection in deep, ice‐free Lake Superior from a set of moorings and an autonomous glider are used to characterize the spatial and temporal scales of the phenomenon. The moored observations show that instability builds at the surface on scales of hours, water near the bottom of the lake begins warming roughly 6 h after sunup, and the water column homogenizes a few hours after sundown. Glider observations suggest the existence of distinct convective chimneys, which carry warmed water to depth with horizontal scales on the order of tens of meters. Patches of photoquenched phytoplankton coincide with patches of anomalously warm water, providing a secondary tracer of water recently in the euphotic zone, and provide insight into the vertical development of convective chimneys. An analysis of the abundance of convective chimneys is used to estimate the lateral scale of convective cells, which appears to be on the order of 50 m.

Very Warm Water Observed Along West Antarctic Ice Shelf

Two years of mooring observations at the edge of the continental shelf show that wind stress and upwelling control the inflow of some of the warmest water observed at an ice shelf front in Antarctica.

Existence, morphology and structure of the Yellow Sea Warm Current Branch approaching waters offshore Qingdao, China

In this study, we focused on full-region cruise survey data, near-bottom continuous mooring observations and sea surface wind products from the western South Yellow Sea in winter; after ensuring the data reliability and accuracy, we processed and analyzed the data. Image resolution experiments were carried out to determine the lowest recognition resolutions for all image types, which represent the resolution characteristics of the data. The existence of a warm water tongue originating from the Yellow Sea Warm Current (YSWC) that approached waters offshore Qingdao was confirmed. For the first time, a high salinity water tongue, corresponding to the warm water tongue, was described and found to be more representative of the YSWC branch path This warm tongue is a sign of the branch originating from the YSWC, which we defined as the Yellow Sea Warm Current Branch approaching waters offshore Qingdao (YSWC-QDB). The pattern of the warm and salty water tongues showed remarkable rear, branching middle, shrinking neck and expanding top regions. These patterns showed a temporal feature of the tongues, and were the result of multi-temporal branches in front of the YSWC main section as well as the YSWC-QDB crossing the southwestward path of the extension of the North Shandong Coastal Current flowing along the southeast coast of the Shandong Peninsula (NSCC-SESE). Analysis using mooring data at a sensitive and representative station also showed the existence of the YSWC-QDB. It is a probabilistic event that manifests as a northwestward flow that decreases gradually from the bottom to the surface in the early cold air transit stage and consistent in the whole water column profile in the later stage. It varies quasi-periodically with weather processes. It also transports some of the YSWC water stored in the entrance area of the Bohai and Yellow seas under winter wind conditions to the western South Yellow Sea as a compensatory current. This current, caused by northerly winds, especially northwest winds and obstruction of the NSCC-SESE, was present, and strong water reduction and compensation caused significant residual sea level oscillations. The compensatory current, if caused by strong northwest wind, began to appear when its direction was opposite to the wind direction. In addition, confirmation of the YSWC-QDB provides an oceanographic basis for the short cooling time and rapid warming in the Qingdao area in winter. This research provides a basis for further studies of the YSWC-QDB at high spatial and temporal resolutions using large sea surface datasets. For monsoon basin dynamics, this study can also be extended to the whole Bohai and Yellow seas and closed or semi-closed basins on the continental margin.

A case study of a Kuroshio main path cut-off event and impacts on the South China Sea in fall-winter 2013–2014

This study examines a Kuroshio main path (KMP) cut-off event east of Taiwan Island occurred in fall-winter 2013–2014 and its impacts on the South China Sea (SCS) by analyzing satellite altimetry and mooring observations. Satellite altimeter sea level anomaly (SLA) images reveal a complete process that a huge cyclonic eddy (CE) from the Pacific collided with the Kuroshio and the western boundary from 15 October 2013 to 15 January 2014. Mooring observations evidenced that the Kuroshio upper ocean volume transport was cut off more than 82% from 17×106 m3/s in September to 3×106 m3/s in November 2013. The KMP cut-off event caused the Kuroshio branching and intruding into the SCS and strengthened the eddy kinetic energy in the northern SCS west of the Luzon Strait. Using the total momentum as a dynamic criterion to determine the role of eddy collision with the Kuroshio reasonably explains the KMP cut-off event.

Effect of Freshwater Discharge on the Nakdong River Estuary: Mooring Observations of Water Temperature and Salinity

Effect of Freshwater Discharge on the Nakdong River Estuary: Mooring Observations of Water Temperature and Salinity

Temporal and spatial evolution of a deep-reaching anticyclonic eddy in the South China Sea

The temporal and spatial evolution of a deep-reaching anticyclonic eddy (AE) is studied using a combination of satellite measurements, moored observations and ocean model reanalysis data in the South China Sea (SCS). Three evolutionary stages in eddy’s lifecycle are identified from changes in eddy dynamical characteristics estimated from satellite altimetry: birth (22 days), growth (64 days), and decay (47 days). Similar patterns are also distinguished from dynamic signals in HYCOM. Further, flows reversal and upwelling of cold water below 1500 m were captured by the in-situ records when this energetic, highly nonlinear and long-lived (over 19 weeks) AE passed by our mooring position. Its detailed vertical structure is examined through temperature anomalies, vertical shear of horizontal velocities, and horizontal streamlines estimated from ocean model reanalysis data. Results from the model reveal a mesoscale AE with first-mode baroclinic structure: a bowl-shaped anticyclonic flow in the upper ocean connected to a slant-cylinder cyclonic flow at depth, with a transition layer at depths between 400 and 700 m. It is in good agreement with moored observations but showing a shallower transition depth, suggesting a slight deficiency in the model due to limited deep-sea observations. Last, we estimate eddy heat transport at different depths and stages along the AE’s path based on the model data. The result reveals that pronounced heat fluxes occur during growth stage (depths 400 m), counting for 73.03% of the total value. In the decay stage, major heat transport occurs at deeper depth (depths > 700–1500 m). Dynamical characteristics suggest that the vertical structure and temporal evolution of the eddy play significant roles in basin-scale movement and heat transferring. Considering that mesoscale eddies are ubiquitous in the SCS, our results support a recently-proposed mechanism, whereby upper ocean flows produce changes in the deep-sea circulation, potentially influencing boundary layer dynamics. For the first time to track and link an individual AE observed by satellite altimetry and ocean model, comparisons indicate that assimilative HYCOM outputs may be useful for examining the deep ocean properties within the SCS, especially under the impact of such an intensified surface-detected eddy.