Mesoscale Anticyclonic Eddies Research Articles

Observed spatiotemporal variation of three-dimensional structure and heat/salt transport of anticyclonic mesoscale eddy in Northwest Pacific

As in-situ observations are sparse, targeted observations of a specific mesoscale eddy are rare. Therefore, it is difficult to study the three-dimensional structure of moving mesoscale eddies. From April to September 2014, an anticyclonic eddy located at 135°E–155°E, 26°N–42°N was observed using 17 rapid-sampling Argo floats, and the spatiotemporal variations in the three-dimensional structure were studied. The results are as follows: (1) the eddy was identified and tracked using satellite altimeter data. It had a lifetime of 269 days and an average radius of 91.5 km. The lifetime of the eddy can be divided into three phases, i.e., the initiation, maturity, and termination phases. The depth of its influence reached 1 000 m; (2) the Argo profiles were divided into seven periods (approximately 20 days in each) for composite analysis, and the composite Argo profiles and CARS2009 (CSIRO Atlas of Regional Seas) climatology data were merged following the data-interpolating variational analysis (DIVA) method to reconstruct the three-dimensional structure. The temperature and salinity anomaly cores of the anticyclonic mesoscale eddy are located from 500 to 600 m. From 800 to 900 m, there is an area of low salinity at the center of the eddy. A high concentration anomaly of dissolved oxygen was located at approximately 250 m; (3) to better understand the features of the eddy and its interaction with the surroundings, we calculated the anomalous velocity of the geostrophic flow and the heat, salt, dissolved oxygen transport anomaly, and discussed the eddy’s origin and its adjustments to topography. The maximum heat, salt, and oxygen transport caused by eddy were 9.37×1011 W, 3.08×103 kg/s, and 2.70×102 kg/s, which all occurred during the termination phase. This study highlights the applicability of using Argo floats to understand the three-dimensional structure thermohaline features of eddies in the North Pacific.

Effects of an anticyclonic eddy on the distribution and community structure of zooplankton in the South China Sea northern slope

Abstract Mesoscale anticyclonic eddies have been frequently observed in the South China Sea (SCS) northern slope. Here we present the first multidisciplinary cruise conducted in an anticyclonic eddy to characterize zooplankton community structure in the northern SCS during March 2017. In-situ net sampling combined with a ZooScan integrated system was used to determine the zooplankton biomass, abundances, and biovolumes in four size classes (0.2–0.5 mm, 0.5–1 mm, 1–2 mm, and 2–5 mm). According to the temperature and salinity, the stations were divided into three groups, corresponding with the inside, edge, and outside of the anticyclonic eddy, respectively. Elevated zooplankton biomass, abundances, and biovolumes were clearly observed at the edge and outside of the anticyclonic eddy compared to the inner eddy area. This was associated with the significantly increased proportions of zooplankton in the 1–2 mm size class. Copepoda were the major contributors to the zooplankton, with 68.7% of the total abundance and 51.5% of the total biovolume. The analysis of similarity revealed that the zooplankton size composition and taxonomic composition were significantly different between the inner and outer areas of the anticyclonic eddy. In addition, we also found obvious diel variations for the zooplankton, particularly in the inner anticyclonic eddy area. Shrimp showed significantly diel variations (0–200 m) in this region. The anticyclonic eddy substantially influenced the zooplankton distribution and community structure in the SCS northern slope during March 2017.

Life cycle of mesoscale eddies in the Gulf of Aden

The Red Sea Water is a warm and salty water produced in the Red Sea by evaporation induced by strong solar radiation. This dense water mass exits the Red Sea through the Strait of Bab El Mandeb, and enters the Gulf of Aden as a density current between 400 and 1000 metre depth. In the Gulf of Aden, in situ and satellites observations have shown the impact of the deeply reaching eddies dominating the mesoscale dynamics, on the spreading of the Red Sea Water. In this paper, we study the life cycle of these mesoscale eddies in the Gulf of Aden by using a regional primitive equation model at mesoscale resolution, and an eddy-tracking algorithm. The mesoscale anticyclonic eddies are formed at the mouth of the Gulf of Aden, and subsequently drift westward into the gulf. Mesoscale anticyclones are long-lived compared to the cyclones. The cyclones result from the interaction of anticyclones with the coast and the sloping topography. The wind stress, the bathymetry and the surrounding eddy field drive the life cycle of eddies. Finally, Kelvin and internal waves are triggered along the northern and southern coasts.

Interaction of Mesoscale and Submesoscale Eddies in the Sea of Okhotsk Based on Satellite Data

We have investigated the interaction between the mesoscale and submesoscale eddies in the deep Kuril Basin of the Sea of Okhotsk. An analysis of the long-term series of satellite altimetry observations (2000–2011) has demonstrated that the circulation in the Kuril Basin is determined by quasi-stationary mesoscale anticyclonic eddies. Based on the QuikSCAT SeaWind scatterometer satellite data, a negative vorticity wind-stress field exists over the deep part of the Sea of Okhotsk in the warm season, which generates anticyclonic circulation composed of mesoscale eddies. An analysis of the satellite infrared images reveals that submesoscale processes determine the interaction of mesoscale eddies with the waters of the zone of intense tidal mixing on the shelf of the Kuril Islands. Submesoscale cyclonic eddies are formed at the fronts of tidal mixing and near the Bussol Strait located in the central part of the Kuril Islands. Applications of Lagrangian analysis have shown that submesoscale eddies move at the periphery of the northern mesoscale eddy within the advective streamer limited by the transport barriers. The formation and destruction of submesoscale eddies caused by horizontal mixing leads to the injection of cold water rich in nutrients from the zone of intense tidal mixing to the deep Kuril Basin. In the summer period, high concentrations of chlorophyll-a are observed in the upper layer of the northeastern part of the Sea of Okhotsk due to the interaction between submesoscale and mesoscale eddies.

Two typical merging events of oceanic mesoscale anticyclonic eddies

Abstract. The long-term theoretical “energy paradox” of whether the final state of two merging anticyclones contains more energy than the initial state is studied by considering two typical merging events of ocean mesoscale eddies. The results demonstrate that the total mass (volume), total circulation (area integration of vorticity), and total angular momentum (AM) are conserved if the orbital AM relative to the center of mass is taken into account as the eddies rotate around the center of mass before merging. For subsurface merging, the mass trapped by the Taylor–Proudman effect above the subsurface eddies should also be included. Both conservation laws of circulation and orbital AM have been overlooked in previous theoretical studies. As a result of fusion during merging, the total eddy kinetic energy decreases slightly. In contrast, the total eddy potential energy (EPE) increases after merging. The increase in EPE is mostly supported by the loss of gravitational potential energy (PE) via eddy sinking below the original level prior to merging. This implies that the merging of eddies requires background gravitational PE to be converted to EPE. In contrast, the vorticity and enstrophy consequently decrease after merging. Thus, the eddy merging effect behaves as a “large-scale energy pump” in an inverse energy cascade. It is noted that eddy conservation and conversion laws depend on the laws of physical dynamics, even if additional degrees of freedom can be provided in a mathematical model.

Building a Maxey–Riley framework for surface ocean inertial particle dynamics

A framework for the study of surface ocean inertial particle motion is built from the Maxey–Riley set. A new set is obtained by vertically averaging each term of the original set, adapted to account for Earth’s rotation effects, across the extent of a sufficiently small spherical particle that floats at an assumed unperturbed air–sea interface with unsteady nonuniform winds and ocean currents above and below, respectively. The inertial particle velocity is shown to exponentially decay in time to a velocity that lies close to an average of seawater and air velocities, weighted by a function of the seawater-to-particle density ratio. Such a weighted average velocity turns out to fortuitously be of the type commonly discussed in the search-and-rescue literature, which alone cannot explain the observed role of anticyclonic mesoscale eddies as traps for marine debris or the formation of great garbage patches in the subtropical gyres, phenomena dominated by finite-size effects. A heuristic extension of the theory is proposed to describe the motion of nonspherical particles by means of a simple shape factor correction, and recommendations are made for incorporating wave-induced Stokes drift and allowing for inhomogeneities of the carrying fluid density. The new Maxey–Riley set outperforms an ocean adaptation that ignored wind drag effects and the first reported adaption that attempted to incorporate them.

Hydrographic and Biological Survey of a Surface‐Intensified Anticyclonic Eddy in the Caribbean Sea

AbstractIn the Caribbean Sea, mesoscale anticyclonic ocean eddies impact the local ecosystem by mixing of low salinity river outflow with the nutrient‐rich waters upwelling along the Venezuelan and Colombian coast. To gain insight into the physics and the ecological impact of these anticyclones, we performed a combined hydrographic and biological survey of one Caribbean anticyclone in February 2018. We found that the anticyclone had a radius of 90 km and was surface intensified with the strongest velocities (0.72 m/s) in the upper 150 m of the water column. Below, isopycnal displacements were found down to 700 dbar. The core of the anticyclone entrained waters from the Orinoco River plume and contained slightly elevated chlorophyll concentrations compared to the surroundings. At the edge of the anticyclone we observed higher densities of flying fish but not higher densities of predators like seabirds and cetaceans. Below the surface, a strong temperature inversion (0.98 °C) was present within a barrier layer. In addition, we found thermohaline staircases that originated from double diffusion processes within Tropical Atlantic Central Water.

Subsurface Mesoscale Eddy Generation in the Ocean off Central Chile

AbstractOff the coast of central Chile, subsurface anticyclonic eddies are a salient feature of the oceanic circulation, transporting a significant fraction of coastal water that is rich in nutrients and poor in dissolved oxygen offshore. In this study, the formation mechanism of these eddies is analyzed through a high‐resolution (~0.3 km) and low‐resolution (~3 km) oceanic model that realistically simulate the regional mean circulation, including the Peru‐Chile Undercurrent (PCUC). An analysis of the vorticity and eddy kinetic energy in both simulations indicated that the subsurface eddies can be triggered through a combination of processes that are associated with instabilities of the PCUC. In the high‐resolution simulation, we observed that the interaction between the PCUC and topographic slope generates anticyclonic vorticity and potential vorticity close to zero in the bottom boundary layer. The separation of the undercurrent from the slope favors the intensification of anticyclonic vorticity. It reaches magnitudes that are larger than the planetary vorticity while kinetic energy is converted from the PCUC to the eddy flow. These processes set the necessary conditions for the development of centrifugal instabilities, which can form submesoscale structures. The coalescence of submesoscale structures generates a subsurface anticyclonic mesoscale eddy. In the low‐resolution simulations (>3 km) centrifugal instabilities are not simulated, and the barotropic conversion of the mean kinetic energy into eddy kinetic energy appears as the main process of eddy formation. We showed that the vertical structure of these eddies is sensitive to the spatial resolution of the model.

Dynamics of Mesoscale Eddies Interacting With a Western Boundary Current Flowing by a Gap

AbstractThe propagation of cyclonic and anticyclonic mesoscale eddies through a wide meridional gap with a crossing western boundary current (WBC) is studied using a 1.5‐layer quasi‐geostrophic ocean circulation model. The results of the numerical experiments suggest that a steady, leaping WBC blocks nearly completely all eddies from propagating westward through the gap. In comparison, both cyclonic and anticyclonic eddies propagate westward nearly uninterrupted through the gap if the WBC is in a penetrating state. Interactions of mesoscale eddies with a critical‐state WBC trigger regime transitions of the WBC between leaping and penetrating states, resulting in dramatic changes of the circulation west of the gap. The radiation of the circulation plumes from the gap into the western basin due to the WBC regime transition is found to be independent of the size and the initial location of the perturbing eddy in the eastern basin, provided that the WBC path shift is induced successfully by the eddy.

Coastal summer eddies in the Peter the Great Bay of the Japan sea: In situ data, numerical modeling and Lagrangian analysis

Coastal mesoscale anticyclonic eddies are regularly generated by the Primorsky Current flowing past the capes in the largest bay of the Japan Sea, the Peter the Great Bay. They were observed in 18 of 21 CTD summer surveys performed from 2001 to 2017. The regional ROMS model is used to study the evolution of the eddy that has been sampled in the oceanographic survey in August 2009. The model is able to reproduce the observed eddy on the days of sampling including its location, size and vertical structure. The Lagrangian maps are computed to investigate and document the generation, deformation and decay of the eddy, to identify the vortex contour and origin of water masses inside it. Such coastal eddies are typical for the summer period in the Bay regulating water exchange between the continental shelf and open sea. They are shown to be generated after intensification of the eastern wind which is sufficiently strong to produce downwelling and increased horizontal density gradients and to generate an anticyclonic vorticity past the capes.

Energy of the Jets of the Antarctic Circumpolar Current and of Their Eddies in the Surface Layer of the Southern Ocean

The kinetic energy of six jets of the Antarctic Circumpolar Current (ACC) and of the cyclonic and anticyclonic mesoscale eddies generated by these jets is studied in application to the surface layer of Antarctic Circle. The study is based on satellite altimetry data for 1993–2015 available at http://aviso.altimetry.fr . Main results of the study are (a) a fivefold excess of the mean energy of jets proper over the mean total (cyclones plus anticyclones) energy of eddies; (b) the prevalence of the energy of the core jet of the Subantarctic Current over the energy of the rest of the ACC jets in the entire Antarctic Circle; (c) a twofold excess of the mean energy of cyclonic eddies over the energy of anticyclones in the upper ocean layer.

The impact of mesoscale eddies on the phytoplankton community in the South Atlantic Ocean: HPLC-CHEMTAX approach

This study describes the pigment-based phytoplankton community within three South Atlantic anticyclonic eddies (at different ages) shed from the Agulhas Current retroflection crossing the South Atlantic Ocean. Seawater samples were collected over these mesoscale structures in June–July 2015 during the Following Ocean Rings in the South Atlantic (FORSA) cruise. Data on phytoplankton pigments, measured with high-performance liquid chromatography (HPLC), were processed using a chemical taxonomy (CHEMTAX) tool to determine and quantify phytoplankton taxonomic groups. In addition, physical (water column structure) and chemical (macronutrient) parameters were determined and related to the biological data. Our results showed that, in general, the community was composed mostly of small flagellates (haptophytes) and prokaryotes (Prochlorococcus) and that pelagophytes were prominent in the younger eddy. This ring, located in the eastern basin of the South Atlantic Ocean, represented a younger and stronger structure, with no evident deep chlorophyll maximum (DCM) depth and an evenly distributed biomass over a well-mixed upper layer, revealing a more diverse phytoplankton community. The weakened structures of the older western eddies presented a pronounced DCM depth below 100 m and similar phytoplankton community composition patterns marked by enhanced Prochlorococcus contributions but also the important occurrences of haptophytes at the DCM depth and Synechococcus and chlorophytes at the surface. The community distributions in all three structures were associated with the distribution of nutrients and acclimation to light conditions. This study contributes to a better understanding of the phytoplankton distribution and its association with the presence of mesoscale anticyclonic eddies in an undersampled and complex region of the South Atlantic Ocean.

Spatiotemporal variations in the summer hypoxia in the Bohai Sea (China) and controlling mechanisms

Based on field observations in the summer of 2006 and long-term series data, this paper investigates the spatiotemporal variations of hypoxia and associated physical-biogeochemical driving mechanisms in the Bohai Sea (BS), China. Results show that the benthic hypoxic zone is mainly distributed in the “V”-shaped trough region in the western BS, and it tends to form two hypoxic centers which generally correspond to the bottom cold-water core. The regional difference in the intensity of stratification has a significant impact on the spatial distribution of hypoxia. The relatively weak stratification and the mesoscale anticyclonic eddy in the central shoal of the BS weaken the connectivity between the southern and northern hypoxic zones. Organic matter decomposition contributes to hypoxia and results in corresponding nutrient pool with a “dual (southern and northern)-core” structure. Intensified eutrophication is the main drive for decreasing in bottom dissolved oxygen and expansion of hypoxic zone in the BS.

Field-observation for an anticyclonic mesoscale eddy consisted of twelve gliders and sixty-two expendable probes in the northern South China Sea during summer 2017

An intensive field observation experiment using 12 Chinese gliders equipped with conductivity-temperature-depth (CTD) sensors and 62 expendable CTD probes (XCTDs) was performed to investigate the 3-D structure and time evolution of an anticyclonic eddy in the northern South China Sea (NSCS). The observed results showed that the anticyclonic eddy had a horizontal radius of about 80 km at surface and a vertical depth of impact of more than 1000 m. The largest temperature and salinity anomalies compared with the averaged values of the temperature and salinity profiles were 3.5°C and 0.4 psu at 120 m depth, respectively. Combined analysis of altimeter sea level and water mass properties indicated that the anticyclonic eddy was shed from the Kuroshio loop current. The vertical axis of the anticyclonic eddy tilted from surface to the observed maximum depth (1000 m) along its translation direction against the 2000 m isobath. The center of the anticyclonic eddy remained in the region east of Dongsha Island for more than half a month. During this time, the long axis direction of the eddy changed from across the slope to along the slope. Then, the eddy moved southward along the 2000 m isobaths. Both the geostrophic current and temperature distribution revealed that the eddy intensity weakened during the observation period gradually. These observations indicated strong interaction between the anticyclonic eddy and the slope topography of Dongsha Island.

Subsurface Fine‐Scale Patterns in an Anticyclonic Eddy Off Cap‐Vert Peninsula Observed From Glider Measurements

AbstractGlider measurements acquired along four transects between Cap‐Vert Peninsula and the Cape Verde archipelago in the eastern tropical North Atlantic during March–April 2014 were used to investigate fine‐scale stirring in an anticyclonic eddy. The anticyclone was formed near 12°N off the continental shelf and propagated northwest toward the Cape Verde islands. At depth, between 100 and –400 m, the isolated anticyclone core contained relatively oxygenated, low‐salinity South Atlantic Central Water, while the surrounding water masses were saltier and poorly oxygenated. The dynamical and thermohaline subsurface environment favored the generation of fine‐scale horizontal and vertical temperature and salinity structures in and around the core of the anticyclone. These features exhibited horizontal scales of O(10–30 km) relatively small with respect to the eddy radius of O(150 km). The vertical scales of O(5–100 m) were associated to density‐compensated gradient. Spectra of salinity and oxygen along isopycnals revealed a slope of around k−2 in the 10‐ to 100‐km horizontal scale range. Further analyses suggest that the fine‐scale structures are likely related to tracer stirring processes. Such mesoscale anticyclonic eddies and the embedded fine‐scale tracers in and around them could play a major role in the transport of South Atlantic Central Water masses and ventilation of the North Atlantic Oxygen Minimum Zone.

One‐Dimensional Turbulence‐Ecosystem Model Reveals the Triggers of the Spring Bloom in Mesoscale Eddies

AbstractA lower trophic ecosystem model coupled with a one‐dimensional physical turbulence closure model was applied to study phytoplankton dynamics and spring bloom initiation in mesoscale anticyclonic eddies (AEs) and cyclonic eddies (CEs). The model simulated ecosystem dynamics between nutrient‐phytoplankton‐zooplankton‐detritus in AEs and CEs, while the physical model provided the seasonal cycle of convective turbulent mixing. The study was motivated by earlier work based on satellite and ship observations, which showed earlier initiation of the spring blooms in CEs with shallow mixed‐layer depths than in AEs with deeper mixed‐layer depths. The model results supported the hypothesis that mixed‐layer depths in eddies play an important role in the dynamics of the spring bloom initiation. Model results revealed that in AEs convective mixing causes light limitation for phytoplankton growth due to deep winter mixing, and the bloom initiation is delayed until relaxation of turbulent convective mixing. Conversely, in the shallow mixed‐layer CEs, blooms initiate before the end of convective mixing due to early improvement in light conditions following the increase in solar radiation. Furthermore, the model showed that the relaxation in zooplankton grazing for the deep mixing contributed to weak winter phytoplankton accumulation in AE, while winter phytoplankton accumulation was faster in the shallow mixed‐layer CE. Overall, the initiation mechanism and the dynamics of the spring phytoplankton blooms are different for AEs and CEs. Therefore, we suggest that, in many parts of the global ocean, eddies play an important role regulating the dynamics of phytoplankton blooms.

An observational analysis of the evolution of a mesoscale anti-cyclonic eddy over the Northern Bay of Bengal during May–July 2014

The Bay of Bengal (BoB) is a distinct oceanic region for mesoscale oceanic eddies. The sea level anomaly from the Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) help to identify an unusual anti-cyclonic eddy (ACE) over head BoB during May–July 2014. Two Indian moored buoys (BD08 and BD09) located over this region aided to study the subsurface thermohaline structures of the ACE. Compared to no-eddy environment, the temperature and salinity showed an increment of ~ 3–4 °C and ~ 1–2 PSU, respectively, during the ACE life period. The temperature and depth of the isothermal layer at genesis (peak) stages are increased to ~ 30 °C (~ 30.7 °C) and ~ 20 m (30 m) when compared with no-eddy conditions (28.2 °C and 10 m). The thermocline depth is deepened to 75 m at the peak stage, while it is 50 m in no-eddy condition. A temperature difference of 3 °C between no-eddy and peak stages of ACE is observed up to 50 m. The ocean heat content (OHC) at BD08 (BD09) during genesis and peak stages has increased by ~ 72% (~ 50%) and ~ 247% (~ 181%), respectively, when compared with no-eddy conditions. Moreover, the MOHC also shows a similar increment of ~ 125% (~ 123%) and ~ 258% (~ 284%), respectively. A noticeable influence is seen in turbulent fluxes and lower atmospheric variables during eddy life. This study highlights the capability of moored buoys in understanding the subsurface thermohaline features of the eddies over northern BoB.

Phytoplankton Size Structure in Association with Mesoscale Eddies off Central-Southern Chile: The Satellite Application of a Phytoplankton Size-Class Model

Understanding the influence of mesoscale and submesoscale features on the structure of phytoplankton is a key aspect in the assessment of their influence on marine biogeochemical cycling and cross-shore exchanges of plankton in Eastern Boundary Current Systems (EBCS). In this study, the spatio-temporal evolution of phytoplankton size classes (PSC) in surface waters associated with mesoscale eddies in the EBCS off central-southern Chile was analyzed. Chlorophyll-a (Chl-a) size-fractionated filtration (SFF) data from in situ samplings in coastal and coastal transition waters were used to tune a three-component (micro-, nano-, and pico-phytoplankton) model, which was then applied to total Chl-a satellite data (ESA OC-CCI product) in order to retrieve the Chl-a concentration of each PSC. A sea surface, height-based eddy-tracking algorithm was used to identify and track one cyclonic (sC) and three anticyclonic (ssAC1, ssAC2, sAC) mesoscale eddies between January 2014 and October 2015. Satellite estimates of PSC and in situ SFF Chl-a data were highly correlated (0.64 < r < 0.87), although uncertainty values for the microplankton fraction were moderate to high (50 to 100% depending on the metric used). The largest changes in size structure took place during the early life of eddies (~2 months), and no major differences in PSC between eddy center and periphery were found. The contribution of the microplankton fraction was ~50% (~30%) in sC and ssAC1 (ssAC2 and sAC) eddies when they were located close to the coast, while nanoplankton was dominant (~60–70%) and picoplankton almost constant (<20%) throughout the lifetime of eddies. These results suggest that the three-component model, which has been mostly applied in oceanic waters, is also applicable to highly productive coastal upwelling systems. Additionally, the PSC changes within mesoscale eddies obtained by this satellite approach are in agreement with results on phytoplankton size distribution in mesoscale and submesoscale features in this region, and are most likely triggered by variations in nutrient concentrations and/or ratios during the eddies’ lifetimes.

Physical connectivity between Pulley Ridge and Dry Tortugas coral reefs under the influence of the Loop Current/Florida Current system

The Pulley Ridge and Dry Tortugas coral reefs are among the most pristine, but also fragile, marine ecosystems of the continental United States. Understanding connectivity processes between them and with surrounding shelf and deep areas is fundamental for their management. This study focuses on the physical processes related to the connectivity of these reefs. Unprecedented in situ time series were used at these specific reef locations, together with satellite observations and numerical simulations, to investigate the dynamics controlling local circulation on the Southwestern Florida Shelf (SWFS) under oceanic influence. The approach of the Loop Current and Florida Current (LC/FC) system to the SWFS slope can induce 0.5 to 1 m/s offshore flows impacting the Pulley Ridge and Dry Tortugas reefs. On the other hand, when the LC/FC system retreats from the slope, onshore flows can carry open-sea waters over the coral reefs. Local formation of cyclonic eddies is possible near the Dry Tortugas reefs in the LC approach case and passage of upstream LC Frontal Eddies is possible in the LC retreat case. Offshore currents ∼1 m/s over the SWFS slope were also found during periods of anticyclonic LC Eddy separation. A novel finding is the shedding and northward propagation of mesoscale anticyclonic eddies from the core of the LC along the West Florida Shelf. Eddy shedding may have a broader effect on the dynamics of the shelf around the study reef areas. Long periods of LC/FC domination over these coral reefs (reaching several weeks to months) are characterized by strong (∼1 m/s) along-shelf currents and continuous upwelling processes, which may weaken the slope stratification and bring colder, deeper waters over the shelf-break and toward the shallower shelf region.

Cases Study of Nonlinear Interaction Between Near‐Inertial Waves Induced by Typhoon and Diurnal Tides Near the Xisha Islands

AbstractNonlinear interaction between near‐inertial waves (NIWs) and diurnal tides (DTs) after nine typhoons near the Xisha Islands of the northwestern South China Sea (SCS) were investigated using three‐year in situ mooring observation data. It was found that a harmonic wave (f + D1, hereafter referred to as fD1 wave), with a frequency equal to the sum of frequencies of NIWs and DTs (hereafter referred to as f and D1, respectively), was generated via nonlinear interaction between typhoon‐induced NIWs and DTs after each typhoon. The fD1 wave mainly concentrates in the subsurface layer, and is mainly induced by the first component of the vertical nonlinear momentum term, the product of the vertical velocity of DT and vertical shear of near‐inertial current (hereafter referred to as Component 1), in which the vertical shear of the near‐inertial current greatly affects the strength of the fD1 current. The larger the Component 1, the stronger the fD1 currents. The background preexisting mesoscale anticyclonic eddy near the mooring site may also enhance the vertical velocity of DT and thus Component 1, which subsequently facilitates the nonlinear interaction‐induced energy transfer to the fD1 wave and enhances the fD1 currents after the passage of a typhoon.