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 spatiotemporal features of submesoscale processes in the northeastern South China Sea

The spatiotemporal features of submesoscale processes （SMPs） in the northeastern South China Sea （SCS） are analyzed based on a high-resolution simulation from 2009 to 2012. The simulation results show that the SMPs with a vertical relative vorticity that matches the local planetary vorticity are ubiquitous in the upper ocean of the northeastern SCS. The SMPs distribution shows an asymmetry due to centrifugal instability, with stronger positive vorticity than negative vorticity. Meanwhile, the SMPs demonstrate an obvious seasonal variation. The SMPs are strong and active in winter but weak and inactive in summer. An investigation of the SMPs generation mechanisms reveals that flow straining and mixed layer depth account for this seasonal variation. The strong flow straining and deep mixed layer depth in winter favor the SMP generation via frontogenesis and mixed layer instability.

Spatial-temporal variability of submesoscale currents in the South China Sea

Spatial and seasonal variabilities of submesoscale currents in the northeastern South China Sea are investigated by employing a numerical simulation with a horizontal resolution of 1km. The results suggest that submesoscale currents are widespread in the surface mixed layer mainly due to the mixed layer instabilities and frontogenesis. In horizontal, submesoscale currents are generally more active in the north than those in the south, since that active eddies, especially cyclonic eddies, mainly occur in the northern area. Specifically, submesoscale currents are highly intensified in the east of Dongsha Island and south of Taiwan Island. In temporal sense, submesoscale currents are more active in winter than those in summer, since the mixed layer is thicker and more unstable in the winter. The parameterization developed by Fox-Kemper et al. is examined in terms of vertical velocity, and the results suggest that it could reproduce the vertical velocity if mixed layer instability dominates there. This study improves our understanding of the submesoscale dynamics in the South China Sea.

Dynamical analysis of submesoscale fronts associated with wind-forced offshore jet in the western South China Sea

This study investigates the submesoscale fronts and their dynamic effects on the mean flow due to frontal instabilities in the wind-driven summer offshore jet of the western South China Sea(WSCS),using satellite observations,a 500 m-resolution numerical simulation,and diagnostic analysis.Both satellite measurements and simulation results show that the submesoscale fronts occupying a typical lateral scale of O(~10)km are characterized with one order of Rossby(Ro)and Richardson(Ri)numbers in the WSCS.This result implies that both geostrophic and ageostrophic motions feature in these submesoscale fronts.The diagnostic results indicate that a net cross-frontal Ekman transport driven by down-front wind forcing effectively advects cold water over warm water.By this way,the weakened local stratification and strong lateral buoyancy gradients are conducive to a negative Ertel potential vorticity(PV)and triggering frontal symmetric instability(SI)at the submesoscale density front.The cross-front ageostrophic secondary circulation caused by frontal instabilities is found to drive an enhanced vertical velocity reaching O(100)m/d.Additionally,the estimate of the down-front wind forcing the Ekman buoyancy flux(EBF)is found to be scaled with the geostrophic shear production(GSP)and buoyancy flux(BFLUX),which are the two primary energy sources for submesoscale turbulence.The large values of GSP and BFLUX at the fronts suggest an efficient downscale energy transfer from larger-scale geostrophic flows to the submesoscale turbulence owing to down-front wind forcing and frontal instabilities.In this content,submesoscale fronts and their instabilities substantially enhance the local vertical exchanges and geostrophic energy cascade towards smaller-scale.These active submesoscale processes associated density fronts and filaments likely provide new physical interpretations for the filamentary high chlorophyll concentration and frontal downscale energy transfer in the WSCS.

Research on submesoscale eddy and front near the South Shetland Islands(Antarctic Peninsula)using seismic oceanography data

The submesoscale processes, including submesoscale eddies and fronts, have a strong vertical velocity, can thus make important supplements to the nutrients in the upper ocean. Using legacy multichannel seismic data AP25 of cruise EW9101 acquired northeast of the South Shetland Islands(Antarctic Peninsula) in February 1991, we identified an oceanic submesoscale eddy with the horizontal scale of ~4 km and a steep shelf break front that has variable dip angles from 5° to 10°. The submesoscale eddy is an anticyclonic eddy, which carries warm core water, can accelerate ice shelves melting. The upwelling induced by shelf break front may play an important role in transporting nutrients to the sea surface. The seismic images with very high lateral resolution may provide a new insight to understand the submesoscale and even small-scale oceanic phenomena in the interior.

Submesoscale motions and their seasonality in the northern Bay of Bengal

The unbalanced submesoscale motions and their seasonality in the northern Bay of Bengal(BoB)are investigated using outputs of the high resolution regional oceanic modeling system.Submesoscale motions in the forms of filaments and eddies are present in the upper mixed layer during the whole annual cycle.Submesoscale motions show an obvious seasonality,in which they are active during the winter and spring but weak during the summer and fall.Their seasonality is associated with the mixed layer instability that depends on the mixed layer depth(MLD).During the winter,the MLD provides a much greater reservoir of the available potential energy,which promotes mixed layer instability to develop active submesoscale motions.The variations of MLD are likely modulated by the larger scale motions and the influxes of freshwater.Further investigations imply that the MLD and the stratified barrier layer are combined to determine the vertical structure of the submesoscale motions.The shallow MLD and strong stratification below during the summer and fall seem to prevent the downward extension of submesoscale motions.But in spring when the weak stratification exists,the penetration depth exceeds the base of the barrier layer.

High-resolution simulation of upper-ocean submesoscale variability in the South China Sea:Spatial and seasonal dynamical regimes

Submesoscale processes in marginal seas usually have complex generating mechanisms,highly dependent on the local background flow and forcing.This numerical study investigates the spatial and seasonal differences of submesoscale activities in the upper ocean of the South China Sea(SCS)and the different dynamical regimes for sub-regions.The spatial and seasonal variations of vertical vorticity,horizontal convergence,lateral buoyancy gradient,and strain rate are analyzed to compare the submesoscale phenomenon within four sub-regions,the northern region near the Luzon Strait(R1),the middle ocean basin(R2),the western SCS(R3),and the southern SCS(R4).The results suggest that the SCS submesoscale processes are highly heterogeneous in space,with different seasonalities in each sub-region.The submesoscale activities in the northern sub-regions(R1,R2)are active in winter but weak in summer,while there appears an almost seasonal anti-phase in the western region(R3)compared to R1 and R2.Interestingly,no clear seasonality of submesoscale features is shown in the southern region(R4).Further analysis of Ertel potential vorticity reveals different generating mechanisms of submesoscale processes in different sub-regions.Correlation analyses also show the vertical extent of vertical velocity and the role of monsoon in generating submesoscale activities in the upper ocean of sub-regions.All these results suggest that the sub-regions have different regimes for submesoscale processes,e.g.,Kuroshio intrusion(R1),monsoon modulation(R2),frontal effects(R3),topography wakes(R4).

Submesoscale-enhanced filaments and frontogenetic mechanism within mesoscale eddies of the South China Sea

Submesoscale activity in the upper ocean has received intense studies through simulations and observations in the last decade,but in the eddy-active South China Sea(SCS)the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood.This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution(∼500 m)simulation.The satellite images show that the elongated highly productive stripes with a typical lateral scale of∼25 km and associated filaments are frequently observed at the periphery of mesoscale eddies.The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching O(100 m/d)due to submesoscale instabilities.The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m^(2).This result implies a significant submesoscale-enhanced vertical exchange between the ocean surface and interior in the filaments.Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity.The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy,but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence.In this context,these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy,as well as enhanced vertical heat transport.

Submesoscale eddies in the East China Sea detected from SAR images

Seven-year(2005-2011)Synthetic Aperture Radar(SAR)images are applied to study oceanic eddies in the East China Sea.It is found that most of these eddies detected from the SAR images are less than 10 km,which are submesoscale eddies.Seasonal differences are evident in the distribution of eddies,with the highest and the lowest number of eddies noted in summer and winter,respectively.Since slick streaks in SAR images look dark,an eddy identified due to the slicks is referred to as“black eddy”.As a result of wave-current interactions in the zones of current shear,it can be seen that an eddy exhibits a bright curve,the eddy is called“white eddy”.During the seven years,95 black eddies and 50 white eddies are identified in the study area.Black eddies are found in the whole study area while white eddies are mainly distributed in the vicinity of the Kuroshio Current.This study suggests that the distribution of the white eddy is denser around the Kuroshio because of the strong shear in the Kuroshio region.In terms of the eddy sizes,white eddies are generally smaller than black eddies.

Physically modulated phytoplankton production and export at submesoscales in the oligotrophic South China Sea Basin

Oceanic submesoscales can significantly influence phytoplankton production and export owing to their similar timescales of days.Based on two-year Biogeochemical Argo(BGC-Argo) observations,this study investigated the development of submesoscale instabilities,particularly symmetric and mixed-layer baroclinic instabilities,and their impacts on biological production and export in the oligotrophic South China Sea basin.In the northern basin,near-surface winter blooms consistently cooccurred with seasonally deepened mixed layers.However,significantly stronger and weaker winter blooms were observed over two consecutive winters within the BGC-Argo observation period.During the first winter,symmetric-instability-induced upward nutrient entrainment played a crucial role in initiating the strong winter bloom in early December,when the mixed layer was approximately 20–30 m shallower than the nutricline.This bloom occurred approximately 20–30 days earlier than that anticipated owing to the contact between the seasonally deepened mixed layer and mesoscale-cyclone-induced uplifted nutricline.The symmetric instability also facilitated the export of fixed phytoplankton carbon from the surface to deeper layers.Conversely,during the second winter,remarkably intense mixed-layer baroclinic instability associated with an intense mesoscale anticyclone led to more significant shoaling of the mixed layer compared to the nutricline,thus increasing the vertical distance between the two layers.Under this condition,upward nutrient injection,phytoplankton bloom,and carbon export were suppressed.In contrast,the BGC-Argo float in the central basin revealed significantly inhibited seasonality of phytoplankton biomass and submesoscale instabilities compared to those in the northern basin,primarily owing to the significantly shallower winter mixed layer.

Subthermocline eddies carrying the Indonesian Throughflow water observed in the southeastern tropical Indian Ocean

We observed a subthermocline eddy(STE)with a cold and fresh core during an observation cruise along a transect of 10°S in the southeastern tropical Indian Ocean(SETIO)in December 2017.The vertical scale,speed radius,and maximum swirl velocity of the STE were about 200 m,55 km,and 0.5 m/s,respectively.The mean Rossby number and Burger number of the STE were then estimated to be about−0.7 and 2.4,indicating the STE was a submesoscale coherent vortex.The STE core water had characteristics of the Indonesian Throughflow(ITF)water and was distinct from that of surrounding areas.By examining Argo float data,another STE was well captured by five successive profiles of the same Argo float.Both STEs showed significant temperature and salinity anomalies at theσ0=26.0-26.5 kg/m3 surfaces.With the assumption that the low-salinity ITF water parcels could be carried only by surface eddies and the STEs,the Argo profiles,which detected low-salinity ITF water and were located outside a surface eddy,were believed to be inside an STE and were used to analyze the distribution,origin,and generation mechanism of the STE.The results suggested that the STEs carrying ITF water may be generated under topography-current interaction at the eastern coastal waters or under front-induced subduction in the area away from coastal waters.Those STEs may be widely distributed in the SETIO and may play a role in ITF water parcel transport.

Analysis of a simplification strategy in a nonhydrostatic model for surface and internal wave problems

This paper examines the simplification strategy of retaining only the nonhydrostatic effect of local acceleration in a three-dimensional fully nonhydrostatic model regarding the submesoscale wave phenomenon in the ocean.Elaborate scale analysis of the vertical component of the Reynold-averaged Navier-Stokes(RANS)equation was performed,confirming the rationalization of this simplification.Then,the simplification was implemented in a RANS equation-based nonhydrostatic model NHWAVE(nonhydrostatic WAVE)to make a simplified nonhydrostatic model.Numerical examples were taken to test its performance,including surface sinusoidal waves propagating on an idealized East China Sea topography,tidally induced internal lee waves and small-scale solitary waves.The results show that in a considerably wide range of nonlinear strengths,the simplified nonhydrostatic model can obtain similar results as those in the fully nonhydrostatic model,even for smaller-scale solitary waves.Nonlinearity influences the applicability of the simplification.The stronger the nonlinearity is,the worse the simplified model describes the nonhydrostatic phenomenon.In general,the simplified nonhydrostatic model can simulate surface waves better than internal waves.Improvement of computational efficiency in the simplified nonhydrostatic model is reasonable,reducing the central processing unit time duration in the fully nonhydrostatic model by 16.4%–20.6%.The specially designed algorithm based on the simplified nonhydrostatic equation can remarkably reduce the computational time.

Seismic Images of Shallow Waters over the Shatsky Rise in the Northwest Pacific Ocean

Recent studies have demonstrated the ability of seismic oceanography to reveal finescale vertical structures of water column in the oceans based on multichannel seismic(MCS)reflection data.Such information can clarify the dynamic processes of mixing,exchange,and translation of water mass and energy.In this study,we present four MCS lines and satellite data to show high-resolution seismic images of shallow waters over the Shatsky Rise in the Northwest Pacific Ocean,where the Kuroshio Exten-sion passes and bifurcates.One of our MCS transects crossed the center of an anticyclonic warm eddy on August 28,2010,confirmed by satellite data such as sea level anomaly(SLA),geostrophic current anomaly(GCA),and sea surface temperature anomaly(SSTa).The seismic image showed that the eddy vertical structure featured a bowl-like shape and onion-like internal layering.The slightly tilted(<0.5°)surface of the eddy was 400m below the sea surface,indicating a subsurface eddy.The eddy was inferred to have a radius of 50 km and a maximum thickness of 500m.Other MCS sections demonstrated the submesoscale structure of oceanfronts,characterized by the dipping reflectors(>2°-3°)at the boundaries between water masses with differing properties.In addition,the discrepancies in SLA,GCA,and SSTa between water masses resulted in different seismic reflectivities.The water masses with high SLA,anticyclonic GCA and positive SSTa featured high-amplitude,continuous,clear-layered,and non-linear reflections,whereas those with low SLA,cyclonic GCA,and negative SSTa were associated with weak,fragmented,less stratification,and more linear reflectors.

Impacts of Oceanic Fronts and Eddies in the Kuroshio-Oyashio Extension Region on the Atmospheric General Circulation and Storm Track

This paper reviews the progress in our understanding of the atmospheric response to midlatitude oceanic fronts and eddies,emphasizing the Kuroshio-Oyashio Extension(KOE)region.Oceanic perturbations of interest consist of sharp oceanic fronts,temperature anomalies associated with mesoscale eddies,and to some extent even higher-frequency submesoscale variability.The focus is on the free atmosphere above the boundary layer.As the midlatitude atmosphere is dominated by vigorous transient eddy activity in the storm track,the response of both the time-mean flow and the storm track is assessed.The storm track response arguably overwhelms the mean-flow response and makes the latter hard to detect from observations.Oceanic frontal impacts on the mesoscale structures of individual synoptic storms are discussed,followed by the role of oceanic fronts in maintaining the storm track as a whole.KOE fronts exhibit significant decadal variability and can therefore presumably modulate the storm track.Relevant studies are summarized and intercompared.Current understanding has advanced greatly but is still subject to large uncertainties arising from inadequate data resolution and other factors.Recent modeling studies highlighted the importance of mesoscale eddies and probably even submesoscale processes in maintaining the storm track but confirmation and validation are still needed.Moreover,the atmospheric response can potentially provide a feedback mechanism for the North Pacific climate.By reviewing the above aspects,we envision that future research shall focus more upon the interaction between smaller-scale oceanic processes(fronts,eddies,submesoscale features)and atmospheric processes(fronts,extratropical cyclones etc.),in an integrated way,within the context of different climate background states.

On the variability of vertical eddy heat flux in the upper ocean

Ocean eddies produce strong vertical heat flux(VHF)in the upper ocean,exerting profound influences on the climate and ecosystem.Currently,mooring array provides a standard way to estimate the eddy-induced VHF(EVHF)based on the adiabatic potential density equation.Apart from the validity of adiabatic assumption,it remains unclear to what extent the estimated EVHF at a single location within a limited time period is representative of its climatological mean value.In this study,we analyzed the above issue by systematically evaluating the variability of EVHF simulated by a 1-km ocean model configured over the Kuroshio Extension.It is found that the EVHF at a single location exhibits pronounced variability.Even averaged over one year that is comparable to the current maintenance capacity of mooring array,the EVHF still deviates significantly from its climatological mean value.For more than 49%of locations in our computational domain(31°-40°N,149°-166°E),the discrepancy between the one-year mean EVHF and its climatological mean value at the peaking depth is larger than the climatological mean itself.The mesoscale eddies play a dominant role in the variability of EVHF but contribute little to the climatological mean EVHF;the opposite is true for submesoscale eddies.Our findings indicate that nested mooring array allowing for isolating the effects of submesoscale eddies will be useful to obtain climatological mean EVHF.

Structure and Temporal Variability of Mediterranean Water in Hydrological and Marine Seismic Data South of Portimao Canyon (Gulf of Cadiz), from 1999 to 2002

Hydrological and marine seismic data, collected in the Gulf of Cadiz (respectively in July 1999, 2000, 2001 and 2002, and in April 2000 and 2001) are analysed to reveal the various structures of Mediterranean Water (MW). Both the hydrological and seismic data clearly identify the MW undercurrents on the Iberian slope, detached MW eddies (meddies and a cyclone) and smaller fragments of MW (filaments and small eddies). Seismic reflectivity and synthetic reflectivity computed from hydrology, indicate that strong acoustic reflectors, associated with 8 - 64 m thick homogeneous water layers, are found above and below meddies and filaments, around the MW undercurrents, but mostly in the lower part of cyclones and below submesoscale eddies. Reflectors are also observed in the near surface layers where thermohaline contrasts are quite pronounced. The successful use of seismic data to locate submesoscale MW structures, superior to that of hydrology, is related to the improved horizontal resolution.