Wind Stirring Research Articles

The Role of Tide and Wind in Modulating Density Stratification in the Pearl River Estuary during the Dry Season

Density stratification plays a crucial role in estuarine hydrodynamics and material transport. In this study, we utilized a well-calibrated numerical model to investigate the stratification processes and underlying mechanisms in the dynamically wide Pearl River Estuary (PRE). In the upper estuary, longitudinal straining governs stratification, enhancing it during ebb tide and reducing it during flood tide. The Coriolis force becomes significant in the lower estuary due to the increased basin width, causing seaward freshwater to be confined to the West Shoal, where a pronounced transverse salinity gradient forms. Interacting with lateral current shear, density stratification is most pronounced in this region. The prevailing northeasterly wind creates a mixed layer near the surface, shifting stratification to the middle layer of the water column in the upper estuary. Wind stirring reduces stratification throughout the estuary. Under the wind’s influence, the seaward outflow is confined to a narrower region and shifts westward, resulting in the most apparent stratification occurring on the West Shoal of the PRE due to lateral straining. These findings on the evolution of freshwater pathways and their role in modulating density stratification have significant implications for other wide estuaries, such as Delaware Bay and the La Plata-Parana estuary.

Response of chlorophyll-a to rainfall event in the basin of the South China sea: Statistical analysis

Chlorophyll-a (Chl-a) is an essential ecological indicator, and affected by processes such as typhoons, mesoscale eddies, and Rossby waves. However, the impact of more frequent and widespread precipitation events on Chl-a seems to be overlooked. This study utilized remote sensing data and reanalysis data to investigate the response of Chl-a to 240 precipitation events in the central South China Sea from 2005 to 2019. The results indicate that precipitation events have a significant impact on Chl-a concentration. Following a precipitation event in 2019, the Chl-a concentration in the affected area increased by approximately 0.22 mg m−³ from the 3rd to the 7th day. The reasons for the increase in Chl-a concentration were the vertical mixing induced by wind stirring and the upwelling caused by wind stress curl, which transported nutrients to the euphotic zone, lowering the sea surface temperature and triggering a proliferation of phytoplankton. Additionally, dissolved nutrients in precipitation provided a nutrient source for Chl-a growth. The contributions of nutrient supply, wind speed, and wind stress curl to the increase in Chl-a concentration during precipitation events were 18%, 37%, and 45%, respectively. Precipitation events enhanced marine primary productivity, playing a crucial role in deepening our understanding of ocean-atmosphere interactions and their impact on marine ecosystem.

The oceanic mixed layer changes along with the state transition of the Beaufort Gyre

Recent state transition of the Beaufort Gyre has drawn great interest in the Arctic research community, but how the upper ocean hydrographic structure varies with this transition remains poorly understood. The upper ocean mixed layer plays an important role in climatic and ecological processes. Therefore, we analyze the Ice-Tethered Profiler (ITP) observations over the last two decades (2004–2022) to investigate the long-term trend of the mixed layer in the Arctic Ocean’s Beaufort Gyre (BG) from an observational perspective. Results show that the linear trend of the BG surface mixed layer depth (MLD) before and after 2015 has changed significantly, characterized by the vanishing or even reversal of the significant deepening trend. This transition is most pronounced in winter. The BG winter mixed layer is significantly cooler, saltier and denser in the mid-transition period (2013–2017) compared to the pre-transition period (2004–2012), but becomes significantly warmer, fresher and lighter in the post-transition period (2018–2022). The transition feature of the depth of maximum buoyancy frequency in the upper BG is similar to that of MLD, while this maximum decreases significantly in both the mid- and post-transition period when compared to their previous period. The deepening signal of MLD is propagated eastward, which coincides with the recent transition of BG position and freshwater distribution. Mechanism analysis further reveals that the reversal of winter MLD trend before and after 2015 may be due to changes in surface wind stirring and Ekman pumping. This study extends the investigation of the recent state transition of BG considering the upper hydrographic structure.

Submesoscale Frontal Instabilities Modulate Large‐Scale Distribution of the Winter Deep Mixed Layer in the Kuroshio‐Oyashio Extension

AbstractIn the mid‐latitude North Pacific, the wintertime ocean heat loss reaches the maximum in the Kuroshio‐Oyashio Extension (KOE) region. However, the mixed layer depth (MLD) there is shallow (<50 m), flanked by a zonal band of deep MLD (>100 m) on either side. Such an observed mixed layer pattern is not reproduced by coarse‐resolution or mesoscale eddy‐resolving (1/10°) models. What causes the observed MLD shoaling along the KOE frontal region? Here we show that the mixed layer shoaling in the front is well reproduced by a 1/30°, submesoscale‐permitting ocean general circulation model, indicating that it is closely related to the submesoscale frontal dynamics. The geostrophic strain rate is strong along the sharp KOE front, favoring submesoscale frontal instabilities. The surface buoyancy loss during winter reduces stratification and enhances the fronts (i.e., with steeper, northward rising isopycnals). The available potential energy stored in the narrow fronts can be released in the form of eddy kinetic energy by the mixed layer baroclinic instabilities (MLI). The associated vigorous ageostrophic motions associated with MLI efficiently slump the vertically oriented isopycnals and restratify the mixed layer. Thus, the MLD is shallow in the KOE frontal region despite the strong surface heat loss and wind stirring during winter. The submesoscale processes revealed here have important implications for improving climate models.

Efficacy and safety of Jiu-Wei-Xi-Feng granules for treating tic disorders in children: study protocol for a randomized controlled equivalence trial

BackgroundTic disorders (TD) is a neuropsychiatric disease with twitch as the main manifestation in childhood. Jiu-Wei-Xi-Feng granules has been marketed in China for treating children with TD. As Long Gu (Os Draconis) in the composition of this Chinese patent medicine is a rare and expensive medicinal material protected by the Chinese government, therefore, we consider replacing it with Mu Li (Concha Ostreae) that has the same effect and is cheaper. This study is designed to evaluate the clinical equivalence between Jiu-Wei-Xi-Feng granules (Os Draconis replaced by Concha Ostreae) (JWXFD) and Jiu-Wei-Xi-Feng granules (original formula) (JWXFO) in children with TD (consumption of renal yin and liver wind stirring up internally syndrome).Methods/designThis is a multicenter, randomized, double-blind, equivalence trial comparing the efficacy and safety of JWXFD and JWXFO in treating Children with tic disorders (consumption of renal yin and liver wind stirring up internally syndrome). A total of 288 patients will be recruited and randomly assigned to two groups in a 1:1 ratio. The treatment course is 6 weeks, with a 2 weeks follow-up. The primary outcome is the mean change value from baseline to 6th week by the Yale Global Tic Severity Scale total tic score (YGTSS-TTS). Secondary outcomes include total effective rate of tic, Yale Global Tic Severity Scale (YGTSS) scores and its factor scores (the degree of motor tics, phonic tics and social function damage), Clinical Global Impression-Severity scale, and TCM syndrome efficacy.DiscussionThe design of this study refers to a large number of similar research design points, and asked for opinions of peer experts, and finally reached a consensus. This trial will provide high-quality evidence on the clinical equivalence between JWXFD and JWXFO and provide a basis for the marketing of JWXFD.Trial registrationChiCTR2000032312Registered on 25 April 2020, http://www.chictr.org.cn/showproj.aspx?proj=52630

Spatial Structure of Vertical Motions and Associated Heat Flux Induced by Mesoscale Eddies in the Upper Kuroshio‐Oyashio Extension

AbstractVertical motions induced by mesoscale eddies in the upper ocean play a vital role in the heat transport over the global ocean. However, the spatial structure of vertical eddy velocity and associated heat flux remains unclear. This study addresses this issue in the Kuroshio‐Oyashio Extension based on an eddy‐rich Community Earth System Model simulation. Adopting a mesoscale eddy composite analysis, we show that the vertical eddy velocity is enhanced at edges of mesoscale eddies with a maximum magnitude of 0.8 m day−1 in the upper ocean for anticyclonic eddies (AEs) and 0.5 m day−1 for cyclonic eddies (CEs). The associated vertical heat flux is upward, reaching 52 and 32 W m−2 along AEs' and CEs' periphery, respectively. Diagnostic analysis suggests that the enhanced vertical eddy motions at eddy edges are primarily attributed to the ageostrophic secondary circulation (ASC) under the turbulent thermal wind balance that accounts for more than 70% of the vertical eddy velocity and 80% of vertical eddy heat flux in the upper ocean. This ASC is stronger in winter than summer due to the intense turbulent mixing induced by strong surface cooling and wind stirring in winter. Accordingly, the vertical eddy velocity and associated heat flux exhibit a distinct seasonal cycle.

Traditional Chinese medicine syndrome differentiation and treatment by stages of Parkinson’s disease: study protocol for a multicentre, randomized, double-blind, placebo-controlled clinical trial

BackgroundParkinson’s disease (PD) is a progressive neurodegenerative disease common in aged populations. Classified by Hoehn & Yahr stages, patients are often divided into mild/early stage, moderate/middle stage, and advanced/late stage. With disease progression, PD shows high heterogeneity in each stage. Based on traditional Chinese medicine (TCM) syndrome differentiation theory and our previous works, we found that during the early stage, the main syndrome is Yin deficiency of the liver and kidney; during the moderate stage, the main syndromes are phlegm heat and wind stirring and blood stasis and wind stirring; and during the late stage, the dominant syndromes are deficiency of Yin and Yang and deficiency of Qi and blood. Hence, we proposed a new model of TCM treatment by the stage of PD. Based on Shudi Pingchan formula, an experimental formula of our team, we developed Ziyin Pingchan formula, Jiedu Pingchan formula, and Fuzheng Pingchan formula to treat each stage. This study is designed to evaluate the therapeutic effect of treating Parkinson’s disease by stages using traditional Chinese medicine and to provide an evidence base for forming a standardized scheme of diagnosis and treatment.MethodsThis study is designed as a multicentre, randomized, double-blind, placebo-controlled clinical trial. Patients will be stratified into 3 subgroups according to Hoehn & Yahr stage; 172, 168, and 72 participants will be required to be in the mild PD, moderate PD, and advanced PD subgroups, respectively, and will be randomized into the treatment or control group at a 1:1 ratio. The mild PD subgroup will receive a 48-week intervention, and the other 2 groups will receive a 24-week intervention. All groups will have a follow-up visit 12 weeks after starting the intervention. The intervention group will receive the Ziyin Pingchan formula, Jiedu Pingchan formula, or Fuzheng Pingchan formula, and the control group will receive the corresponding placebo. The primary outcomes will be the first addition of levodopa for the mild PD subgroup, the duration of the “OFF” period for the moderate PD subgroup, and the Parkinson's Disease Questionnaire (PDQ-39) for the advanced PD subgroup. The secondary outcomes will also be verified by subgroups, including the Unified Parkinson’s Disease Rating Scale (UPDRS), Parkinson’s Disease Sleep Scale-2 (PDSS-2), scales for Outcomes in Parkinson’s Disease—Autonomic (SCOPA-AUT), and the nonmotor symptom scale (NMSS).Expected outcomesTo our knowledge, this is the first trial to combine TCM syndrome differentiation with PD clinical stages and put it into clinical practice. The results of this trial will provide clinical evidence for the therapeutic effect of TCM formulas on PD patients of all stages and help build a new TCM treatment by stage model of PD.Trial registration: This trial is registered at the Chinese Clinical Trial Registry (http://www.chictr.org.cn/). Registration number: ChiCTR2200056373, Date: 2022–02-04, version 1.

An ocean modeling study to quantify wind forcing and oceanic mixing effects on the tropical North Pacific subsurface warm bias in CMIP and OMIP simulations

Sea surface temperature (SST) bias in the climate models has been a focus in the past, but subsurface temperature biases have not received much attention yet. In this study, subsurface temperature biases in the tropical North Pacific (TNP) are investigated by analyzing the CMIP6, CMIP5 and OMIP products, and by performing ocean model simulations. It is found that almost all the CMIP and OMIP simulations have a pronounced subsurface warm bias (SWB) in the northeastern tropical Pacific (NETP), and the model developments over the past decade do not indicate obvious improvements in bias pattern and magnitude from CMIP5 to the latest version CMIP6. This SWB is primarily caused by the model deficiencies in the simulated surface wind stress curl (WSC) in the NETP, which is too weak to produce a sufficient Ekman upwelling, a bias that also exists in OMIP simulations. The uncertainties in the parameterizations of the oceanic vertical mixing processes also make a great contribution, and it is demonstrated that the estimated oceanic vertical diffusivities are overestimated both in the upper boundary layer and the interior in the CMIP and OMIP simulations. The relationships between the SWB and the misrepresented oceanic vertical mixing processes are investigated by conducting several ocean-only experiments, in which the upper boundary layer mixing is modified by reducing the wind stirring effect in the Kraus-Turner type bulk mixed-layer scheme, and the interior mixing is constrained by using the Argo-derived diffusivity. By applying these modifications to oceanic vertical mixing schemes, the SWB is greatly reduced in the NETP. The consequences of this SWB are further analyzed. Because the thermal structure in the NETP can influence the simulations of oceanic circulations and equatorial upper-ocean thermal structure, the large SWB in the CMIP6 models tends to produce a weak equatorward water transport in the subsurface TNP, a weak equatorial upwelling and a warm equatorial upper ocean.

CO2‐Induced Decoupling of Tropical Surface and Thermocline Water Temperature at the Onset of Interglacials

AbstractTropical air‐sea interaction is important in global climate change; its behavior over geological history is poorly understood but can be explored by examining reconstructed sea surface temperature (SST) and thermocline water temperature (TWT). Here, deglacial‐interglacial air‐sea interactions over the past 500 ka were studied by comparing UK'37‐derived SST0‐30m and TEXH86‐derived TWT75m in a sediment core from the southern South China Sea. During deglacials, SST0‐30m and TWT75m varied synchronously toward interglacial peaks, while during the peak interglacials, TWT75m decreased earlier than SST0‐30m. These changes have been found ubiquitously in tropical oceans during the last two glacial‐interglacial cycles. We propose that the prolonged warm interglacial SST was probably sustained by the natural CO2 “overshoot” greenhouse effect at the end of most terminations. The early interglacial TWT decrease following local insolation is driven by the decreasing downward heat transport efficiency induced by weakening wind stirring.

Dissolved Oxygen Content in the Reservoirs of the Volga River

Dissolved oxygen content determined by the Winkler method and water saturation by oxygen were studied in the reservoirs of the Volga River in summer 2015 and 2016 in the surface and near bottom layers of water. In the reservoirs of the upper reaches of the Volga River, including the Uglich and Ivankovo reservoirs, oxygen content in the water rather often was lower than its limiting permissible concentration for water bodies used for fishery. Significant oxygen deficiency was observed in hypolimnion at all stations of the Uglich Reservoir. Rather often (in 10-45% of cases) its content was lower than 2 mg/L, whereas in the Ivankovo Reservoir - in 75% of cases. In the reservoirs of the middle and lower reaches of the Volga River, dissolved oxygen content was within the normal range favorable to the development of hydrobionts, which was conditioned by intensive wind stirring and high current velocity, especially in the lower reaches of the river. During the period of investigations in the surface layer of water of the lake section of the Gorky Reservoir, dissolved oxygen content was higher than that in its river section. In the other studied reservoirs, the distribution of dissolved oxygen over their area of water was rather uniform.

Relationship between the southern Yellow Sea Cold Water Mass and the distribution and composition of suspended particulate matter in summer and autumn seasons

The Yellow Sea is an important channel for transporting terrigenous materials from the East Asian continent to the western Pacific Ocean. Within the Yellow Sea, the Yellow Sea Cold Water Mass (YSCWM) is the most typical hydrological phenomenon during the warm half-year. Clarifying the relationship between suspended particulate matter and the YSCWM is a prerequisite for understanding the source-to-sink sedimentary system in the Yellow Sea. This study analyzed the suspended particulate matter concentrations in the southern Yellow Sea in the summer and autumn of 2012 and used conductivity, temperature and depth data collected at 32 hydrographic stations to study the impact of the southern YSCWM (SYSCWM) on suspended particulate matter. In summer, the tidal mixing front, induced by the SYSCWM and tidal mixing, limited the spread of suspended particulate matter from coastal areas to offshore areas, which led to the development of a suspended particulate matter concentration front in sloping areas. The pycnocline obstructed the vertical diffusion of nutrients in the SYSCWM, resulting in a subsurface chlorophyll maximum layer forming in the pycnocline. In autumn, abundant nutrients were released from the SYSCWM due to northerly wind stirring, which promoted the growth of phytoplankton. The wind stirring also deepened the pycnocline, and the pycnocline blocked the upward diffusion of resuspended particulate matter from the bottom. This paper provides a mechanistic model that reveals the relationship between the SYSCWM and suspended particulate matter based on the above conclusions.

Geostrophic Spirals Generated by the Horizontal Diffusion of Vortex Stretching in the Yellow Sea

Horizontal velocity spirals with a clockwise rotation (downward looking) rate of 1.7° m−1, on average, were observed in the western and northern Yellow Sea from December 2006 to February 2007. With the observed thermal wind relation, the beta-spiral theory was used to explain the dynamics of spirals. It was found that the horizontal diffusion of geostrophic vortex stretching is likely to be a major mechanism for generating geostrophic spirals. Vertical advection associated with surface/bottom Ekman pumping and topography-induced upwelling is too weak to support these spirals. Strong wind stirring and large heat loss in wintertime lead to weak stratification and diminish the effects of vertical advection. The cooling effect and vertical diffusion are offset by an overwhelming contribution of horizontal diffusion in connection with vortex stretching. The Richardson number-dependent vertical eddy diffusivity reaches a magnitude of 10−4 m2 s−1 on average. An eddy diffusivity of 2870 m2 s−1 is required for dynamic balance by estimating the residual term. This obtained value of 10−4 m2 s−1 is in good agreement with the estimation in terms of observed eddy activities. The suppressed and unsuppressed diffusivities in the observation region are 2752 and 2881 m2 s−1, respectively, which supports a closed budget for velocity rotation.

Scaling wind stirring effects in an oceanic bulk mixed layer model with application to an OGCM of the tropical Pacific

The mixed layer depth (MLD) plays an important role in the climate system through its influences on sea surface temperature (SST). The Kraus–Turner–Niiler (hereafter referred to as KTN) bulk mixed layer (ML) model is designed for describing the MLD and has been adopted widely by many ocean modeling. However, large biases exist in the MLD simulation using the original KTN model in the tropical Pacific. This is partly due to the uncertainties in representing wind stirring effect in the model, which is scaled by a parameter (m0). Traditionally, m0 is taken as a constant uniformly in space. In this study, the m0 is estimated as spatially and seasonally varying through its inverse calculation from a balance equation describing the turbulent kinetic energy (TKE) budget of the ML. It is illustrated that the m0 is spatially varying over the tropical Pacific. The derived m0 fields are then embedded into an ocean general circulation model (OGCM). Compared with the observations and the global ocean data assimilation system (GODAS) analyses, the MLD simulations in the OGCM with varying m0 are substantially improved in the tropical Pacific Ocean on seasonal and interannual time scales. Additionally, the Pacific subtropical cells (STCs) become intensified, accompanied with the strengthening of upwelling in the eastern equatorial Pacific; thus, more realistic simulations are obtained in spatially and seasonally varying m0 case compared with the constant m0 case. As the related cooling effect from the upwelling is enhanced, the simulated SST is slightly cooled down in the eastern equatorial Pacific. Further applications and implications are also discussed.

Traditional Chinese Medicine symptom pattern analysis for Parkinson's disease

ObjectiveTo use the theory of Traditional Chinese Medicine (TCM) to identify the major symptom patterns of Parkinson's disease. MethodsJournal databases were searched for relevant articles in the last 30 years. Articles were reviewed for symptom patterns of Parkinson's disease and analyzed using frequency analysis, cluster analysis, and other methods of data extraction. ResultsThe analyses indicated that the most frequent symptom patterns of Parkinson's disease are yin deficiency of kidney and liver, deficiency of Qi and blood, phlegm heat and wind stirring, blood stasis and wind stirring, and deficiency of yin and yang. ConclusionTaken together, the analyses identified the primary symptom patterns of Parkinson's disease as Yin deficiency of kidney and liver, deficiency of Qi and blood, phlegm heat and wind stirring, and blood stasis and wind stirring.

Role of barrier layer in the developing phase of “Category 6” super typhoon Haiyan

With the remarkable intensity of 170 knots, Typhoon Haiyan started as a tropical depression on November 3 and developed to the peak as super tropical cyclone (TC) on November 7 in 2013. This intensity makes Haiyan one of the strongest TCs record ever observed and 35 knots higher than the maximum of the existing highest category. Haiyan originated from the eastern part of the Northwest Pacific Warm Pool and moved westward over warm water with a thick barrier layer (BL). The BL reduced the vertical mixing and entrainment caused by Haiyan and prevented the cold thermocline water into the mixed layer (ML). As a result, sea temperature cooling associated with wind stirring was suppressed. Relative high sea surface temperature (SST) kept fueling Haiyan via latent heat flux release, which favored the rapid development of a “Category 6” super typhoon

Role of barrier layer in the developing phase of “Category 6” super typhoon Haiyan

With the remarkable intensity of 170 knots, Typhoon Haiyan starts as a tropical depression on November 3 and develops to the peak as super tropical cyclone (TC) on November 7 in 2013. This intensity makes Haiyan one of the strongest TCs record ever observed and 35 knots higher than the maximum of the existing highest category. Haiyan originated from the eastern part of the Northwest Pacific Warm Pool and moved westward over warm water with a thick barrier layer (BL). The BL reduced the vertical mixing and entrainment caused by Haiyan and prevented the cold thermocline water into the mixed layer (ML). As a result, sea temperature cooling associated with wind stirring was suppressed. Relative high sea surface temperature (SST) kept fueling Haiyan via latent heat flux release, which favored the rapid development of a “Category 6” super typhoon.

Erratum to: Inhibition of mixed-layer deepening during winter in the northeastern Arabian Sea by the West India Coastal Current

Though the deep mixed layers (MLs) that form in the northeastern Arabian Sea (NEAS) during the winter monsoon (November–February) have been attributed to convective mixing driven by dry, cool northeasterly winds from the Indian subcontinent, data show that the deepest MLs occur in the northern NEAS and the maxima of latent-heat and net heat fluxes in the southern NEAS. We use an oceanic general circulation model to show that the deep MLs in the NEAS extend up to ~20°N till the end of December, but are restricted poleward of ~22°N (~23°N) in January (February). This progressive restriction of the deep mixed layers within the NEAS is due to poleward advection of water of lower salinity by the West India Coastal Current (WICC). The deep MLs are sustained till February in the northern NEAS because convective mixing deepens the ML before the waters of lower salinity reach this region and the wind stirring and convective overturning generate sufficient turbulent energy for the ML to maintain the depth attained in January. Though the atmospheric fluxes tend to cool the ML in the southern NEAS, this cooling is countered by the warming due to horizontal advection. Likewise, the cooling due to entrainment, which continues in the southern NEAS even as the ML shallows during January–February, is almost cancelled by the warming caused by a downwelling vertical velocity field. Therefore, the SST changes very little during December–February even as the ML shallows dramatically in the southern NEAS. These deep MLs of the NEAS also preclude a strong intraseasonal response to the intraseasonal variability in the fluxes. This role of horizontal advection implies that the ML depth in the NEAS is determined by an interplay of physical processes that are forced differently. The convective mixing depends on processes that are local to the region, but the advection is due to the WICC, whose seasonal cycle is primarily forced by remote winds. By inhibiting the formation of deep MLs in the southern NEAS, the WICC limits the region of formation of the high-salinity water masses of this region. Since the deep MLs in the NEAS have been linked to the high chlorophyll concentration there, our results imply that the conventional approach of averaging over boxes for studying the impact of physics on biogeochemistry can mask important details that are due to advection because it is the advective component of any budget that is most affected by the averaging process.

Dynamics of the periphery current in Rhode Island Sound

Observations reveal a near-surface circulation around the periphery of Rhode Island Sound (RIS) that occurs in summer stratified conditions and disappears in winter when weak solar insolation and wind stirring result in strong vertical mixing. According to a series of numerical simulations and theoretical analysis, we attribute the summer intensification of this “periphery current” to a circulation produced by seasonal bottom thermal fronts often observed from May to September. The strength of the thermal fronts is proportional to the surface solar radiation. Meanwhile, the simulations capture a continuous topographically rectified tidal residual current in RIS and a pair of opposite-sign headland eddies around Montauk Point in Block Island Sound (BIS). Our analysis suggests the importance of nonlinear vorticity advection and frictional torques in BIS. On the other hand, the vorticity balance shows the importance of velocity torque by contributing to the opposite headland gyres, while the effect by planetary vorticity stretching is negligible over BIS but important in RIS.

Cruise observation and numerical modeling of turbulent mixing in the Pearl River estuary in summer

The turbulent mixing in the Pearl River estuary and plume area is analyzed by using cruise data and simulation results of the Regional Ocean Model System (ROMS). The cruise observations reveal that strong mixing appeared in the bottom layer on larger ebb in the estuary. Modeling simulations are consistent with the observation results, and suggest that inside the estuary and in the near-shore water, the mixing is stronger on ebb than on flood. The mixing generation mechanism analysis based on modeling data reveals that bottom stress is responsible for the generation of turbulence in the estuary, for the re-circulating plume area, internal shear instability plays an important role in the mixing, and wind may induce the surface mixing in the plume far-field. The estuary mixing is controlled by the tidal strength, and in the re-circulating plume bulge, the wind stirring may reinforce the internal shear instability mixing.

Analyses of intermittent mixing and stratification within the North Passage of the Changjiang (Yangtze) River estuary, China: A three-dimensional model study

The TELEMAC-3D, incorporating a stability function, and the potential energy anomaly equation (ϕ-equation), are used to analyze neap-spring tidal and intratidal variability of intermittent mixing and stratification within the North Passage of the Changjiang (Yangtze) River estuary in the wet season. Eight terms in the ϕ-equation are used to examine physical mechanisms and the relative importance of each term for the lower reach of the North Passage. As revealed by the gradient Richardson number (Ri), the Simpson number (Si) and the potential energy anomaly (ϕ), weak mixing and persistent stratification appear on a neap tide, while strong mixing and periodic stratification on a spring tide within the main channel in the middle and lower reaches of the North Passage. The landward subtidal flow is much stronger on a neap tide than that on a spring tide. Within the main channel in the lower reach, large magnitude of longitudinal ϕ-advection (Au) reflects the important effect of saltwedge movement on stratification. Large magnitude of lateral ϕ-advection (Av) may be enhanced by large lateral gradient of ϕ due to the complex bathymetry and artificial structures. Both longitudinal (Au) and lateral ϕ-advections (Av) are temporally and spatially intermittent. Large longitudinal depth-mean straining (Bu) overlays the combined effect of tidal straining, circulation and river discharge. Large lateral depth-mean straining (Bv) is generated by large lateral density gradient interacting with the shear flow. The magnitude of integrated vertical turbulent buoyancy flux (E) mainly depends on tidal stirring at the bottom, while wind stirring at the surface and shear instability at the pycnocline are secondary contributors. The magnitudes of the other physical mechanisms including longitudinal non-mean straining (Cu), lateral non-mean straining (Cv) and vertical advection (D) are relatively smaller than those above. Neap-spring tidal variability of mixing and stratification mainly results from combined effect of three principal physical mechanisms, i.e. tidal stirring, longitudinal (Bu) and lateral depth-mean strainings (Bv). Intratidal variability of mixing and stratification is apparent on a spring tide. It seems that Advection, Straining and Stirring Induced Periodic Stratification (ASSIPS), rather than Advection and Straining Induced Periodic Stratification (ASIPS) and Straining Induced Periodic Stratification (SIPS), controls intratidal variability of mixing and stratification within the North Passage.