scholarly journals Temporal evolution of the momentum balance terms and frictional adjustment observed over the inner shelf during a storm

Ocean Science ◽  
2016 ◽  
Vol 12 (1) ◽  
pp. 137-151 ◽  
Author(s):  
M. Grifoll ◽  
A. L. Aretxabaleta ◽  
J. L. Pelegrí ◽  
M. Espino
Keyword(s):  
Coriolis Force ◽  
Wind Stress ◽  
Water Depth ◽  
Momentum Balance ◽  
Wave Radiation ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Inner Shelf ◽  
Coastal Waves ◽  
Nw Mediterranean

Abstract. We investigate the rapidly changing equilibrium between the momentum sources and sinks during the passage of a single two-peak storm over the Catalan inner shelf (NW Mediterranean Sea). Velocity measurements at 24 m water depth are taken as representative of the inner shelf, and the cross-shelf variability is explored with measurements at 50 m water depth. During both wind pulses, the flow accelerated at 24 m until shortly after the wind maxima, when the bottom stress was able to compensate for the wind stress. Concurrently, the sea level also responded, with the pressure-gradient force opposing the wind stress. Before, during and after the second wind pulse, there were velocity fluctuations with both super- and sub-inertial periods likely associated with transient coastal waves. Throughout the storm, the Coriolis force and wave radiation stresses were relatively unimportant in the along-shelf momentum balance. The frictional adjustment timescale was around 10 h, consistent with the e-folding time obtained from bottom drag parameterizations. The momentum evolution at 50 m showed a larger influence of the Coriolis force at the expense of a decreased frictional relevance, typical in the transition from the inner to the mid-shelf.

scholarly journals Shifting momentum balance and frictional adjustment observed over the inner-shelf during a storm

2015 ◽  
Vol 12 (3) ◽  
pp. 897-924
Author(s):  
M. Grifoll ◽  
A. Aretxabaleta ◽  
J. L. Pelegrí ◽  
M. Espino
Keyword(s):  
Wind Stress ◽  
Water Depth ◽  
Momentum Balance ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Inner Shelf ◽  
Bottom Stress ◽  
Nw Mediterranean ◽  
High Kinetic Energy ◽  
Nw Mediterranean Sea

Abstract. We investigate the rapidly changing equilibrium between the momentum sources and sinks during the passage of a two-peak storm over the Catalan inner-shelf (NW Mediterranean Sea). Velocity measurements at 24 m water depth are taken as representative of the inner shelf, and the cross-shelf variability is explored with additional measurements at 50 m water depth. At 24 m, as the storm-related wind stress accelerated the flow, velocity increased throughout the water column, resulting in bottom stress starting to become important. The sea level also responded, with the pressure gradient force opposing the wind stress. In particular, during the second wind pulse, there were rapid oscillations in the acceleration and advective terms, apparently reflecting the incapacity of the bottom stress to dissipate the high kinetic energy of the system. The Coriolis and wave induced terms (via radiation stresses) were less important in the momentum balance. The frictional adjustment time scale was around 10 h, consistent with the e-folding time obtained from bottom drag parameterizations. Estimates of the frictional time and Ekman depth confirm the prevailing frictional response at 24 m. The momentum evolution in deeper parts of the shelf (50 m) showed an increase in the Coriolis force at the expense of the frictional term, typical in the transition from the inner to the mid-shelf.

scholarly journals Zonal Momentum Balance in the Tropical Atmospheric Circulation during the Global Monsoon Mature Months

2013 ◽  
Vol 70 (2) ◽  
pp. 583-599 ◽  
Author(s):  
Wenchang Yang ◽  
Richard Seager ◽  
Mark A. Cane
Keyword(s):  
Angular Momentum ◽  
Pressure Gradient ◽  
Atmospheric Circulation ◽  
Coriolis Force ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Global Monsoon ◽  
Upper Troposphere ◽  
Nonlinear Advection ◽  
Zonal Momentum

Abstract In this paper, zonal momentum balances of the tropical atmospheric circulation during the global monsoon mature months (January and July) are analyzed in three dimensions based on the ECMWF Interim Re-Analysis (ERA-Interim). It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure gradient force, the Coriolis force, and friction. The nonlinear advection term plays a significant role only in the Asian summer monsoon regions within the ABL. In the upper troposphere, the pressure gradient force, the Coriolis force, and the nonlinear advection are the dominant terms. The transient eddy force and the residual force (which can be explained as convective momentum transfer over open oceans) are secondary, yet cannot be neglected near the equator. Zonal-mean equatorial upper-troposphere easterlies are maintained by the absolute angular momentum advection associated with the cross-equatorial Hadley circulation. Equatorial upper-troposphere easterlies over the Asian monsoon regions are also controlled by the absolute angular momentum advection but are mainly maintained by the pressure gradient force in January. The equivalent linear Rayleigh friction, which is widely applied in simple tropical models, is calculated and the corresponding spatial distribution of the local coefficient and damping time scale are estimated from the linear regression. It is found that the linear momentum model is in general capable of crudely describing the tropical atmospheric circulation dynamics, yet the caveat should be kept in mind that the friction coefficient is not uniformly distributed and is even negative in some regions.

scholarly journals The Deflection of the China Coastal Current over the Taiwan Bank in Winter

2018 ◽  
Vol 48 (7) ◽  
pp. 1433-1450 ◽  
Author(s):  
Enhui Liao ◽  
Lie Yauw Oey ◽  
Xiao-Hai Yan ◽  
Li Li ◽  
Yuwu Jiang
Keyword(s):  
Pressure Gradient ◽  
Wind Stress ◽  
Large Scale ◽  
Numerical Models ◽  
Gradient Force ◽  
Coastal Current ◽  
Pressure Gradient Force ◽  
Bottom Stress ◽  
In Situ Data ◽  
Offshore Flow

AbstractIn winter, an offshore flow of the coastal current can be inferred from satellite and in situ data over the western Taiwan Bank. The dynamics related to this offshore flow are examined here using observations as well as analytical and numerical models. The currents can be classified into three regimes. The downwind (i.e., southward) cold coastal current remains attached to the coast when the northeasterly wind stress is stronger than a critical value depending on the upwind (i.e., northward) large-scale pressure gradient force. By contrast, an upwind warm current appears over the Taiwan Bank when the wind stress is less than the critical pressure gradient force. The downwind coastal current and upwind current converge and the coastal current deflects offshore onto the bank during a moderate wind. Analysis of the vorticity balance shows that the offshore transport is a result of negative bottom stress curl that is triggered by the positive vorticity of the two opposite flows. The negative bottom stress curl is reinforced by the gentle slope over the bank, which enhances the offshore current. Composite analyses using satellite observations show cool waters with high chlorophyll in the offshore current under moderate wind. The results of composite analyses support the model findings and may explain the high productivity over the western bank in winter.

Do Eddies Play a Role in the Momentum Balance of the Leeuwin Current?

2005 ◽  
Vol 35 (6) ◽  
pp. 964-975 ◽  
Author(s):  
Ming Feng ◽  
Susan Wijffels ◽  
Stuart Godfrey ◽  
Gary Meyers
Keyword(s):  
Kinetic Energy ◽  
Pressure Gradient ◽  
Reynolds Stress ◽  
Wind Stress ◽  
Mesoscale Eddies ◽  
Eddy Kinetic Energy ◽  
Momentum Balance ◽  
Gradient Force ◽  
Leeuwin Current ◽  
Eastern Boundary

Abstract The Leeuwin Current is a poleward-flowing eastern boundary current off the western Australian coast, and alongshore momentum balance in the current has been hypothesized to comprise a southward pressure gradient force balanced by northward wind and bottom stresses. This alongshore momentum balance is revisited using a high-resolution upper-ocean climatology to determine the alongshore pressure gradient and altimeter and mooring observations to derive an eddy-induced Reynolds stress. Results show that north of the Abrolhos Islands (situated near the shelf break between 28.2° and 29.3°S), the alongshore momentum balance is between the pressure gradient and wind stress. South of the Abrolhos Islands, the Leeuwin Current is highly unstable and strong eddy kinetic energy is observed offshore of the current axis. The alongshore momentum balance on the offshore side of the current reveals an increased alongshore pressure gradient, weakened alongshore wind stress, and a significant Reynolds stress exerted by mesoscale eddies. The eddy Reynolds stress has a −0.5 Sv (Sv ≡ 106 m3 s−1) correction to the Indonesian Throughflow transport estimate from Godfrey’s island rule. The mesoscale eddies draw energy from the mean current through mixed barotropic and baroclinic instability, and the pressure gradient work overcomes the negative wind work to supply energy for the instability process. Hence the anomalous large-scale pressure gradient in the eastern Indian Ocean drives the strongest eddy kinetic energy level among all the midlatitude eastern boundary currents.

scholarly journals Rapid dynamical evolution of ITCZ events over the east Pacific

2021 ◽  
pp. 1-48
Author(s):  
Marie C. McGraw ◽  
James G. Larson
Keyword(s):  
Coriolis Force ◽  
Dynamical Evolution ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Surface Dynamics ◽  
Near Surface ◽  
East Pacific ◽  
Dynamical Features ◽  
East Pacific Ocean ◽  
Meridional Advection

Abstract The latitudinal location of the east Pacific Ocean intertropical convergence zone (ITCZ) changes on time scales of days to weeks during boreal spring. This study focuses on tropical near-surface dynamics in the days leading up to the two most frequent types of ITCZ events, nITCZ (Northern Hemisphere) and dITCZ (double). There is a rapid, daily evolution of dynamical features on top of a slower, weekly evolution that occurs leading up to and after nITCZ and dITCZ events. Zonally-elongated bands of anomalous cross-equatorial flow and off-equatorial convergence rapidly intensify and peak one day before or the day of these ITCZ events, followed one or two days later by a peak in near-equatorial zonal wind anomalies. In addition, there is a wide region north of the southeast Pacific subtropical high where anomalous northwesterlies strengthen prior to nITCZ events and southeasterlies strengthen before dITCZ events. Anomalous zonal and meridional near-surface momentum budgets reveal that the terms associated with Ekman balance are of first-order importance preceding nITCZ events, but that the meridional momentum advective terms are just as important before dITCZ events. Variations in cross-equatorial flow are promoted by the meridional pressure gradient force (PGF) prior to nITCZ events and the meridional advection of meridional momentum in addition to the meridional PGF before dITCZ events. Meanwhile, variations in near-equatorial easterlies are driven by the zonal PGF and the Coriolis force preceding nITCZ events and the zonal PGF, the Coriolis force, and the meridional advection of zonal momentum before dITCZ events.

scholarly journals Responses of estuarine circulation to the morphological evolution in a convergent, microtidal estuary

2021 ◽  
Author(s):  
Rui Zhang ◽  
Bo Hong ◽  
Lei Zhu ◽  
Wenping Gong ◽  
Heng Zhang
Keyword(s):  
Human Activities ◽  
Morphological Evolution ◽  
Estuarine Circulation ◽  
Longitudinal Vortex ◽  
Vertical Shear ◽  
Momentum Balance ◽  
Dominant Factor ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Lateral Circulation

Abstract. The Huangmaohai Estuary (HE) is a funnel-shaped microtidal estuary in the west of the Pearl River Delta (PRD) in southern China. Since China's reform and opening up in 1978, extensive human activities have occurred and greatly changed the estuary's topography, and modified its hydrodynamics. In this study, we examined the morphological evolution by analyzing remote sensing data with ArcGIS tools and studied the responses of hydrodynamics to the changes in topography from 1977 to 2010 by using the Delft3d model. We took the changes in estuarine circulation during neap tides in dry seasons as an example. The results show that human reclamation caused a narrowing of the estuary, and channel dredging deepened the estuary. These human activities changed both the longitudinal and lateral estuarine circulations. The longitudinal circulation was observed to increase with the deepening and narrowing of the estuary. The lateral circulation experienced changes in both the magnitude and pattern. The momentum balance analysis shows that when the depth and width changed simultaneously, the longitudinal estuarine circulation was modulated by both the channel deepening and width reduction, in which the friction, pressure gradient force, and advection terms were altered. The analysis of the longitudinal vortex dynamics indicates that the changes in the vertical shear of the longitudinal flow, lateral salinity gradient, and vertical mixing were responsible for the change in the lateral circulation. The changes in water depth are the dominant factor affecting lateral circulation intensity. This study has implications for sediment transport and morphological evolution in estuaries heavily impacted by human interventions.

Modeling Study of Variable Upwelling Circulation in the East China Sea: Response to a Coastal Promontory

2014 ◽  
Vol 44 (4) ◽  
pp. 1078-1094 ◽  
Author(s):  
Zhiqiang Liu ◽  
Jianping Gan
Keyword(s):  
East China Sea ◽  
East China ◽  
Momentum Balance ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
The East China Sea ◽  
Bottom Stress ◽  
China Sea ◽  
Vorticity Advection ◽  
Nonlinear Advection

Abstract A three-dimensional, high-resolution numerical model is used to investigate processes and dynamics of an intensified upwelling that is induced by a coastal promontory over the East China Sea (ECS) shelf. The center of the intensified upwelling around the promontory has been constantly observed, but, so far, it has been dynamically unexplained. Forced by an idealized southeasterly wind stress, the model results well capture the observed upwelling at the lee of the coastal promontory. The intensified upwelling is formed by a strengthened shoreward transport downstream of the promontory as the upwelling jet veers shoreward. The jet is mainly controlled by a cross-shore geostrophic balance and is largely modulated by both centrifugal acceleration associated with nonlinear advection and by bottom stress. The strengthened shoreward transport is mainly attributed to the cross-shore geostrophic current that is induced by a countercurrent (negative) pressure gradient force (PGF) and partly attributed to the bottom Ekman transport. Based on the analyses of the momentum balance and depth-integrated vorticity dynamics, the authors provide a new explanation for the origin of negative PGF. It is found that the countercurrent PGF is generated by negative bottom stress curl and strengthened by negative vorticity advection downstream of the promontory. While the negative bottom stress curl arises from bottom shear vorticity, the source of negative advection downstream of the promontory is the negative shear vorticity on the seaside of the shoreward-bent jet. Nevertheless, cyclonic curvature vorticity at the bottom and positive vorticity advection in the water column at the promontory weakens the negative PGF. Although nonlinear advection strengthens vorticity advection, it weakens bottom stress curl and has little net effect on the countercurrent PGF.

scholarly journals Coastal Trapped Waves, Alongshore Pressure Gradients, and the California Undercurrent*

2014 ◽  
Vol 44 (1) ◽  
pp. 319-342 ◽  
Author(s):  
Thomas P. Connolly ◽  
Barbara M. Hickey ◽  
Igor Shulman ◽  
Richard E. Thomson
Keyword(s):  
Pressure Gradient ◽  
Wind Stress ◽  
West Coast ◽  
Current System ◽  
Ocean Model ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Pressure Gradients ◽  
Transport Pathway ◽  
California Current System

Abstract The California Undercurrent (CUC), a poleward-flowing feature over the continental slope, is a key transport pathway along the west coast of North America and an important component of regional upwelling dynamics. This study examines the poleward undercurrent and alongshore pressure gradients in the northern California Current System (CCS), where local wind stress forcing is relatively weak. The dynamics of the undercurrent are compared in the primitive equation Navy Coastal Ocean Model and a linear coastal trapped wave model. Both models are validated using hydrographic data and current-meter observations in the core of the undercurrent in the northern CCS. In the linear model, variability in the predominantly equatorward wind stress along the U.S. West Coast produces episodic reversals to poleward flow over the northern CCS slope during summer. However, reproducing the persistence of the undercurrent during late summer requires additional incoming energy from sea level variability applied south of the region of the strongest wind forcing. The relative importance of the barotropic and baroclinic components of the modeled alongshore pressure gradient changes with latitude. In contrast to the southern and central portions of the CCS, the baroclinic component of the alongshore pressure gradient provides the primary poleward force at CUC depths over the northern CCS slope. At time scales from weeks to months, the alongshore pressure gradient force is primarily balanced by the Coriolis force associated with onshore flow.

scholarly journals A discussion on the existence of the anomalous high and the anomalous low

2015 ◽  
Vol 33 (10) ◽  
pp. 1253-1261
Author(s):  
N. Li
Keyword(s):  
Pressure Gradient ◽  
Centrifugal Force ◽  
Coriolis Force ◽  
Air Flow ◽  
Formation Mechanisms ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Gradient Wind ◽  
Reasonable Explanation ◽  
Classic Literature

Abstract. The air flow in a three-way balance between the Coriolis force, the centrifugal force and the pressure gradient force, i.e., the gradient wind, is discussed. The author studies formation mechanisms and possible existence of four types of gradient wind (the normal high, the normal low, the anomalous high and the anomalous low), and proposes reasonable explanation of the evolution of the gradient wind, especially for the anomalous high and the anomalous low, both of which are considered to be pure mathematical solutions and are overlooked in classic literature.

scholarly journals Axial Wind Effects on Stratification and Longitudinal Salt Transport in an Idealized, Partially Mixed Estuary*

2009 ◽  
Vol 39 (8) ◽  
pp. 1905-1920 ◽  
Author(s):  
Shih-Nan Chen ◽  
Lawrence P. Sanford
Keyword(s):  
Wind Stress ◽  
Salinity Gradient ◽  
Ocean Model ◽  
Salt Transport ◽  
Salt Flux ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Shear Dispersion ◽  
Wind Events ◽  
Wind Mixing

Abstract A 3D hydrodynamic model [Regional Ocean Model System (ROMS)] is used to investigate how axial wind influences stratification and to explore the associated longitudinal salt transport in partially mixed estuaries. The model is configured to represent a straight estuarine channel connecting to a shelf sea. The results confirm that wind straining of the along-channel salinity gradient exerts an important control on stratification. Two governing parameters are identified: the Wedderburn number (W) defined as the ratio of wind stress to axial baroclinic pressure gradient force, and the ratio of an entrainment depth to water depth (hs/H). Here W controls the effectiveness of wind straining, which promotes increases (decreases) in stratification during down-estuary (up-estuary) wind. The ratio hs/H determines the portion of the water column affected by direct wind mixing. While stratification is always reduced by up-estuary wind, stratification shows an increase-then-decrease transition when down-estuary wind stress increases. Such transition is a result of the competition between wind straining and direct wind mixing. A horizontal Richardson number modified to include wind straining/mixing is shown to reasonably represent the transition, and a regime diagram is proposed to classify the wind’s role on stratification. Mechanisms driving salt flux during axial wind events are also explored. At the onset and end of the wind events, barotropic adjustment drives strong transient salt fluxes. Net salt flux is controlled by the responses of subtidal shear dispersion to wind forcing. Moderate down-estuary winds enhance subtidal shear dispersion, whereas up-estuary winds always reduce it. Supporting observations from upper Chesapeake Bay are presented.

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