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1.
Dmitry V. Stepanov 《Ocean Dynamics》2018,68(10):1353-1370
Based on an eddy-permitting numerical model, the mesoscale variability in the East-Sakhalin Current is investigated during the winter-spring period. Analysis of necessary conditions for the development of baroclinic instability showed that the nearshore component of the East-Sakhalin Current is potentially baroclinic unstable in the first half-year. The simulated circulation uncovered a generation of anticyclonic eddies on the eastern Sakhalin shelf. It was established that a spatial scale of these eddies and the first baroclinic Rossby radius of deformation are values of the same order; a lifetime of these eddies varies from 4 to 6 weeks, given the Rossby number varies from 0.05 to 0.2. Analysis of the rate of eddy energy conversion on the eastern Sakhalin shelf showed that the generation of the revealed mesoscale eddies results from, mainly, baroclinic instability, whereas barotropic instability can be both favoring and preventing to the generation of these eddies. 相似文献
2.
In order to determine the maintenance mechanisms of the currents of the global ocean, this study investigates the budget of
the annual mean kinetic energy (KE) in a high-resolution (0.1° × 0.1°) semi-global ocean simulation. The analysis is based
on a separation of the mean KE using the barotropic (i.e., depth-averaged) and baroclinic (the residual) components of velocity.
The barotropic and baroclinic KEs dominate in higher and lower latitudes, respectively, with their global average being comparable
to each other. The working rates of wind forcing on the barotropic and baroclinic circulations in the global ocean are 243
and 747 gigawatts, respectively. This study presents at least three new results for the budget of the barotropic KE. Firstly,
an energy diagram is rederived to show that the work of the barotropic component of the horizontal pressure gradient (HPG)
is connected to the work related to the joint effect of baroclinicity and bottom relief (JEBAR), and then to the budget of
potential energy (PE). Secondly, the model analysis shows that the globally averaged work of the barotropic HPG (which is
connected to the work related to JEBAR and then to the budget of the PE) is nearly zero. This indicates that the wind- and
buoyancy-induced barotropic circulations in the global ocean are of the same strength with opposite sign. Thirdly, it is found
that the work of the wind forcing on the barotropic component of the simulated Antarctic Circumpolar Current (ACC) is canceled
by the combined effect, in equal measure, of the work of the barotropic HPG and the work of dissipative processes for mean
KE. This result makes a significant contribution to the discussion on the depth-integrated momentum balance of the ACC. The
barotropic KE is dissipated by the effects of bottom frictional stress, lateral frictional stress, and the Reynolds stress,
of which more than half is attributed to an unexpectedly large contribution from biharmonic horizontal friction. Future studies
should pay more attention to the role of biharmonic friction used in high-resolution numerical models. 相似文献
3.
Numerical model experiments have been performed to analyze the low-latitude baroclinic continental shelf response to a tropical cyclone. The theory of coastally trapped waves suggests that, provided appropriate slope, latitude, stratification and wind stress, bottom-intensified topographic Rossby waves can be generated by the storm. Based on a scale analysis, the Nicaragua Shelf is chosen to study propagating topographic waves excited by a storm, and a model domain is configured with simplified but similar geometry. The model is forced with wind stress representative of a hurricane translating slowly over the region at 6 km h−1. Scale analysis leads to the assumption that baroclinic Kelvin wave modes have minimal effect on the low-frequency wave motions along the slope, and coastal-trapped waves are restricted to topographic Rossby waves. Analysis of the simulated motions suggests that the shallow part of the continental slope is under the influence of barotropic topographic wave motions and at the deeper part of the slope baroclinic topographic Rossby waves dominate the low-frequency motions. Numerical solutions are in a good agreement with theoretical scale analysis. Characteristics of the simulated baroclinic waves are calculated based on linear theory of bottom-intensified topographic Rossby waves. Simulated waves have periods ranging from 153 to 203 h. The length scale of the waves is from 59 to 87 km. Analysis of energy fluxes for a fixed volume on the slope reveals predominantly along-isobath energy propagation in the direction of the group velocity of a topographic Rossby wave. Another model experiment forced with a faster translating hurricane demonstrates that fast moving tropical cyclones do not excite energetic baroclinic topographic Rossby waves. Instead, robust inertial oscillations are identified over the slope. 相似文献
4.
Energy distributions of the large-scale horizontal currents caused by wind in the baroclinic ocean 总被引：3，自引：0，他引：3
ZHOU Lei TIAN Jiwei & WANG Dongxiao . Physical Oceanography Laboratory Ocean University of China Qingdao China . Key Laboratory of Tropical Marine Environmental Dynamics South China Sea Institute of Oceanology Chinese Academy of Sciences Guangdong China 《中国科学D辑(英文版)》2005,48(12):2267-2275
Wind is the main energy source for the generation of the internal waves and the ocean mixing. Wunsch[1] estimated that about 1 TW (1 TW = 1012 W) energy was transported into the ocean from the winds by us-ing the altimeter data. Watanabe et al.[2] numerically calculated that the mixing processes obtained 0.7 TW energy from the global wind, which afforded most of the energy needed by the maintenance of the Merid-ional Overturning Circulation (MOC). During the past 50 years, in the Norther… 相似文献
5.
A cross-sectional model of an idealised constant depth gulf with a sill at its entrance, connected to a deep ocean, is used
to examine the barotropic and baroclinic response of the region to wind forcing. The role of the oceanic boundary condition
is also considered. Calculations show that in the case of a tall sill, where the pycnocline intersects the sill, the baroclinic
response of the gulf is similar to that of a lake, and internal waves cannot radiate energy out of the gulf. The barotropic
response shows free surface oscillations, with nodes located close to the centre of the oceanic basin and entrance to the
gulf, with associated barotropic resonant periods. As the sill height is reduced, baroclinic wave energy is radiated from
the gulf into the ocean, and the form of the baroclinic response changes from a standing wave (tall sill) as in a lake to
a progressive wave (no sill). The location of sea surface elevation nodes and resonant periods changes as the sill height
is reduced. Calculations of the barotropic resonant periods with and without stratification could not determine if they were
influenced by the presence of stratification, although published analytical theory suggests that they should be able to when
energy is lost from the gulf by internal wave radiation. This inability to detect changes in barotropic resonant period due
to stratification effects is due to the small change in resonant frequency produced by baroclinic effects, as shown by analytical
results, and the broad peak nature of the computed resonant frequency. In the case of a closed offshore boundary (an offshore
island), there is a stronger and narrower energy peak at the resonant frequency than when a barotropic radiation condition
is applied. However, the influence of stratification upon the resonant frequency could not be accurately determined. Although
the offshore boundary was well removed from the gulf to such an extent that any baroclinic waves reflected from it could not
reach the gulf within the integration period, it did, however, slightly influence the gulf baroclinic response due to its
influence on the barotropic response. 相似文献
6.
Kyoko Ohashi Jinyu Sheng Keith R. Thompson Charles G. Hannah Harold Ritchie 《Continental Shelf Research》2009
A numerical shelf circulation model was developed for the Scotian Shelf, using a nested-grid setup consisting of a three-dimensional baroclinic inner model embedded inside a two-dimensional barotropic outer model. The shelf circulation model is based on the Princeton Ocean Model and driven by three-hourly atmospheric forcing provided by a numerical weather forecast model and by tidal forcing specified at the inner model's open boundaries based on pre-calculated tidal harmonic constants. The outer model simulates the depth-mean circulation forced by wind and atmospheric pressure fields over the northwest Atlantic Ocean with a horizontal resolution of 1/12°. The inner model simulates the three-dimensional circulation over the Gulf of St. Lawrence, the Scotian Shelf, and the adjacent slope with a horizontal resolution of 1/16°. The performance of the shelf circulation model is assessed by comparing model results with oceanographic observations made along the Atlantic coast of Nova Scotia and in the vicinity of Sable Island (on the Scotian Shelf) during two periods: October 2000–March 2001 and April–June 2002. Analysis of model results on Sable Island Bank indicates that tidal currents account for as much as ∼80% of the total variance of near-bottom currents, and currents driven by local winds account for ∼30% of the variance of the non-tidal near-bottom currents. Shelf waves generated remotely by winds and propagating into the region also play an important role in the near-bottom circulation on the bank. 相似文献
7.
Observations of semidiurnal internal tidal currents from three moorings deployed on the continental shelf off central Chile during summer and winter of 2005 are reported. The spectra of the baroclinic currents showed large peaks at the semidiurnal band with a dominant counterclockwise rotation, which was consistent with internal wave activity. The amplitude of the barotropic tidal currents varied according to the spring–neap cycle following the sea level fluctuations. In contrast, the amplitudes of the internal tide showed high spatial-temporal variability not directly related to the spring–neap modulation. Near the middle of the continental shelf and near the coast (San Vicente Bay) the variance of the semidiurnal baroclinic current is larger than the variance of its barotropic counterpart. The vertical structure of the baroclinic tidal current fluctuations was similar to the structure of the first baroclinic internal wave mode. In general, in the three study sites the variance of the baroclinic current was larger near the surface and bottom and tended to show a minimum value at mid depths. Kinetic energy related to semidiurnal internal waves was larger in winter when stratification of the water column was stronger. During summer, upwelling and the decrease of freshwater input from nearby rivers reduced the vertical density stratification. The amplitude of the semidiurnal internal tide showed a tendency to be enhanced with increasing stratification as observed in other upwelling areas. The continental shelf break and submarine canyons, which limit the continental shelf in the alongshore direction, represent near-critical slopes for the semidiurnal period and are suggested to be the main internal tide generation sites in the study region. 相似文献
8.
The Camamu Bay (CMB) is located on the narrowest shelf along the South American coastline and close to the formation of two major Western Boundary Currents (WBC), the Brazil/North Brazil Current (BC/NBC). These WBC flow close to the shelf break/slope region and are expected to interact with the shelf currents due to the narrowness of the shelf. The shelf circulation is investigated in terms of current variability based on an original data set covering the 2002-2003 austral summer and the 2003 austral autumn. The Results show that the currents at the shelf are mainly wind driven, experiencing a complete reversal between seasons due to a similar change in the wind field. Currents at the inner-shelf have a polarized nature, with the alongshore velocity mostly driven by forcings at the sub-inertial frequency band and the cross-shore velocity mainly supra-inertially forced, with the tidal currents playing an important role at this direction. The contribution of the forcing mechanisms at the mid-shelf changes between seasons. During the summer, forcings in the two frequency bands are important to drive the currents with a similar contribution of the tidal currents. On the other hand, during the autumn season, the alongshore velocity is mostly driven by sub-inertial forcings and tidally driven currents still remain important in both directions. Moreover, during the autumn when the stratification is weaker, the response of the shelf currents to the wind forcing presents a barotropic signature. The meso-scale processes related to the WBC flowing at the shelf/slope region also affect the circulation within the shelf, which contribute to cause significant current reversals during the autumn season. Currents at the shelf-estuary connection are clearly supra-inertially forced with the tidal currents playing a key role in the generation of the along-channel velocities. The sub-inertial forcings at this location act mainly to drive the weak ebb currents which were highly correlated with both local and remote wind forcing during the summer season. 相似文献
9.
William R. Holland 《地球物理与天体物理流体动力学》2013,107(1):187-210
AbstractOne of the central unsolved theoretical problems of the large scale ocean circulation is concerned with explaining the very large transports measured in western boundary currents such as the Gulf Stream and the Kuroshio. The only theory up to now that can explain the size of these transports is that of non-linear recirculation in which the advective terms in the momentum equations became important near the western boundary. In this paper an alternative explanation is suggested. When bottom topography and baroclinic effects are included in a wind-driven ocean model it is shown that the western boundary current can have a transport larger than that predicted from the wind stress distribution even when the nonlinear advective terms are ignored. The explanation lies in the presence of pressure torques associated with bottom topography which can contribute to the vorticity balance in the same sense as the wind stress curl.Three numerical experiments have been carried out to explore the nature of this process using a three dimensional numerical model. The first calculation is done for a baroclinic ocean of constant depth, the second for a homogeneous ocean with an idealized continental slope topography, and the third for a baroclinic ocean with the same continental slope topography. The nature of the vorticity balance and of the circulation around closed paths is examined in each case, and it is shown that bottom pressure torques lead to enhanced transport in the western boundary current only for the baroclinic case with variable depth. 相似文献
10.
Synoptic scale variability of the Southern Ocean wind field in the high-frequency range of barotropic Rossby waves results
in transport variations of the Antarctic Circumpolar Current (ACC), which are highly coherent with the bottom pressure field
all around the Antarctic continent. The coherence pattern, in contrast to the steady state ACC, is steered by the geostrophic
f/h contours passing through Drake Passage and circling closely around the continent. At lower frequencies, with interannual
and decadal periods, the correlation with the bottom pressure continues, but baroclinic processes gain importance. For periods
exceeding a few years, variations of the ACC transport are in geostrophic balance with the pressure field associated with
the baroclinic potential energy stored in the stratification, whereas bottom pressure plays a minor role. The low-frequency
variability of the ACC transport is correlated with the baroclinic state variable in the entire Southern Ocean, mediated by
baroclinic topographic–planetary Rossby waves that are not bound to f/h contours. To clarify the processes of wave dynamics and pattern correlation, we apply a circulation model with simplified
physics (the barotropic–baroclinic-interaction model BARBI) and use two types of wind forcing: the National Centers for Environmental
Prediction (NCEP) wind field with integrations spanning three decades and an artificial wind field constructed from the first
three empirical orthogonal functions of NCEP combined with a temporal variability according to an autoregressive process.
Experiments with this Southern Annular Mode type forcing have been performed for 1,800 years. We analyze the spin-up, trends,
and variability of the model runs. Particular emphasis is placed on coherence and correlation patterns between the ACC transport,
the wind forcing, the bottom pressure field and the pressure associated with the baroclinic potential energy. A stochastic
dynamical model is developed that describes the dominant barotropic and baroclinic processes and represents the spectral properties
for a wide range of frequencies, from monthly periods to hundreds of years. 相似文献
11.
Linear Rossby wave dispersion relationships suggest that Jupiter’s Great Red Spot (GRS) is a baroclinic structure embedded in a barotropic shearing zonal flow. Quasi-geostrophic (QG) two-layer simulations support the theory, as long as an infinitely deep zonal flow is assumed. However, once a finite depth of the lower layer is assumed, a self-interaction of the baroclinic eddy component produces a barotropic radiating field, so that the GRS-like eddy can no longer remain compact. Compactness is recovered by explicitly introducing a deep dynamics of the interior for the lower layer, instead of the shallow QG formulation. An implication of the result is a strong coupling of the GRS to a convectively active interior.Paper presented to the NP Symposia of the 1991 Wiesbaden EGS Assembly on “Nonlinear processes in Geophysics” 相似文献
12.
本文将夏季气候平均的基本气流分解为正压和斜压分量,使用一个线性斜压模式,研究了不同斜压基本气流对热带西北太平洋地区初始气旋性环流扰动低频发展演变的重要作用.其中,控制试验较好地模拟出初始气旋扰动向西北方向传播、在西北太平洋季风槽附近停滞增强、在东亚地区出现经向波列和在南海到海洋大陆地区形成西北—东南向波列等特征.改变斜压分量的敏感性试验结果表明,正压基流不能为西传的初始扰动供给足够的能量;海陆热力差异引起东亚地区的纬向温度梯度和北风垂直切变,是东亚太平洋型经向波列形成和维持的重要因素;当基本气流中的斜压纬向偏差部分线性增大时,扰动的能量会呈e指数迅速增强,提示在气候变化的背景下,基本气流微小的改变可能带来天气或季节内扰动强度的剧烈响应. 相似文献
13.
Two-layer equatorial primitive equations for the free troposphere in the presence of a thin atmospheric boundary layer and thermal dissipation are developed here. An asymptotic theory for the resonant nonlinear interaction of long equatorial baroclinic and barotropic Rossby waves is derived in the presence of such dissipation. In this model, a self-consistent asymptotic derivation establishes that boundary layer flows are generated by meridional pressure gradients in the lower troposphere and give rise to degenerate equatorial Ekman friction. That is to say, the asymptotic model has the property that the dissipation matrix has one eigenvalue which is nearly zero: therefore the dynamics rapidly dissipates flows with pressure at the base of the troposphere and creates barotropic/baroclinic spin up/spin down. The simplified asymptotic equations for the amplitudes of the dissipative equatorial barotropic and baroclinic waves are studied by linear theory and integrated numerically. The results indicate that although the dissipation slightly weakens the tropics to midlatitude connection, strong localized wave packets are nonetheless able to exchange energy between barotropic and baroclinic waves on intraseasonal timescales in the presence of baroclinic mean shear. Interesting dissipation balanced wave-mean flow states are discovered through numerical simulations. In general, the boundary layer dissipation is very efficient for flows in which the barotropic and baroclinic components are of the same sign at the base of the free troposphere whereas the boundary layer dissipation is less efficient for flows whose barotropic and baroclinic components are of opposite sign at the base of the free troposphere. 相似文献
14.
The baroclinic response of a stratified coastal embayment (Lunenburg Bay of Nova Scotia) to the observed wind forcing is examined using two numerical models. A linear baroclinic model based on the normal mode approach shows skill at reproducing the observed isotherm movements and sub-surface currents during a time of strong stratification in the bay. The linear model also shows that the isotherm movement in Lunenburg Bay is influenced by the wind forcing and propagation of baroclinic Kelvin waves from neighbouring Mahone Bay. The effects of nonlinearity and topography are investigated using a three-dimensional nonlinear coastal circulation model. The nonlinear model results demonstrate that the nonlinear advection terms generate a gyre circulation at the entrance of Lunenburg Bay, and the slope bottom topography at the mouth of the bay strengthens the sub-surface time-mean inflow on the southern side of the bay. A comparison of model-calculated currents in different numerical experiments clearly shows that baroclinicity plays a dominant role in the dynamics of wind-driven circulation in Lunenburg Bay. 相似文献
15.
The sea level and the barotropic, frictional circulation response for the New York Bight are used to demonstrate the effects of external sea-level forcing, bathymetry, and variable friction. The governing equation is the steady, integrated vorticity equation and is computed by finite differencing over a curvilinear grid conforming to the 10- and 100-m isobaths and extending for 250 km alongshore. The boundary conditions are based on the hypothesis that the dynamics of the shelf are driven by the external sea-level gradient and the coastal no-flux condition; and consequently the conditions at the lateral boundaries are dependent thereon. Therefore, the external sea-level slope must be independently specified, and the lateral boundary conditions must be dependently generated. The diabathic component of the external sea slope forces the calm wind circulation by its effect on the transport through the upstream boundary; and the parabathic component has also an important modifying effect by forcing a shelf convergent transport. The parabathic sea slope at the coast is independent of its offshore value, being instead a direct product of the coastal boundary condition.The bottom friction is expressed as related to the sea level through a bottom length parameter and a veer angle, both of which are taken to increase shoreward. An additional bottom stress component, related to the surface stress, is determined for bottom depths less than the Ekman depth. Such bottom stress variability produces significant alterations in the nearshore flow field, over the constant bottom stress formulation, by reducing it and causing it to veer downgradient and downwind in the nearshore.The model is forced by different wind directions and the results are discussed. The circulations generally conform to the observed mean flow patterns, but with several smaller-scale features. The strong bathymetric feature of the Hudson Shelf Valley causes a polarized up- and downvalley flow for winds with an eastward or westward component, respectively. Under mean westerly winds, there is a divergence in the shelf valley flow at about the 60-m isobath. The Apex gyre existing off the western tip of Long Island becomes more extensive for winds changing from northeast to southwest. Mean flow reversals (to the northeast) occur off both Long Island and New Jersey for wind directions changing counterclockwise from northwest to southeast and from west to east, respectively. Southeastward transport over the outer New Jersey shelf tends to be enhanced by wind and external sea-level conditions; and the transport over the New Jersey midshelf, particularly in the lee of the shelf valley, tends to be weak and variable also under these mean conditions. 相似文献
16.
17.
Parker MacCready Neil S. Banas Barbara M. Hickey Edward P. Dever Yonggang Liu 《Continental Shelf Research》2009
A numerical simulation of circulation in the Columbia River estuary and plume during the summer of 2004 is used to explore the mixing involved as river water is transformed into shelf water. The model is forced with realistic river flow, tides, wind stress, surface heat flux, and ocean boundary conditions. Simulated currents and water properties on the shelf near the mouth are compared with records from three moorings (all in 72 m of water) and five CTD sections. The model is found to have reasonable skill; statistically significant correlations between observed and modeled surface currents, temperature, and salinity are all 0.42–0.72 for the mooring records. Equations for the tidally averaged, volume-integrated mechanical energy budget (kinetic and potential) are derived, with attention to the effects of: (i) Reynolds averaging, (ii) a time varying volume due to the free surface, and (iii) dissipation very close to the bottom. It is found that convergence of tidal pressure work is the most important forcing term in the estuary. In the far field plume (which has a volume 15 times greater than that of the estuary), the net forcing is weaker than that in the estuary, and may be due to either tidal currents or wind stress depending on the time period considered. These forcings lead to irreversible mixing of the stratification (buoyancy flux) that turns river water into shelf water. This occurs in both the plume and estuary, but appears to be more efficient (17% vs. 5%), and somewhat greater (4.2 MW vs. 3.3 MW), in plume vs. estuary. This demonstrates the importance of both wind and tidal forcing to watermass transformation, and the need to consider the estuary and plume as part of a single system. 相似文献
18.
Large scale, stationary features of the Benguela Current system have been observed on satellite infrared images. This paper attempts to establish the most likely dynamical framework required to explain the features. Although the observed features are clearly baroclinic, the wavelength of the 2.2 day period barotropic shelf wave (Buchwald and Adams, 1968, Proceedings of the Royal Society of London, A305, 235–250) fits the observed longshore spatial structure of the features well. The barotropic shelf wave model fails to explain the baroclinic nature and the stationary aspect of the observed features. More observational and theoretical work is necessary to understand the dynamics of these features. 相似文献
19.
Brian DzwonkowskiKyeong Park Ho Kyung Ha William M. Graham Frank J. HernandezSean P. Powers 《Continental Shelf Research》2011,31(9):939-950
Hydrographic variability on the Alabama shelf just outside of Mobile Bay, a major source of river discharge into the Gulf of Mexico, is examined using time series of water column temperature and surface and bottom salinity from a mooring site with a depth of 20 m in conjunction with a series of across-shelf CTD surveys. The time series data show variability in a range of time scales. The density variation is affected by both salinity and temperature, with its relatively strong annual signal mostly determined by temperature and its year to year variability mostly determined by salinity. Seasonal mean structures of temperature, salinity, and density show a transition from estuarine to shelf conditions in which three regions with distinct seasonal characteristics in their horizontal and vertical gradient structures are identified. Correlation analysis with the available forcing functions demonstrates the influence of Mobile Bay on the variability at the mooring site. At low frequencies, river discharge from Mobile Bay has a varying influence on salinity, which is absent during the periods with unusually low discharge. At shorter synoptic time scales, both the estuarine response to the across-shelf wind stress and the shelf response to the along-shelf wind stress are significantly correlated with temperature/salinity variability: the former becoming important for the surface layer during winter whereas the latter for the bottom layer during both winter and summer. These forcing functions are important players in determining the estuarine-shelf exchange, which in turn is found to contribute to the shelf hydrographic structure. 相似文献
20.
Fourteen acoustic Doppler current profilers (ADCPs) were deployed on the shelf and slope for 1 year just west of the DeSoto Canyon in the Northeastern Gulf of Mexico by the Naval Research Laboratory (NRL) as part of its Slope to Shelf Energetics and Exchange Dynamics (SEED) project. The winter and spring observations are discussed here in regards to the low-frequency current variability and its relation to wind and eddy forcing. Empirical orthogonal function (EOF) analyses showed that two modes described most of the current variability. Wind-forced variability of the along-shelf flow was the main contributor in Mode 1 while eddies contributed much of the variability in Mode 2. Wind-stress controlled currents on the shelf and slope at time scales of about a week. On longer time scales, variations in the currents on both the outer shelf and slope appear to be related to seasonal variations in the time-cumulated wind stress curl. Winds were dominant in driving the along-shelf transports, particularly along the slope. However, the effective wind stress component was found to be aligned with the west Florida shelf direction rather than the local shelf direction. Eddy intrusions, which were more numerous in winter and spring than in summer and fall, and winds were found to contribute significantly to cross-shelf exchange processes. 相似文献