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1.
Charles A. Lin 《地球物理与天体物理流体动力学》2013,107(4):227-259
Abstract A high vertical resolution model is used to examine the instability of a baroclinic zonal flow and a finite amplitude topographically forced wave. Two families of unstable modes are found, consisting of zonally propagating most unstable modes, and stationary unstable modes. The former have time scale and spatial structure similar to baroclinic synoptic disturbances, but are localized in space due to interaction with the zonally asymmetric forcing. These modes transport heat efficiently in both the zonal and meridional directions. The second family of stationary unstable modes has characteristics of modes of low frequency variability of the atmosphere. They have time scales of 10 days and longer, and are of planetary scale with an equivalent barotropic vertical structure. The horizontal structure resembles blocking flows. They are maintained by available potential energy of the basic wave, and have large zonal heat fluxes. The results for both families of modes are interpreted in terms of an interaction between forcing and baroclinic instability to create favoured regions for eddy development. Applications to baroclinic planetary waves are also considered. 相似文献
2.
Two prognostic experiments taking into account real atmospheric forcing for 2006 and 2011 were carried out based on the eddy-resolving numerical model with a horizontal resolution of 1.6 km for the Black Sea. The main dynamic features such as the Rim Current, the Sevastopol, and Batumi anticyclones are reproduced in both experiments. The model results are confirmed via observation data. We accomplished the analysis of simulated circulation and energetics. The results demonstrate that both the vertical viscosity and vertical diffusion along with the energy inflow from the wind have been the main contributors to the annual and seasonal budgets of kinetic and potential energies of the Black Sea circulation. It is shown that two regimes of the Black Sea general circulation are implemented depending on a magnitude of wind contribution to the kinetic energy in winter. Intensive mesoscale eddy formation was observed along the Anatolian, Caucasian, and Crimean coasts. The analysis of the Black Sea circulation and eddy energetics allowed us to conclude that the generation and development of the mesoscale coastal eddies is associated with the barotropic instability in case of intensive coastal currents and is associated with both the barotropic and baroclinic instability in case of weak coastal currents. 相似文献
3.
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
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.
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
6.
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
7.
Initially the development of shallow sea three-dimensional barotropic tidal models is briefly reviewed with a view to determining what were the key measurements that allowed progress in this field and rigorous model validation. Subsequently this is extended to a brief review of baroclinic tidal models to try to determine a “way forward” for baroclinic model development. The difficulty of high spatial variability, and wind influence are identified as possibly important issues that must be considered in validating baroclinic tidal models. These are examined using a three-dimensional unstructured grid model of the M2 internal tide on the shelf edge region off the west coast of Scotland. The model is used to investigate the spatial variability of the M2 internal tide, and associated turbulence energy and mixing in the region. Initial calculations are performed with tidal forcing only, with subsequent calculations briefly examining how the tidal distribution is modified by down-welling and up-welling favourable winds. Calculations with tidal forcing only, show that there is significant spatial variability in the internal tide and associated mixing in the region. In addition, these are influenced by wind effects which may have to be taken into account in any model validation exercise. The paper ends by discussing the comprehensive nature of data sets that need to be collected to validate internal tidal models to the same level currently attained with three dimensional barotropic tidal models. 相似文献
8.
Abstract Merilees and Warn's (1975) nonlinear interaction analysis of two-dimensional nondivergent flow is extended to examine the quasi-geostrophic two-layer model. Two sets of triads exist in this model (Salmon, 1978). The purely barotropic triads are the same as the triads examined by Merilees and Warn. Baroclinic-barotropic triads are found to exchange more energy or potential enstrophy with smaller or larger scales depending on the scale of motion as compared with the internal Rossby deformation radius and the relative wavenumber position of baroclinic and barotropic components. 相似文献
9.
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
10.
Andrew J. Willmott 《地球物理与天体物理流体动力学》2013,107(1-4):39-63
Abstract This paper investigates the generation of linear baroclinic Rossby waves by meridional oscillations of a climatological zonal wind stress in a reduced gravity ocean bounded by an eastern coastline. Using a power series technique an analytical solution is derived for the interfacial displacement. It is found that for a given period of oscillation of the zonal winds, a finite number of propagating Rossby waves will be generated with frequencies equal to a harmonic of the forcing frequency. The number of propagating modes increases with increasing period of the wind stress. In addition to the propagating waves the complete solution for the interfacial displacement consists of a rapidly convergent infinite sum of evanescent terms. The displacement field is calculated for atmospheric forcing parameters typical of those found at mid-latitudes. Further, it is shown that a near resonant response can be generated using atmospheric parameter values typical of those found over the North Pacific. 相似文献
11.
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
12.
A weakly nonlinear model is used to examine the mean transverse circulation (cross-isobath) driven by tidal-induced buoyancy flux. The mean Eulerian flows driven by both the barotropic and baroclinic tide are presented for a semi-infinite wedge. The mean flow driven by the barotropic tide is significant only near the apex where the thickness of the frictional boundary layer is comparable to the water depth. The mean flow there is characterized by a single-cell circulation with offshore flow near the bottom, and its magnitude can reach a few percentage or a significant fraction of the tidal velocity in oceanic applications. The mean flow driven by the baroclinic tide, on the other hand, is characterized by pairs of half-open (on the seaward side) counter-rotating cells, the number of which equals the vertical mode number. For a baroclinic tide propagating onshore, the mean flow near the top and bottom surfaces is always directed offshore and its magnitude can reach a large fraction of the tidal velocity. Taken together, the model thus predicts a mean offshore flow near the bottom while higher up in the water column the mean flow direction is less definite due to the contribution from different tidal components. The model results are consistent with some current measurements over the Georges Bank. 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.
Abstract Starting from Euler's equations of motion a nonlinear model for internal waves in fluids is developed by an appropriate scaling and a vertical integration over two layers of different but constant density. The model allows the barotropic and the first baroclinic mode to be calculated. In addition to the nonlinear advective terms dispersion and Coriolis force due to the Earth's rotation are taken into account. The model equations are solved numerically by an implicit finite difference scheme. In this paper we discuss the results for ideal basins: the effects of nonlinear terms, dispersion and Coriolis force, the mechanism of wind forcing, the evolution of Kelvin waves and the corresponding transport of particles and, finally, wave propagation over variable topography. First applications to Lake Constance are shown, but a detailed analysis is deferred to a second paper [Bauer et al. (1994)]. 相似文献
15.
A. M. Treguier 《地球物理与天体物理流体动力学》2013,107(1-4):43-68
Abstract Steady currents develop in oceanic turbulence above topography even in the absence of steady forcing. Mesoscale steady currents are correlated with mesoscale topography with anticyclonic eddies above topographic bumps, and large scale westward flows develop when β is non-zero. The relationship between those two kinds of steady currents, as well as their dependence on various parameters, is studied using a barotropic quasi-geostrophic channel model. The percentage of steady energy is found to depend on the forcing, friction and topography in a non-monotonic fashion. For example, the percentage of steady currents grows with the energy level in the linear regime (low energies) and decreases when the energy level increases in the nonlinear regime (high energies). Mesoscale steady currents are the energy source for the steady westward flow U, and therefore U is the maximum when large scale and mesoscale currents are of the same order of magnitude. This happens when the ratio S of the large scale slope βH/f 0 and the mesoscale rms topographic slope α is of order one. U decreases for both small and large values of S. 相似文献
16.
The scattering of first mode linear baroclinic Rossby waves by a top-hat ridge in a continuously stratified ocean, with Brunt-Väisälä frequency that decays exponentially with depth below a surface mixed layer, is the subject of this study. A numerical mode matching technique is used to calculate the transmission coefficients for the propagating modes over the ridge. It is found that the scattered field depends crucially upon the stratification. For example, when the majority of the density variation is confined to a thin thermocline, corresponding to a small e-folding scale, gamma ?1, for the Brunt-Väisälä frequency, a large amount of the incident wave energy is reflected by a small amplitude ridge. Appreciable energy conversion between the propagating barotropic and baroclinic modes takes place in this case. An asymptotic analysis for a small amplitude ridge is presented that confirms these numerical results. In the limit gamma ?1→ 0, it is demonstrated that the scattered field in the continuously stratified ocean model differs markedly from the two-layer solution. The latter does not exhibit appreciable reflection of the incident wave energy for a small amplitude ridge. In conclusion, the application of a two-layer ocean model to describe Rossby wave scattering by ridges in place of a continuously stratified model cannot be recommended. 相似文献
17.
The Mediterranean Sea is a region of intense air–sea interactions, with in particular strong evaporation over sea which drives
the thermohaline circulation. The Mediterranean region is also prone to strong precipitation events characterized by low spatial
extent, short duration, and high temporal variability. The impacts of intense offshore precipitation over sea, in the Gulf
of Lions which is a spot for winter deep convection, are investigated using four sensitivity simulations performed at mesoscale
resolution with the eddy-resolving regional ocean model NEMO-MED12. We use various atmospheric fields to force NEMO-MED12,
downscaled from reanalyses with the non-hydrostatic mesoscale Weather Research and Forecasting model but differing in space
resolutions (20 and 6.7 km) or in time frequencies (daily and three-hourly). This numerical study evidences that immediate,
intense, and rapid freshening occurs under strong precipitation events. The strong salinity anomaly induced extends horizontally
(≃50 km) as vertically (down to 50 m) and persists several days after strong precipitation events. The change in the space
resolution of the atmospheric forcing modifies the precipitating patterns and intensity, as well as the shape and the dynamics
of the low-salinity layer formed are changed. With higher forcing frequency, shorter and heavier precipitation falls in the
ocean in the center of the Gulf of Lions, and due to a stronger vertical shear and mixing, the low-salinity anomaly propagates
deeper. 相似文献
18.
This study investigates transient eddy activity anomalies in the mid-latitude upper troposphere associated with intensity variability of the wintertime North Pacific subtropical front. Our results show that the meridional gradient of air temperature and baroclinic instability in the mid-latitude atmosphere become stronger as the subtropical front intensifies, and the mid-latitude westerly jet accelerates with barotropic structure. We further divide the mid-latitude atmospheric eddy activities into high-(2–7 days) and low-frequency(10–90 days) eddy activities according to their life periods. We find that, when the oceanic subtropical front intensifies, the high-frequency atmospheric eddy activity in the mid-latitudes strengthens while the low-frequency eddy activity weakens. The stronger high-frequency eddy activity tends to moderate the air temperature gradient and baroclinicity in the mid-latitudes. High-frequency eddy anomalies accelerate the westerly jet on the northern side and downstream of the westerly jet, and enhance the jet with equivalent barotropic structure. In contrast, the weaker low-frequency eddy activity has a negative contribution to zonal wind speed tendency and attenuates the zonal homogenization of the jet. The anomalous thermodynamic forcing of the low-frequency eddy activity helps maintain the meridional gradient of air temperature in the mid-troposphere. 相似文献
19.
Based on eddy-permitting ocean circulation model outputs, the mesoscale variability is studied in the Sea of Okhotsk. We confirmed that the simulated circulation reproduces the main features of the general circulation in the Sea of Okhotsk. In particular, it reproduced a complex structure of the East-Sakhalin current and the pronounced seasonal variability of this current. We established that the maximum of mean kinetic energy was associated with the East-Sakhalin Current. In order to uncover causes and mechanisms of the mesoscale variability, we studied the budget of eddy kinetic energy (EKE) in the Sea of Okhotsk. Spatial distribution of the EKE showed that intensive mesoscale variability occurs along the western boundary of the Sea of Okhotsk, where the East-Sakhalin Current extends. We revealed a pronounced seasonal variability of EKE with its maximum intensity in winter and its minimum intensity in summer. Analysis of EKE sources and rates of energy conversion revealed a leading role of time-varying (turbulent) wind stress in the generation of mesoscale variability along the western boundary of the Sea of Okhotsk in winter and spring. We established that a contribution of baroclinic instability predominates over that of barotropic instability in the generation of mesoscale variability along the western boundary of the Sea of Okhotsk. To demonstrate the mechanism of baroclinic instability, the simulated circulation was considered along the western boundary of the Sea of Okhotsk from January to April 2005. In April, the mesoscale anticyclonic eddies are observed along the western boundary of the Sea of Okhotsk. The role of the sea ice cover in the intensification of the mesoscale variability in the Sea of Okhotsk was discussed. 相似文献
20.
On the dynamics of strait flows: an ocean model study of the Aleutian passages and the Bering Strait
A high-resolution numerical ocean circulation model of the Bering Sea (BS) is used to study the natural variability of the BS straits. Three distinct categories of strait dynamics have been identified: (1) Shallow passages such as the Bering Strait and the Unimak Passage have northward, near barotropic flow with periodic pulses of larger transports; (2) wide passages such as Near Straits, Amukta Pass, and Buldir Pass have complex flow patterns driven by the passage of mesoscale eddies across the strait; and (3) deep passages such as Amchitka Pass and Kamchatka Strait have persistent deep return flows opposite in direction to major surface currents; the deep flows persist independent of the local wind. Empirical orthogonal function analyses reveal the spatial structure and the temporal variability of strait flows and demonstrate how mesoscale variations in the Aleutian passages influence the Bering Strait flow toward the Arctic Ocean. The study suggests a general relation between the barotropic and baroclinic Rossby radii of deformations in each strait, and the level of flow variability through the strait, independent of geographical location. The mesoscale variability in the BS seems to originate from two different sources: a remote origin from variability in the Alaskan Stream that enters the BS through the Aleutian passages and a local origin from the interaction of currents with the Bowers Ridge in the Aleutian Basin. Comparisons between the flow in the Aleutian passages and flow in other straits, such as the Yucatan Channel and the Faroe Bank Channel, suggest some universal topographically induced dynamics in strait flows. 相似文献