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1.
Air–sea interaction over ocean fronts and eddies   总被引：1，自引：0，他引：1  
Air–sea interaction at ocean fronts and eddies exhibits positive correlation between sea surface temperature (SST), wind speed, and heat fluxes out of the ocean, indicating that the ocean is forcing the atmosphere. This contrasts with larger scale climate modes where the negative correlations suggest that the atmosphere is driving the system. This paper examines the physical processes that lie behind the interaction of sharp SST gradients and the overlying marine atmospheric boundary layer and deeper atmosphere, using high resolution satellite data, field data and numerical models. The importance of different physical mechanisms of atmospheric response to SST gradients, such as the effect of surface stability variations on momentum transfer, pressure gradients, secondary circulations and cloud cover will be assessed. The atmospheric response is known to create small-scale wind stress curl and divergence anomalies, and a discussion of the feedback of these features onto the ocean will also be presented. These processes will be compared and contrasted for different regions such as the Equatorial Front in the Eastern Pacific, and oceanic fronts in mid-latitudes such as the Gulf Stream, Kuroshio, and Agulhas Return Current.  相似文献   

2.
A model for the structure and motion of baroclinic solitary waves in the atmosphere or ocean is presented. Like gravity wave solitons, these planetary wave solutions are both weakly nonlinear and weakly dispersive. The dispersion effects, induced by β, are small because the scale of the wave is large compared to the deformation radius. The steepening effects are provided by the interaction of the wave with exterior mean shear flow, which may be either barotropic or baroclinic. The solutions have two properties which suggest that such theories may be useful in modelling solitary disturbances in the atmosphere or ocean: radial symmetry and fluid speeds which exceed the phase speed of the wave itself. As an example, we apply the model to Gulf Stream Rings.  相似文献   

3.
The direction normal to the Earth spherical (or ellipsoidal) surface is not vertical (called deflected vertical) since the vertical direction is along the true gravity g (= igλ+ jgφ+ kgz). Here, (λ, φ, z) are (longitude, latitude, depth), and (i, j, k) are the corresponding unit vectors. The spherical (or ellipsoidal) surfaces are not horizontal surfaces (called deflected-horizontal surfaces). The most important body force g (true gravity) has been greatly simplified without justification in oceanography to the standard gravity (-g0k) with g0 = 9.81 m/s2. Impact of such simplification on ocean dynamics is investigated in this paper using the Ekman layer model. In the classical Ekman layer dynamic equation, the standard gravity (-g0k) is replaced by the true gravity g(λ, φ, z) with a constant eddy viscosity and a depth-dependent-only density ρ(z) represented by an e-folding near-inertial buoyancy frequency. New Ekman spiral and in turn new formulae for the Ekman transport are obtained for ocean with and without bottom. With the gravity data from the global static gravity model EIGEN-6C4 and the surface wind stress data from the Comprehensive Ocean-Atmosphere Data Set (COADS), large difference is found in the Ekman transport using the true gravity and standard gravity.  相似文献   

4.
The effects on the convective boundary layer (CBL) of shading due to shallow cumulus clouds are investigated. The main question is to see whether clouds are able to produce secondary circulations by shading of the surface (dynamic heterogeneities) and how these dynamic heterogeneities interact with static heterogeneities in terms of the production of secondary circulations. Also the effects of cloud shadows on cloud-field characteristics are analyzed. The effects are studied using large-eddy simulations of a cloud-topped CBL with an idealized surface. Over a homogeneous surface, shadows trigger secondary circulations with different strengths depending on the solar zenith angle \(\vartheta \), with large \(\vartheta \) favouring the development of secondary circulations. Over a static heterogeneous surface with a simple striped pattern, the strength of secondary circulations is effectively reduced by dynamic heterogeneities at small \(\vartheta \). At large \(\vartheta \), however, the effect on secondary circulations depends on the orientation of the striped static heterogeneities to the shadow-casting direction of the clouds. The influence of shadows is only small if they are cast perpendicular to the striped heterogeneity, but if stripes and the shadow-casting direction are parallel, secondary circulations are reduced in strength also for large \(\vartheta \). Shadow effects on the cloud-field characteristics vary with \(\vartheta \) as well. The results show that small \(\vartheta \) favours the development of small clouds with a reduced lifetime while large \(\vartheta \) promotes the development of larger clouds with an extended lifetime compared to non-shading clouds.  相似文献   

5.
Experiments using ferrofluid, magnetically attached to a horizontal cylinder rotating about a vertical axis through its center-point, demonstrate that this system is capable of simulating important aspects of ocean basin circulations. In this note, westward intensification and the generation of thermal wind jets are illustrated, and examples with both thermal and mechanical forcing are given. Axially periodic anomalies in the magnetic field are reflected in the observations, in qualitative agreement with theory, and this suggests further refinement of the parameters of the experiment, such as increasing the magnetic fluid's saturation magnetization. As is, the experiment shows promise as an effective demonstration of some of the basic ideas of physical oceanography.  相似文献   

6.
Responses of global ocean circulation and temperature to freshwater runoff from major rivers were studied by blocking regional runoff in the global ocean general circulation model(OGCM)developed at the Massachusetts Institute of Technology.Runoff into the tropical Atlantic,the western North Pacific,and the Bay of Bengal and northern Arabian Sea were selectively blocked.The blocking of river runoff first resulted in a salinity increase near the river mouths(2 practical salinity units).The saltier and,therefore,denser water was then transported to higher latitudes in the North Atlantic,North Pacific,and southern Indian Ocean by the mean currents.The subsequent density contrasts between northern and southern hemispheric oceans resulted in changes in major ocean currents.These anomalous ocean currents lead to significant temperature changes(1°C-2°C)by the resulting anomalous heat transports.The current and temperature anomalies created by the blocked river runoff propagated from one ocean basin to others via coastal and equatorial Kelvin waves.This study suggests that river runoff may be playing an important role in oceanic salinity,temperature,and circulations;and that partially or fully blocking major rivers to divert freshwater for societal purposes might significantly change ocean salinity,circulations,temperature,and atmospheric climate.Further studies are necessary to assess the role of river runoff in the coupled atmosphere-ocean system.  相似文献   

7.
An integral theorem is proved which establishes the sufficient conditions for the stability with respect to small perturbations of inviscid and unforced, non-geostrophic, inertial, mean currents in a fluid comprised of homogeneous layers of different densities within an enclosed ocean basin. The derivation is by a generalization of a technique employed by Drazin and Howard (1966) and Blumen (1973). The relevance of such solutions to the mean sub-tropical ocean gyres may be that they represent stable reservoirs of mean energy and, as such, might serve as “building blocks” for the general circulation. Several examples are discussed which support this hypothesis.  相似文献   

8.
Stochastic wind forcing of ocean gyre circulations is examined using the ideas of generalized linear stability theory applied to the barotropic vorticity equation of a idealized ocean. The barotropic vorticity equation is linearized about a time-evolving basic state flow, and the spatial patterns of stochastic surface wind stress curl that are optimal for increasing the variability of the ocean are computed. The most disruptive pattern of stochastic forcing is found to be insensitive to: measures of variance, the optimization time, the temporal decorrelation time of the stochastic forcing, the time evolution of the basic state flow, the stability of the basic state flow, basin size, gyre symmetry, and the presence of bathymetry. In addition, the most disruptive pattern of wind stress curl is reminiscent of that which would be associated with individual large-scale weather systems in the atmosphere, and changes in the amplitude of the atmospheric teleconnection patterns. The response of a nonlinear model to stochastic forcing described by the optimal patterns is examined, and the dynamics of the response discussed.  相似文献   

9.
Summary Tropical ocean thermocline variability is studied using gridded data assimilated by an ocean model in the period 1950–2000. The dominant patterns and variability are identified using EOF analysis applied to E–W depth slices of sea temperatures averaged over the tropics. After removing the annual cycle, an east–west ‘see-saw’ with an interannual to decadal rhythm is the leading mode in each of the tropical basins. In the case of the leading mode in the Pacific, the thermocline oscillation forms a dipole structure, but in the (east) Atlantic and (southwest) Indian Ocean there is a single center of action. The interaction of the ocean thermocline and atmospheric Walker circulations is studied through cross-modulus analysis of wavelet-filtered EOF time scores. Our study demonstrates how tropical ocean thermocline variability contributes to zonal circulation anomalies in the atmosphere. The equatorial Pacific thermocline oscillation explains 62 and 53% of the variability of the Pacific and Atlantic zonal overturning circulations, the latter driving convective polarity between North Africa and South America. The Pacific sea-saw leads the Atlantic zonal circulation by a few months.  相似文献   

10.
The interaction between non-uniform near-surface currents and long surface waves is shown to produce large-scale secondary circulations. The circulations are caused by the Craik–Leibovich vortex force imposed on the existing non-uniform current by the surface waves. The current could be produced by different types of sources, such as by ship wakes or by river and sewer outflows. In this paper the circulations are considered for three representative types of currents: a near-surface jet, a shear current, and an underwater jet. A model similar to the model of Langmuir circulations is formulated and studied numerically. The general model takes into account the effect of viscosity on the main current as well as the effect of the circulation-related advection on the main current and secondary flow itself. A simplified model that describes the initial stage of the development of circulations was used in order to demonstrate the strength of the phenomenon and its dependence on some parameters of the problem. At this initial stage, the effect of viscosity on the main current as well as the effect of advection caused by the circulations was neglected (under assumption that the perturbation velocity is small). The effect of the viscosity on the circulations was included in the solution, and it was shown that initial development of the circulations is practically independent of the viscosity. This fact simplifies the solution of the problem and removes the uncertainty related to the value of the turbulent viscosity at the initial stage of the circulations. The results obtained demonstrate that strong circulations are generated under very realistic assumptions regarding the parameters of the current and the surface waves. The maximum velocity at the surface produced by such circulations can easily reach several centimeters per second. A circulatory flow with this magnitude of velocity at the surface can significantly affect short surface waves and, correspondingly, radar and optical signatures produced by the initial currents on the sea surface. Some important conclusions about the nature of these signatures are made based on numerical results and simple qualitative arguments. Theoretical predictions include, for example, the asymmetry of centerline ship wakes and the difference in the width and length between images of two wakes of similar ships moving in opposite directions when ambient surface waves are present.  相似文献   

11.
By using an ageostrophic shallow water model, it is pointed out that a kind of lateral boundary meso-scales jet can be established near the plateau or coast. The characteristic width of this kind of jet is proportional to the scale ofL c=L0(C0/Vg), whereL 0=C 0/f is the radius of Rossby deformation,C 0=(g * H)1/2 the speed of gravity wave and g* the reduced gravity. In general,L c is of the order of one hundred kilometes and tens of kilometers in the atmosphere and in the ocean respectively. The large-scale geostrophic current is an important background condition for forming this kind of jet. From this view point it seems that this kind of atmospheric meso-scale jet only occurs in late spring and summer in the eastern part of Asia, because there is a large-scale south monsoon over there. For the ocean, this kind of meso-scale jet seems to be a semi-persistant system and not to show a significant seasonal variation, and it can be established on both sides of the ocean.  相似文献   

12.
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13.
Meridional, linear, and free modes of global, primitive-equation, ocean-atmosphere models were analyzed to see if they contain multi-year, especially decadal ( 10–30 years), oscillation time scale modes. A two-layer model of the global ocean and a two-level model of the global atmosphere were formulated. Both models were linearized around axially-symmetric basic states containing mean meridional circulations. The linearized perturbation system was solved as an eigenvalue problem. The operator matrix was discretized in the north-south direction with centered finite differences. Uncoupled, meridional modes of oscillation of the ocean and the atmosphere models were calculated. Calculations were performed at three grid spacings (5°, 2.5° and 1.25°) and for two types of basic states (symmetric and asymmetric). Uncoupled, free oceanic modes in the presence of mean meridional circulations have oscillation time scales ranging from two years to several centuries. Such low frequency meridional modes do not exist in the ocean model if there are no mean meridional circulations. A large number of oceanic modes are grouped around decadal oscillation time scales. All the oceanic modes have neutral growth rates. The spatial structures of some of the oceanic modes are comparable to observed spatial structures of sea surface temperature variations in the Pacific Ocean. Most years to decades variability of meridional modes of the ocean model is contained in tropical and midlatitude modes. Some oceanic modes with years to decades periods have standing oscillations in the tropics and poleward propagation of zonal velocity and layer thickness outside the tropics. Uncoupled, free atmospheric modes in the presence of mean meridional circulations have oscillation time scales ranging from a week to several decades. Such low-frequency meridional modes do not exist in the atmospere model if there are no mean meridional circulations. A large number of modes are grouped around intraseasonal time scales. Unlike the oceanic modes, the atmospheric modes are weakly unstable. Most of the intraseasonal variability of atmospheric modes is contained in tropical, midlatitude, and polar modes. Atmospheric modes with oscillation periods longer than about one year have global extent. Meridional ocean-atmospheric modes exist in the models wherever there are mean meridional circulations, i.e., tropical, midlatitude, polar, and global. Oceanic and atmospheric eigenvectors have symmetric (assymetric) latitudinal structures if their basic states are symmetric (asymmetric) around the equator. For both models, models calculated at coarser than 2.5° grid spacing do not accurately represent low-frequency variability. Scale analysis shows taht advection by tge basic state meridional velocities is the primary cause of the meridional oscillations on time scales longer than two years in the ocean model and longer than a few weeks in the atmosphere model. Meridional modes of the coupled ocean-atmosphere models are the subject of a subsequent paper.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil  相似文献   

14.
A turbulent stratified shear flow is generated in a towing tank by towing a grid or a circular cylinder through a tank of stratified salt water. The internal waves and turbulence generated in these flows are visualized with shadowgraphs and measured with quartz-coated hot-film probes (up to four probes for velocity fluctuations) and single-electrode conductivity probes (up to four probes for salinity fluctuations) which are towed at the same speed as the obstacle. The velocity and salinity signals are recorded on magnetic tapes. A portion of these signals is processed directly-on-line with a digital computer. From these shadowgraphs and probe measurements, we observe that
  1. Far downstream of the obstacle where the turbulence has already subsided, the stratified fluid always has a layered structure. This layered structure persists for a long time, and is a result of the convection of turbulently mixed layers by the mean flow. These results indicate that in the regions of a stably stratified atmosphere and ocean where the turbulence has subsided, one could often find layered structure.
  2. There are spectral peaks and valleys in the measured velocity and salinity autospectra when the stratifications are sufficiently strong. Under certain conditions, these spectral peaks tend to lift up the spectral curves to show substantialf ?5/3 subranges, although the turbulence Reynolds numbers are too low for the flows to have recognizable inertial subranges. This anomalousf ?5/3 subrange demonstrates the pitfalls of using spectral measurements in thef ?5/3 subrange to predict the turbulent energy dissipation rate through the Kolmogorov hypothesis.
  3. A diagnostic method is developed for distinguishing internal waves from turbulence, utilizing their phase characteristics. The phase characteristics can be conveniently examined from the cospectra and quadrature spectra measurements of: (a), two vertically separated velocity probes; (b), two vertically separated density probes; and (c), a velocity probe and a density probe. This method is demonstrated to be useful in the laboratory and can be applied directly to atmospheric and oceanic measurements to distinguish internal waves from turbulence.
  4. From the coherency measurements, it is found that the entire turbulent stratified wake is actually whipping up and down at a frequency corresponding to the Brunt-Väisälä frequency. This indicates that similar stratified shear flows in the atmosphere and in the ocean, such as the jet streams in the atmosphere and the Cromwell current in the ocean, may oscillate vertically, which in turn can induce horizontal oscillation and meandering.
  相似文献   

15.
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16.
Large-eddy simulation is used to study secondary circulations in the convective boundary layer modulated as a result of horizontally varying surface properties and surface heat fluxes over flat terrain. The presence of heat flux heterogeneity and its alignment with respect to geostrophic wind influences the formation, strength and orientation of organized thermals. Results show boundary-attached roll formation along heat flux maxima in the streamwise direction. The streamwise organization of the updrafts and downdrafts formed downwind of heterogeneities leads to counter-rotating secondary circulations in the crosswind plane. The distribution of resolved-scale pressure deviations shows large pressure gradients in the crosswind plane. Spanwise and vertical velocity variances and heat flux profiles depict considerable spatial variability compared to a homogeneous forest simulation. Secondary circulations are observed for various ambient wind scenarios parallel and perpendicular to heterogeneities. In the presence of increased wind speed, thermals emerging from the heat flux heterogeneity are elongated, and organize along and downwind of large-scale heterogeneity in the streamwise direction. Simulation with a reduced heat flux shows a shallower circulation with a lower aspect ratio. Point measurements of heat flux inside the roll circulation could be overestimated by up to 15–25% compared to a homogeneous case.  相似文献   

17.
This study documents simulated oceanic circulations and sea ice by the coupled climate system model FGOALS-f3-L developed at the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, under historical forcing from phase 6 of the Coupled Model Intercomparison Project (CMIP6). FGOALS-f3-L reproduces the fundamental features of global oceanic circulations, such as sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), vertical temperature and salinity, and meridional overturning circulations. There are notable improvements compared with the previous version, FGOALS-s2, such as a reduction in warm SST biases near the western and eastern boundaries of oceans and salty SSS biases in the tropical western Atlantic and eastern boundaries, and a mitigation of deep MLD biases at high latitudes. However, several obvious biases remain. The most significant biases include cold SST biases in the northwestern Pacific (over 4°C), freshwater SSS biases and deep MLD biases in the subtropics, and temperature and salinity biases in deep ocean at high latitudes. The simulated sea ice shows a reasonable distribution but stronger seasonal cycle than observed. The spatial patterns of sea ice are more realistic in FGOALS-f3-L than its previous version because the latitude–longitude grid is replaced with a tripolar grid in the ocean and sea ice model. The most significant biases are the overestimated sea ice and underestimated SSS in the Labrador Sea and Barents Sea, which are related to the shallower MLD and weaker vertical mixing.  相似文献   

18.
Abstract

We look at the development of the first plumes that emerge from a convectively unstable boundary layer by modelling the process as the instability of a fluid with a time‐dependent mean density field. The fluid is semi‐infinite, rotating, dissipative ‐ characterized by the ratio of its viscosity to thermal diffusivity (Prandtl number Pr = ν/κ) ‐ and initially homogeneous. A constant destabilizing heat flux is applied at the boundary and the stability of the evolving density field is investigated both mathematically and in laboratory experiments.

Using a “natural convective” scaling, we show that the behaviour of the non‐dimensional governing equations depends on Pr and the parameter γ = f(ν/B)1/2, where f is the Coriolis parameter, and B is the applied buoyancy flux. For the ocean, γ ≈ 0.1, whilst for the atmosphere γ ≈ 0.01. In the absence of rotation, the behaviour of the differential equations is independent of B, depending only on Pr. The boundary‐layer Rayleigh number (Rabl) is also independent of B. We show that Rabl, evaluated at the onset of rapid vertical motion, depends on the form of the perturbation.

Due to the time‐dependence of the mean density field, analytic instability analysis is difficult, so we use a numerical technique. The governing equations are transformed to a stretched vertical coordinate and their stability investigated for a particular form of perturbation function. The model predictions are, for the ocean: instability time ～2–4 h, density difference ～0.002–0.013 kg m‐3, boundary‐layer thickness ～50–75 m and horizontal scale ～200–300 m; and for the atmosphere: instability time ～10 min, temperature difference ～2.0–3.0°C, boundary‐layer thickness ～400–500 m and horizontal scale ～1.5–2.0 km.

Laboratory experiments are performed to compare with the numerical predictions. The time development of the mean field closely matches the assumed analytic form. Furthermore, the model predictions of the instability timescale agree well with the laboratory measurements. This supports the other predictions of the model, such as the lengthscales and buoyancy anomaly.  相似文献   

19.
Several numerical experiments have been undertaken with a three-dimensional mesoscale model in order to determine to what extent a water surface such as a lake can influence mesoscale flow patterns.It is shown that the influence of the lake is important when cumulus clouds are present. These clouds, generated by evaporation from the water surface are small but induce significant secondary circulations which disrupt the flow field on the mesoscale.Artificial suppression of cloud activity results in a situation where the lake exerts little influence on the atmospheric environment in comparison to the control experiment where the body of water is absent. Atmospheric stability controls the intensity of perturbations to the mean flow when clouds are present.The study is of interest when modeling a number of complex phenomena simultaneously; the results shown here indicate that under certain stability conditions, a small lake can be ignored as to its dynamic and thermodynamic influence on atmospheric processes, thus leading to a neglect of a number of equations taking into account moisture terms explicitly.  相似文献   

20.
RAMS 2001: Current status and future directions   总被引：21，自引：0，他引：21  
Summary ?An overview of the Regional Atmospheric Modeling System (RAMS) is presented. We focus on new developments in the RAMS physics and computational algorithms since 1992. We also summarize some of the recent applications of RAMS that includes synoptic-scale weather systems and climate studies, to small-scale research using RAMS configured as a large eddy simulation model or to even flow around urban buildings. The applications include basic research on clouds, cloud systems, and storms, examination of interactions between tropical deep convective systems and ocean circulations, simulations of tropical cyclones, extreme precipitation estimation, regional climatic studies of the interactions between the atmosphere and the biosphere or snow-covered land-surfaces, prototype realtime mesoscale numerical weather prediction, air pollution applications, and airflow around buildings. Received March 12, 2001; revised August 28, 2001  相似文献   

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