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1.
The response time of the large-scale oceanic circulation due to freshwater perturbations is investigated with models of different complexity. A three-dimensional ocean general circulation model (OGCM) and a zonally averaged ocean model are employed. In order to distinguish advection and diffusion from fast baroclinic processes (e.g. waves in the OGCM) a color tracer is injected at the same time and location as the freshwater discharges. In spite of the inability of the zonally averaged model to represent wave processes in a realistic way similarities with the OGCM are found for the propagation patterns of density anomalies and of color tracer. In the OGCM as well as in the zonally averaged model, density anomalies propagate faster than anomalies of a passive color tracer in the case of vertical density stratification. The progression of density anomalies leads to changes of the oceanic circulation, and both oceanic models exhibit circulation changes in areas distant from the discharge places long before the passively entrained color tracer has reached these regions. The fact that a zonally averaged model simulates baroclinic processes faster than advection even if internal gravity waves are not represented due to neglected acceleration terms, is clarified with a conceptual box model. 相似文献
2.
A new method of assimilating sea surface height (SSH) data into ocean models is introduced and tested. Many features observable by satellite altimetry are approximated by the first baroclinic mode over much of the ocean, especially in the lower (but non-equatorial) and mid latitude regions. Based on this dynamical trait, a reduced-dynamics adjoint technique is developed and implemented with a three-dimensional model using vertical normal mode decomposition. To reduce the complexity of the variational data assimilation problem, the adjoint equations are based on a one-active-layer reduced-gravity model, which approximates the first baroclinic mode, as opposed to the full three-dimensional model equations. The reduced dimensionality of the adjoint model leads to lower computational cost than a traditional variational data assimilation algorithm. The technique is applicable to regions of the ocean where the SSH variability is dominated by the first baroclinic mode. The adjustment of the first baroclinic mode model fields dynamically transfers the SSH information to the deep ocean layers. The technique is developed in a modular fashion that can be readily implemented with many three-dimensional ocean models. For this study, the method is tested with the Navy Coastal Ocean Model (NCOM) configured to simulate the Gulf of Mexico. 相似文献
3.
《Ocean Modelling》2002,4(2):121-135
Numerical studies of surface ocean fronts forced by inhomogeneous buoyancy loss show nonhydrostatic convective plumes coexisting with baroclinic eddies. The character of the vertical overturning depends sensitively on the treatment of the vertical momentum equation in the model. It is less well known how the frontal evolution over scales of O(10 km) is affected by these dynamics. Here, we compare highly resolved numerical experiments using nonhydrostatic and hydrostatic models and the convective-adjustment parametrization. The impact of nonhydrostatic processes on average cross-frontal transfer is weak compared to the effect of the O(1 km) scale baroclinic motions. For water-mass distribution and formation rate nonhydrostatic dynamics have similar influence to the baroclinic eddies although adequate resolution of the gradients in forcing fluxes is more important. The overall implication is that including nonhydrostatic surface frontal dynamics in ocean general circulation models will have only a minor effect on scales of O(1 km) and greater. 相似文献
4.
The generation of small meanders of the Kuroshio off southern Kyûshû is investigated. Basing on the fact that the small meanders tend to follow an increase in velocity of the Kuroshio in the Satsunan region (Sekine andToba, 1981), the influence of this velocity increase on the quasi-stationary path of the Kuroshio is studied numerically. Simplified bottom and coastal topographies are employed in a two layer model ocean. A quasi-stationary numerical solution with a constant inflow is used for the initial condition, and a temporal increase in the inflow with corresponding leakage is employed as the boundary condition to investigate nonlinear effects due to the increase in current velocity. Experiments for four different physical models are carried out to determine the specific roles of the continental slope, the planetary-effect, and density stratification. Temporal increase in the inflow tends to cause offshore shift of the current path. But the topographic effect of the continental slope is strong enough that no significant shift of the current path occurs in the case of the barotropic ocean. However, in the case of a baroclinic ocean, temporal increase in the inflow does cause generation of small meanders, because density stratification diminishes the topographic effect. A larger density stratification provides more favorable condition for the appearance of the small meander, and a cyclonic eddy is formed on the continental side of the small meander path. 相似文献
5.
Much has been written of the error in computing the baroclinic pressure gradient (BPG) with sigma coordinates in ocean or atmos- pheric numerical models. The usual way to reduce the error is to subtract area-averaged density stratification of the whole computa- tion region. But if there is great difference between the area-averaged and the local averaged density stratification, the error will be obvious. An example is given to show that the error from this method may be larger than that from no correction sometimes. The definition of local area is put forward. Then, four improved BPG difference schemes of subtracting the local averaged density strat- ification are designed to reduce the error. Two of them are for diagnostic calculation (density field is fixed), and the others are for prognostic calculation (density field is not fixed). The results show that the errors from these schemes all significantly decrease. 相似文献
6.
《Oceanologica Acta》2003,26(5-6):597-607
A three-dimensional baroclinic shelf sea model is employed to simulate the tidal and non-tidal residual current in the South China Sea. The four most significant constituents, M2, S2, K1 and O1, are included in the experiments with tidal effect. At most stations, the computed harmonic constants agree well with the observed ones. The circulations of the South China Sea in summer (August) and winter (December) are mainly discussed. It is shown that the barotropic tidal residual current is too weak to affect the South China Sea circulation, whilst the contribution of the baroclinic tidal residual current to the South China Sea circulation would be important in the continental shelf sea areas, especially in the Gulf of Thailand and Gulf of Tonkin. In the deep-sea areas, the upper barotropic or baroclinic tidal residual current is relatively very weak, however, the speed order of the deep baroclinic tidal residual current can be the same as that of the mean current without tidal effect. Moreover, the baroclinic tidal residual current seems to be related to the different seasonal stratification of ocean. 相似文献
7.
The satellite altimeter data reveal that intraseasonal long Rossby wave is amplified in the western part of subtropical ocean. Based on a two and half layer ocean model we infer that the intraseasonal long Rossby wave may be amplified by the baroclinic instability. According to the baroclinic instability criterion derived from the two and half layer model, we calculate the baroclinic instability area of the Subtropical North Pacific Ocean based on Levitus98 data. The baroclinic instability area is well in accord with the amplification area of the intraseasonal long Rossby wave, and this also proves that the baroclinic instability is the main amplification mechanism of the intraseasonal long Rossby wave in the subtropical ocean. The consistency between the baroclinic instability area and potential vorticity (PV) pool is further proved in this paper, therefore, we have confidence that the intraseasonal long Rossby wave is amplified in the PV pool. Due to the relatively large ocean basin and weak ventilation, the PV pool is much larger in the North Pacific Ocean than in the North Atlantic Ocean, and this is the reason for the difference of wave amplification areas of these two Oceans. 相似文献
8.
1Introduction Planetary waves involve the main processes bywhich perturbation signal in one part of the ocean istransferred to another part.In the1970s,mostof theprevious theoretical studies focus on the local Ekmanpumping and the nondispersive long baroc… 相似文献
9.
Nobuo Suginohara 《Journal of Oceanography》1973,29(6):236-250
Effect of the typhoon passage on the western boundary region of a two-layer ocean with bottom topography is studied. The ocean is initially at rest and is set in motion by a typhoon passing parallel to the west coast. Equations that represent barotropic and baroclinic modes of motions are solved numerically by means of the method of finite differences. Motions of the barotropic mode are assumed to be horizontally non-divergent. In this mode, an elongated vortex is produced by the typhoon and propagates toward the south after passage of the typhoon. Behavior of the vortex may be interpreted as continental shelf waves. It is found that the formation and propagation of continental shelf waves are hardly affected by the density stratification. As for the baroclinic response, the typhoon causes considerable interface displacements along its track. The interface displacements are associated with geostrophic motions and remain for long time, though they are formed on the continental slope. Besides the large scale baroclinic response, internal Kelvin waves are induced along the artificial east wall. 相似文献
10.
B. A. Kagan E. V. Sofina A. A. Timofeev 《Izvestiya Atmospheric and Oceanic Physics》2010,46(2):224-231
In order to reproduce the diapycnal mixing induced by internal tidal waves (ITWs) in the Arctic Ocean, we use a modified version
of the three-dimensional finite-element hydrothermodynamic model QUODDY-4. We found that the average (over the tidal cycle)
and integral (by depth) baroclinic tidal energy dissipation rate in individual areas of the Siberian continental shelf and
in the straits between the Canadian Arctic archipelago are much higher than in the open ocean and its values on ridges and
troughs are qualitatively similar to one another. Moreover, in the area of open-ocean ridges, the baroclinic tidal energy
dissipation rate increases as it approaches the bottom, but only in the bottom boundary layer; on the Mid-Atlantic and Hawaii
ridges, such an increase is observed within a few hundreds of meters away from the bottom. The average (in area and depth
of the open ocean) coefficient of diapycnal mixing defined by the baroclinic tidal energy dissipation rate is higher than
the coefficient of molecular kinematic viscosity and only a few times lower than the canonical value of the coefficient of
vertical turbulent viscosity, which is used in models of global oceanic circulation. Coupled with the reasoning on the localization
of baroclinic tidal energy dissipation, this fact leads to the conclusion that disregarding the contribution that ITW-induced
diapycnal mixing makes to the ocean-climate formation is hardly justified. 相似文献
11.
针对中、低纬太平洋ARGO测站的4个实际层结个例,计算了无背景流下准地转海洋Rossby内波的垂直模态。结果表明,海水层结垂直方向的不均匀和跃层的存在,使该垂直模态的波型发生了歪曲变形,致使其偏离了余弦波：跃层越强,这种变形也越大：层结最强的层次对应着最大的波动振幅,多极值的层结对应着多个极大振幅：该内波垂直结构函数的零点随模态序数依次递增,且其零点在海水层结较强的层次要比较弱的层次密集。 相似文献
12.
《Ocean Modelling》2010,33(3-4):175-187
This paper presents a five-year global simulation of HYCOM, the HYbrid Coordinate Ocean Model, that simultaneously resolves the eddying general circulation, barotropic tides, and baroclinic tides with 32 layers in the vertical direction and 1/12.5° (equatorial) horizontal grid spacing. A parameterized topographic wave drag is inserted into the model and tuned so that the surface tidal elevations are of comparable accuracy to those in optimally tuned forward tide models used in previous studies. The model captures 93% of the open-ocean sea-surface height variance of the eight largest tidal constituents, as recorded by a standard set of 102 pelagic tide gauges spread around the World Ocean. In order to minimize the impact of the wave drag on non-tidal motions, the model utilizes a running 25-h average to approximately separate tidal and non-tidal components of the near-bottom flow. In contrast to earlier high-resolution global baroclinic tide simulations, which utilized tidal forcing only, the simulation presented here has a horizontally non-uniform stratification, supported by the wind- and buoyancy forcing. The horizontally varying stratification affects the baroclinic tides in high latitudes to first order. The magnitude of the internal tide perturbations to sea surface elevation amplitude and phase in a large box surrounding Hawai’i is quite similar to that observed in satellite altimeter data, although the exact locations of peaks and troughs in the modeled perturbations differ from those in the observed perturbations. 相似文献
13.
The response of the thermocline to changes in atmospheric forcing are explored in two 50-year hindcast North Pacific model runs. The model runs only differ in their resolution and horizontal viscosity values. The thermocline response is explored through a modal decomposition. The first baroclinic mode response is qualitatively similar in both model runs, with a somewhat smaller response in the lower resolution model. This mode is primarily wind driven. The second baroclinic mode shows a larger response at midlatitudes in the low-resolution model than in the higher resolution model. This is consistent with the presence of very large-scale baroclinic instability in the return flow of the subtropical gyre at low-resolution, and represents a spurious response of the ocean model to large horizontal viscosity. This spurious mode of decadal variability in the thermocline is maintained even when there is variability in atmospheric forcing. This result suggests that care must be taken in interpretation of the realism of mid-latitude modes of variability centered in western boundary current extensions of coupled ocean–atmosphere models. 相似文献
14.
A study was conducted to understand the mechanisms driving observed subtidal variability in the stratification of Saldanha Bay, located in the southern Benguela system. It was found that the 6–8 day period variability in bay stratification was caused by the inflow and outflow of cold upwelled water driven by changing baroclinic pressure gradients between the coastal and bay domains. The direction and magnitude of the pressure gradients were governed by coastal upwelling activity and a lag in the response of the bay to changes in density structure in the coastal ocean. When the pressure gradients were bayward and cold water was being driven into the bay the cycle was termed to be in an ‘ active phase ’ and the reverse was termed the ‘ relaxation phase ’. The upwelling-favourable equatorward wind stress impacted the bay stratification in two ways: on the regional scale, wind drives upwelling and governs the inflow–outflow of cold upwelled bottom water, which strengthens stratification; conversely, on the local bay scale, wind drives vertical mixing, which weakens stratification. A four-phase model is used to describe the observed variability in stratification in the bay. The associated density-driven exchange flows are capable of flushing the bay in 6–8 days, about one-third of the time for tidal exchange alone (c. 25 days). These inflows of cold bottom water are ecologically critical as they supply nutrients to the bay and thus impose a control on new production within the bay environment. Further ecological implications of this bay–ocean exchange include export of phytoplankton new production to the coast, limitation of the risk of harmful algal blooms (HABs) and the division of the system into two distinct ecosystems (bay and lagoon). 相似文献
15.
Hao Liu 《Journal of Oceanography》2007,63(1):67-75
In general, competition between buoyancy mechanisms and mixing dynamics largely determines the water column structure in a
shelf sea. A three dimensional baroclinic ocean model forced by surface heat fluxes and the 2.5 order Mellor-Yamada turbulence
scheme is used to simulate the annual cycle of the temperature in the Bohai Sea. The difference between the sea surface temperature
(SST) and sea bottom temperature (SBT) is used to examine the evolution of its vertical stratification. It is found that the
water column is well-mixed from October to March and that the seasonal thermocline appears in April, peaks in July and then
weakens afterwards, closely following the heat budget. In addition, the Loder parameter based on the topography and tidal
current amplitude is also computed in order to examine tidal fronts in the BS, which are evident in summer months when the
wind stirring mechanism is weak. 相似文献
16.
This paper reports on the ocean level seasonal variability in the tropical Atlantic calculated within the framework of a non-linear baroclinic model allowing for the bottom topography and the coastline when the real distributions of wind stress, temperature, and salinity in the upper ocean are preset. The results of the numerical experiment are compared with altimeter data and computations of the dynamic topography of the ocean surface.Translated by Mikhail M. Trufanov. 相似文献
17.
C. Hansen E. Kvaleberg A. Samuelsen 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(9):1079-1091
Altimetry and ocean color observations are used in combination with a coupled physical-primary production ocean model to investigate anticyclonic eddies at two locations in the Norwegian Sea. Of particular interest are the formation of the anticyclonic eddies, and their influence on primary production. The formation of these anticyclonic eddies are due to baroclinic instabilities set up by shifts in the wind in north/south direction, leading to simultaneously formation of eddies throughout the area. After a density stratification develops in the upper 100 m of the water column, the anticyclones become a subsurface lens of well mixed water with the characteristics of intra-thermocline eddies. The deep mixed layer inside anticyclonic eddies delay phytoplankton bloom by approximately two weeks compared to the surrounding areas. As the mixed layer within the anticyclones become smaller than the critical depth, the combination of this and sufficiently high nutrient levels support a phytoplankton bloom. From the satellite observations, there is an evidence of phytoplankton being advected toward the center of the eddies, but also of isolated phytoplankton blooms within the intra-thermocline eddies. The combined use of a numerical model and satellite observations provides three-dimensional information on the structure and properties of both eddies and primary production. The presented model is particularly useful in cloud-covered areas where ocean color images are frequently unavailable. 相似文献
18.
可渗透结构具有使波浪作用减弱的效应,而海水的层化及水波的非线性使结构的波绕射产生多层复杂机制。将可渗透结构应用于复杂海况条件中,海水的层化性、波浪的非线性及结构的透空性构成了波绕射的一个十分复杂的数学问题。该问题存在理论研究的必要性,而文章则着重探讨其数学分析的可能性。通过引入二层海的层化海模式及Stokes二阶波的非线性波模式,给出了二阶多色波对透空结构的波绕射的定解问题提法,提出了复合形式的二阶多色波辐射条件式及可渗透结构的二阶物面条件式,应用特征函数解法与积分法推导了多色波对结构绕射的一阶势解与二阶作用的耦合积分解式,并讨论了解式所涉及无穷积分的算法。 相似文献
19.
浪致混合对亚热带冬季海洋混合强度的影响 总被引：1，自引：1，他引：0
上层海洋在全球气候系统中起着至关重要的作用。对上层海洋层结及混合的模拟偏差一直是海洋和气候数值模式发展中悬而未决的问题。本文首先评估了CMIP5中45个模式对上层海洋层结模拟的偏差,确认了冬季亚热带地区海洋模式垂向混合偏强。随后,基于自然资源部第一海洋研究所地球系统模式(FIO-ESM v1.0),分别开展了1986?2005年期间包含和关闭海浪垂向混合情况下的数值实验,分析浪致混合对亚热带冬季海洋混合强度模拟的影响及机制。发现浪致混合使得气候模式中亚热带海域冬季的海洋层结增强,增强的层结使上层海洋更加稳定。首次揭示了增加浪致混合反而降低了海洋总体的垂向混合率:浪致混合使北半球冬季亚热带海域混合率从无浪实验的227 cm2/s降低到有浪实验的178 cm2/s,降低了21.6%;南半球冬季亚热带海域混合率从无浪实验的189 cm2/s降低到有浪实验的165 cm2/s,降低了12.7%。进一步分析发现,浪致混合主要是通过增加冬季亚热带海域上层海洋的热含量从而强化了海洋的层结,最终改善了气候模式对上层海洋混合的模拟。 相似文献
20.
Kinjiro Kajiura 《Journal of Oceanography》1974,30(6):271-281
The effect of stratification on very long-period waves trapped on a straight continental shelf of constant depth is examined for a two-layer model. There are 4 modes in this system. The characteristics of the mode with the largest phase velocity can be approximated by the barotropic mode. The mode corresponding to the barotropic shelf-wave mode is modified by the baroclinic motions significantly, and in the limit of very narrow shelf width, the mode characteristics are transformed from those of the barotropic shelf-wave to the baroclinic Kelvin wave if the long-shore wave length is larger than the internal deformation radius. In this case, the stratification has an apparent effect of increasing phase velocity of barotropic shelf-waves. The remaining two modes are dominated by baroclinic motions with significant contribution from barotropic motions: among which the one has a shelf-wave characteristics for small values of the shelf width and approaches the mode corresponding to the baroclinic Kelvin wave in shallower water for large shelf width and the other is a stationary mode. If the long-shore wave length is much shorter than the internal deformation radius, the motions in the upper and lower layers are decoupled: the surface and bottom modes analogous to those discussed byRhines (1970) appears.If the interface is deeper than the shelf depth, the stationary mode is absent and the characteristics of the third mode approaches those of the baroclinic double Kelvin wave mode as the shelf width increases. 相似文献