Description of coastline variations in an ocean general circulation model

A wetting and drying algorithm is considered and implemented in a three-dimensional sigma–z coordinate model of ocean thermo- and hydrodynamics. The algorithm is tested in two idealized experiments simulating the run-up of a tidal wave on the coast and in a realistic experiment simulating the evolution of the Caspian Sea coastline in the 20th century.     

Delaunay mesh generation for an unstructured-grid ocean general circulation model

An incremental method is presented to generate automatically boundary-fitted Delaunay triangulations of the global ocean. The method takes into account Earth curvature and allows local mesh refinement in order to resolve topological or dynamical features like midocean ridges or western boundary currents. Crucial issues like the nodes insertion process, the boundary integrity problem or the creation of inner nodes are explained. Finally, the quality of generated triangulations is discussed.     

Interannual to decadal Gulf Stream variability in an eddy-resolving ocean model

Meridional shifts of the Gulf Stream (GS) jet on interannual to decadal timescales and the corresponding oceanic changes around the GS are investigated using a near global eddy-resolving ocean model hindcast from 1960 to 2003. The simulated variability in the shifts of the GS jet axis shows good agreement with observations, and lags atmospheric fluctuations characterized by the North Atlantic Oscillation by about 2 years. This lagged response of the GS jet to the atmospheric variations is attributed to the westward propagation of the undulation of the jet axis from 45°W to 75°W, which has a wavelength of about 4000 km and a displacement of 0.5°. The propagation direction and phase speed of about 2.8 cm s⁻¹ are consistent with the thin-jet theory. The shifts of the jet axis in the downstream region are likely induced by wind fluctuations through Ekman convergence over the central North Atlantic. Associated with the northward (southward) shift of the jet axis, sea surface temperature is warming (cooling) around and north of the jet, and the former warming has a deep and meridionally narrow subsurface structure, consistent with the northward shift of the jet. The meridional shifts of the jet accompany coherent meridional shifts of energetic eddy activity regions around the GS. Our numerical results suggest that the GS jet brings the atmospheric signals from the central to the western North Atlantic, and the resultant meridional shift of the jet induces the notable oceanic changes around the GS.     

Implementation of variable time stepping in an ocean general circulation model

Ocean general circulation models (OGCMs) which represent the governing equations on a finite difference grid require shorter time steps with increasing resolution. Thus, until now, in the absence of filtering, the time step length has been determined by the smallest grid spacing within the model domain. Here we present a method for reducing the time step length (and increasing the number of time steps taken) at selected points in the grid, so as to minimise the computational cost of integrating the OGCM, whilst achieving numerical stability throughout the model domain without filtering. This variable time stepping method can be used to overcome numerical constraints associated with the convergence of longitude–latitude grids at the poles, and also to allow efficient integration of model domains with variable resolution. Examples of the computational saving are given.     

涡分辨率模式模拟的南海中尺度涡旋

Mesoscale eddies(MEs) in the South China Sea(SCS) simulated by a quasi-global eddy-resolving ocean general circulation model are evaluated against satellite data during 1993–2007. The modeled ocean data show more activity than shown by the satellite data and reproduces more eddies in the SCS. A total of 345(428) cyclonic eddies(CEs) and 330(371) anti-cyclonic eddies(AEs) generated for satellite(model) data are identified during the study period, showing increase of ~24% and ~12% for the model data, respectively. Compared with eddies in satellite, the simulated eddies tend to have smaller radii, larger amplitudes, a slightly longer lifetime, faster movement and rotation speed, a slightly larger nonlinear properties(U/c) in the model. However, the spatial distribution of generated eddies appears to be inhomogeneous, with more CEs in the northern part of SCS and fewer AEs in the southern part. This is attributed to the exaggerated Kuroshio intrusion in the model because the small islands in the Luzon Strait are still not well resolved although the horizontal resolution reaches(1/10)°. The seasonal variability in the number and the amplitude of eddies generated is also investigated.     

Assessment of mixed layer models embedded in an ocean general circulation model

The atmospheric mixed layer obtained using the Mellor–Yamada model grows slower and becomes shallower than observed, which motivated Nakanishi and Niino (J Meteorol Soc Jpn 87:895–912, 2009) to present a modified version of the Mellor–Yamada model. In this study, incorporating each of the Mellor–Yamada and the Nakanishi–Niino models into an ocean general circulation model, we evaluate its performance in the ocean. Comparing the numerical results with the observed ones in the western North Pacific, the Nakanishi–Niino model is shown to exhibit a better performance than the Mellor–Yamada model under strong wind forcing and sea surface cooling during winter and after passage of typhoons during summer.     

涡分辨率海洋模式及其海气耦合模式中吕宋海峡处的黑潮入侵

比较了准全球涡分辨率海洋模式（简记为LICOMH）及其海气耦合模式（简记为LICOMHC）中的黑潮入侵南海与观测中黑潮入侵的差异。我们发现在单独海洋模式中黑潮入侵与观测相比过强,而在其海气耦合模式中这一差异得到了改善。冬季的吕宋海峡输送（LST）在LICOMH中为-8.8×10⁶ m³s^-1,而在LICOMHC中则下降到-6.0×10⁶ m³s^-1 。进一步的研究表明是大尺度风场,局地风应力和吕宋海峡以东中尺度涡旋的共同作用导致了黑潮入侵在两个模式中的不同。LICOMH中吕宋岛东北部相对较强的气旋导致了较弱的黑潮输送及吕宋海峡处较强的黑潮入侵。以上三者共同作用造成的LST差异大约是2.0×10⁶ m³s^-1,与两个模式间的LST差异大小基本相当。进一步对LICOMH与LICOMHC中的EKE收支进行分析表明,LICOMH中更强的EKE输送及斜压转换项导致了黑潮以东存在更强的气旋,而海表风场对两个模式中的涡旋差异贡献极小。     

Low-frequency variations of the Eastern Subtropical Front in the North Pacific in an eddy-resolving ocean general circulation model: roles of central mode water in the formation and maintenance

Temporal variations (1960–2005) of the Eastern Subtropical Front (ESTF) in the North Pacific are investigated using historical-run output data of the eddy-resolving Meteorological Research Institute Community Ocean Model, forced by atmospheric reanalysis dataset. Simulated ESTF is distributed in a region of zonal band of 24°N–30°N east of the International Date Line, and is located at the southern boundary of the central mode water (CMW) north of the front. The ESTF intensity clearly shows an interdecadal variation with a timescale of about 20?years. This variation is associated with that in the potential vorticity of CMW, which originates in the CMW formation region farther north about 3?years earlier due to changes in the surface wind forcing.     

全球海洋环流模式中上层海洋对表面强迫的响应和调整 I. 年际变率

利用一个较高分辨率的全球海洋环流模式在COADS1945～1993年逐月平均资料的强迫下对海温和环流场进行了模拟试验,研究了全球热带海洋(主要是热带太平洋)海温和环流场的年际变化特征及模式ENSO冷暖事件演变的控制机理.结果表明,模式成功地再现了和观测一致的海温和环流的年际变化以及ENSO演变特征.其中热带印度洋年际SST变率的主要模态表现为与ENSO相联系的海盆尺度的一致性增暖或变冷现象,次级模态为热带印度洋偶极子模态;热带大西洋的SST年际变率表现为类ENSO的年际振荡现象.在热带太平洋,SST年际变化主要表现为ENSO型,环流的年际变率表现为与ENSO相对应的热带海洋质量循环圈的年际异常.对应于暖(冷)事件,前期赤道海洋垂直环流圈显示出减弱(增强)的特征.其中南赤道流异常的位相较Nino3区海温总体要超前5个月左右的时间;赤道上翻流异常的位相在表层要超前4个月,并随时间由上至下扩展;赤道潜流的异常则显示出东传特征,其中最早的较为显著的异常发生ENSO成熟前3个月180°附近.在模式ENSO冷暖事件的演变过程中,次表层海温异常沿赤道的东传起了关键作用,模式的ENSO模态主要表现为"时滞振子"模态.     

Recent changes in the North Atlantic circulation simulated with eddy-permitting and eddy-resolving ocean models

Three eddy-permitting (1/4°) versions and one eddy-resolving (1/12°) version of the OCCAM ocean model are used to simulate the World Ocean circulation since 1985. The first eddy-permitting simulation has been used extensively in previous studies, and provides a point of reference. A second, improved, eddy-permitting simulation is forced in the same manner as the eddy-resolving simulation, with a dataset based on a blend of NCEP re-analysis and satellite data. The third eddy-permitting simulation is forced with a different dataset, based on the ERA-40 re-analysis data. Inter-comparison of these simulations in the North Atlantic clarifies the relative importance of resolution and choice of forcing dataset, for simulating the mean state and recent variability of the basin-scale circulation in that region. Differences between the first and second eddy-permitting simulations additionally reveal an erroneous influence of sea ice on surface salinity, dense water formation, and the meridional overturning circulation. Simulations are further evaluated in terms of long-term mean ocean heat transport at selected latitudes (for which hydrographic estimates are available) and sea surface temperature errors (relative to observations). By these criteria, closest agreement with observations is obtained for the eddy-resolving simulation. In this simulation, there is also a weak decadal variation in mid-latitudes, with heat transport strongest, by around 0.2 PW, in the mid-1990s. In two of the eddy-permitting simulations, by contrast, heat transport weakens through the study period, by up to 0.4 PW in mid-latitudes. The most notable changes of heat transport in all simulations are linked to a weakening of the subpolar gyre, rather than changes in the meridional overturning circulation. It is concluded that recent changes in the structure of mid-latitude heat transport in the North Atlantic are more accurately represented if eddies are explicitly resolved.     

Tidally driven mixing in a numerical model of the ocean general circulation

Astronomical data reveals that approximately 3.5 terawatts (TW) of tidal energy is dissipated in the ocean. Tidal models and satellite altimetry suggest that 1 TW of this energy is converted from the barotropic to internal tides in the deep ocean, predominantly around regions of rough topography such as mid-ocean ridges. A global tidal model is used to compute turbulent energy levels associated with the dissipation of internal tides, and the diapycnal mixing supported by this energy flux is computed using a simple parameterization.The mixing parameterization has been incorporated into a coarse resolution numerical model of the global ocean. This parameterization offers an energetically consistent and practical means of improving the representation of ocean mixing processes in climate models. Novel features of this implementation are that the model explicitly accounts for the tidal energy source for mixing, and that the mixing evolves both spatially and temporally with the model state. At equilibrium, the globally averaged diffusivity profile ranges from 0.3 cm² s⁻¹ at thermocline depths to 7.7 cm² s⁻¹ in the abyss with a depth average of 0.9 cm² s⁻¹, in close agreement with inferences from global balances. Water properties are strongly influenced by the combination of weak mixing in the main thermocline and enhanced mixing in the deep ocean. Climatological comparisons show that the parameterized mixing scheme results in a substantial reduction of temperature/salinity bias relative to model solutions with either a uniform vertical diffusivity of 0.9 cm² s⁻¹ or a horizontally uniform bottom-intensified arctangent mixing profile. This suggests that spatially varying bottom intensified mixing is an essential component of the balances required for the maintenance of the ocean’s abyssal stratification.     

A reduced grid for a parallel global ocean general circulation model

A limitation of many global climate models with explicit finite-difference numerics is the timestep restriction caused by the decrease in cell size associated with the convergence of meridians near the poles. To keep the longitudinal width of model cells as uniform as possible, we apply a “reduced” grid to a three-dimensional primitive equation ocean-climate model. With this grid the number of cells in the longitudinal direction is reduced at high latitudes. The grid consists of subgrids which interact at interfaces along their northern and southern boundaries, where the resolution changes by a factor of three. We extend the finite-difference techniques to these interfaces, focusing on the conservation required to perform long time integrations, while preserving the staggered spatial arrangement of variables and the numerics used on subgrids. The common alternative used to reduce the timestep restriction caused by the spherical grid is the filtering of high-frequency modes from the high-latitude solution. The reduced grid allows an increased timestep while eliminating the need for filtering and reduces execution time per model step by roughly 20%. We implement the reduced grid model for parallel computer architectures with two-dimensional domain decomposition and message passing, with speedup results similar to those of the original model. We present results of model runs showing small effects on the solution and sizable improvements to the execution time.     

Structure of the subsurface counter current beneath the Tsushima warm current simulated by an ocean general circulation model

The subsurface counter current beneath the Tsushima Warm Current is simulated using a three-dimensional circulation model. The model well reproduces the counter current beneath the Tsushima Warm Current on the shelf break. The counter current appears as nearshore parts of the subsurface clockwise circulations from spring to early winter. The clockwise circulations are separated by developed shelves such as the Oki Spur and the Noto Peninsula, thus the counter current is not a continuous flow along the Japanese coast in this model. The vertical structure of the counter current can be explained by a density structure with the thermal wind relationship. The permanent and seasonal pycnoclines form mutually opposite horizontal density gradients near the Japanese coast in summer. Such a density structure results in a speed maximum of the counter current away from the bottom. It is remarkable that the second baroclinic mode is dominant in nearshore parts of the subsurface clockwise circulations in summer, which are attributed to the density structure. Similar density structures are also found in some coastal regions of the world oceans where subsurface counter currents are expected.     

Towards the general circulation of the North Atlantic ocean

After reviewing the inverse method, we apply it to deducing the general circulation of the North Atlantic ocean. We argue that the method is purely classical in nature, being nothing more than a mathematical statement of the principles upon which nearly all previous circulation schemes have been based. The ‘smoothed’ solution is shown to represent the components of the flow field that are determinable independently of the initial reference level. We then produce two circulation schemes based upon two different initial reference levels — 2000 decibars and the bottom — called North Atlantic-1A and North Atlantic-1B respectively. The models share many features in common and are strikingly similar to several previous schemes, most notably those of Jacobsen and Defant in the region west of Bermuda. No simple level-of-no-motion emerges in the flow fields; rather the velocity sections exhibit a complex cellular structure. Zonally integrated meridional cells of models and of the uniquely determined components are very similar, showing a poleward movement of warm saline water compensated at depth by a return flow of cold, fresher water. The magnitudes of the implied polar sea overflows and the heat fluxes are in good agreement with previous estimates. Finally, it is argued that neither these model circulations nor any other circulation pattern based upon the existing data can be regarded as actually representing the true time average ocean circulation because the data are aliased in time; the frequency/wavenumber spectrum of the ocean is inadequately known to determine the resulting errors.     

Buoy measured wind, wind stress and wind stress curl over the shelf off Bodega Bay, California

An array of five buoys and three coastal stations is used to characterize the winds, stress, and curl of the wind stress over the shelf off Bodega Bay, California. The wind and wind stress are strong and persistent in the summer and weak in the winter. In the summer, wind and stress decrease strongly across the shelf, toward the coast. Combinations of buoys are used to compute the curl of the wind stress over different portions of the shelf. The mean summer 2001 curl of the wind stress over the array depends upon the area selected, varying between −1.32×10⁻⁶ and +7.80×10⁻⁶ Pa m⁻¹. The winter 2002 wind-stress curl also depends on location, varying from −2.06×10⁻⁶ to +2.78×10⁻⁶ Pa m⁻¹. Mean monthly curl of the wind stress is a maximum in the summer and a minimum near zero in the winter. In both the summer and the winter, the correlation between the wind-stress curl for different portions of the shelf varies between moderate negative, though insignificance, to high positive. A wind measurement at a single point can be poorly related to the measured curl of the wind stress at other locations over the shelf. The measurements show that the use of one wind measurement to characterize the curl of the wind stress over the shelf without further investigation of the local wind-stress curl structure is risky.     

一个高分辨率太平洋-印度洋海盆环流模式的初步结果

利用LASG/IAP发展的一个0.25°×0.25°高分辨率太平洋-印度洋海盆环流模式,初步分析了模式在太平洋区域的模拟结果,并与海洋同化资料以及前人的研究结果作比较,检验此模式对该区域平均气候态、年际变化的模拟能力。分析表明,模式较好地再现了海表温度（SST）分布、赤道温跃层和纬向流结构、赤道流系分布形态、海表高度以及正压流函数空间分布特征;同时,对显著的El Ni?o和La Ni?a事件的模拟等方面与Simple Ocean Data Assimilation（SODA）2.0.2版本结果相近。此外,模式模拟北赤道流（NEC）分叉点位置的季节和年际变化以及吕宋海峡流量的年际变化与已有研究结果基本一致。进一步分析还发现,在年际尺度上,NEC分叉点位置和吕宋海峡流量与ENSO密切相关。     

Concurrent simulation of the eddying general circulation and tides in a global ocean model

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.     

风场对全球海洋CFC-11 吸收的影响

为了在中国科学院大气物理研究所的全球海洋模式(LASG/IAP Climate Ocean Model,LICOM)中研究不同的风场引起的海气传输速度对三氯一氟甲烷(CFC-11)在海洋中的分布和吸收产生的影响,同时选出更适用于LICOM模式模拟海洋对气体吸收的风场,本文做了4组对比实验,即传输速度为常数(实验-C)以及依赖3个不同风场(Esbensen and Kushnir观测风场(实验-EK)、National Centers for Environmental Prediction(NCEP)再分析风场(实验-NP)、QuikSCAT卫星风场(实验-QS))的实验。在对比分析中重点考察了CFC-11的海气通量、海表浓度、水柱总量、传输过程等。结果显示,不同风场带来的传输速度差异会造成模拟结果在局部海域存在显著差异,但在大部分海域的差异并不显著。而且随着积分时间的增加,从海气通量、存储量两方面可以看出,不同模拟结果之间的差异有着减小的趋势。如1955年1月北大西洋局部地区,不同的风场下的海气通量模拟结果间的差异达到20%左右,到了1995年1月这种差异下降到15%左右。此外,传输速度的选取依赖风场计算值的全球平均值(试验-C)会造成模拟结果相对偏小,而风速相对较大的QuikSCAT卫星风场资料在一定程度上使得模式模拟结果与观测资料更为接近。