Effect of land–ocean contrast on the structure of anomaly tropical circulation

While the important role of land–ocean contrast (LOC) in the mean atmospheric circulation is well-known, an intriguing question remains as to whether LOC can also significantly influence the anomaly circulation. This question is particularly important in the tropics, where a large part of the variabilities is known to be due to convective internal dynamics, which in turn can be significantly affected by LOC. In the present work, we investigate this question using a model of convectively driven anomaly circulation in the tropics. Emphasizing the lower tropospheric flow, we adopt a model describing the horizontal dynamics of the first baroclinic mode on an equatorial β-plane, in the presence of moist feedbacks. We introduce LOC in both latitudinal and longitudinal directions. Land surface dryness is taken into account in the moisture budget through the control of evaporative flux. A constant non-latent heating term is used over land in order to represent radiative and sensible heating effects. First, a control run is performed, numerically, without any LOC using random initial perturbations. The gravest mode that emerges from the control run, which is a wave-2 feature with a period around 20 days, is then used as the initial field for the model runs with LOC. The results show that LOC can significantly influence even the tropical variabilities. A latitudinal LOC, with a land mass in the northern hemisphere (north of 10°N), tends to shift the region of maximum precipitation slightly north of the equator with accompanying steeper gradients near the land–ocean boundary. The implications of this result for our understanding of Asian summer monsoon conditions are discussed. When LOC is only in the longitudinal direction, the dominant wave structure that emerges from the model run has wavenumber one and a period of about 35 days, very similar to the observed 30–60 day oscillation. In our final experiment, which is nearer to the realistic land–ocean configuration in the tropics, it is found that both latitudinal and longitudinal effects of LOC are important aspects of the tropical anomaly circulation. It is suggested that some of the patterns in the precipitation distribution, observed in the tropics and simulated using general circulation models are results of convectively induced internal dynamics, modulated by LOC.     

The transmission of spatially-compact internal wave packets through a critical level

Results are presented from numerical simulations of the incidence of a spatially-compact beam of internal waves on a shear layer containing a critical level. It is found that a significant fraction of the energy of the incident beam is transmitted across the critical level when the packet is narrow in physical space and when the energy density of the packet is not too high so that nonlinear effects are weak. As either the packet width or its energy density increases, the presence of a critical level renders the shear layer increasingly opaque to the transmission of energy. The familiar exponentially small value of the transmission coefficient is recovered as the spatial width of the packet exceeds (roughly) eight to ten wave lengths.     

On the inertial motion of a homogeneous ocean

An investigation of properties of the solutions of the steady state inviscid barotropic vorticity equation in a rectangular basin was performed for various functional relationships between potential vorticity and the streamfunction. All computed solutions which have slow interior flow and satisfy Arnol'd-Blumen condition are qualitatively similar to Fofonoff's inertial gyre. A new class of solutions with a small vortex near the boundary current is described. Such solutions are not stable under finite amplitude perturbations and instability is manifested in the drift of the vortex. It is shown that the vortex is robust and decays only after collision with the boundary or under the influence of very large perturbations. We also show that the inertial boundary current may be much wider than in Fofonoff's model due to the appearance of a countercurrent on its seaward side.     

On the bottom relief effect on wind-driven currents in a homogeneous ocean

The three-dimensional model of stationary wind-driven currents in a homogeneous ocean of a variable depth is investigated. The model is linear but includes horizontal and vertical turbulent mixings. Two cases of the behaviour of the isolines of the function $\text{?/H}$ are considered, namely: (1) all isolines $\text{?/H}$ start at one part of the coastline and end in another part of it, and (2) a certain isoline $\text{?/H}$ exists which is tangential to the coastline. Here ? is the Coriolis parameter, and H is the depth of the ocean. The first case is the simplest one; it arises in particular if H = constant and the coasts are meridional. The second case is marked by the boundary current separation from the coast. The paper deals with the boundary layers which arise at the surface, bottom, side boundary and inside the ocean.     

A laboratory model of a convectively driven ocean

Many large-scale flows in the ocean are driven by an imposed horizontal density gradient and the resulting circulation is strongly influenced by the Earth's rotation. Some of the essential features of such flows have been incorporated into a laboratory model by differentially heating and cooling the vertical end walls of a shallow rectangular cavity rotating about a vertical axis. Buoyancy driven boundary currents produced by the heating and cooling were unstable and the resulting eddy structures eventually dominated the system. A broad mean flow, perpendicular to the side walls, developed in the central region of the cavity. The resultant steady-state flow had a barotropic component consisting of two large-scale gyres of cyclonic and anticyclonic senses. In this paper, the evolution of the flow towards its final steady state is described. Measurements of the time-scales for the establishment of stratification in the cavity, point to the existence of two dynamical density adjustment modes.     

A mechanistic model of isopycnal diffusion in the ocean

A two-dimensional (xz) box ocean model, is formulated to examine the mechanistic role of isopycnal diffusion (i.e. diffusion along a constant density surface) as compared to that of horizontal/vertical diffusion. A large-scale surface temperature anomaly forces a steady solution. The presence of isopycnal diffusion substantially increases the vertical penetration in the steady state: the vertical heat flux is increased by an order of magnitude in some locations. The time scale of the transient response is also modified, and the nature of this response is scale dependent. It is thus not possible in general to reproduce the transient response with isopycnal diffusion by adjusting diffusitives in a lateral diffusion formulation. Implications of the results to climate modelling are considered.     

A look at the ocean in the EC-Earth climate model

EC-Earth is a newly developed global climate system model. Its core components are the Integrated Forecast System (IFS) of the European Centre for Medium Range Weather Forecasts (ECMWF) as the atmosphere component and the Nucleus for European Modelling of the Ocean (NEMO) developed by Institute Pierre Simon Laplace (IPSL) as the ocean component. Both components are used with a horizontal resolution of roughly one degree. In this paper we describe the performance of NEMO in the coupled system by comparing model output with ocean observations. We concentrate on the surface ocean and mass transports. It appears that in general the model has a cold and fresh bias, but a much too warm Southern Ocean. While sea ice concentration and extent have realistic values, the ice tends to be too thick along the Siberian coast. Transports through important straits have realistic values, but generally are at the lower end of the range of observational estimates. Exceptions are very narrow straits (Gibraltar, Bering) which are too wide due to the limited resolution. Consequently the modelled transports through them are too high. The strength of the Atlantic meridional overturning circulation is also at the lower end of observational estimates. The interannual variability of key variables and correlations between them are realistic in size and pattern. This is especially true for the variability of surface temperature in the tropical Pacific (El Ni?o). Overall the ocean component of EC-Earth performs well and helps making EC-Earth a reliable climate model.     

A one-dimensional model for the parameterization of deep convection in the ocean

A one-dimensional penetrative plume model has been constructed to parameterize the process of deep convection in ocean general circulation models (OGCMs). This research is motivated by the need for OGCMs to better model the production of deep and intermediate water masses. The parameterization scheme takes the temperature and salinity profiles of OGCM grid boxes and simulates the subgrid-scale effects of convection using a one-dimensional parcel model. The model moves water parcels from the surface layer down to their level of neutral buoyancy, simulating the effect of convective plumes. While in transit, the plumes exchange water with the surrounding environment; however, the bulk of the plume water mass is deposited at e level of neutral buoyancy. Weak upwelling around the plumes is included to maintain an overall mass balance. The process continues until the negative buoyant energy of the one-dimensional vertical column is minimized. The parameterized plume entrainment rate, which plays a central role in the parameterization, is calculated using modified equations based on the physics of entraining buoyant plumes. This scheme differs from the convective adjustment techniques currently used in OGCMs, because the parcels penetrate downward with the appropriate degree of mixing until they reach their level of neutral stability.     

海洋模式中虚假越中性层混合研究综述

在现代全球海洋数值模式的开发研究中,需要在空间尺度为海盆尺度上,时间尺度在几十年、几百年甚至涡流翻转数百次的时间内保持水团属性不变,而湍流混合又是维持层结和海洋环流循环所需要的.在海洋数值模型的开发中,需要将方程组在全球网格上进行离散处理,这势必会产生截断误差,从而产生虚假越中性层混合.因此将目前国内外学者和研究人员对虚假混合的来源、诊断方法、控制方法整理做出简要综述,以供读者参考学习.     

Grid transformation for incorporating the Arctic in a global ocean model

A grid transformation is described which isolates the Arctic and North Atlantic, rotates the spherical grid to pass an equator up the Atlantic through the north pole and remaps the Coriolis parameter. Boundary condition information is exchanged along the equatorial Atlantic so that the Arctic-Atlantic model is dynamically coupled to a model of the rest of the global ocean (which remains on the geographic spherical grid). The transform produces a more regular grid over the Arctic and eliminates the need for filtering or special treatment at the pole. The transform has been implemented in the GFDL Modular Ocean Model. After testing with idealized geometry, a 300 y global integration is compared to an integration using the geographic spherical grid and Fourier filtering. Results are similar, with differences in the Arctic and western North Atlantic regions leading to smaller air-sea heat flux near the Gulf Stream separation latitude for the transform case. Use of the transform also leads to a reduction in computation time.     

Chaos in a simple model of the three-dimensional, salt-dominated ocean circulation

 A simple moments model used in studying the large-scale thermally driven ocean circulation, in one hemisphere, is extended with a set of evolution equations for the basin-averaged salinity gradients. Natural formulations of the boundary conditions for the heat flux and the (virtual) salt flux are given, the latter based on the SST-evaporation feedback. Stommel’s box model result, a coexisting thermal and saline solution, is retrieved in the limit of no rotation. Including rotation in a salt-dominated setting, a steady circulation is found which bifurcates for higher Rayleigh numbers in a periodic solution which becomes chaotic through a cascade of subharmonic bifurcations. Periodic motion results from two different mechanisms. First, the stable stationary state bifurcates into a periodic solution where anomalously saline water is advected by the overturning circulation. Second, this periodic solution bifurcates into a state which is dominated, during the larger part of the cycle, by diffusion and inertia, characterized by a decreasing overturning rate, and, during the subsequent shorter part of the cycle, by rapid advection and restratification of the entire basin. The basin-averaged vertical density field is stably stratified in the steady and the periodic regimes and remains statically stable in the chaotic regime. Received: 16 July 1996 / Accepted: 29 December 1997     

Steady-state frictional geostrophic circulation in a one-layer ocean model with thermodynamics

The governing equations are developed for a steady-state frictional geostrophic inhomogeneous 1.5-layer ocean model, with horizontal velocity field that is linearly sheared in the vertical coordinate. We show that in the adiabatic, thermally non-diffusive limit there are an infinite number of solutions for the temperature and depth fields of the subtropical gyre even with the constraint of identical mass within each temperature range. In the non-adiabatic case, a unique subtropical gyre solution exists that can exhibit a temperature front, containing an unbounded meridional gradient, in the northwest corner of the solution domain. The role of mixing of enthalpy in the western boundary layer (WBL) region was investigated by comparing the two extreme cases of no mixing and complete mixing of enthalpy in this region. Also investigated was the dependence of the meridional heat transport on the air–sea heat exchange coefficient, κ. The temperature field was found to be strongly influenced by mixing. However, both qualitatively and quantitatively, the heat transport is similar in the model with and without mixing. The heat transport attains a single local maximum at κ=κ_c, that lies within values that are oceanographically relevant.     

Response Process of Oceanto AtmosphericForcing and Optimal Response Frequency in the CZ Ocean Model

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A degradation approach to accelerate simulations to steady-state in a 3-D tracer transport model of the global ocean

 We have developed a new method to accelerate tracer simulations to steady-state in a 3-D global ocean model, run off-line. Using this technique, our simulations for natural ¹⁴C ran 17 times faster when compared to those made with the standard non-accelerated approach. For maximum acceleration we wish to initialize the model with tracer fields that are as close as possible to the final equilibrium solution. Our initial tracer fields were derived by judiciously constructing a much faster, lower-resolution (degraded), off-line model from advective and turbulent fields predicted from the parent on-line model, an ocean general circulation model (OGCM). No on-line version of the degraded model exists; it is based entirely on results from the parent OGCM. Degradation was made horizontally over sets of four adjacent grid-cell squares for each vertical layer of the parent model. However, final resolution did not suffer because as a second step, after allowing the degraded model to reach equilibrium, we used its tracer output to re-initialize the parent model (at the original resolution). After re-initialization, the parent model must then be integrated only to a few hundred years before reaching equilibrium. To validate our degradation-integration technique (DEGINT), we compared ¹⁴C results from runs with and without this approach. Differences are less than 10‰ throughout 98.5% of the ocean volume. Predicted natural ¹⁴C appears reasonable over most of the ocean. In the Atlantic, modeled Δ¹⁴C indicates that as observed, the North Atlantic Deep Water (NADW) fills the deep North Atlantic, and Antartic Intermediate Water (AAIW) infiltrates northward; conversely, simulated Antarctic Bottom Water (AABW) does not penetrate northward beyond the equator as it should. In the Pacific, in surface eastern equatorial waters, the model produces a north–south assymetry similar to that observed; other global ocean models do not, because their resolution is inadequate to resolve equatorial dynamics properly, particularly the intense equatorial undercurrent. The model’s oldest water in the deep Pacific (at −239‰) is close to that observed (−248‰), but is too deep. Surface waters in the Southern Ocean are too rich in natural ¹⁴C due to inadequacies in the OGCM’s thermohaline forcing. Received: 18 March 1997 / Accepted: 27 July 1997     

The impact of a downslope water-transport parametrization in a global ocean general circulation model

The impact of a downslope water-transport parametrization on the circulation and water mass characteristics of a global depth-level ocean general circulation model is investigated. The spreading of dense water from the formation regions into the deep ocean is known to be poorly represented in depth-level models with no bottom boundary layer resolved or attached. The new scheme is simple and intends to parametrize the effects of various oceanographic processes (rather than the processes themselves) that help dense water to descend topographic slopes by which the formation regions are separated from the world ocean. The new scheme significantly improves the large scale properties of the North Atlantic Deep Water. Changes in the North Atlantic circulation, however, are rather small. In the Southern Ocean, the exchange between the dense water formation regions on the continental shelves and the deep ocean is strengthened at the expense of deep water mass formation by open ocean convection. In all three ocean basins, the density of the deep and bottom water is higher with the new parametrization, which brings the simulations closer to observations in the Atlantic and Indian Oceans. In the Pacific Ocean, however, where the density has already been well reproduced without the downslope transport, it becomes slightly too high. The results are in agreement with those from other model studies.     

Sensitivity of a global ice–ocean model to the Bering Strait throughflow

 To understand the influence of the Bering Strait on the World Ocean’s circulation, a model sensitivity analysis is conducted. The numerical experiments are carried out with a global, coupled ice–ocean model. The water transport through the Bering Strait is parametrized according to the geostrophic control theory. The model is driven by surface fluxes derived from bulk formulae assuming a prescribed atmospheric seasonal cycle. In addition, a weak restoring to observed surface salinities is applied to compensate for the global imbalance of the imposed surface freshwater fluxes. The freshwater flux from the North Pacific to the North Atlantic associated with the Bering Strait throughflow seems to be an important element in the freshwater budget of the Greenland and Norwegian seas and of the Atlantic. This flux induces a freshening of the North Atlantic surface waters, which reduces the convective activity and leads to a noticeable (6%) weakening of the thermohaline conveyor belt. It is argued that the contrasting results obtained by Reason and Power are due to the type of surface boundary conditions they used. Received: 27 October 1995/Accepted: 20 November 1996     

A comparison of shelf observation platforms for assimilation in an eddy-resolving ocean model

An assessment of the likely benefits of assimilating in situ temperature (T) and salinity (S) observations from repeat glider transects and surface velocity observations from high-frequency radar arrays into an eddy-resolving ocean model is presented. The deployment of new shelf observation platforms around Australia is being undertaken through the Australian Integrated Marine Observing System program. In this study, various options for an observing system along the coast of New South Wales, Australia, are assessed for their benefits to an ocean forecast and reanalysis system. The forecast system considered here uses ensemble optimal interpolation (EnOI) for data assimilation. Using error estimates from the EnOI scheme, estimates of the theoretical analysis errors are calculated for different observing systems that include a range of remotely sensed and in situ observations. The results demonstrate that if HF radar observations are assimilated along with the standard components of the global ocean observing system, the analysis errors are likely to reduce by as much as 80% for velocity and 60% for T, S and sea-level in the vicinity of the observations. Owing to the relatively short along-shore decorrelation length-scales for T and S near the shelf, the glider observations are likely to provide the forecast system with a more modest gain.     

The influence of the Bering Strait on the circulation in a coarse resolution global ocean model

An ocean general circulation model of global domain, full continental geometry and bottom topography, is used to study the influence of the Bering Strait on the general circulation by comparing equilibrium solutions obtained with and without a land-bridge between Siberia and Alaska. The model is integrated with restoring boundary conditions (BC) on temperature and salinity, and later, with mixed BC in which a restoring BC on temperature is maintained but a specified flux condition on salinity is imposed. In both cases, the effect of the Bering Strait is to allow a flow of about 1.25–1.5 Sv from the North Pacific to the Arctic Ocean and, ultimately, back to the North Pacific along the western boundary current regions of the Atlantic and Indian Oceans. When a restoring BC on salinity is used, the overturning associated with North Atlantic Deep Water and Antarctic Intermediate Water formation are increased if the Bering Strait is present in the model geometry. The result of switching to a specified flux BC on salinity is to cause a transition in the THC in which the overturning associated with North Atlantic Deep Water formation increases from about 12 Sv to about 22 Sv. This transition occurs in an essentially smooth fashion with no significant variability and is about 12% smaller in magnitude if the Bering Strait is present in the model geometry. Because the Bering Strait appears to exert some influence on the general circulation and the formation of deep water masses, it is recommended that this Strait be included in the geometry of similar resolution models designed to study the deep ocean and potential changes in climate. Correspondence to: CJC Reason     

Impact of ocean optical properties on seasonal SST: results with a surface ocean model coupled to the LMD AGCM

As the accuracy of ocean models improves, determination of the solar irradiance within the ocean may become important to simulate precisely the seasonal evolution of the SST. As ocean optical properties are not well documented in space and time, we have undertaken a sensitivity study to measure the corresponding SST uncertainties at a global scale using a model coupling the LMD AGCM with an integral mixed layer model and a thermodynamic sea ice representation. The downwelling irradiance formulation is that of Paulson and Simpson which has been tuned for the five water types of the Jerlov classification. Two sensitivity, and academic, experiments corresponding to a uniformly clear ocean or turbid ocean are carried out. Turbid waters exhibit, in general, a stronger seasonal cycle of the SST of about 2°C. The sensitivity is far from uniform, with a maximum in the subtropics and the mid-latitudes of the summer hemisphere. It corresponds precisely to the area in which the observed optical properties present a large temporal variability which is therefore likely to have an action on the seasonal cycle of the ocean surface temperatures. We perform a decomposition of the model sensitivity in four terms, corresponding to the direct impact of the water type change, feedback due to the mixed layer change, feedback due to the surface solar irradiance change, and feedback due to the non solar heat fluxes change. The first two terms dominate the SST change. The direct effect tends to increase the warming of the mixed layer. In addition, the mixed layer depth diminishes because of a higher stabilizing effect of solar radiation on the TKE budget. This tends to increase further summer warming of the SST as well as their winter cooling.