Rapid changes of the oceanic circulation in a hierarchy of ocean models

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.     

Vertical Thermohaline Structure,Water Masses,and Large-Scale Fronts in the Southwest Atlantic and Neighboring Antarctic Water Areas

The vertical profiles of temperature and salinity are classified by using the contemporary array of actual and monthly average archive oceanographic data. A scheme of decomposition of the investigated water area into zones according to the typical features of vertical stratification of the thermohaline field is proposed. The criterion of maximum of the horizontal temperature and salinity gradients is used to select and map the principal large-scale frontal boundaries. The parameters of fronts and some regularities of their seasonal variability are described and the data on water masses interacting in the analyzed system of fronts are presented.     

Thermohaline Anomalies in the Spring and Early Summer of 2000 in the Gulf of Trieste

Abstract. In the framework of the Interreg II Project (July 1998 - June 2001), hydro-logical, chemical and biological data were collected in the Gulf of Trieste.
During spring and summer 2000, some particular thermohaline anomalies were observed in the Gulf of Trieste. Especially in May and June the water body showed: a very strong thermohaline stratification, an increase of advective salt water coming from the south and the presence of sharp pycnoclines. In July the temperature was higher than usual in the whole water column. Moreover, in late May and in June, massive mucilaginous aggregates were observed along the water column and at the surface.
In order to highlight these particular thermohaline features the hydrological data of 16 stations were analysed (Fig. 1). Two stations, in particular, were considered: one offshore (St. AA1, average depth 20 m) and one close to the coast (St. C1, average depth 17 m). For these two stations a best-fit analysis, computed over 11 and 7 years, respectively, was performed on temperature, salinity and density excess data.
Moreover, the hydrological features were compared with the rainfall, air temperature, wind speed data (Istituto Sperimentale Talassografico di Trieste - ISTT) and the Isonzo River's flow rate (Direzione Regionale dell'Ambiente - Regione F. V. G.) collected from January 1998 to December 2000.     

About some morphological characteristics of the oceanic fields' vertical structure

A set of parameters is considered which allow us to determine the morphological structure of the vertical temperature, salinity, and density fields. If the parameters are integrated over the layer chosen, their computing does not call for imposing conditions of the fluctuation components' statistical heterogeneity. The parameters can be applied to describe both fine structure stratification and background stratification. Presumably, they may be used for mapping diverse types of thermohaline water stratification and the vertical structure of other fields when large sets of data covering the areas with marked spatial variability of the relevant conditions are available. Examples of simulations andin situ calculations are provided.Translated by Vladimir A. Puchkin.     

The different behaviour of modeled ocean circulation under an atmosphere with different heat capacity

We examine the difference in modeled thermohaline circulation under an atmosphere with no heat capacity (NHC) and infinite heat capacity (IHC) in a series of numerical experiments using the Bryan/Cox OGCM. An NHC atmosphere allows ocean sea surface temperatures to respond to changes in oceanic poleward heat transport, inferring an atmosphere that is allowed to seek its equilibrium temperature, whereas an IHC atmosphere does not. This is responsible for the following different behaviour patterns under the two atmospheres: 1) under NHC atmosphere, oceanic thermal oscillation persists, whereas under IHC atmosphere it does not; 2) under NHC atmosphere, the oceanic thermohaline circulation is less sensitive to high latitude freshening than under IHC atmosphere; 3) under either atmosphere, multiple equilibrium solutions are possible. However, under NHC atmosphere, two equilibria of the thermohaline circulation are generated in the same way as in the GFDL fully coupled model, while under IHC atmosphere, they are not.     

大洋细结构混合参数化方法在世界大洋环流模式中的应用

在大洋环流模式中,铅直混合的参数化方法起着关键性的作用。将大洋细结构混合参数化方法首次应用于世界大洋环流模式中。使用中科院大气所(LASG)发展的20层世界大洋环流模式(OGCM)ML20,月平均风场作为强迫场,利用ML20模式在稳定初始状态下运行300a后的计算结果作为本实验进行数值模拟的初始场。该参数化方法对世界大洋环流模式的影响主要表现为：永久性温度跃层的厚度明显变薄;对深层水和底层水的模拟有改进;对南极中层水的模拟比较成功;但是对赤道海区的模拟结果欠佳。     

三种垂向混合方案对HYCOM模拟北太平洋西边界流系的影响研究

基于HYCOM(Hybrid Coordinate Oceanic Circulation Model),以OFES(OGCM for the Earth Simulator)资料为参考,分析了KPP、MY2.5、KT三种不同混合方案对北太平洋西边界流系的模拟结果的影响。结果表明:三种不同混合方案模拟的上层海洋平均流场与OFES资料相似,但在流向和流幅上略有差异,其中KPP方案模拟的流速与OFES资料最为接近,MY2.5方案次之,KT方案与其差别最大。通过代表性断面上流速的对比分析,对模式就北赤道流、棉兰老流、棉兰老潜流、黑潮的模拟效果进行比较,KPP方案模拟的效果同前人的观测和研究最为接近。分析了北赤道流,棉兰老流,棉兰老潜流,黑潮的流量的季节变化特征,其中KPP方案与OFES资料计算的棉兰老流和棉兰老潜流的季节变化特征与前人描述比较一致,表现为春强秋弱。KPP方案和OFES资料的计算结果表明,北赤道流和棉兰老流大致上是同向变化的,而在冬季棉兰老流同黑潮的变化基本上是一致的。     

Staircase and inversion structures of thermocline in the north-west Tropical Atlantic

The peculiarities of the vertical fine thermohaline structure of waters in the north-west Tropical Atlantic are considered on the data of STD surveys recorded in winter-spring 1984. The variability of the characteristics of staircase and inversion elements of stratification with depth is analysed over the horizontal as well as related to the mesoscale and large-scale dynamics of waters. The coefficients of horizontal turbulent exchange are estimated within the framework of Joyce's hypothesis on quasi-compensation of vertical and turbulent horizontal transport. The effects of double diffusion are considered to dominate in vertical transport.UDK 551.465.15Translated by Mikhail M. Trufanov.     

Mixed Layer Depth and its Variability in the Eastern Equatorial Indian Ocean as Revealed by Observations and Model Simulations

Mixed layer depth (MLD) variability in the Eastern Equatorial Indian Ocean (EEIO) from a hindcast run of an Ocean General Circulation Model (OGCM) forced by daily winds and radiative fluxes from NCEP-NCAR reanalysis from 2004 to 2006 is investigated. Model MLD compares well with the ～20,000 observations from Argo floats and a TRITON buoy (1.5°S and 90°E) in the Indian Ocean. Tests with a one-dimensional upper ocean model were conducted to assess the impact on the MLD simulations that would result from the lack of the diurnal cycle in the forcing applied to the OGCM. The error was of the order of ～12 m. MLD at the TRITON buoy location shows a bimodal pattern with deep MLD during May–June and December–January. MLD pattern during fall 2006 was significantly different from the climatology and was rather shallow during December–January both in the model and observation. An examination of mixed layer heat and salt budget suggested salinity freshening caused by the advective and vertical diffusive mixing to be the cause of shallow MLD.     

Freshwater forcing as a booster of thermohaline circulation

Making use of a simple two‐layer model, we analyze the impact of freshwater forcing on the thermohaline circulation. We consider the forward‐type circulation dominated by thermal forcing, implying that the freshwater forcing acts to reduce the density contrast associated with the equator‐to‐pole temperature contrast (prescribed in the model). The system is described by two variables: the depth of the upper layer ( H ) and the density contrast between the upper and lower layer (Δρ), which decreases with salinity contrast. The rate of poleward flow of light surface water and the diapycnal flow (i.e., upwelling) driven by widespread small‐scale mixing are both modeled in terms of H and Δρ. Steady states of thermohaline circulation are found when these two flows are equal. The representation of the diapycnal flow ( M_D ) is instrumental for the dynamics of the system. We present equally plausible examples of a physically based representation of M_D for which the thermohaline circulation either decreases or increases with density contrast. In the latter case, contrary to the traditional wisdom, the freshwater forcing amplifies the circulation and there exists a thermally dominated equilibrium for arbitrary intensity of freshwater forcing. Here, Stommel's famous feedback between circulation and salinity contrast is changed from a positive to a negative feedback. The interaction of such a freshwater boosted thermohaline circulation with the climate system is fundamentally different from what is commonly assumed, an issue which is briefly addressed.     

Effects of a continental slope along the western boundary on the abyssal circulation

To investigate effects of a continental slope along the western boundary on the abyssal circulation, numerical experiments using multi-level models were carried out. An ocean which extends over the northern and southern hemispheres is forced by cooling inside the ocean at the southwest corner of the basin and uniform heating through the sea surface. When the reference density for the cooling is vertically uniform, effects of the slope emerge clearly for the slope with considerably broad width. The deep western boundary current flowing over the slope feeds no bottom flows in the southern hemisphere, and carries the warmed deep water into the northern hemisphere. This leads to the increased meridional density gradient, which results in the modification of deep flow patterns. When the reference density is vertically distributed, the upper and lower northward flowing western boundary currents form in the deep layer. As the density stratification relaxes the topographic control, the westward intensification of the upper boundary current is achieved over the slope. The intensified flow is accompanied by the countercurrent and they form the horizontal recirculation over the slope. However, the effects are confined around the slope region and the interior flow patterns do not change. The lower boundary current is not significantly affected by the slope and has the large width with no countercurrent. It is found that the actual continental slope does not have significant effects on the gross feature of the thermohaline circulation.     

On the variation of thermohaline characteristics of lenses of Mediterranean water at a distance from the source

Seven intrathermocline lenses (ITLs) of Mediterranean origin have been found during three cruises of the R/VArnold Veimer in the eastern north Atlantic Ocean. As ITL moved off its source, relative conservativity of some thermohaline characteristics such as the density range occupied by the lenses, theT, S indices, the core's vertical structure, and the form of the thermohaline front were observed. In the region of mixing, the intensity of the finestructure intrusion stratification increases with the age of lenses when they propagate to the colder freshened background waters. The systems of the step-like structures below the ITL core are formed at a considerable distance between the lenses and the source.Translated by Mikhail M. Trufanov.     

Sensitivity of the modelled thermohaline circulation to the parameterisation of mixing across the Greenland–Scotland ridge

The Hadley Centre climate model HadCM3 simulates a stable thermohaline circulation driven by deep water formation in the Norwegian and Labrador Seas without the need for flux adjustments. It has however been suggested that this result is the fortuitous consequence of the local use of the Roussenov convection scheme in this region, and that the model simulation may depend sensitively on this parameterisation. Here we investigate the sensitivity of the thermohaline circulation (THC) to the model’s treatment of the overflows from the Nordic Seas for both pre-industrial and increasing greenhouse gas forcings. We find that although the density structure in the Labrador Sea does depend upon the specifics of how the overflows are modelled, the global thermohaline circulation and climate responses are not sensitive to these details. This result gives credibility to previously published modelling studies on the response of the thermohaline circulation to anthropogenic greenhouse gas forcing, and implies that research may profitably be focussed on the large scale transports, where models are known to disagree.     

The principal factors contributing to the flux of salt in a narrow, partially stratified estuary

Observations of the velocity and salinity structure of the Tees estuary were made at eight stations along the estuary axis between Victoria Bridge and the sea during the summer of 1975. The measurements were made on ten separate tidal periods covering neap and spring tides.The data were collected over a period of relatively low freshwater flows and the residual current was found to have a strong dependence on the Stokes drift. At the upstream stations, the residuals were more than an order of magnitude greater than the currents anticipated from the freshwater discharge. Although the mean stratification decreased as the tidal range increased, the vertical circulation was stronger on spring tides than on neaps. Vertical variations in the amplitude and phase of the tidal current results in a current which strengthens the vertical circulation. However, this effect only made a relatively small contribution to the observed vertical circulation.The relative contribution of the individual salt flux terms to the net upstream transport of salt varies along the estuary. As the estuary narrows, the contribution by the oscillatory terms dominates that from the shear in the steady state flow. Of these oscillatory terms, the correlation of velocity and salinity fluctuations plays a key rôle in the salt transport. The depth mean values make a greater contribution than deviations from the depth mean and the flux due to phase variations over depth is smaller than either of these. Since the Stokes drift is compensated by a down-stream steady state flow, it does not contribute to the tidal mean transport of salt.At the seaward end of the estuary, the salt fluxes due to the steady state vertical shear and the convariance of the tidal fluctuations act in a complementary way to counter the seaward transport of salt by the freshwater flow. With the possible exceptions of the wide or narrow reaches of the Tees, the longitudinal fluxes of salt due to transverse variations in velocity, salinity and depth and turbulent fluctuations are of secondary importance as contributors to the estuary salt budget.On both neap and spring tides, the computed total salt transports at the Newport and Victoria bridges did not match the values required for a salt balance with the corresponding freshwater flows. These fluxes were probably the cause of the observed downstream displacement of the tidal mean salinity distribution between neap and spring tides.     

Surface manifestations of internal waves investigated by a subsurface buoyant Jet: 1. The mechanism of internal-wave generation

In a large test reservoir with artificial temperature stratification at the Institute of Applied Physics, Russian Academy of Sciences, we have performed a major laboratory simulation of the nonstationary dynamics of buoyant turbulent jets generated by wastewater flows from underwater collector diffusers. The interaction of buoyant jets with the pycnocline leads to an active generation of internal waves. An analysis of the dependence of wave amplitude on the control parameter proportional to the rate of liquid flow from the collector diffuser has indicated that this dependence is adequately described by a function that is characteristic for the presence in the Hopf bifurcation system, which occurs for a soft actuation mode of self-oscillations of the globally instable mode. To check the conditions for the actuation of the globally instable mode, we have performed an auxiliary experiment in a small reservoir with a salt stratification formulated similar to the experiment in the big reservoir. Using the particle image velocimetry (PIV) method, we have measured the velocity field in the buoyant jet and constructed the profiles of transverse velocity in several sections. When the jet approaches the pycnocline, a counterflow is generated at the edges. A stability analysis for the resulting profiles of flow velocities performed by the method of normal modes has revealed that, for the jet portions with counterflow, the condition of absolute instability by the Briggs criterion for axisymmetric jet oscillations is satisfied, which testifies to the fact that the globally instable mode is actuated. The estimates for oscillation frequencies of the globally instable mode are well consistent quantitatively with the measured spectrum of jet oscillations.     

Three-dimensional numerical prediction of vertical temperature structure of the Huanghai and the Bohai Seas

In the paper, the sea is divided into two layers with density jumping, assuming that the physical parameters in each layer are independent of depth. Two-layer flow field with tide and wind currents is calculated with extended ADI method, after the calculation for flow field is stable , coupled with temperature diffusion equations and thermohaline depth prediction equation, a four-day time prediction of the surface, bottom temperature and thermohaline depth of the Huanghai and the Bohai Seas. At the same time, three dimensional temperature field of sea water is predicted through vertical temperature distribution function. The result indicates that the prediction quality of the whole model and the fitting degree between the predicted result and the measured values are satisfactory.     

Controlling spatial oscillations in bed level update schemes

A frequent problem with process-based coastal morphological models is the appearance of high wave number spatial oscillations in the simulated bed levels with time. After a sufficiently long time, these oscillations become dominant and mask the large-scale features of the bed level evolution.The equation for conservation of sediment mass is used to show that the spatial oscillations are generated by the dependence of the bed celerity (celerity of the bed level oscillations) with bed levels, which is due to the non-linear relationship between sediment transport and bed levels. This breeds higher spatial harmonics of the bed level oscillations with time. In this situation, using a Finite Difference (FD) scheme that does not damp oscillations with high wave numbers leads to the generated harmonics being kept in the solution. These generate further harmonics until the entire solution is dominated by high wave number oscillations.In this paper, a finite difference scheme, in combination with a filtering procedure, is used to dissipate high wave number oscillations. Analysis of the amplification portraits show that the filtering procedure in combination with a Lax–Wendroff scheme does not affect oscillations with lower wave numbers (larger scale features resolved with seven or more grid points). Some examples are also presented to illustrate these features.     

On the dynamics and effects of double-diffusively driven intrusions

Some of the characteristics of intrusive finestructure observed in oceanic fronts suggest that salt fingering is active in the intrusions. An extension of Stern's 1967 study of the stability of a thermohaline front to intrusive finestructure driven by salt fingers is presented to investigate this hypothesis. Intrusions are found to grow due to the salt finger induced density fluxes even in the presence of small scale momentum dissipation. The presence of this dissipation defines a fastest growing mode for the disturbance which has physical characteristics not unlike those observed in oceanic fronts. The model results do not agree well with the laboratory studies of intrusions. For many fronts, the model requires initial perturbations in the front which can support salt fingers. Linear internal waves are investigated to determine if they can provide the initial perturbation. It is shown that, away from the equator, the interaction between internal waves and intrusions is weak because of a mismatch of time scales. Finally, growing intrusions are shown to flux heat, salt and density across fronts. The senses of these fluxes are to run down the lateral gradients of many oceanic fronts.     

Simulation of the Circulation and Space Structure of Thermohaline Fields in the Sevastopol Bay with Regard for the Actual External Data (Winter, 1997)

We discuss the results of the numerical experiment aimed at the simulation of the behavior of currents and transformations of the temperature and salt modes in the Sevastopol Bay in January–February 1997. In the numerical analysis, we use actual data on the velocity and direction of the wind, sea surface temperature, and the discharge of River Chernaya. It is shown that the circulation and structure of hydrological fields are mainly connected with the direction of the wind, its intensity, and variability in the course of time. Since the analyzed water area is shallow, the currents inside the bay undergo rapid transformations (less than for an hour after changes in the wind). At the same time, the transformations of the thermohaline fields are slower. Due to the inflow of fresh waters of River Chernaya and salt waters from the open sea through the strait, the structure of thermohaline fields formed in the bay is nonuniform (both in the vertical and horizontal directions). The distribution of salinity plays the main role in the formation of the vertical stratification, which is natural for the winter season. Due to the process of freshening of water, a quite high vertical salinity gradient is formed in the upper layer of the sea. As a result, the process of cooling does not lead to the appearance of convection and inversions of temperature are formed in the case where warmer waters are located in the bottom layers. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 60–76, March–April, 2005.     

On several mechanisms of the formation of the compensated thermohaline inhomogeneities in the ocean

The process of the geostrophic adjustment in the stably stratified two-component medium is studied in the framework of a linear approximation. We demonstrate that, at the final stage of that process, a stationary trace is generated by the distribution of the temperature and salinity, whose horizontal inhomogeneities mutually compensate in the field of the density. The compensation level for the stationary thermohaline distributions forming during the geostrophic adjustment is estimated. The origination mechanism of compensated thermohaline inhomogeneities in hydrodynamically stable shear flows is examined. We show that, in such flows, the disturbances of the fields of buoyancy (density), pressure, and velocity damp with time, whereas the compensated disturbances of the fields of the temperature and salinity are carried off by the flow without damping. Based on the explicit solutions of the dynamic equations, it is shown that the evolution of the compensated distribution of the temperature and salinity in the shear flows usually results in the sharpening of the spatial gradients. This feature may be, among others, related to one of the factors of the origination of the fine structure of the ocean: the small-scale thermohaline inhomogeneities, which exist against the background of the smooth vertical distribution of the density.