Effects of the hydroelectric developments on the oceanographic surface parameters of Hudson Bay

Abstract

A one‐dimensional oceanic mixed‐layer model was used to simulate the annual surface layer properties of Hudson Bay. The model reproduces the sparse available data well and shows the equal importance of seasonal ice cover and run‐off on the pycnocline pattern. In spring, the large freshwater input from run‐off and local ice melt followed by summer heating slows the deepening of the pycnocline depth by wind mixing. As these stabilizing effects decrease and the wind strength increases, the pycnocline depth increases in the fall and continues to increase in the winter when the salt rejection effect during ice growth replaces the cooling effect. In the spring the salt rejection reduces and run‐off increases; the large pycnocline depth cannot be maintained and a shallow pycnocline is formed, starting a new seasonal cycle.

When the run‐off cycle includes the effects of hydroelectric developments, the results indicate that a new shallow surface pycnocline is formed earlier in the spring. This causes a decrease in surface layer temperature and salinity, thus stimulating more ice growth. On the other hand, in the summer the surface layer salinity is higher and the temperature lower. This decreases the stability, thus further deepening the pycnocline and increasing the deviations from normal conditions.     

The relation of meridional pressure gradients to North Atlantic deep water volume transport in an ocean general circulation model

We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic.     

Stability dependent parameterisations of vertical mixing in ocean general circulation models

Summary Parameterisations of mixing induced through shear instability, internal wave breaking, and double diffusion are investigated in simulations of ocean climate using a global ocean general circulation model (OGCM). Focus is placed on the sensitivity of the large scale circulation, water mass formation and transport of heat as measures of the model's ability to represent current climate. The model resolution is typical of OGCMs being coupled to atmospheric. GCMs in climate models and the parameterisations investigated are all computationally inexpensive enough to allow for integrations on long time scales. Under the assumption of constant vertical eddy coefficients (the control case), the model climatology displays acceptable values of North Atlantic Deep Water formation, Antarctic Circumpolar Current (ACC) transport, and Indonesian through-flow but an excessively deep and diffuse pycnocline structure with weak stratification in the deep ocean. It is found that various circulation and water mass properties are sensitive to the choice of parameterisation of vertical mixing and that determining a scheme which works satisfactorily over all regions (tropical, mid-latitude, and polar) of the domain is not straightforward. Parameterisations of internal wave breaking or upper ocean shear instability lead to some improvements in the model water mass formation. ACC and poleward heat transport when compared to the control case whereas parameterisations of double diffusive processes did not. Based on these and other results, various recommendations are made for mixing parameterisations in ocean climate models.With 8 Figures     

CLIMBER-2: a climate system model of intermediate complexity. Part II: model sensitivity

 A set of sensitivity experiments with the climate system model of intermediate complexity CLIMBER-2 was performed to compare its sensitivity to changes in different types of forcings and boundary conditions with the results of comprehensive models (GCMs). We investigated the climate system response to changes in freshwater flux into the Northern Atlantic, CO₂ concentration, solar insolation, and vegetation cover in the boreal zone and in the tropics. All these experiments were compared with the results of corresponding experiments performed with different GCMs. Qualitative, and in many respects, quantitative agreement between the results of CLIMBER-2 and GCMs demonstrate the ability of our climate system model of intermediate complexity to address diverse aspects of the climate change problem. In addition, we used our model for a series of experiments to assess the impact of some climate feedbacks and uncertainties in model parameters on the model sensitivity to different forcings. We studied the role of freshwater feedback and vertical ocean diffusivity for the stability properties of the thermohaline ocean circulation. We show that freshwater feedback plays a minor role, while changes of vertical diffusivity in the ocean considerably affect the circulation stability. In global warming experiments we analysed the impact of hydrological sensitivity and vertical diffusivity on the long-term evolution of the thermohaline circulation. In the boreal and tropical deforestation experiments we assessed the role of an interactive ocean and showed that for both types of deforestation scenarios, an interactive ocean leads to an additional cooling due to albedo and water vapour feedbacks. Received: 28 May 2000 / Accepted: 9 November 2000     

Mechanisms controlling seasonal mixed layer temperature and salinity in the Southwestern Tropical Indian Ocean

We use a heat- and salt-conserving ocean state estimation product to study the seasonal cycles of the mixed layer (ML) temperature (MLT) and salinity (MLS) balances over the southwestern tropical Indian Ocean (SWTIO) thermocline ridge (STR; 50°–75°E, 12°–5°S). For seasonal MLT, surface heat flux and ocean processes are both important. They tend to re-enforce each other during peak cooling (May–June) and warming (November) periods, but not during transition periods. The dominant ocean process is wind-driven vertical mixing. It is modulated by the variable strength of the monsoon winds (which affect the vertical diffusivity), and to a lesser extent by variability of thermocline depth (which influences the vertical stratification across the ML base). For example, thermocline shoaling in April–July alters the vertical stratification near the ML base; thus, when the monsoon winds heighten (June–September) and the vertical diffusivity increases (deepening the ML base), relatively cool subsurface water is near the ML base and easily incorporated into the ML by vertical mixing. However, vertical advection as a direct response to thermocline shoaling has little affect on MLT. This explains why MLT and thermocline depth are not positively correlated here on the seasonal timescale (as they are on the interannual timescale). Meridional advection associated with Ekman transport driven by the monsoon winds plays a secondary role. Seasonal MLS, however, is dominated by meridional advection. Vertical process effects on MLS are small, due to a weak salinity gradient near the ML base throughout the year.     

Vertical eddy diffusivity estimations in Swan river estuary

A field experiment is described in which diapycnal diffusivity is estimated by direct and indirect methods in Swan river estuary, Perth, Western Australia. The microstructure profiles were collected in a narrow and straight part of this estuary using a portable flux profiler (PFP). The profiles were segmented into stationary parts and the rate of vertical eddy diffusivity was estimated for the segments within the pycnocline. The direct measurement showed that instantaneous flux could be positive or negative with a low net rate of vertical eddy diffusivity for mass of about 6.5 × 10⁻⁸ m² s⁻¹. All the indirect methods overestimated this rate. However, within the indirect methods, the method of Osborn yielded the poorest estimate while the method of Osborn and Cox gave the closest estimate.     

Skill assessment of three earth system models with common marine biogeochemistry

We have assessed the ability of a common ocean biogeochemical model, PISCES, to match relevant modern data fields across a range of ocean circulation fields from three distinct Earth system models: IPSL-CM4-LOOP, IPSL-CM5A-LR and CNRM-CM5.1. The first of these Earth system models has contributed to the IPCC 4th assessment report, while the latter two are contributing to the ongoing IPCC 5th assessment report. These models differ with respect to their atmospheric component, ocean subgrid-scale physics and resolution. The simulated vertical distribution of biogeochemical tracers suffer from biases in ocean circulation and a poor representation of the sinking fluxes of matter. Nevertheless, differences between upper and deep ocean model skills significantly point to changes in the underlying model representations of ocean circulation. IPSL-CM5A-LR and CNRM-CM5.1 poorly represent deep-ocean circulation compared to IPSL-CM4-LOOP degrading the vertical distribution of biogeochemical tracers. However, their representations of surface wind, wind stress, mixed-layer depth and geostrophic circulations (e.g., Antarctic Circumpolar Current) have been improved compared to IPSL-CM4-LOOP. These improvements result in a better representation of large-scale structure of biogeochemical fields in the upper ocean. In particular, a deepening of 20–40 m of the summer mixed-layer depth allows to capture the 0–0.5 μgChl L^?1 concentrations class of surface chlorophyll in the Southern Ocean. Further improvements in the representation of the ocean mixed-layer and deep-ocean ventilation are needed for the next generations of models development to better simulate marine biogeochemistry. In order to better constrain ocean dynamics, we suggest that biogeochemical or passive tracer modules should be used routinely for both model development and model intercomparisons.     

Comparison of vertical mixing schemes embedded in a global ocean GCM. Part II: Large scale circulation and water mass formation

Summary The sensitivity of the circulation and water mass properties in a global ocean circulation model (OGCM) to the stability dependent vertical mixing parameterisations of Pacanowski and Philander (1981) and Henderson-Sellers (1985) is investigated. The work extends a previous study which examined upper ocean charateristics and mixed layer evolution resulting from these schemes incorporated in the OGCM and made a recommendation as to the appropriateness of the latter scheme for global models. Under the assumption of constant vertical eddy coefficients (the control case), the model climatology displays acceptable values of North Atlantic Deep Water formation, Antarctic Circumpolar Current strength, and Indonesian throughflow but an excessively deep and diffuse pycnocline structure with weak stratification in the deep ocean. It is found that these circulation and water mass properties are sensitive to the choice of parametrisation of vertical mixing and that the two stability dependent schemes are unable to perform satisfactorily over the global domain, instead being better suited to the tropics. Under conditions optimal for representing the tropical current and temperature structure, these schemes result in significant weakening of major currents (particularly, the ACC) and reductions in the rates of deep water formation and poleward heat transports. These deficiencies can only be remedied at the expense of the improvements to the simulation in the tropical part of the domain. The results presented indicate that the suggestions made in the previous study do not extend to situations where the deep ocean, and particularly, the global thermohaline circulation is important.With 8 Figures     

Impact of vertical mixing induced by small vertical scale structures above and within the equatorial thermocline on the tropical Pacific in a CGCM

Oceanic vertical mixing is known to influence the state of the equatorial ocean which affects the climate system, including the amplitude of El Niño/Southern Oscillation (ENSO). Recent measurements of ocean currents at high vertical resolution capture numerous small vertical scale structures (SVSs) within and above the equatorial thermocline that contribute significantly to vertical mixing but which are not sufficiently resolved by coarse resolution ocean models. We investigate the impact of the vertical mixing induced by the SVSs on the mean state and interannual variability in the tropical Pacific by using a coupled general circulation model. The vertical mixing induced by the SVSs is represented as an elevated vertical diffusivity from the surface down to the 20 °C isotherm depth, a proxy for the depth of the thermocline. We investigate different forms for the elevated mixing. It is found that the SVS-induced mixing strongly affect the mean state of the ocean leading to a warming of sea surface temperature (SST) and associated deepening and sharpening of the thermocline in the eastern equatorial Pacific. We find that the SST warming induced by the elevated mixing is further strengthened through the Bjerknes feedback and SST-shortwave flux feedback. We also find a reduction in the number of large amplitude ENSO events and in certain cases an increase in the skewness of ENSO.     

Estimates of anthropogenic CO<Subscript>2</Subscript> uptake in a global ocean model

A global ocean general circulation model (L30T63) is employed to study the uptake and distribution of anthropogenic CO₂ in the ocean. A subgrid-scale mixing scheme called GM90 is used in the model. There are two main GM90 parameters including isopycnal diffusivity and skew (thickness) diffusivity. Sensitivities of the ocean circulation and the redistribution of dissolved anthropogenic CO₂ to these two parameters are examined. Two runs estimate the global oceanic anthropogenic CO₂ uptake to be 1.64 and 1.73 Pg C yr^-1 for the 1990s, and that the global ocean contained 86.8 and 92.7 Pg C of anthropogenic CO₂ at the end of 1994, respectively. Both the total inventory and uptake from our model are smaller than the data-based estimates. In this presentation, the vertical distributions of anthropogenic CO₂ at three meridional sections are discussed and compared with the available data-based estimates. The inventory in the individual basins is also calculated. Use of large isopycnal diffusivity can generally improve the simulated results, including the exchange flux, the vertical distribution patterns, inventory, storage, etc. In terms of comparison of the vertical distributions and column inventory, we find that the total inventory in the Pacific Ocean obtained from our model is in good agreement with the data-based estimate, but a large difference exists in the Atlantic Ocean, particularly in the South Atlantic. The main reasons are weak vertical mixing and that our model generates small exchange fluxes of anthropogenic CO₂ in the Southern Ocean. Improvement in the simulation of the vertical transport and sea ice in the Southern Ocean is important in future work.     

Onset of convection in a rotating semi‐infinite fluid: Theory and experiment

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 (Ra_bl) is also independent of B. We show that Ra_bl, 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.     

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.     

The North Atlantic thermohaline circulation simulated by the GISS climate model during 1970–99

Abstract

Evidence based on numerical simulations is presented for a strong correlation between the North Atlantic Oscillation (NAO) and the North Atlantic overturning circulation. Using an ensemble of numerical experiments with a coupled ocean‐atmosphere model including both natural and anthropogenic forcings, it is shown that the weakening of the thermohaline circulation (THC) could be delayed in response to a sustained upward trend in the NAO, which was observed over the last three decades of the twentieth century, 1970–99. Overall warming and enhanced horizontal transports of heat from the tropics to the subpolar North Atlantic overwhelm the NAO‐induced cooling of the upper ocean layers due to enhanced fluxes of latent and sensible heat, so that the net effect of warmed surface ocean temperatures acts to increase the vertical stability of the ocean column. However, the strong westerly winds cause increased evaporation from the ocean surface, which leads to a reduced fresh water flux over the western part of the North Atlantic. Horizontal poleward transport of salinity anomalies from the tropical Atlantic is the major contributor to the increasing salinities in the sinking regions of the North Atlantic. The effect of positive salinity anomalies on surface ocean density overrides the opposing effect of enhanced warming of the ocean surface, which causes an increase in surface density in the Labrador Sea and in the ocean area south of Greenland. The increased density of the upper ocean layer leads to deeper convection in the Labrador Sea and in the western North Atlantic. With a lag of four years, the meridional overturning circulation of the North Atlantic shows strengthening as it adjusts to positive density anomalies and enhanced vertical mixing. During the positive NAO trend, the salinity‐driven density instability in the upper ocean, due to both increased northward ocean transports of salinity and decreased atmospheric freshwater fluxes, results in a strengthening overturning circulation in the North Atlantic when the surface atmospheric temperature increases by 0.3°C and the ocean surface temperature warms by 0.5° to 1°C.     

Stabilization of thermohaline circulation by wind-driven and vertical diffusive salt transport

 The stability of the thermohaline circulation is investigated using an ocean general circulation model coupled to a simple atmospheric model. The atmospheric model is so developed that it represents the wind stress and the freshwater flux more realistically than existing energy balance models. The coupled model can reproduce the realistic deep ocean circulation without any flux adjustment. Effects of the wind stress and the vertical diffusion on the thermohaline circulation are studied by conducting various experiments with the coupled model. The Ekman upwelling between 60^∘N and 90^∘N brings up salt to the sea surface, while the compensation flow of the Ekman transport and the wind-driven gyre circulation between 30^∘N and 60^∘N carry salt horizontally to the high latitudes. By carrying out experiments where the wind stress is completely or partly removed, it is demonstrated that either of the vertical or the horizontal salt transport prevents the halocline formation at high latitudes and maintains the thermohaline circulation. For an experiment in which the vertical diffusivity is enhanced at high latitudes, it is shown that the vertical diffusion at high latitudes also prevents the halocline formation and stabilizes the thermohaline circulation. It is also shown that the value of the vertical diffusivity at high latitude affects the existence of the multiple equilibria of the thermohaline circulation. Received: 26 April 2000 / Accepted: 10 January 2001     

On the horizontal density ratio in the upper ocean

The horizontal density ratio in the upper ocean is examined using SeaSoar data collected over the last 15 years in the Pacific, Atlantic, and Indian Oceans. The horizontal density ratio R is defined to be the ratio of the relative effect of temperature and salinity on density. A front with a horizontal density ratio of 1 is said to be compensated since temperature and salinity gradients compensate in their effect on density. The statistics of density ratio are examined through calculation of conditional probability density functions. Case studies from each of the oceans elucidate processes affecting the density ratio. Global distributions of density ratio are calculated as functions of mixed-layer depth, distance below the mixed layer, and magnitude of thermohaline variability. Compensation is found in all oceans, on 3–4 km horizontal scales, when the mixed layer is deep and significant thermohaline variability exists. The tendency for compensation is stronger as mixed-layer depth increases. Conversely, compensation is not typical in shallow mixed layers, or when thermohaline variability is weak. The thermocline density ratio is found to be 2, in agreement with previous observational studies, and consistent with the process of salt fingering. The transition from R=1 in the mixed layer to R=2 in the thermocline is sharp when the mixed layer is deep. The ubiquity of compensation in the mixed layer is consistent with recent theory that suggests horizontal eddy diffusivity is a growing function of density gradient.     

Steering of upper ocean currents and fronts by the topographically constrained abyssal circulation

A two-layer theory is used to investigate (1) the steering of upper ocean current pathways by topographically constrained abyssal currents that do not impinge on the bottom topography and (2) its application to upper ocean – topographic coupling via flow instabilities where topographically constrained eddy-driven deep mean flows in turn steer the mean pathways of upper ocean currents and associated fronts. In earlier studies the two-layer theory was applied to ocean models with low vertical resolution (2–6 layers). Here we investigate its relevance to complex ocean general circulation models (OGCMs) with high vertical resolution that are designed to simulate a wide range of ocean processes. The theory can be easily applied to models ranging from idealized to complex OGCMs, provided it is valid for the application. It can also be used in understanding some persistent features seen in observed ocean frontal pathways (over deep water) derived from satellite imagery and other data. To facilitate its application, a more thorough explanation of the theory is presented that emphasizes its range of validity. Three regions of the world ocean are used to investigate its application to eddy-resolving ocean models with high vertical resolution, including one where an assumption of the two-layer theory is violated. Results from the OGCMs with high vertical resolution are compared to those from models with low vertical resolution and to observations. In the Kuroshio region upper ocean – topographic coupling via flow instabilities and a modest seamount complex are used to explain the observed northward mean meander east of Japan where the Kuroshio separates from the coast. The Japan/East Sea (JES) is used to demonstrate the impact of upper ocean – topographic coupling in a relatively weak flow regime. East of South Island, New Zealand, the Southland Current is an observed western boundary current that flows in a direction counter to the demands of Sverdrup flow and counter to the direction simulated in nonlinear global flat bottom and reduced gravity models. A model with high vertical resolution (and topography extending through any number of layers) and a model with low vertical resolution (and vertically compressed but otherwise realistic topography confined to the lowest layer) both simulate a Southland Current in the observed direction with dynamics depending on the configuration of the regional seafloor. However, the dynamics of these simulations are very different because the Campbell Plateau and Chatham Rise east and southeast of New Zealand are rare features of the world ocean where the topography intrudes into the stratified water column over a relatively broad area but lies deeper than the nominal 200 m depth of the continental shelf break, violating a limitation of the two-layer theory. Observations confirm the results from the high vertical resolution model. Overall, the model simulations show increasingly widespread upper ocean – topographic coupling via flow instabilities as the horizontal resolution of the ocean models is increased, but fine resolution of mesoscale variability and the associated flow instabilities are required to obtain sufficient coupling. As a result, this type of coupling is critical in distinguishing between eddy-resolving and eddy-permitting ocean models in regions where it occurs.     

Vertical heat transports in the ocean and their effect on time-dependent climate change

In response to increasing atmospheric concentrations of greenhouse gases, the rate of time-dependent climate change is determined jointly by the strength of climate feedbacks and the efficiency of processes which remove heat from the surface into the deep ocean. This work examines the vertical heat transport processes in the ocean of the HADCM2 atmosphere–ocean general circulation model (AOGCM) in experiments with CO₂ held constant (control) and increasing at 1 per year (anomaly). The control experiment shows that global average heat exchanges between the upper and lower ocean are dominated by the Southern Ocean, where heat is pumped downwards by the wind-driven circulation and diffuses upwards along sloping isopycnals. This is the reverse of the low-latitude balance used in upwelling–diffusion ocean models, the global average upward diffusive transport being against the temperature gradient. In the anomaly experiment, weakened convection at high latitudes leads to reduced diffusive and convective heat loss from the deep ocean, and hence to net heat uptake, since the advective heat input is less affected. Reduction of deep water production at high latitudes results in reduced upwelling of cold water at low latitudes, giving a further contribution to net heat uptake. On the global average, high-latitude processes thus have a controlling influence. The important role of diffusion highlights the need to ensure that the schemes employed in AOGCMs give an accurate representation of the relevant sub-grid-scale processes. Received: 8 July 1999 / Accepted: 17 November 1999     

Constraining ocean diffusivity from the 8.2 ka event

Greenland ice-core data containing the 8.2 ka event are utilized by a model-data intercomparison within the Earth system model of intermediate complexity, CLIMBER-2.3 to investigate their potential for constraining the range of uncertain ocean diffusivity properties. Within a stochastic version of the model (Bauer et al. in Paleoceanography 19:PA3014, 2004) it has been possible to mimic the pronounced cooling of the 8.2 ka event with relatively good accuracy considering the timing of the event in comparison to other modelling exercises. When statistically inferring from the 8.2 ka event on diffusivity the technical difficulty arises to establish the related likelihood numerically per realisation of the uncertain model parameters: while mainstream uncertainty analyses can assume a quasi-Gaussian shape of likelihood, with weather fluctuating around a long term mean, the 8.2 ka event as a highly nonlinear effect precludes such an a priori assumption. As a result of this study the Bayesian Analysis leads to a sharp single-mode likelihood for ocean diffusivity parameters within CLIMBER-2.3. Depending on the prior distribution this likelihood leads to a reduction of uncertainty in ocean diffusivity parameters (e.g. for flat prior uncertainty in the vertical ocean diffusivity parameter is reduced by factor 2). These results highlight the potential of paleo data to constrain uncertain system properties and strongly suggest to make further steps with more complex models and richer data sets to harvest this potential.     

Wind-driven changes in Southern Ocean residual circulation, ocean carbon reservoirs and atmospheric CO2

The effect of idealized wind-driven circulation changes in the Southern Ocean on atmospheric CO₂ and the ocean carbon inventory is investigated using a suite of coarse-resolution, global coupled ocean circulation and biogeochemistry experiments with parameterized eddy activity and only modest changes in surface buoyancy forcing, each experiment integrated for 5,000 years. A positive correlation is obtained between the meridional overturning or residual circulation in the Southern Ocean and atmospheric CO₂: stronger or northward-shifted westerly winds in the Southern Hemisphere result in increased residual circulation, greater upwelling of carbon-rich deep waters and oceanic outgassing, which increases atmospheric pCO₂ by ～20 μatm; weaker or southward-shifted winds lead to the opposing result. The ocean carbon inventory in our model varies through contrasting changes in the saturated, disequilibrium and biogenic (soft-tissue and carbonate) reservoirs, each varying by O(10–100) PgC, all of which contribute to the net anomaly in atmospheric CO₂. Increased residual overturning deepens the global pycnocline, warming the upper ocean and decreasing the saturated carbon reservoir. Increased upwelling of carbon- and nutrient-rich deep waters and inefficient biological activity results in subduction of unutilized nutrients into the ocean interior, decreasing the biogenic carbon reservoir of intermediate and mode waters ventilating the Northern Hemisphere, and making the disequilibrium carbon reservoir more positive in the mode waters due to the reduced residence time at the surface. Wind-induced changes in the model carbon inventory are dominated by the response of the global pycnocline, although there is an additional abyssal response when the peak westerly winds change their latitude, altering their proximity to Drake Passage and changing the depth extent of the southward return flow of the overturning: a northward shift of the westerly winds isolates dense isopycnals, allowing biogenic carbon to accumulate in the deep ocean of the Southern Hemisphere, while a southward shift shoals dense isopycnals that outcrop in the Southern Ocean and reduces the biogenic carbon store in the deep ocean.     

中尺度涡旋混合参数空间变化对物理场以及CFC-11模拟的影响

本文选用中国科学院大气物理研究所全球海洋模式(LICOM),对中尺度涡旋参数化方案(GM90方案)中等密度扩散系数和等密度面厚度扩散系数(统称为涡旋扩散系数A_ρ)对物理场及CFC-11(一氟三氯甲烷)分布的影响进行了研究。本文做了两个试验,即涡旋扩散系数采用常系数方式(对照试验)和采用在非绝热层以下A_ρ随海洋浮力频率垂直变化的参数化方案(浮力试验)。模拟结果表明,依浮力频率垂直变化的方案对模式物理场的模拟能力有一定程度的提升,如南极绕极流的输送强度比常系数方案增大了约20%~30%,与观测事实更接近;浮力试验对对照试验中过强的南极中层水有一定的削弱作用,使得模式对南大洋高纬次表层位密度的模拟有一定的改善。稍有不足的是,浮力试验对南极底层水也有一定的削弱,使得2000~3000 m深度位密度模拟较常系数方案偏低。通过对CFC-11分布、存储以及输送的研究发现,次网格参数取值的不同对南大洋CFC-11模拟情况有较大影响。浮力试验加大了南北高纬CFC-11海表的吸收通量,对南极大陆周边海域向南大洋主储藏区(34°S~60°S)的CFC-11输送能力有一定的增强,使得南大洋对CFC-11储藏量增大,大部分海区与观测资料更接近。通过CFC-11断面分析,浮力试验对南大洋上层海洋位密度模拟的改善使得CFC-11垂直结构与观测更接近。