Daniel G. Wright

A Zonally Averaged, Coupled Ocean-Atmosphere Model for Paleoclimate Studies

Abstract A zonally averaged ocean model for the thermohaline circulation is coupled to a zonally averaged, one-layer energy balance model of the atmosphere to form a climate model for paleoclimate studies. The emphasis of the coupled model is on the oceans thermohaline circulation in the Pacific, Atlantic, and Indian oceans. Each basin is individually resolved, and they are connected by the Southern Ocean through which mass, heat, and salt are exchanged. Under present-day conditions, the global conveyor belt is simulated: deep water is formed in the North Atlantic and the Southern Ocean, whereas both Pacific and Indian oceans show broad upwelling. Latitude-depth structures of modeled temperature and salinity fields, as well as depth-integrated meridional transports of heat and freshwater, compare well with estimates from observations when wind stress is included. Ekman cells are present in the upper ocean and contribute substantially to the meridional fluxes at low latitudes, bringing them to close agree...

A Zonally Averaged Ocean Model for the Thermohaline Circulation. Part I: Model Development and Flow Dynamics

Abstract A two-dimensional latitude–depth ocean model is developed on the basis of the zonally averaged balance equations of mass, momentum, heat, and salt. Its purpose is to investigate the dynamics and variability of the buoyancy-forced thermohaline circulation. For the time scales of interest an annually averaged model is selected, and the momentum balance is taken to be diagnostic. The east-west pressure gradient, which arises upon zonally averaging the momentum equations, is parameterized in terms of the meridional pressure gradient. The thermohaline circulation is driven by mixed surface boundary conditions, i.e., temperatures are relaxed to prescribed values while the salt flux is held constant. The dynamics of the flow is investigated in hemispheric and global geometries for both short and long time integrations, the latter extending over many thousands of years. As has been noted by previous investigators, it is possible to perturb a steady state such that a diffusively dominated regime results. ...

The influence of high‐latitude surface forcing on the global thermohaline circulation

The basin-averaged, latitude-depth ocean model of Wright and Stocker (1992) is used to simulate the deep circulation of the world ocean. Under present-day surface forcing, sinking occurs in the North Atlantic and the southern ocean, and realistic temperature and salinity structures are obtained in the Atlantic, Pacific, and Indian oceans. “Color” tracers and radiocarbon are used to identify the composition of the deepwater masses and the associated renewal time scales. While broad agreement with observations is found in all basins, the water masses in the southern ocean are too young. The global thermohaline circulation and the composition of the deepwater masses are sensitive to the buoyancy contrast between the southern ocean and the North Atlantic. This contrast can be modified by changing relaxation values of temperature and salinity at the northern and southern high latitudes. If the model is forced with the zonal averages of the observed surface salinity, North Atlantic Deep Water is the dominant deep ocean water mass, and hardly any Antarctic Bottom Water flows into the Atlantic. Choosing instead the observed salinities of the newly formed deep water as the restoring values, the model realistically simulates the penetration of Antarctic Bottom Water into the different ocean basins. This has a global effect through reducing both strength and depth of North Atlantic Deep Water formation. If higher surface salinity values are applied in the southern ocean, a steady state is obtained whose tracer distributions and overturning are consistent with reconstructions of the deep circulation during the last glacial maximum. The two states are stable also under mixed boundary conditions and transitions are possible by smoothly varying the surface freshwater flux of one state to that of the other. These experiments suggest the importance of modified high-latitude forcing in glacial-to-interglacial transitions.

Interpretations of the JEBAR Term

Abstract In diagnostic calculations of the oceans circulation, the so-called JEBAR (joint effect of baroclinicity and relief) term may induce a significant depth-average current field, as has been noted in the oceanographic literature. Here we present two consistent interpretations of this term. In the equation governing the vorticity of the depth-averaged current, a topographic vortex-stretching term proportional to the dot product of depth-averaged velocity and the depth gradient arises. We show that JEBAR corrects this term by removing the contribution of the geostrophic flow referenced to the bottom, which cannot generate topographic vortex stretching. In the evolution equation for the vorticity of the depth-integrated flow, a bottom-torque term is present. We show that the JFBAR term discussed above enters here as a contribution to the bottom torque, emphasizing the role of JEBAR as a forcing term. We also show that when a diagnostic calculation is formulated completely in terms of the transport str...

Accurate and Computationally Efficient Algorithms for Potential Temperature and Density of Seawater

Abstract An equation of state for seawater is presented that contains 25 terms and is an excellent fit to the Feistel and Hagen equation of state. It is written in terms of potential temperature (rather than in situ temperature), as required for efficient ocean model integrations. The maximum density error of the fit is 3 × 10–3 kg m–3 in the oceanographic ranges of temperature, salinity, and pressure. The corresponding maximum error in the thermal expansion coefficient is 4 × 10–7 °C–1, which is a factor of 12 less than the corresponding maximum difference between the Feistel and Hagen equation of state and the widely used but less accurate international equation of state. A method is presented to convert between potential temperature and in situ temperature using specific entropy based on the Gibbs function of Feistel and Hagen. The resulting values of potential temperature are substantially more accurate than those based on the lapse rate derived from the international equation of state.

Algorithms for Density, Potential Temperature, Conservative Temperature, and the Freezing Temperature of Seawater

Algorithms are presented for density, potential temperature, conservative temperature, and the freezing temperature of seawater. The algorithms for potential temperature and density (in terms of potential temperature) are updates to routines recently published by McDougall et al., while the algorithms involving conservative temperature and the freezing temperatures of seawater are new. The McDougall et al. algorithms were based on the thermodynamic potential of Feistel and Hagen; the algorithms in this study are all based on the “new extended Gibbs thermodynamic potential of seawater” of Feistel. The algorithm for the computation of density in terms of salinity, pressure, and conservative temperature produces errors in density and in the corresponding thermal expansion coefficient of the same order as errors for the density equation using potential temperature, both being twice as accurate as the International Equation of State when compared with Feistel’s new equation of state. An inverse function relating potential temperature to conservative temperature is also provided. The difference between practical salinity and absolute salinity is discussed, and it is shown that the present practice of essentially ignoring the difference between these two different salinities is unlikely to cause significant errors in ocean models.

SENSITIVITIES OF A ZONALLY AVERAGED GLOBAL OCEAN CIRCULATION MODEL

A global ocean circulation model is constructed in which the Pacific, Atlantic, Indian, and Southern oceans are separately resolved, each being represented by zonally averaged equations expressing conservation of momentum, mass, heat, and salt. Results are presented and compared with relevant zonally averaged observations. The sensitivity of the stream function and the meridional fluxes of heat and water are examined as functions of the horizontal and vertical diffusion coefficients, and as functions of a closure parameter introduced in averaging the equations. The results are sensitive to changes in the vertical diffusion coefficient and the closure parameter. The sensitivities to changes in the vertical diffusion coefficient are similar to those of a three-dimensional ocean general circulation model. Results are relatively insensitive to the value of the horizontal diffusion coefficient provided it is of the order of 103 m2 s−1 or smaller. However, for larger values, a northward heat flux throughout the Atlantic basin, as observed, cannot be obtained. Wind stress significantly improves the comparison with observational estimates of the meridional heat and water fluxes, particularly near the equator, where there are large flux divergences associated with the Ekman transport. Results are further improved when horizontal diffusion is modified to include a contribution proportional to the local current speed. The effects of the Mediterranean and Red seas are examined and shown to be important in redistributing salt vertically. The model is then generalized to allow for depth-integrated flow through the Bering Strait and the Indonesian passages. Both effects improve the comparison with observational estimates, but neither effect appears to be crucial in determining the global circulation or water mass properties in this model. The inclusion of a barotropic flow introduces the necessity to specify a reference temperature in order to calculate the heat flux directly from velocity-temperature correlations. Simply neglecting the barotropic flow may contribute to large discrepancies between flux estimates based on integrated surface fluxes and “direct” correlation methods.

Tidal rectification and frontal circulation on the sides of Georges Bank

Using Wright and Loders (l985a,b) depth-dependent tidal rectification model and Garrett and Loders (1981) diagnostic frontal circulation model, predictions of the residual circulation associated with the topographic rectification of tidal currents and the summertime density field on the northwestern and open ocean sides of Georges Bank are made and compared with observations. In general, the estimates of both wintertime and summertime along-isobath currents are in qualitative agreement with observations, but the agreement between predicted and observed cross-isobath currents is poor. The circulation associated with tidal rectification is primarily along isobaths in an anticyclonic sense around the Bank at all depths. The cross-isobath circulation is much weaker and, in the Eulerian specification, is dominated by two cells with opposing current directions. However, a significant Stokes velocity is predicted such that the along-isobath Lagrangian current is generally less than its Eulerian counterpart, and the cross-isobath Lagrangian current is sometimes in the opposite direction to its Eulerian counterpart. Both the along-isobath and cross-isobath currents associated with tidal rectification are predicted to be significantly stronger in summer than in winter due to a reduction in the strength of friction as a result of reduced wind stress and increased density stratification. An additional contribution to the anticyclonic circulation around Georges Bank is associated directly with the summertime tidal front around the Bank. This flow component is predicted to form a second intense along-isobath jet on the northwestern side, slightly off-bank of that due to tidal rectification, and a broader flow on the open ocean side. The associated cross-isobath circulation is predicted to be much weaker than the along-isobath circulation, with a general on-bank bottom flowon both sides of the Bank.

Atlantic Climate Variability and Predictability: A CLIVAR Perspective

Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.

CANDIE: A New Version of the DieCAST Ocean Circulation Model

Abstract The development and verification of a new version of the DieCAST ocean circulation model to be referred to as CANDIE (Canadian Diecast) are considered. Both CANDIE and DieCAST have many features in common with the well-known Modular Ocean Model (MOM) of the Geophysical Fluid Dynamics Laboratory. Of particular relevance to the present study are the rigid-lid approximation and the use of standard Cartesian coordinates. The DieCAST formulation in terms of the surface pressure, rather than the volume transport streamfunction, is also used in CANDIE to reduce numerical sensitivity to ocean depth variations. The major difference between MOM and DieCAST is the use of a mixed C and A grid formulation in DieCAST rather than the B grid formulation used in MOM. CANDIE differs from DieCAST in the use of a standard C grid formulation and a reduction in the magnitude of the time truncation error associated with the implicit treatment of the Coriolis force. The implementation of the rigid-lid approximation is r...