Mechanisms affecting the overturning response in global warming simulations

Climate models used to produce global warming scenarios exhibit widely diverging responses of the thermohaline circulation (THC). To investigate the mechanisms responsible for this variability, a regional Atlantic Ocean model driven with forcing diagnosed from two coupled greenhouse gas simulations has been employed. One of the coupled models (MPI) shows an almost constant THC, the other (GFDL) shows a declining THC in the twenty-first century. The THC evolution in the regional model corresponds rather closely to that of the respective coupled simulation, that is, it remains constant when driven with the forcing from the MPI model, and declines when driven with the GFDL forcing. These findings indicate that a detailed representation of ocean processes in the region covered by the Atlantic model may not be critical for the simulation of the overall THC changes in a global warming scenario, and specifically that the coupled model’s rather coarse representation of water mass formation processes in the subpolar North Atlantic is unlikely to be the primary cause for the large differences in the THC evolution. Sensitivity experiments have confirmed that a main parameter governing the THC response to global warming is the density of the intermediate waters in the Greenland–Iceland–Norwegian Seas, which in turn influences the density of the North Atlantic Deep Water, whereas changes in the air–sea heat and freshwater fluxes over the subpolar North Atlantic are only of moderate importance, and mainly influence the interannual–decadal variability of THC. Finally, as a consequence of changing surface fluxes, the Labrador Sea convection ceases by about 2030 under both forcings (i.e., even in a situation where the overall THC is stable) indicating that the eventual breakdown of the convection is likely but need not coincide with substantial THC changes

On the determination of internal-wave directional spectra from moored instruments

Presented here is a model for determining the characteristic parameters of an internalwave field using cross spectra of data obtained from vertically separated and/or horizontally separated moored instruments. The model is based on the assumption that the motion consists mainly of linear free internal waves having random phase relationships. The internalwave energy is supposed to be distributed among a finite number of modes having continuous directional distribution...

A technique for the determination of surface heat and freshwater fluxes from hydrographic observations, using an approximate adjoint ocean circulation model

A technique to determine the large-scale time-averaged ocean circulation from hydrographic observations and surface flux estimates is described. It is based on an inversion of the Bryan-Cox ocean general circulation model. We have constructed an approximate adjoint to that model which is computationally simpler and more economic than the exact adjoint. The optimization algorithm, although not optimal in a statistical sense, allows calculation of all state variables such that they are consistent with the equilibrium dynamics of the circulation model and agree as closely as possible with the observed data. To verify the technique, we have performed identical twin experiments with the circulation model in an idealized geometry. It is found that in principle the true model state including surface fluxes can be recovered with acceptable accuracy, even if no information on the surface fluxes is available. Under ideal conditions, the resulting rms errors of the surface fluxes were as low as 3 W/m2 and 0.2 m/year for heat and freshwater, respectively. Regions of deep-water formation due to convection show however larger errors on a small spatial scale, depending on the nonlinear, threshold-like nature of convective adjustment. The optimized solutions are distinctly sensitive to the integration time interval, with optimal values around three years. The results suggest that the procedure is suitable to obtain a consistent description of the oceanic state, and in particular more accurate estimates of the air-sea heat and freshwater fluxes

Recent advances in modelling the ocean circulation and its effects on climate

The authors aim to acquaint the reader with the current state of ocean circulation models, their ability to model the present climate state and its variability, and their major shortcomings and uncertainties. They limit the discussion to three-dimensional models of the physical system. They begin by describing the basic structure of circulation models, and discussing various problems with their implementation. They give a brief overview of the types of observational data in oceanography, and the ways in which the data are used. Some results from models of the wind-driven circulation are discussed, with particular emphasis on the dynamics of mesoscale eddies. Considerable progress has been made in understanding short-term variability associated with ENSO, and the authors describe ocean-atmosphere interactions in the tropics as well as results from coupled ocean-atmosphere models for ENSO variability. Models of the thermohaline circulation are described and some emerging ideas regarding long-term changes are given

Friedrich A. Schott (1939-2008)

The physical oceanography community recently lost one of its most influential and productive scientists. Friedrich A. (“Fritz”) Schott, who had been fighting leukemia for about a year, died on 30 April 2008 at the age of 69

The influence of numerical advection schemes on the results of ocean general circulation models

The dependence of results from coarse-resolution models of the North Atlantic circulation on the numerical advection algorithm is studied. In particular, the sensitivity of parameters relevant for climate simulations as e.g., meridional transport of mass and heat and main thermocline thickness is investigated. Three algorithms were considered: (a) a central difference scheme with constant values for horizontal and vertical diffusion, (b) an upstream scheme with no explicit diffusion, and (c) a flux-corrected transport (FCT) scheme with constant and strictly isopycnal diffusion. The temporal evolution of the three models on time scales of centuries is markedly different, the upstream scheme resulting in much shorter adjustment time whereas the central difference scheme is slower and controlled by vertical diffusion rather than advection. In the steady state, the main thermocline structure is much less diffusive in the FCT calculation which also has much lower heat transport. Both horizontal circulation and overturning in the meridional-vertical plane are strongest in the upstream-model. The results are discussed in terms of the effective vertical (diapycnal) mixing in the different models. A significant increase in vertical resolution would be required to eliminate the high sensitivity due to the numerical algorithms, and allow physically motivated mixing formulations to become effective

Assimilation of Geosat altimeter data into an eddy-resolving primitive equation model of the North Atlantic Ocean

We present a new method for assimilating observations of sea surface height (SSH) into a high‐resolution primitive equation model. The method is based on the concept of reinitialization. First, the surface velocity increments necessary to adjust the model forecast to the observed geostrophic surface currents are projected onto deep velocity increments by a linear regression method. Second, changes in the density field required to balance the changes in the velocity field geostrophically are obtained from an inversion of the thermal wind equation. A unique partition of the density increments into corresponding temperature and salinity changes is realized by conserving the local θ‐S relation of the model forecast. In contrast to pure statistical methods that infer temperature and salinity changes from correlations with SSH anomalies, our approach explicitly conserves water mass properties on isopycnals. For the assimilation experiment we use optimally interpolated maps of Geosat SSH anomalies (the mean topography is taken from the model), which are assimilated into the World Ocean Circulation Experiment (WOCE) Community Modeling Effort (CME) model of the North Atlantic Ocean at 5‐day intervals covering the year 1987. It is shown that the assimilation significantly improves the model's representation of eddy activity, with the hydrographic structure of individual eddies agreeing well with independent hydrographic observations. The importance of a careful treatment of water mass properties in the assimilation process is discussed and further illustrated by comparing different assimilation schemes