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Detlef Stammer

Abstract

Oceanic state estimation is a powerful tool to estimate internal model parameters simultaneously with the model’s initial conditions and surface forcing field that jointly would bring a model into consistency with time-varying large-scale ocean observations. Here an attempt to estimate geographically varying fields of horizontal and vertical viscosity and diffusivity within a 9-yr-long estimation procedure is presented. The estimated coefficients are highly efficient in preserving watermass characteristics and frontal structures by reducing the model temperature and salinity drift, especially around the Southern Ocean. The estimated mean circulation results in stronger transports of western boundary currents and of the Antarctic Circumpolar Current. Moreover, an increase of about 10% in the strength of the meridional overturning circulation and in the poleward heat transport can be found. Estimated changes in the horizontal mixing coefficients seem to agree with the notion that diapycnal mixing is superfically high with Laplacian mixing formulations, especially close to frontal structures in the ocean. In comparison with adjustments in tracer diffusivities (vertically and horizontally), adjustments of viscosity coefficients are fairly minor outside lateral boundary regions, suggesting that state estimation attempts might be most successful in providing enhanced insight into tracer mixing.

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Detlef Stammer

Abstract

Estimates of eddy energy and eddy scales obtained previously from TOPEX/POSEIDON (T/P) altimeter data are interpreted in the context of a baroclinically unstable flow field. From the observations an integral timescale T alt can be defined that—combined with estimates of the eddy kinetic energy—sets a mixing length scale. Results are compared with theories of a baroclinically unstable flow field. For such conditions, the Eady theory predicts a timescale T bc = Ri/f from the mean-flow Richardson number Ri, which shows some qualitative agreement with T/P results in terms of a geographical distribution. A factor of 2 difference between the timescales can be explained in terms of a systematic difference between the specific definitions of scale estimates. Although transfer length and velocity scales emerging out of scaling arguments lack resemblance with observations, a transfer length scale based on T bc and the observed eddy kinetic energy is strikingly consistent with observed eddy scales. Primarily independent of the energetic state of the ocean, they are to first order largest in low latitudes and decrease toward high latitudes. Invoking a “mixing length” hypothesis, an eddy transfer κ for a scalar tracer in the ocean can be estimated from eddy statistics as a function of geographical position. Two different estimates of κ can be obtained from altimetric data: (i) κ = α KE L bc and (ii) κ̃ = α′(g/f)σ(SSH), where α and α′ are scaling factors, and σ(ζ) is the rms sea-surface height variability estimated from T/P data. The approaches lead to similar estimates of a meridional eddy-induced heat and salt flux inferred from climatological meridional temperature and salinity gradient F T = −c p κT/∂y and F S = −κS/∂y. Results are consistent with previous knowledge but, because estimates are based on mean meridional gradients, they have to be considered a lower bound on instantaneous eddy transports in the ocean.

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Detlef Stammer

Abstract

An attempt is made to determine the three-dimensional ocean circulation from satellite altimeter measurements by assimilating Geosat sea surface height data into an eddy-resolving quasigeostrophic (QG) model of the eastern North Atlantic Ocean. Results are tested against independent information from hydrographic field observations and moored current meter data collected during the Geosat ERM. The comparison supports the concept of inferring aspects of the three-dimensional flow field from sea surface height observations by combining altimetric measurements with the dynamics of ocean circulation models.

A Holland-type QG model with open boundaries is set up on a 2000 km × 2000 km domain of the eastern North Atlantic between 25° and 45°N, 32° and 8°W. By using a simple nudging technique, about two years of Geosat altimeter data are assimilated into the model every five days as space–time objective analyses on the model grid. The error information resulting from the analysis is used during the assimilation procedure to account for data uncertainties. Results show an intense eddy field, which in the surface layer interacts with a meandering Azores Front. Compared to Geosat, the model leads to smoothed fields that follow the observations.

Model simulations are significantly correlated with hydrographic data from March 1988 and June 1989, both close to the surface and in the subsurface. Good agreement is also found between the model velocity fields and moored current meter data in the top two model layers. The agreement is visually weak in the bottom layer, although a coherence analysis reveals an agreement between the model simulation and current meter data over the full water column at periods exceeding 80 days.

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Detlef Stammer

Abstract

Three years of altimetric data from the TOPEX/POSEIDON spacecraft have been used to study characteristics of eddy variability over the World Ocean. The nature of the variability and its spatial structure are characterized in terms of the geographical distribution of eddy energy, as simple approximations of observed regional frequency and wavenumber spectra, and in terms of associated eddy time and space scales of sea surface height (SSH) variability and geostrophic velocity. Emphasis is put on summarizing characteristics typical for dynamically distinct regions of the World Ocean. This effort results in an attempt to describe the observed ocean variability in terms of universal spectral relations that depend only on few mean flow parameters such as the first-mode Rossby radius of deformation. Regional peculiarities follow naturally as deviations from the fundamental frequency and wavenumber spectra presented here.

Frequency spectra of both variables can be summarized by three basic types representing (i) the energetic boundary currents, (ii) the bulk of the extratropical basins, and (iii) the tropical interior oceans. Extratropical wavenumber spectra suggest a geostrophically turbulent ocean. They are basically uniform in shape and show a plateau on long wavelength for SSH and a steep spectral decay close to k −5 toward smaller wavelengths. The transition between both regimes shifts toward longer cutoff wavenumbers from low to high latitudes, and related spatial eddy scales can be described in terms of the first-mode Rossby deformation radius of the mean flow field. Although consistent with estimated Rhines scales in low latitudes, inferred eddy scales are up to a factor of 0.3 smaller at high latitudes.

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Armin Köhl and Detlef Stammer

Abstract

An important part of ocean state estimation is the design of an observing system that allows for the efficient study of climate related questions in the ocean. A solution to the design problem is presented here in terms of optimal observations that emerge as singular vectors of the modified data resolution matrix. The actual computation is feasible only for scalar quantities and in the limit of large observational errors. Identical twin experiments performed in the framework of a 1° North Atlantic primitive equation model demonstrate that such optimal observations, when applied to determining the heat transport across the Greenland–Scotland ridge, perform significantly better than traditional section data. On seasonal to interannual time scales, optimal observations are located primarily along the continental shelf and information about heat transport, wind stress, and stratification is being communicated through boundary waves and advective processes. On time scales of about 1 month, sea surface height observations appear to be more efficient in reconstructing the cross-ridge heat transport than hydrographic observations. Optimal observations also provide a tool for understanding changes of ocean state associated with anomalies of integral quantities such as meridional heat transport.

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Youyu Lu and Detlef Stammer

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The vorticity budget of the vertically integrated circulation from two global ocean simulations is analyzed using a horizontal spacing of 2° × 2° in longitude/latitude. The two simulations differ in their initial hydrographic conditions and surface wind and buoyancy forcing. The constrained simulation obtains optimal initial condition and surface forcing through assimilating observational data using the model's adjoint, whereas the unconstrained simulation uses Levitus climatological conditions for initialization and is driven by NCEP–NCAR reanalysis forcing, plus restoring to the monthly surface temperature and salinity climatological conditions. The goal is to examine the dynamics that sets the time-mean circulation and to understand the differences between the constrained and unconstrained simulations. It is found that, similar to eddy-permitting simulations, the bottom pressure torque (BPT) in coarse-resolution models plays an important role in the western boundary currents and in the Southern Ocean, and largely balances the difference between wind stress curl and βV for the depth-integrated flow. BPT is a controlling factor of the interior abyssal flow. The geostrophic vorticity relation holds in the interior basins in intermediate and deep layers and breaks down in the upper ocean toward the surface. In the upper layer of the interior basins, the model simulations show statistically significant deviation from the Sverdrup balance. In the subtropical gyre regions, the deviation from Sverdrup balance is confined to zonal bands that are balanced by the curls of lateral friction and nonlinear advection. The differences between the constrained and unconstrained simulations are significant in vorticity terms. The adjustment to Levitus hydrographic climatological data as the model's initial condition causes the most significant changes in BPT, which is the main reason for changes in abyssal flow. The analysis also points to needs for further improvement of models and controlling the influence of data errors in ocean state estimation.

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Dietmar Dommenget and Detlef Stammer

Abstract

Simulations and seasonal forecasts of tropical Pacific SST and subsurface fields that are based on the global Consortium for Estimating the Circulation and Climate of the Ocean (ECCO) ocean-state estimation procedure are investigated. As compared to similar results from a traditional ENSO simulation and forecast procedure, the hindcast of the constrained ocean state is significantly closer to observed surface and subsurface conditions. The skill of the 12-month lead SST forecast in the equatorial Pacific is comparable in both approaches. The optimization appears to have better skill in the SST anomaly correlations, suggesting that the initial ocean conditions and forcing corrections calculated by the ocean-state estimation do have a positive impact on the predictive skill. However, the optimized forecast skill is currently limited by the low quality of the statistical atmosphere. Progress is expected from optimizing a coupled model over a longer time interval with the coupling statistics being part of the control vector.

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Armin Köhl and Detlef Stammer

Abstract

An estimate of the time-varying ocean circulation, obtained over the period 1952–2001, is analyzed here with respect to its decadal and longer-term changes in sea level. The estimate results from a synthesis of most of the ocean datasets available during this 50-yr period with the Estimating the Circulation and Climate of the Ocean/Massachusetts Institute of Technology (ECCO/MIT) ocean circulation model. Over the period 1992 through 2001, the increase in thermosteric sea level rise on average amounts to 1.2 mm yr−1 over the top 750 m and 1.8 mm yr−1 over the total water column. This corresponds to an increase in upper-ocean heat content of 1.5 × 1022 J yr−1 and is in agreement with the estimates of Willis et al. However, over the period 1962 through 2001 the global net thermosteric sea level rise is estimated as 0.66 mm yr−1 over the top 750 m, which is twice the recent estimate from Antonov et al. (0.33 mm yr−1). The corresponding trend over the total water column of 0.92 mm yr−1 is also about twice their value for the layer of 0–3000 m (0.40 mm yr−1). For the last decade, the global heat flux into the ocean of 1.5 W m−2 is twice as large as the recent estimate by Willis et al. due to the heat content change in deeper layers. Regional changes in sea level are predominantly associated with an intensification of the subtropical gyre circulation and a corresponding redistribution of heat. The horizontal advection of heat due to an increase in wind stress curl is found to explain a major fraction of the estimated regional sea level trends over the last 40 years. However, the mechanisms appear different during the last decade when in some regions changes in surface heat flux may explain as much as 50% of the sea level changes.

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Armin Köhl and Detlef Stammer

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The German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) provided a dynamically consistent estimate of the time-varying ocean circulation over the 50-yr period 1952–2001. The GECCO synthesis combines most of the data available during the entire estimation period with the ECCO–Massachusetts Institute of Technology (MIT) ocean circulation model using its adjoint. This GECCO estimate is analyzed here for the period 1962–2001 with respect to decadal and longer-term changes of the meridional overturning circulation (MOC) of the North Atlantic. A special focus is on the maximum MOC values at 25°N. Over this period, the dynamically self-consistent synthesis stays within the error bars of H. L. Bryden et al., but reveals a general increase of the MOC strength. The variability on decadal and longer time scales is decomposed into contributions from different processes. Changes in the model’s MOC strength are strongly influenced by the southward communication of density anomalies along the western boundary originating from the subpolar North Atlantic, which are related to changes in the Denmark Strait overflow but are only marginally influenced by water mass formation in the Labrador Sea. The influence of density anomalies propagating along the southern edge of the subtropical gyre associated with baroclinically unstable Rossby waves is found to be equally important. Wind-driven processes such as local Ekman transport explain a smaller fraction of the variability on those long time scales.

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Detlef Stammer and Christian Dieterich

Abstract

A method is presented that can provide high-resolution (in space and time) satellite measurements of the absolute and time-varying surface geostrophic flow field. Based on the analysis of a high-resolution circulation model of the North Atlantic, it is demonstrated that a tandem satellite mission as anticipated from the French–U.S. Jason and Ocean Topography Experiment (TOPEX)/Poseidon missions flown along parallel tracks would be suitable to measure the velocity of the geostrophic surface flow field and its higher statistical moments, such as kinetic energy and Reynolds stresses, with a space and time resolution similar to that obtained currently for sea surface height data from the TOPEX/Poseidon mission. The anticipated remote geostrophic velocity observations would allow unprecedented studies of the ocean general circulation, including its mean and eddy energies, eddy–eddy, and eddy–mean flow interactions.

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