Rolf H. Käse

Interannual changes in the overflow from the Nordic Seas into the Atlantic Ocean through Denmark Strait

The global thermohaline circulation is an important part of Earths climate system. Cold, dense water formed in the Nordic Seas enters the Atlantic Ocean as overflows across the sills of the Greenland-Scotland Ridge. The Denmark Strait Overflow (DSO) is one of the main sources of North Atlantic Deep Water. Until now the DSO has been believed to be stable on interannual timescales. Here, for the first time, evidence is presented from a 4-year program of observations showing that overflow transports in 1999/2000 were approximately 30% higher than previous estimates. Later, transports decreased remarkably during the observation period, coincident with a temporary temperature increase of about 0.5°C.

Synoptic sections of the Denmark Strait Overflow

We report on a rapid high-resolution survey of the Denmark Strait overflow (DSO) as it crosses the sill, the first such program to incorporate full-water-column velocity profiles in addition to conventional hydrographic measurements. Seven transects with expendable profilers over the course of one week are used to estimate volume transport as a function of density. Our observations reveal the presence of a strongly barotropic flow associated with the nearly-vertical front dividing the Arctic and Atlantic waters. The seven-section mean transport of water denser than σθ=27.8 is 2.7±0.6Sv, while the mean transport of water colder than 2.0°C is 3.8±0.8 Sv. Although this is larger than the 2.9 Sv of θ < 2°C water measured by a 1973 current meter array, we find that a sampling of our sections equivalent to the extent of that array also measures 2.9Sv of cold water. Both the structure and magnitude of the measured flow are reproduced well by a high-resolution numerical model of buoyancy-driven exchange with realistic topography.

Can meddies be detected by satellite altimetry

Geosat Exact Repeat Mission altimeter data have been used to estimate the sea surface height anomaly relative to a 2-year mean reference surface in the eastern North Atlantic Ocean. Horizontal maps of the sea surface height anomaly for various periods were constructed by means of an objective analysis procedure. The mean sea level was approximated by the dynamic topography from climatological hydrographic data. Results were compared to the dynamic topography calculated from hydrographic ground truth data sampled during March 1988 on an eddy-resolving station network in the Iberian Basin. Despite the low signal-to-noise ratio of the altimeter data in this part of the ocean, the agreement of the altimeter signal with hydrographic measurements is found to be significant. Individual cyclonic and anticyclonic eddies in the Mediterranean outflow are identified in the altimetric maps. The information visible in the sea surface height is significantly correlated with the dynamic topography of the layer 500/3000 dbar containing the core of the Mediterranean outflow and maximum velocities of the baroclinic eddies. The successive development of the identified eddies as studied from a sequence of sea surface height maps indicates a slight westward drift of the most pronounced feature with approximately 1.9 cm/s propagation velocity. The inferred path of that feature is consistent with additional information from moored current meter data.

Reconstructed Mediterranean Salt Lens Trajectories

The existence of energetic anticyclonic mid-depth vortices of Mediterranean Water (meddies) questions the validity of a conventional advective–diffusive balance in the eastern Atlantic subtropical gyre. A mesoscale experiment in the Azores–Madeira region reveals a link of these meddies to large-scale subsurface meanders. For the first time it is shown that meddies may have strong surface vorticity, indicative of a generation process involving the Azores Current—a deep reaching near-surface jet.

Structure and variability of the Denmark Strait Overflow: Model and observations

We report on a combined modeling and observational effort to understand the Denmark Strait Overflow (DSO). Four cruises over the course of 3 years mapped hydrographic properties and velocity fields with high spatial resolution. The observations reveal the mean path of the dense water, as well as the presence of strong barotropic flows, energetic variability, and strong bottom friction and entrainment. A regional sigma coordinate numerical model of interbasin exchange using realistic bottom topography and an overflow forced only by an upstream reservoir of dense fluid is compared with the observations and used to further investigate these processes. The model successfully reproduces the volume transport of dense water at the sill, as well as the 1000-m descent of the dense water in the first 200 km from the sill and the intense eddies generated at 1–3 day intervals. Hydraulic control of the mean flow is indicated by a region supercritical to long gravity waves in the dense layer located approximately 100 km downstream of the sill in both model and observations. In addition, despite the differences in surface forcing, both model and observations exhibit similar transitions from mostly barotropic flow at the sill to a bottom-trapped baroclinic flow downstream, indicating the dominant role of the overflow in determining the full water column dynamics.

Flow through Denmark Strait

On the basis of hydrographic observations taken in the vicinity of Denmark Strait, a primitive equation model is used to investigate physical mechanisms that control the exchange through the strait. The dense water transport is topographically controlled and predictions by Whitehead [1998] and Killworth and McDonald [1993] are consistent with numerical model results. The distribution of temperature and thickness of the modeled plume is in good agreement with the high-resolution hydrographic data.

The Sensitivity of the Greenland–Scotland Ridge Overflow to Forcing Changes

Processes that influence the volume and heat transport across the Greenland–Scotland Ridge system are investigated in a numerical model with ° horizontal resolution. The focus is on the sensitivity of cross-ridge transports and the reaction of the subpolar North Atlantic Ocean circulation to changes in wind stress and buoyancy forcing on seasonal to interannual timescales. A general relation between changes in wind stress or cross-ridge density contrasts and the overturning transport of Greenland–Iceland–Norwegian Seas source water is established from a series of idealized experiments. The relation is used subsequently to interpret changes in an experiment over the years 1992–97 with realistic forcing. On seasonal and interannual timescales there is a clear correlation between heat flux and wind stress curl variability. The realistic model suggests a steady decrease in the strength of the cyclonic subpolar gyre of the North Atlantic with a corresponding decrease in heat transport during the 1990s

Numerical Modeling of Meander and Eddy Formation in the Azores Current Frontal Zone

Abstract Numerical experiments with an 11-level primitive equation, finite-difference model in a periodic channel are performed to analyze the properties of unstable finite-amplitude disturbances in an idealized Azores Current. Release of available potential energy due to baroclinic instability occurs preferentially on scales of about 100 km with a theoretical growth time of 8 days. At larger times, the combined effect of friction and nonlinear transfer between internal and external (depth integrated) mode and the distribution of energy among different wavenumbers of the initial disturbance determine the scale of the meandering jet. Cold water tongues with a meridional scale of several hundred km found in satellite images and hydrographic surveys east of the Azores are prescribed as initial disturbances. They develop into pairs of troughs and ridges dominated by cyclonic vortices on the poleward flank of the jet. Phase propagation is downstream at 2–4 km day−1. Extremely strong frontogenetic enhancement o...

Causes of Changes in the Denmark Strait Overflow

Abstract The warming Nordic seas potentially tend to decrease the overflow across the Greenland–Iceland–Scotland Ridge (GISR) system. Recent observations by Macrander et al. document a significant drop in the intensity of outflowing Denmark Strait Overflow Water of more than 20% over 3 yr and a simultaneous increase in the temperature of the bottom layers of more than 0.4°C. A simulation of the exchange across the GISR with a regional ocean circulation model is used here to identify possible mechanisms that control changes in the Denmark Strait overflow and its relations to changed forcing condition. On seasonal and longer time scales, the authors establish links of the overflow anomalies to a decreasing capacity of the dense water reservoir caused by a change of circulation pattern north of the sill. On annual and shorter time scales, the wind stress curl around Iceland determines the barotropic circulation around the island and thus the barotropic flow through Denmark Strait. For the overlapping time sc...

Cyclogenesis in the Denmark Strait Overflow Plume

A densely spaced hydrographic survey of the northern Irminger Basin together with satellite-tracked near-surface drifters confirm the intense mesoscale variability within and above the Denmark Strait overflow. In particular, the drifters show distinct cyclonic vortices over the downslope edge of the outflow plume. Growing perturbations such as these can be attributed to the baroclinic instability of a density current. A primitive equation model with periodic boundaries is used to simulate the destabilization of an idealized dense filament on a continental slope that resembles the northeastern Irminger Basin. Unstable waves evolve rapidly if the initial temperature profile is perturbed with a sinusoidal anomaly that exceeds a certain cutoff wavelength. As the waves grow to large amplitudes isolated eddies of both signs develop. Anticyclones form initially within the dense filament and are rich in overflow water. In contrast, cyclones form initially with their center in the ambient water but wrap outflow water around their center, thus containing a mixture of both water types. The nonlinear advection of waters that were originally located within the front between both water masses contributes most significantly to the stronger intensification of the cyclones in comparison with anticyclones. The frontal waters carry positive relative vorticity into the center of the cyclone. The process bears therefore some resemblance to atmospheric frontal cyclogenesis. After saturation there is a bottom jet of overflow water that is confined by counterrotating eddies: anticyclones upslope and cyclones downslope of the overflow core. The parameter dependence of the maximum growth rate is studied, and the implications of eddy-induced mixing for the water mass modification is discussed.