The Rossby radius in the Arctic Ocean

The first (and second) baroclinic deformation (or Rossby) radii are presented north of ~60° N, focusing on deep basins and shelf seas in the high Arctic Ocean, the Nordic seas, Baffin Bay, Hudson Bay and the Canadian Arctic Archipelago, derived from climatological ocean data. In the high Arctic Ocean, the first Rossby radius increases from ~5 km in the Nansen Basin to ~15 km in the central Canadian Basin. In the shelf seas and elsewhere, values are low (1–7 km), reflecting weak density stratification, shallow water, or both. Seasonality strongly impacts the Rossby radius only in shallow seas, where winter homogenization of the water column can reduce it to below 1 km. Greater detail is seen in the output from an ice–ocean general circulation model, of higher resolution than the climatology. To assess the impact of secular variability, 10 years (2003–2012) of hydrographic stations along 150° W in the Beaufort Gyre are also analysed. The first-mode Rossby radius increases over this period by ~20%. Finally, we review the observed scales of Arctic Ocean eddies

Isopycnal averaging at constant height. Part II: relating to the residual streamfunction in eulerian space

In Part I , the “vertical” transport streamfunction was defined as resulting from isopycnic averaging at constant height in the same way that the meridional streamfunction results from averaging at constant latitude. Part II here discusses the relationship between these two isopycnic streamfunctions and the Eulerian residual streamfunction that arises from the transformed Eulerian mean (TEM). It is known that the meridional isopycnic streamfunction can be approximated by a Taylor expansion to give an Eulerian residual streamfunction involving the horizontal eddy flux. This Taylor expansion approximation works well in the interior, removing the spurious mixing associated with the simple Eulerian-averaged streamfunction. However, it fails near the surface where isopycnals outcrop to the surface. It can be shown in a similar way that the vertical isopycnic streamfunction can formally be approximated by a residual streamfunction involving the vertical eddy flux. However, if horizontal isopycnal displacements are large, this approximation fails even in the ocean interior. Inspired by the two different residual streamfunctions, a more general form of TEM formulation is explored. It is shown that the different TEM residual streamfunctions arise from decomposing the eddy flux into a component along isopycnals, which leads to advective flow, and a remaining diffusive component, which is oriented either vertically or horizontally. In theory the diffusive flux can be oriented in any direction, although in practice the orientation should be such that neither the advective flow nor the diffusive flux cross any boundary (surface, sidewalls, and bottom). However, it is not clear how to merge the continuously changing orientation in a physically meaningful way. A variety of approaches are discussed

Isopycnal averaging at constant height. Part I: the formulation and a case study

Simple Eulerian averaging of velocities, density, and tracers at constant position is the most natural way of averaging. However, Eulerian averaging gives incorrect watermass distributions and properties as well as spurious diabatic circulations such as the Deacon cell. Instead of averaging at constant height, averaging along isopycnals removes such fictitious mixing and diabatic circulations. Such isopycnal averaging is normally performed at constant latitude, that is, averaging along isopynals as they heave up and down. As a result, height information is lost and the sea surface becomes much warmer (or lighter) than with simple Eulerian averaging. In fact, averaging can be performed along arbitrarily aligned surfaces. This study considers a particular case in which isopycnal averaging is performed at constant height. Thus, this new isopycnal averaging follows isopycnals as they meander horizontally at constant z. Height information is now retained at the cost of losing latitudinal information. The advantage of this averaging is that it avoids the problem of giving a surface that is too warm. Associated with this new isopycnal averaging, a “vertical” transport streamfunction in (ρ, z) space can be defined, in analogy to the conventional meridional overturning streamfunction in (y, ρ) space. Here in Part I, this constant-height isopycnal averaging is explained and illustrated in an idealized zonal channel model. In Part II the relationship between the two different isopycnal averagings and the Eulerian mean eddy flux divergence is explored

Spin-up of upwelling circulation over a continental shelf

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Formation of an Azores Current due to Mediterranean overflow in a modeling study of the North Atlantic

A mechanism for the formation of the Azores Current is proposed. On the basis of observations and model results, it is argued that the primary cause of the Azores Current is the water mass transformation associated with the Mediterranean overflow in the Gulf of Cadiz. Observations show that the transport of the Mediterranean outflow water through the Strait of Gibraltar increases significantly as it descends the continental slope by entraining the overlying North Atlantic Central Water. This entrainment process introduces a sink at the eastern boundary to the ocean upper layer in addition to the inflow into the Mediterranean. Such a sink is capable of inducing strong zonal flows such as the Azores Current. This mechanism is confirmed by numerical experiments with and without the representation of the Mediterranean overflow process. The numerical model is based on the Miami Isopycnic Coordinate Ocean Model. The model does not include the Mediterranean overflow explicitly, but restores the model density fields in the Gulf of Cadiz toward the observations. This restoring condition produces a reasonable representation of the water mass transformation deduced from observations. The formation of the Azores Current in response to the water mass transformation in the Gulf of Cadiz suggests that the Mediterranean overflow is not only a source of warm and saline water at depth, but also has a strong dynamic impact on the ocean upper layer. This study emphasizes the need to improve the representation of the Mediterranean overflow process in general circulation models in order to capture the correct characteristics of the flow fields and water masses in the subtropical eastern North Atlantic

Global rate and spectral characteristics of internal gravity wave generation by geostrophic flow over topography

The rate of generation of internal gravity waves in the lee of small length scale topography by geostrophic flow in the World Ocean was estimated using linear theory with corrections for finite amplitude topography. Several global data sets were combined for the calculation including an ocean circulation model for the near-bottom geostrophic flow statistics, over 500 abyssal current meter records, historical climatological data for the buoyancy frequency, and two independent estimates of the small scale topographic statistical properties. The first topography estimate was based on an empirically-derived relationship between paleo-spreading rates and abyssal hill roughness, with corrections for sedimentation. The second estimate was based on small-scale (<100 km) roughness of satellite altimetry-derived gravity field, using upward continuation relationships to derive estimates of abyssal hill roughness at the seafloor at scales less than approximately 20 km. The lee wave generation rate was found to be between 0.34 to 0.49 TW. The Southern Hemisphere produced 92% of the lee wave energy, with the Southern Ocean dominating. Strength of the bottom flow was the most important factor in producing the global pattern of generation rate, except in the Indian Ocean where extremely rough topography produced strong lee wave generation despite only moderate bottom flows. The results imply about one half of the mechanical power input to the ocean general circulation from the extra-equatorial wind stress of the World Ocean results from abyssal lee wave generation. Topographic length scales between 176 m and 2.5 km (horizontal wavelengths between 1 and 16 km) accounted for 90% of the globally integrated generation