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April 17, 2008 - Ocean - URL of this posting: http://www.atmosocean.org/preprints/#2008.04.17.Berloff.et.al - Request to remove this posting
On formation of multiple zonal jets in the oceans.
Pavel Berloff, Igor Kamenkovich, and Josef Pedlosky
submitted to Journal of Fluid Mechanics, pdf

Multiple alternating zonal jets observed in the ocean are studied with an idealized quasigeostrophic model with the background flow imposed. Formation of the jets is governed by a spatially nonlocal mechanism that involves basin-scale instabilities. Energy of the background flow is released to the primary unstable mode with long meridional and short zonal lengthscale. This mode undergoes secondary instability that sets meridional scale of the multiple zonal jets. In a zonal channel, eddies generated by the instabilities maintain several weakly damped annular modes that significantly modify the jets and feed back on the primary instability.

It is found that the jets are driven by the mixed, barotropic-baroclinic dynamics and maintained by either Reynolds or form stress forcing, depending on the direction of the background flow. The underlying dynamical mechanism is illuminated both with statistical analysis of the nonlinear equilibrium solutions and with linear stability analysis of the flow components. Finally, we find that the jets are associated with alternating weak barriers to the meridional material transport, but locations of these barriers are not unique.


March 20, 2008 - Ocean - URL of this posting: http://www.atmosocean.org/preprints/#2008.03.20.Lapeyre - Request to remove this posting
What mesoscale signal does the altimeter reflect? On the decomposition in baroclinic modes and on a surface-trapped mode
Guillaume Lapeyre
submitted to Journal of Physical Oceanography, pdf

This study is motivated by the ongoing debate on the dynamical properties of surface motions at mesoscales that are measured by altimetry (for SSH) and microwave (for SST). The mesoscale signal seen by the altimeter is often considered to be associated with the first baroclinic mode, but recent results indicate that SST spectra and kinetic energy spectra derived from SSH have the same slope which is not consistent with this hypothesis. Moreover baroclinicmodes are associated by definition with vanishing buoyancy anomalies at the ocean surface which is obviously not the case. Here a careful derivation of the vertical modes is done using the concepts of quasigeostrophic potential vorticity (QG PV) theory. It is shown that the surface condition linking the streamfunction derivative and surface buoyancy necessitates to add a surface-trapped mode with no interior QG PV. The decomposition of a geostrophic flow on baroclinic modes alone is therefore incomplete and a complete decomposition involves both the surface mode and the barotropic/baroclinic modes. The surface mode is the generalization of a Surface QG (SQG) solution and is not orthogonal in the standard sense with baroclinic modes as it strongly projects at mesoscales on the first baroclinic mode. These results are illustrated with analytical examples and with a realistic simulation of the North Atlantic ocean. The surface mode is shown to be as energetic as the interior modes in the OGCM simulation. Moreover it dominates the surface mesoscale signal in most of the active regions of the Atlantic. On the other hand, the first baroclinic mode becomes dominant at depth as expected by previous results of the literature. The dominance of the surface mode at the surface is shown to be determined at first order by the large-scale forcing of PV and surface buoyancy. These results point out the necessity of a new interpretation of the surface dynamics and its coupling with the ocean interior for turbulent flows at mesoscales.


October 31, 2007 - Ocean - URL of this posting: http://www.atmosocean.org/preprints/#2007.10.31.Thompson.Heywood - Request to remove this posting
Frontal structure and transport in the northwestern Weddell Sea
Andrew F. Thompson and Karen J. Heywood
submitted to Deep-Sea Research I, pdf

Hydrographic data from the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project are analyzed to determine the frontal structure and transport along a section across the continental shelf and slope in the northwestern Weddell Sea. The flow is dominated by three barotropic northward flowing currents: the Antarctic Coastal Current, the Antarctic Slope Front and the Weddell Front. The strongest baroclinic flows are confined to the region between the Slope Front and the Weddell Front over the steepest part of the continental slope. The Antarctic Coastal Current flows over the continental shelf near a local steepening in the bathymetry and has a transport of approx. 1.3 Sv. The Antarctic Slope Front is found approximately 25 km offshore of the shelf break in 800 m of water. The Slope Front, which is associated with a transport of approx. 4 Sv, exhibits peak velocities at the sea bed that reach 35 cm s^(-1) as detected by lowered acoustic Doppler profiler (LADCP) measurements. A third, previously unreported northward current is found centered between the 2500 m and 3000 m isobath corresponding to a local break in the topography. There is evidence that this is the same feature known as the Weddell Front traditionally associated with flow over the 3000 m isobath in the northern Weddell Sea. The absence of the Weddell Front in data from pervious field programs is explained by too coarse or too shallow sampling. The Weddell Front accounts for approx. 17 Sv of northward transport. A deep outflow is observed all along the continental slope between the Slope Front and the Weddell Front. The deep outflow is localized in two to three distinct cores that are tied to topographical features. The total transport across the section is 46 6 Sv. This value exceeds previous estimates because the full-depth and de-tided LADCP measurements allowed the narrow (approx. 20 km) frontal currents to be resolved, leading to more accurate estimates of the barotropic component of the flow. We discuss the physical processes that may lead to the formation and maintenance of these fronts.


August 3, 2007 - Ocean - URL of this posting: http://www.atmosocean.org/preprints/#2007.08.03.Spall.et.al - Request to remove this posting
Western Arctic shelfbreak eddies: Formation and transport
Michael A. Spall, Robert S. Pickart, Paula S. Fratantoni, and Albert J. Plueddemann
submitted to Journal of Physical Oceanography, pdf

The mean structure and time-dependent behavior of the shelfbreak jet along the southern Beaufort Sea, and its ability to transport properties into the basin interior via eddies, are explored using high resolution mooring data and an idealized numerical model. The analysis focuses on springtime, when weakly stratified winter-transformed Pacific water is being advected out of the Chukchi Sea. When winds are weak, the observed jet is bottom trapped with a low potential vorticity core, has maximum mean velocities of O( 25 cm/s), and an eastward transport of 0.42 Sv. Despite the absence of winds, the current is highly time dependent, with relative vorticity and twisting vorticity often important components of the Ertel potential vorticity. An idealized primitive equation model forced by dense, weakly stratified waters flowing off a shelf produces a mean mid-depth boundary current similar in structure to that observed at the mooring site. The model boundary current is also highly variable, and produces numerous strong, small, anticyclonic eddies that transport the shelf water into the basin interior. Analysis of the energy conversion terms in both the mooring data and the numerical model indicates that the eddies are formed via baroclinic instability of the boundary current. The structure of the eddies in the basin interior compares well with observations from drifting ice platforms. The results suggest that eddies shed from the shelfbreak jet contribute significantly to the offshore flux of heat, salt, and other properties, and are likely important for the ventilation of the halocline in the western Arctic Ocean. Interaction with an anticyclonic basin-scale circulation, meant to simulate the Beaufort Gyre, enhances the offshore transport of shelf water and results in a loss of mass transport from the shelfbreak jet.


July 24, 2007 - Ocean - URL of this posting: http://www.atmosocean.org/preprints/#2007.07.24.Fox-Kemper.et.al - Request to remove this posting
Parameterization of mixed layer eddies. I: Theory and diagnosis
Baylor Fox-Kemper, Raffaele Ferrari, and Robert Hallberg
submitted to Journal of Physical Oceanography, pdf

Ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean. In this paper a parameterization for the restratification driven by finite amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal. The streamfunction is proportional to the product of the horizontal density gradient, the mixed layer depth squared, and the inertial period. Hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence. In this paper the parameterization is theoretically motivated, confirmed to perform well for a wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. In particular it is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification. Two companion papers discuss the numerical implementation and the climate impacts of this parameterization.


July 6, 2007 - Ocean - URL of this posting: http://www.atmosocean.org/preprints/#2007.07.06.Chelton.et.al - Request to remove this posting
Global observations of large oceanic eddies
Dudley B. Chelton, Michael G. Schlax, Roger M. Samelson, and Roland A. de Szoeke
submitted to Geophysical Research Letters (accepted), pdf

Ten years of sea-surface height (SSH) fields constructed from the merged TOPEX/Poseidon (T/P) and ERS-1/2 altimeter datasets are analyzed to investigate mesoscale variability in the global ocean. The higher resolution of the merged dataset reveals that nearly 60% of the variability over much of the World Ocean is accounted for by eddies with amplitudes of 5-25 cm and diameters of 100-200 km. These eddies propagate nearly due west at approximately the phase speed of nondispersive baroclinic Rossby waves with preferences for slight poleward and equatorward deflection of cyclonic and anticyclonic eddies, respectively. The vast majority of the eddies are found to be nonlinear.


June 6, 2007 - Atmospheric Science - URL of this posting: http://www.atmosocean.org/preprints/#2007.06.06.Vallis.Gerber - Request to remove this posting
Local and hemispheric dynamics of the North Atlantic Oscillation, annular patterns and the zonal index
Geoffrey K. Vallis and Edwin P. Berger
submitted to Dynamics of Atmospheres and Oceans (in press), pdf

In this paper we discuss the atmospheric dynamics of the North Atlantic Oscillation (NAO), the zonal index, and annular patterns of variability (also known as annular modes). Our goal is to give a unified treatment of these related phenomena, to make explicit how they are connected and how they differ, and to illustrate their dynamics with the aid of an idealized primitive equation model. Our focus is on tropospheric dynamics.

We first show that the structure of the empirical orthogonal functions (EOFs) of the NAO and annular modes follows, at least in part, from the structure of the baroclinic zone. Given a single baroclinic zone, and concomitantly a single eddy-driven jet, the meridional structure of the EOFs follows from the nature of the jet variability, and if the jet variability is constrained to conserve zonal momentum then the observed structure of the EOF can be explained with a simple model. In the zonal direction, if the baroclinic zone is statistically uniform then so is the first EOF, even though there may be little correlation of any dynam- ical fields in that direction. If the baroclinic activity is zonally concentrated, then so is the first EOF. Thus, at the simplest order of description, the NAO is a consequence of the pres- ence of an Atlantic storm track; the strong statement of this would be that the NAO is the variability of the Atlantic storm track. The positive phase of the NAO corresponds to eddy momentum fluxes (themselves a consequence of wave breaking) that push the eddy-driven jet polewards, separating it distinctly from the subtropical jet. The negative phase of the NAO is characterized by an equatorial shift and, sometimes, a weakening of the eddy fluxes and no separation between sub-tropical and eddy-driven jets. Variations in the zonal index (a measure of the zonally averaged zonal flow) also occur as a consequence of such activ- ity, although the changes occurring are not necessarily synchronous at different longitudes, and the presence of annular modes (i.e., the associated patterns of variability) does not necessarily indicate zonally symmetric dynamics.

The NAO, is not, however, a consequence of purely local dynamics, for the storm tracks depend for their existence on patterns of topographic and thermal forcing of near hemi- spheric extent. The Atlantic storm track in particular is a consequence of the presence of the Rocky mountains, the temperature contrast between the cold continent and warm ocean, and the lingering presence of the Pacific storm track. The precise relationship between the NAO and the storm tracks remains to be determined, as do a number of aspects of storm track dynamics, including their precise relation to the stationary eddies and to the regions of largest baroclinicity. Similarly, the influences of the stratosphere and of sea-surface tem- perature anomalies, and the causes and predictability of the inter-annual variability of the NAO remain open problems.