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L. S. Graff and J. H. LaCasce
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Annalisa Bracco, J. H. LaCasce, and Antonello Provenzale

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Probability density functions (PDFs) of daily velocities from subsurface floats deployed in the North Atlantic and equatorial Atlantic Oceans are examined. In general, the PDFs are approximately Gaussian for small velocities, but with significant exponential tails for large velocities. Correspondingly, the kurtoses of the distributions are greater than three. Similar PDFs are found in both western and eastern regions, above and below 1000-m depth, with more significant non-Gaussianity in the North Atlantic than at the equator. Analogously, Lagrangian statistics in decaying two-dimensional turbulence also display non-Gaussian velocity PDFs with approximately exponential tails, in the limit of large Reynolds number.

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L. S. Graff and J. H. LaCasce

Abstract

The impact of changes in sea surface temperature (SST) on the statistics of extratropical cyclones is investigated. The cyclones were identified in an atmospheric general circulation model (AGCM) using an objective Lagrangian tracking algorithm, applied to the 850-hPa relative vorticity. The statistics were generated for several 20-yr simulations, in which the SSTs were warmed or cooled by 2 K in latitudinal bands. The response was studied in both hemispheres, during summer and winter.

Changes in the position of the storm tracks are largely consistent with those seen in previous studies. Increasing SSTs uniformly or increasing the midlatitude SST gradient results in a poleward shift in the storm tracks, with the clearest trends seen in the Southern Hemisphere (SH). Here it is demonstrated that the SST modifications alter the cyclone characteristics as well. When the warming includes the low latitudes and/or the midlatitude gradient is increased, there are more short-lived cyclones. These are also on average more intense and translate faster, both poleward and eastward.

The poleward displacement is correlated with cyclone intensity, so that stronger cyclones translate to higher latitudes. This is suggestive of vortex self-advection in the presence of a mean potential vorticity (PV) gradient. The increased eastward translation is correlated with the depth-averaged zonal velocity, and so is likely related to an increase in the steering-level velocity. These changes in cyclone translation probably contribute to the changes in the storm tracks seen previously.

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Lise Seland Graff and J. H. LaCasce

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A poleward shift in the extratropical storm tracks has been identified in observational and climate simulations. The authors examine the role of altered sea surface temperatures (SSTs) on the storm-track position and intensity in an atmospheric general circulation model (AGCM) using realistic lower boundary conditions.

A set of experiments was conducted in which the SSTs where changed by 2 K in specified latitude bands. The primary profile was inspired by the observed trend in ocean temperatures, with the largest warming occurring at low latitudes. The response to several other heating patterns was also investigated, to examine the effect of imposed gradients and low- versus high-latitude heating. The focus is on the Northern Hemisphere (NH) winter, averaged over a 20-yr period.

Results show that the storm tracks respond to changes in both the mean SST and SST gradients, consistent with previous studies employing aquaplanet (water only) boundary conditions. Increasing the mean SST strengthens the Hadley circulation and the subtropical jets, causing the storm tracks to intensify and shift poleward. Increasing the SST gradient at midlatitudes similarly causes an intensification and a poleward shift of the storm tracks. Increasing the gradient in the tropics, on the other hand, causes the Hadley cells to contract and the storm tracks to shift equatorward. Consistent shifts are seen in the mean zonal velocity, the atmospheric baroclinicity, the eddy heat and momentum fluxes, and the atmospheric meridional overturning circulation. The results support the idea that oceanic heating could be a contributing factor to the observed shift in the storm tracks.

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J. H. LaCasce and P. E. Isachsen

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The southwest Indian Ocean is distinguished by discontinuities in the wind-driven Sverdrup circulation. These connect the northern and southern tips of Madagascar with Africa and the southern tip of Africa with South America. In an analytical barotropic model with a flat bottom, the discontinuities produce intense westward jets. Those off the northern tip of Madagascar and the southern tip of Africa are always present, while the strength of that off southern Madagascar depends on the position of the zero curl line in the Indian Ocean (the jet is strong if the line intersects Madagascar but weak if the line is north of the island). All three jets are barotropically unstable by the Rayleigh–Kuo criterion. The authors studied the development of the instability using a primitive equation model, with a flat bottom and realistic coastlines. The model produced westward jets at the three sites and these became unstable after several weeks, generating 200–300-km scale eddies. The eddies generated west of Madagascar are in accord with observations and with previous numerical studies. The model’s Agulhas eddies are similar in size to the observed eddies, both the anticyclonic rings and the cyclones that form to the west of the tip of South Africa. However, the model’s Agulhas does not retroflect, most likely because of its lack of stratification and topography, and so cannot capture pinching-off events. It is noteworthy nevertheless that a retroflection is not required to produce eddies here.

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P. E. Isachsen, J. H. LaCasce, and J. Pedlosky

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The stability of baroclinic Rossby waves in large ocean basins is examined, and the quasigeostrophic (QG) results of LaCasce and Pedlosky are generalized. First, stability equations are derived for perturbations on large-scale waves, using the two-layer shallow-water system. These equations resemble the QG stability equations, except that they retain the variation of the internal deformation radius with latitude. The equations are solved numerically for different initial conditions through eigenmode calculations and time stepping. The fastest-growing eigenmodes are intensified at high latitudes, and the slower-growing modes are intensified at lower latitudes. All of the modes have meridional scales and growth times that are comparable to the deformation radius in the latitude range where the eigenmode is intensified. This is what one would expect if one had applied QG theory in latitude bands. The evolution of large-scale waves was then simulated using the Regional Ocean Modeling System primitive equation model. The results are consistent with the theoretical predictions, with deformation-scale perturbations growing at rates inversely proportional to the local deformation radius. The waves succumb to the perturbations at the mid- to high latitudes, but are able to cross the basin at low latitudes before doing so. Also, the barotropic waves produced by the instability propagate faster than the baroclinic long-wave speed, which may explain the discrepancy in speeds noted by Chelton and Schlax.

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J. H. LaCasce, J. Escartin, Eric. P. Chassignet, and Xiaobiao Xu

Abstract

The stability of a horizontally and vertically sheared surface jet is examined, with a focus on the vertical structure of the resultant eddies. Over a flat bottom, the instability is mixed baroclinic/barotropic, producing strong eddies at depth that are characteristically shifted downstream relative to the surface eddies. Baroclinic instability is suppressed over a large slope for retrograde jets (with a flow antiparallel to topographic wave propagation) and to a lesser extent for prograde jets (with flow parallel to topographic wave propagation), as seen previously. In such cases, barotropic (lateral) instability dominates if the jet is sufficiently narrow. This yields surface eddies whose size is independent of the slope but proportional to the jet width. Deep eddies still form, forced by interfacial motion associated with the surface eddies, but they are weaker than under baroclinic instability and are vertically aligned with the surface eddies. A sinusoidal ridge acts similarly, suppressing baroclinic instability and favoring lateral instability in the upper layer. A ridge with a 1-km wavelength and an amplitude of roughly 10 m is sufficient to suppress baroclinic instability. Surveys of bottom roughness from bathymetry acquired with shipboard multibeam echo sounding reveal that such heights are common beneath the Kuroshio, the Antarctic Circumpolar Current, and, to a lesser extent, the Gulf Stream. Consistent with this, vorticity and velocity cross sections from a 1/50° HYCOM simulation suggest that Gulf Stream eddies are vertically aligned, as in the linear stability calculations with strong topography. Thus, lateral instability may be more common than previously thought, owing to topography hindering vertical energy transfer.

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A. Gjermundsen, J. H. LaCasce, and L. S. Graff

Abstract

In numerous studies, midlatitude storm tracks have been shown to shift poleward under global warming scenarios. Among the possible causes, changes in sea surface temperature (SST) have been shown to affect both the intensity and the position of the tracks. Increased SSTs can increase both the lateral heating occurring in the tropics and the midlatitude temperature gradients, both of which increase tropospheric baroclinicity.

To better understand the response to altered SST, a simplified energy balance model (EBM) is used. This employs the principal of maximum entropy production (MEP) to determine the meridional heat fluxes in the atmosphere. The model is similar to one proposed by but represents only the atmospheric response (the surface temperatures are fixed). The model is then compared with a full atmospheric general circulation model [Community Atmosphere Model, version 3 (CAM3)].

In response to perturbed surface temperatures, EBM exhibits similar changes in (vertically integrated) air temperature, convective heat fluxes, and meridional heat transport. However, the changes in CAM3 are often more localized, particularly at low latitudes. This, in turn, results in a shift of the storm tracks in CAM3, which is largely absent in EBM. EBM is more successful, however, at representing the response to changes in high-latitude heating or cooling. Therefore, MEP is evidently a plausible representation for heat transport in the midlatitudes, but not necessarily at low latitudes.

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L. S. Graff, S. Guttu, and J. H. LaCasce

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The dispersion of pairs of synthetic particles, advected with ECMWF winds, is examined. The particles were deployed at three latitudes and on three potential temperature surfaces in both hemispheres. Separation statistics are calculated and evaluated in relation to 2D turbulence theory and to Eulerian structure functions calculated directly from the wind data.

At the smallest sampled scales (100–1000 km), the pair-separation velocities are correlated, and the dispersion is laterally isotropic, at least at the higher latitudes. At larger scales, the dispersion is zonally anisotropic, and the pair velocities are uncorrelated. In all cases, the dispersion grows exponentially in time, and the second-order Eulerian structure functions consistently increase as separation squared. This implies nonlocal dispersion, which obtains with energy spectra at least as steep as K −3.

Regional variations are seen in the parameters however. The e-folding times and the maximum scales for exponential growth are significantly larger on the 430-K surface than on the 315-K surface, and the dispersion is anisotropic at low latitudes, even at the smallest scales. Therefore, 2D homogeneous turbulence theory is applicable at best at subdeformation scales at the higher latitudes.

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J. H. LaCasce, O. A. Nøst, and P. E. Isachsen

Abstract

Steady flows in regions of closed geostrophic contours, such as in ocean basins or over seamounts, are examined by calculating solutions to the nonlinear quasigeostrophic and shallow-water equations with topography. For oceanically realistic choices of parameters, the solutions are asymmetric in that those with cyclonic circulation resemble the topography while those with anticyclonic circulation exhibit small-scale structure and cross-isobath flow. These small-scale structures reflect topographic wave modes, which are stationary with the anticyclonic circulation. The implication of the asymmetry is that random wind forcing is much more likely to excite persistent cyclonic than anticyclonic flow. This may explain why the circulation in the Norwegian and Greenland Gyres is most often cyclonic.

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