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K. Ngan, P. Bartello, and D. N. Straub

Abstract

Although it is now accepted that imbalance in the atmosphere and ocean is generic, the feedback of the unbalanced motion on the balanced flow has not received much attention. In this work the parameterization problem is examined in the context of rotating stratified turbulence, that is, with a nonhydrostatic Boussinesq model. Using the normal modes as a first approximation to the balanced and unbalanced flow, the growth of ageostrophic perturbations to the quasigeostrophic flow and the associated feedback are studied. For weak stratification, there are analogies with the three-dimensionalization of decaying 2D turbulence: the growth rate of the ageostrophic perturbation follows a linear estimate, geostrophic energy is extracted from the base flow, and the associated damping on the geostrophic base flow (the “eddy viscosity”) is peaked at large horizontal scales. For strong stratification, the transfer spectra and eddy viscosities maintain this structure if there is synoptic-scale motion and the buoyancy scale is adequately resolved. This has been confirmed for global Rossby and Froude numbers of O(0.1). Implications for atmospheric and oceanic modeling are discussed.

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K. Ngan, P. Bartello, and D. N. Straub

Abstract

Although predictability represents one of the fundamental problems in atmospheric science, gaps in our knowledge remain. Theoretical understanding of the inverse error cascade is limited mostly to homogeneous, isotropic turbulence, whereas numerical simulations have focused on highly complex numerical weather prediction models. These results cannot be easily reconciled.

This paper describes selected aspects of the predictability behavior of rotating stratified turbulence. The objective is to determine how the predictability varies with scale when the dynamics are more realistic than the idealized models that underlie the classical picture of predictability and yet are free of the parameterizations that complicate interpretation of NWP models. Using a numerical model of the nonhydrostatic Boussinesq equations, it is shown that the predictability decay, as diagnosed by the relative error, is slower for subsynoptic flow. The dependence on the deformation radius, differences between balanced and unbalanced modes, and implications for NWP models are discussed.

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David N. Straub, Peter D. Killworth, and Mitsuhiro Kawase

Abstract

The classic Stommel–Arons problem is revisited in the context of a basin with a general bottom topography containing an equator. Topography is taken to be smoothly varying and, as such, there are no vertical side walls in the problem. The perimeter of the abyssal basin is thus defined as the curve along which the layer depth vanishes. Because of this, it is not required that the component of horizontal velocity perpendicular to the boundary curve vanish on the boundary. Planetary geostrophic dynamics leads to a characteristic equation for the interface height field in which characteristics typically originate from a single point located on the eastern edge of the basin at the equator. For a simple choice of topography it is possible to solve the problem analytically. In the linear limit of weak forcing, the solution exhibits an intensified flow on the western edge of the basin. This flow is pan of the interior solution and is thus not a traditional, dissipative western boundary current.

When the fully nonlinear continuity equation is considered, the characteristics form a caustic on the western side of the basin. Characteristics cannot be integrated through the caustic so that a boundary layer involving higher-order dynamics is required. Approaching the caustic from the cast, velocities predicted by the interior solution become infinite. Because of this and because of global man budget considerations, the boundary layer is shown to lie east of the caustic. The position of the boundary layer is, however, not unique. Away from the equator, it is straightforward to append a Stommel boundary layer to the interior solution.

Next, localized mass sources are considered. Their dynamics (planetary geostrophy is assumed, except where boundary currents are required) include caustics and boundary layers in a manner similar to the sink-driven problem described above. For topography such that characteristics have a positive westward component, a caustic lies on the poleward side of the source and extends to the equator. Again, to balance the mass budget, a dissipative boundary layer is required inside the caustic. This, together with all the characteristics leaving the source region, intersects the equator at a finite angle. There, velocities become infinite, while the layer depth goes to zero. Although the details of the implied boundary layer are left unresolved, it is argued that the boundary current crosses the equator at a small angle to the eastward direction.

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G. C. Smith, F. J. Saucier, and D. Straub

Abstract

Mostly because of a lack of observations, fundamental aspects of the St. Lawrence Estuary’s wintertime response to forcing remain poorly understood. The results of a field campaign over the winter of 2002/03 in the estuary are presented. The response of the system to tidal forcing is assessed through the use of harmonic analyses of temperature, salinity, sea level, and current observations. The analyses confirm previous evidence for the presence of semidiurnal internal tides, albeit at greater depths than previously observed for ice-free months. The low-frequency tidal streams were found to be mostly baroclinic in character and to produce an important neap tide intensification of the estuarine circulation. Despite stronger atmospheric momentum forcing in winter, the response is found to be less coherent with the winds than seen in previous studies of ice-free months. The tidal residuals show the cold intermediate layer in the estuary is renewed rapidly (14 days) in late March by the advection of a wedge of near-freezing waters from the Gulf of St. Lawrence. In situ processes appeared to play a lesser role in the renewal of this layer. In particular, significant wintertime deepening of the estuarine surface mixed layer was prevented by surface stability, which remained high throughout the winter. The observations also suggest that the bottom circulation was intensified during winter, with the intrusion in the deep layer of relatively warm Atlantic waters, such that the 3°C isotherm rose from below 150 m to near 60 m.

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S. A. Venegas, L. A. Mysak, and D. N. Straub

Abstract

The climate variability of the South Atlantic region is determined from 40 yr (1953–92) of Comprehensive Ocean–Atmosphere Data Set monthly sea surface temperature (SST) and sea level pressure (SLP) data using the empirical orthogonal function (EOF) and the singular value decomposition (SVD) analysis methods. The EOF method is applied to each field separately, whereas the SVD method is applied to both fields simultaneously. The significance of the atmosphere–ocean interaction is revealed by a strong resemblance between individual (EOF) and coupled (SVD) modes of SST and SLP. The three leading modes of coupled variability on interannual and interdecadal timescales are discussed in some detail.

The first coupled mode, which accounts for 63% of the total square covariance, represents a 14–16-yr period oscillation in the strength of the subtropical anticyclone, accompanied by fluctuations of a north–south dipole structure in the SST. The atmosphere–ocean coupling is strongest during the southern summer. The second coupled mode (20% of the total square covariance) is characterized by east–west shifts of the anticyclone center, in association with 6–7-yr period fluctuations of SST off the coast of Africa. The coupling depicted by this mode is weaker than that found in the first and third modes. The third coupled mode (6% of the total square covariance) is characterized by north–south displacements of the anticyclone, accompanied by SST fluctuations over a latitudinal band in the central South Atlantic. These oscillations occur on a relatively short interannual timescale (∼4 yr). As with the first mode, the atmosphere–ocean coupling is strongest during the southern summer. This mode is found to be temporally and spatially correlated with the El Niño–Southern Oscillation phenomenon. The statistical robustness of the results is tested by using a Monte Carlo approach, which indicates that the presented results are highly significant.

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Bjorn Stevens, Donald H. Lenschow, Gabor Vali, Hermann Gerber, A. Bandy, B. Blomquist, J. -L. Brenguier, C. S. Bretherton, F. Burnet, T. Campos, S. Chai, I. Faloona, D. Friesen, S. Haimov, K. Laursen, D. K. Lilly, S. M. Loehrer, Szymon P. Malinowski, B. Morley, M. D. Petters, D. C. Rogers, L. Russell, V. Savic-Jovcic, J. R. Snider, D. Straub, Marcin J. Szumowski, H. Takagi, D. C. Thornton, M. Tschudi, C. Twohy, M. Wetzel, and M. C. van Zanten

The second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study is described. The field program consisted of nine flights in marine stratocumulus west-southwest of San Diego, California. The objective of the program was to better understand the physics a n d dynamics of marine stratocumulus. Toward this end special flight strategies, including predominantly nocturnal flights, were employed to optimize estimates of entrainment velocities at cloud-top, large-scale divergence within the boundary layer, drizzle processes in the cloud, cloud microstructure, and aerosol–cloud interactions. Cloud conditions during DYCOMS-II were excellent with almost every flight having uniformly overcast clouds topping a well-mixed boundary layer. Although the emphasis of the manuscript is on the goals and methodologies of DYCOMS-II, some preliminary findings are also presented—the most significant being that the cloud layers appear to entrain less and drizzle more than previous theoretical work led investigators to expect.

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