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  • Author or Editor: Albert I. Barcilon x
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Albert I. Barcilon

Abstract

The solution of buoyant jets in a calm stratified atmosphere is considered. The Morton model yields a set of differential equations that can be solved in a closed form in a phase space having for coordinates the integral with height of the mass flux, the mass flux, and the derivative with height of the mass flux. By using an approximate expression that relates the mass flux integral with height to a given height, we are able to transfer knowledge of the phase space solution to physical space.

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Zhijin Li
,
Albert Barcilon
, and
I. M. Navon

Abstract

This work describes the dynamics of adjoint sensitivity perturbations that excite block onsets over the Pacific and Atlantic Oceans. Appropriate functions are derived for the blocking indices for these two regions and the model basic flow is constructed from Northern Hemisphere climatological data. The concepts of sensitivity analysis are extended to forced problems. This tool is used to investigate block onset due to atmospheric forcing, such as that resulting from tropical sea surface temperature anomalies. These linear studies are carried out in a hemispherical, primitive equations, θ-coordinate, two-layer model.

Results show that wind sensitivity perturbations less than 10 m s−1 and sensitivity forcing of vorticity sources of the order of 1.5 × 10−10 s−2 are sufficient to excite block onset. Both for the Pacific and Atlantic blocking, sensitivity perturbations and forcing perturbations, when expressed in terms of vertical vorticity, display a Rossby wave train structure mainly found on the southward flanks of the Pacific and Atlantic jets, that is, near the Philippines and the Caribbean regions.

From inferences based on the flow evolution of these sensitivity perturbations and with the help of potential vorticity analyses on the two constant potential temperature surfaces in this model, a dynamical framework that may explain Pacific and Atlantic block onsets is proposed. The nonuniform potential vorticity distribution in the jets, in particular the concentration of these gradients on potential vorticity waveguides, and the Lagrangian advection of potential vorticity by the eddies making up the stationary Rossby wave train and their energy propagation and convergence all conspire to play a key role in the growth of the synoptic-scale eddies supported by baroclinic as well as barotropic processes. It is proposed that the structural modification of the eddies in the wave train leads to the planetary structures that become associated with block onset. More specifically, the wave train in the Pacific evolves into a blocking dipole while the Atlantic block is found at the leading edge of the Rossby wave train across the Atlantic. Furthermore, this study shows that at the initial time the Pacific block displays a clear baroclinic structure while the wave train associated with the Atlantic block has a much more barotropic structure.

The significance of these results and their potential applications to predictions of blocking are discussed.

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Jeremy P. Grist
,
Sharon E. Nicholson
, and
Albert I. Barcilon

Abstract

Differences in the basic state over West Africa between wet and dry years are well documented. This study investigates whether there are also observable differences in the easterly waves between wet and dry years and if these differences might be attributed to the changes in the basic state.

Contrasting basic states from the rainy seasons of the four wet years (1958–61) and four dry years (1982–85) were derived from the NCEP reanalysis. The basic states served as input for the linear instability model. The model results indicated faster growth rates and greater phase speeds in the wet years. These results were consistent with a wavelet analysis of the 600-mb meridional wind. This analysis showed that waves were stronger and tended to have a greater contribution from the longer periods during the wet years.

The differences in the waves appear to be due to the greater horizontal and vertical shear in wet years. The relative importance of these two were assessed using the Charney–Stern necessary condition for instability. It appears that the horizontal shear is more important in causing the differences. Although the baroclinic and barotropic terms were of similar magnitude in dry years, in wet years the barotropic term increased significantly, whereas the baroclinic term did not. Implications of the results for the understanding of interannual and interdecadal rainfall variability over West Africa are discussed.

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Kwang-Yul Kim
,
James J. O'Brien
, and
Albert I. Barcilon

Abstract

The structures of the two principal modes of sea surface temperature (SST) variability were extracted by conducting cyclostationary EOF (CSEOF) analysis and regression analysis on several key variables. The CSEOF analysis extracts two dominant modes of SST variability that are distinct in nature. The first CSEOF is stochastic in nature and represents a standing mode of SST variability associated with a basinwide change in the surface wind. The second CSEOF exhibits a strong deterministic component describing a biennial oscillation between a warm phase and a cold phase. The surface wind directional change in the far-western Pacific appears to be instrumental for the oscillation between the two phases. Because of the distinct nature of evolution, dynamical and thermodynamical responses of the two modes are different. Further, the predictability of the two modes is different. Specifically, the biennial mode is more predictable because of the strong deterministic component associated with its evolution. The distinction of the two modes, therefore, may be important for predicting ENSO. The irregular interplay of the two modes seems to explain some inter-ENSO variability, namely, variable duration of ENSO events, approximate phase-locking property, and irregular onset and termination times.

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