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Ernesto H. Berbery and Julia Nogués-Paegle

Abstract

The mechanics of the interaction between tropical beating [estimated from outgoing longwave radiation (OLR)] and Southern Hemisphere (SH) subtropical and extratropical circulations on intraseasonal time scales are discussed. Base points are selected from teleconnectivity and teleconnection maps between OLR and zonal wind, heights, and meridional component of the divergent wind. Then, composites are formed for pentads with OLR anomalies at the base point greater in magnitude than one standard deviation.

Enhanced convection over Indonesia is found to be associated with increases of both the southward component of the meridional divergent wind and of the westerly, zonal wind to the south of the heating region during the SH summer. The increased westerly wind gradients, resulting to a certain extent from strengthened northerly flow, together with increased values of the southward component of the divergent wind, lead to an enhancement of the Rossby wave source in the vorticity equation in the vicinity of Australia. Streamfunction anomalies indicate that a wave train evolves from this region, following the typical ray path expected from linear theory.

Tropical-extratropical connections are less pronounced during SH winter than during summer, though an increase of westerly winds in the SH is found associated with enhanced convective activity in the Northern Hemisphere. The increase of the zonal wind during winter is again explained by meridional overturnings that emanate from the heating regions. Isentropic trajectories are used to show that the zonal accelerations caused by the poleward motion at upper levels are in agreement with observed values. The enhancement of convective activity is also related to a southward increase of the meridional component of the divergent wind that maximizes near the equator. However, since the latitudes of maximum southward component of the meridional divergent wind differ from those with maximum changes in the gradient of absolute vorticity, no increase of the Rossby wave source or excitation of Rossby waves due to tropical heating is found during this season.

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Ernesto H. Berbery and Carolina S. Vera

Abstract

The structure and evolution of the fluctuations in synoptic scales in the Southern Hemisphere (SH) during winter are discussed using six years of European Centre for Medium-Range Weather Forecasts analyses.

It is shown that patterns from unfiltered meridional wind series in the SH display all the features needed to represent the synoptic-scale waves. Typical periods and wavelengths are similar to those observed in the Northern Hemisphere (4 days, 4000 km), although over the Pacific Ocean they can be as high as 7–8 days and 4700 km, respectively. As in the Northern Hemisphere, tilts are not geographically fixed but change with the stage of the evolution of the wave. The phase speed of the waves agrees with the low-level winds in extensive areas of the middle latitudes and ranges from 12 m s −1 in the Indian Ocean to 6 m s−1 in the Pacific Ocean. The estimated group velocities achieve maximum values of about 38 m s −1, also in the Indian Ocean, and agree with the upper-level maximum winds, in accord with reported model results for the leading fringe of the wave packets.

The wave packets show a decay of upstream centers as new ones grow downstream, suggesting that down-stream development contributes to the evolution of the synoptic-scale waves in the SH storm track. This process is observed both in the subpolar and subtropical jets, but the sequence of centers developing downstream is more coherent in the latter, probably due to the weaker baroclinicity.

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Ernesto H. Berbery, Julia Nogués-Paegle, and John D. Horel

Abstract

The dynamical basis of intraseasonal oscillations of the Southern Hemisphere summer and winter seasons is studied with a combination of observed diagnostics and simplified prognostic models. High-frequency oscillations, zonal mean variations, and seasonal and interannual variabilities are removed from the six-year dataset in an effort to reduce the effect of high-frequency dynamical instabilities and long-period forced fluctuations. The diagnoses focus upon those processes that have most frequently been explained in terms of Rossby-wave propagation through atmospheres with variable refractive indices. It is useful to study both winter and summer seasons simultaneously because of the large changes in the seasonally averaged state and large consequent changes in atmospheric waveguides between these seasons. A nonlinear shallow-water model slowly relaxed toward the time-averaged winter and summer observed mean fields is used to describe the characteristics of wave propagation in a horizontally varying basic state. Perturbations are introduced in four different regions corresponding to points where observed atmospheric teleconnectivities are relatively large, and the signal propagation is analyzed using averaging procedures similar to those employed for the observational study. Furthermore, differences between stationary and nonstationary patterns are also discussed.

The four general regions selected for the observational study are Australia, New Zealand, South America, and the Atlantic Ocean. Differences from winter to summer are related to concomitant changes of the background latitudinal gradient of absolute vorticity. During winter and summer meridional propagation is toward the tropics. Winter wave patterns have mainly zonal paths and show a slow phase velocity on the order of 3 m s−1, while during summer, patterns tend to be geographically fixed. During winter, regions of imaginary refractive index flank the subtropical and polar jet streams. These jet streams seem to act as waveguides for disturbances emanating from the southern Indian Ocean and western Australia, where two wave trains exist. Wave activity flux vectors suggest that these disturbances originate in the subtropical southern Indian Ocean and that equatorward propagation prevails at the exit region of the subpolar jet stream and over South America and the Atlantic Ocean. During summer, observed wave patterns tend to have a more meridional component, again in agreement with the background latitudinal gradient of absolute vorticity.

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