Observational studies of low-frequency oscillations in the Southern Hemisphere
This work examines 40-50 day fluctuations in the Southern Hemisphere middle and high latitudes. The existence of these fluctuations are established in rawinsonde data at Easter Island (27°S, 109°W) and in global geopotential heights fields. These fluctuations behave as quasi-stationary wavetrains that propagate energy along an elliptical path from low latitudes in the Indian Ocean to the coast of Antarctica, near 70°W. The wavetrains exhibit vertical phase tilts from the surface to about 500 mb; above that, they are vertically in phase. However, the wavetrains are not observed in the middle and upper stratosphere. Over the Indian Ocean, there is also evidence suggesting that the wavetrains interact with the tropical 40-50 day oscillation. In addition, the extratropical 40-50 day fluctuations extract energy from the mean zonal flow through baroclinic processes but lose energy to the mean zonal flow through barotropic processes;The results revealed here increase the understanding of the global 40-50 day phenomena and may provide an important effect in the evolution of the tropical 40-50 day oscillation.