Latitudinal height couplings between single tropopause and 500 and 100 hpa within the Southern Hemisphere

Abstract

In order to provide further insights into the relationships between the tropopause and different mandatory levels, this paper discusses the coupling between standardized tropopause height anomalies (STHAs) and standardized 500-hPa and 100-hPa height anomalies (S5HAs and S1HAs, respectively) within the 'climatic year' for three sets of upper-air stations located approximately along 20°S, 30°S and 45 °S. Data used in this research consists in a radiosonde database spanning the period 1973-2007. The mandatory levels are supposed to be included in each radiosonde profile. The tropopause, on the other hand, is calculated from the significant levels available for each sounding using the lapse rate definition. After applying a selection procedure, a basic statistical analysis combined with Fourier analysis is carried out in order to build up the standardized variables. Empirical orthogonal functions (EOFs) in S-mode are used to get the normal modes of oscillation as well as their time evolution, for STHA/S5HA as well as for STHA/S1HA coupling, separately, within the aforementioned latitudes. Overall, there are definite cycles in the time evolution associated with each EOF structure at all three latitudes, the semiannual wave playing the most important role in most of the cases. Nevertheless, 20°S seems to be the only latitude driven by diabatic heating cycles in the middle atmosphere. Certainly, EOF1 at this latitude has a semiannual behaviour and seems to be strongly influenced by the tropical convection seasonality. Apparently, the convectively driven release of latent heat in the middle troposphere affects the time evolution of the EOF1 structure. By contrast, the vertical propagation of planetary waves is raised as a possible explanation for the EOF1 and EOF2 behaviour at latitudes beyond 20°S, in view of the close connection existent between the semiannual oscillation (SAO) and the reversion in the direction of the zonal wind.