| Summary: | This thesis concerns an intraseasonal periodic behavior of the Southern Hemisphere storm track, which is particularly conspicuous during the summer months. It is shown that surface eddy kinetic energy (EKE) and rain rates inferred from remote sensing data exhibit remarkable periodicity around 25 days in the austral summer. This is consistent with subseasonal climate variability, baroclinic annular mode (BAM), a subseasonal oscillation in the storm track activity, discovered recently in the meteorological reanalysis products. With the assumption that the interaction between eddies and the mean flow plays an important role in this periodic behavior, the thesis begins by developing a new theoretical framework of the eddy - mean flow interaction. It consists of three coupled equations for the interior and surface finite-amplitude wave activity (FAWA) and the barotropic zonal-mean zonal flow. The theory provides an accurate latitude-by-latitude description of atmospheric angular momentum - wave activity budget that captures the storm track dynamics.
In the mid-latitude austral summer, the wave activity budget reveals a largely adiabatic, antiphase covariation of FAWA and the mean flow. A marked periodicity is found for FAWA around 20-30 days, but not for the mean flow. The former is primarily driven by the low-level meridional eddy heat flux, which also exhibits a sharp spectral peak around 25 days, whereas the latter is primarily driven by the meridional eddy momentum flux. The difference in the spectra of FAWA and the mean flow arises from (i) distinct spectra of low-level meridional eddy heat flux and the barotropic eddy momentum flux convergence and (ii) a strong thermal damping of surface wave activity.
The thesis demonstrates that the 25-day periodicity in FAWA and the eddy heat flux is particularly robust during the warm season. It is shown that the essence of the periodicity is reproduced in a hierarchy of numerical models, including the state-of-the-art climate models, and that the results are largely insensitive to the representation of the oceans. A dry GCM reproduces qualitatively BAM-like eddy heat flux spectra if the zonal-mean state resembles that of the austral summer and if the surface thermal damping is sufficiently strong. The two-layer quasi-geostrophic model captures the gist of oscillation when baroclinicity is weak and the bottom layer is chosen sufficiently thinner than the top layer.
The observed eddy heat flux cospectra in summer contain a few dominant frequencies for each of the energy-containing zonal wavenumbers (4-6). As these modes travel at different phase speeds they interfere with each other and produce an amplitude modulation to the eddy heat flux with a periodicity consistent with the BAM. The meridionally confined baroclinic zone in the mean state of the austral summer provides a waveguide that directs the mode propagation and interference along the latitude circle. Somewhat surprisingly, the emerging picture champions the linear wave dynamics as a driving mechanism of the 25-day periodicity. This stands in a stark contrast to the prevailing interpretation of the BAM based on the nonlinear oscillator model, in which the feedback between the eddy heat flux and background baroclinicity is deemed essential for the observed periodicity. This thesis demonstrates that such feedback is not corroborated by data for the austral storm track.
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