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Abstract
This PhD thesis investigates the relative importance of oceanic and atmospheric
influences on extremes, long-term trends, and seasonal to interannual variability
of precipitation for different regions in the Southern Hemisphere in observations,
reanalysis data, and output from general circulation models (GCM).
Examination of interannual rainfall extremes over southwest Western Australia
(SWWA) reveals a characteristic dipole pattern of Indian Ocean sea surface temperature anomalies (SSTA). This coincides with a large-scale reorganization of the wind field over the tropical/subtropical Indian Ocean changing SSTA, via anomalous Ekman transport in the tropical Indian Ocean and via anomalous air-sea heat fluxes in the subtropics, and altering moisture advection onto SWWA. The potential impact of these Indian Ocean SSTA in driving modulations of mid-latitude
precipitation across southern and western regions of Australia is assessed in atmospheric GCM simulations. The SSTA give rise to changes in the thermal properties of the atmosphere, meridional thickness gradient, subtropical jet, thermal wind, and baroclinicity over southern regions of Australia, thus modulating precipitation. In addition, links between anomalous wet conditions over East Africa and these characteristic Indian Ocean SSTA are explored during the "short rain" season in October-November.
Interannual extremes m New Zealand rainfall and their modulation by modes
of Southern Hemisphere climate variability, namely the Southern Annular Mode
(SAM) and El Nino-Southern Oscillation (ENSO), are investigated. Late twentieth
Century trends in New Zealand precipitation are examined for the period 19792006
to quantify the relative impact of recent changes in the large-scale atmospheric
circulation related to the SAM and ENSO. Increasingly drier conditions over much
of New Zealand can be partially explained by the SAM and ENSO.
Cool season rainfall variability in southeastern Australia is investigated via a
classification and characterization of the predominant types of synoptic systems
occurring in the region, focusing on frontal and cutoff low systems. Two definitions
of the autumn break developed for northwestern Victoria are employed to produce
a synoptic climatology of the break phenomenon. Trends in characteristics of the
autumn break indicate that the most recent drought in southeastern Australia is
comparable in severity with the two major droughts in the twentieth Century.