This work identifies and documents a suite of large-scale drivers of rainfall variability in the Australian region. The key driver in terms of broad influence and impact on rainfall is the El Niño–Southern Oscillation (ENSO). ENSO is related to rainfall over much of the continent at different times, particularly in the north and east, with the regions of influence shifting with the seasons. The Indian Ocean dipole (IOD) is particularly important in the June–October period, which spans much of the wet season in the southwest and southeast where IOD has an influence. ENSO interacts with the IOD in this period such that their separate regions of influence cover the entire continent. Atmospheric blocking also becomes most important during this period and has an influence on rainfall across the southern half of the continent. The Madden–Julian oscillation can influence rainfall in different parts of the continent in different seasons, but its impact is strongest on the monsoonal rains in the north. The influence of the southern annular mode is mostly confined to the southwest and southeast of the continent. The patterns of rainfall relationship to each of the drivers exhibit substantial decadal variability, though the characteristic regions described above do not change markedly. The relationships between large-scale drivers and rainfall are robust to the selection of typical indices used to represent the drivers. In most regions the individual drivers account for less than 20% of monthly rainfall variability, though the drivers relate to a predictable component of this variability. The amount of rainfall variance explained by individual drivers is highest in eastern Australia and in spring, where it approaches 50% in association with ENSO and blocking.

The Indian Ocean zonal dipole is a mode of variability in sea surface temperature that seriously affects the climate of many nations around the Indian Ocean rim, as well as the global climate system. It has been the subject of increasing research, and sometimes of scientific debate concerning its existence/nonexistence and dependence/independence on/from the El Niño–Southern Oscillation, since it was first clearly identified in Nature in 1999. Much of the debate occurred because people did not agree on what years are the El Niño or La Niña years, not to mention the newly defined years of the positive or negative dipole. A method that identifies when the positive or negative extrema of the El Niño–Southern Oscillation and Indian Ocean dipole occur is proposed, and this method is used to classify each year from 1876 to 1999. The method is statistical in nature, but has a strong basis on the oceanic physical mechanisms that control the variability of the near-equatorial Indo-Pacific basin. Early in the study it was found that some years could not be clearly classified due to strong decadal variation; these years also must be recognized, along with the reason for their ambiguity. The sensitivity of the classification of years is tested by calculating composite maps of the Indo-Pacific sea surface temperature anomaly and the probability of below median Australian rainfall for different categories of the El Niño–Indian Ocean relationship.

Since 1995, a large region of Australia has been gripped by the most severe drought in living memory, the so‐called “Big Dry”. The ramifications for affected regions are dire, with acute water shortages for rural and metropolitan areas, record agricultural losses, the drying‐out of two of Australia's major river systems and far‐reaching ecosystem damage. Yet the drought's origins have remained elusive. For Southeast Australia, we show here that the “Big Dry” and other iconic 20th Century droughts, including the Federation Drought (1895–1902) and World War II drought (1937–1945), are driven by Indian Ocean variability, not Pacific Ocean conditions as traditionally assumed. Specifically, a conspicuous absence of Indian Ocean temperature conditions conducive to enhanced tropical moisture transport has deprived southeastern Australia of its normal rainfall quota. In the case of the “Big Dry”, its unprecedented intensity is also related to recent higher temperatures.

Daily rainfall during the April–October growing season in a major cropping region of southeastern Australia has been related to particular types of synoptic weather systems over a period of 33 yr. The analysis reveals that cutoff lows were responsible for at least 50% of all growing-season rainfall and accounted for 80% of daily rainfall events exceeding 25 mm per station. The proportion of rainfall contributed by cutoff lows varies throughout the growing season. It is highest in austral autumn and spring (55% and 57%, respectively) and falls to a minimum in July (42%). By way of contrast, the total contribution of all types of frontal systems to growing-season rainfall is about 32%, although the monthly value reaches a maximum of 41% in July when mean cutoff rainfall reaches a minimum. Rainfall associated with fronts is strongly concentrated in the lower range of daily falls (less than 10 mm per station). Frontal rainfall is found to be more consistent from year to year than is cutoff rainfall. The number of cutoff lows per season is highly variable, and there is a significant correlation between the number of cutoff days and atmospheric blocking in the region south of Australia in each month of the growing season. The mean amount of rainfall per cutoff day is also variable and has declined by approximately 0.8 mm over the analysis period. An understanding of the mechanisms controlling year-to-year variability of cutoff rainfall is therefore an important step in improving seasonal forecasts for agriculture in southeastern Australia.

Exceptional sea ice conditions occurred in the West Antarctic Peninsula (WAP) region from September 2001 to February 2002, resulting from a strongly positive atmospheric pressure anomaly in the South Atlantic coupled with strong negative anomalies in the Bellingshausen-Amundsen and southwest Weddell Seas. This created a strong and persistent north-northwesterly flow of mild and moist air across the WAP. In situ, satellite, and NCEP-NCAR Reanalysis (NNR) data are used to examine the profound and complex impact on regional sea ice, oceanography, and biota. Extensive sea ice melt, leading to an ocean mixed layer freshening and widespread ice surface flooding, snow-ice formation, and phytoplankton growth, coincided with extreme ice deformation and dynamic thickening. Sea ice dynamics were crucial to the development of an unusually early and rapid (short) retreat season (negative ice extent anomaly). Strong winds with a dominant northerly component created an unusually compact marginal ice zone and a major increase in ice thickness by deformation and over-rafting. This led to the atypical persistence of highly compact coastal ice through summer. Ecological effects were both positive and negative, the latter including an impact on the growth rate of larval Antarctic krill and the largest recorded between-season breeding population decrease and lowest reproductive success in a 30-yr Adélie penguin demographic time series. The unusual sea ice and snow cover conditions also contributed to the formation of a major phytoplankton bloom. Unexpectedly, the initial bloom occurred within compact sea ice and could not be detected in Sea-Viewing Wide Fieldof-View Sensor (SeaWiFS) ocean color data. This analysis demonstrates that sea ice extent alone is an inadequate descriptor of the regional sea ice state/conditions, from both a climatic and ecological perspective; further information is required on thickness and dynamics/deformation.

Cool season rainfall variability in southeastern Australia is investigated via classification and characterization of the predominant types of synoptic systems occurring in the region. These types are frontal systems, cut-off low systems, and other systems. Rainfall in the region is dominated by cut-off systems and these systems are the main influence on the interannual variability of rainfall. Both cut-off systems and frontal systems display an enhancement of thermal (thickness) gradient as rainfall increases, but the mechanisms for intensification differ. Cut-off systems intensify in the region in association with local increases in baroclinicity and the subtropical jet, whereas frontal systems tend to intensify via a confluence of subtropical and polar jets. Interannual rainfall variability is examined for groupings of years based on both clustering of continental rainfall patterns and on El Nino/Southern Oscillation (ENSO)/Indian Ocean Dipole (IOD) years. Cut-off systems exhibit consistent enhancements of thermal gradients for groupings of years in which they produce more rainfall. For ENSO/IOD groupings, the cut-off thermal gradients are consistent with the underlying sea surface temperature (SST) anomalies. Wet years in southeastern Australia are usually produced by cut-off systems, but can also be produced by frontal systems. In those cases the mid-tropospheric flow pattern is reminiscent of the negative Southern Annular Mode (SAM) pattern. The positive SAM pattern is also associated with enhanced rainfall in the southeast via local intensification of blocking and cut-off systems.

The relative influences of Indian and Pacific Ocean modes of variability on Australian rainfall and soil moisture are investigated for seasonal, interannual, and decadal time scales. For the period 1900-2006, observations, reanalysis products, and hindcasts of soil moisture during the cool season (June-October) are used to assess the impacts of El Niñ o-Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD) on southeastern Australia and the Murray-Darling Basin, two regions that have recently suffered severe droughts. A distinct asymmetry is found in the impacts of the opposite phases of both ENSO and IOD on Australian rainfall and soil moisture. There are significant differences between the dominant drivers of drought at interannual and decadal time scales. On interannual time scales, both ENSO and the IOD modify southeastern Australian soil moisture, with the driest (wettest) conditions over the southeast and more broadly over large parts of Australia occurring during years when an El Niñ o and a positive IOD event (La Niñ a and a negative IOD event) co-occur. The atmospheric circulation associated with these responses is discussed. Lower-frequency variability over southeastern Australia, however, including multiyear drought periods, seems to be more robustly related to Indian Ocean temperatures than Pacific conditions. The frequencies of both positive and negative IOD events are significantly different during periods of prolonged drought compared to extended periods of ''normal'' rainfall. In contrast, the frequency of ENSO events remains largely unchanged during prolonged dry and wet periods. For the Murray-Darling Basin, there appears to be a significant influence by La Niñ a and both positive and negative IOD events. In particular, La Niñ a plays a much more prominent role than for more southern regions, especially on interannual time scales and during prolonged wet periods. For prolonged dry (wet) periods, positive IOD events also occur in unusually high (low) numbers.

Antarctic fast ice is of key climatic and ecological importance, yet its distribution and variability are poorly understood. We present a detailed analysis of fast ice along the Adélie Land coast (East Antarctica) using satellite data from 1992 to 1999. Fast ice formation along this coastline is intimately linked to grounded iceberg distribution in waters of < 350 m depth. Considerable interannual variability occurs in areal extent and formation/break-up; the variability is related to wind direction. Distance to the fast ice edge and its extent are major determinants of emperor penguin Aptenodytes forsteri breeding success at Pointe Géologie. Of crucial importance are the frequency and duration of fast ice break-out events in the deep-water trough north-northwest of the colony. Successful penguin breeding seasons in 1993, 1998 and 1999 ([number of fledged chicks in late November / number of breeding pairs] > 75% success) coincided with lower-than-average fast ice extents and persistently short distances to nearest open water (foraging grounds), and corresponded to a strong positive phase of the Southern Annular Mode. Poor breeding seasons in 1992, 1994 and 1995 (success <15%) coincided with average to slightly higher-than-average ice extents and persistently long distances to foraging grounds. Poor-to-moderate breeding years (success ~40 to 50%), e.g. 1996 and 1997, occurred with above-average ice extents combined with fairly long distances from breeding to foraging grounds during the chick nurturing season. The overall correlation between breeding success and distance was high (r 2 = 0.89), albeit based on a limited number of years (n = 8). Substantially less fast ice was present in two Argon satellite photographs taken in August and October 1963. This coincided with a highly successful breeding season and appears to have been related to stronger and more southerly winds.