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Michael J. Pook, Peter C. McIntosh, and Gary A. Meyers

Abstract

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.

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Warren B. White, Gary A. Meyers, Jean Rene Donguy, and Stephen E. Pazan

Abstract

Short-term climatic variability in both sea surface temperature (SST) and vertically averaged temperature over the upper 400 m of ocean (T av) is mapped over the Pacific from 20°S to 50°N each bimonth for four years from 1979 to 1982, leading up to the 1982–83 ENSO (El Nino–Southern Oscillation) event. This mapping was made possible by the collection of approximately 85 000 temperature/depth observations in the Pacific Ocean by volunteer observing ships. Anomalies of SST and T av were approximately the same magnitude at midlatitude as in the tropics, with the exception of large changes occurring in the tropics during the 1982–83 ENSO event. During the ENSO event, (T av variability was largest in the western tropical North Pacific and SST variability was largest in the eastern equatorial Pacific. Both parameters had spatial patterns which were of opposite phase on either side of the ocean, indicating an eastward shift of warm waters during the ENSO event. Correlation studies determined that on average during the four years extraequatorial T av anomalies propagated from east to west at baroclinic long-wave speeds, travelling faster near the equator (i.e., 25–75 cm s−1 at 10°N and 10°S) and slower at higher latitudes (i.e., 2 cm s−1 new 40°N). At the equator, T av anomaly propagation was to the east at internal Kelvin wave speeds (i.e., 50–250 cm s−1). On the other hand, SST anomalies propagated on average during the four years in the direction of mean surface currents, so that, for example, at 5°N anomalies propagated to the east with the North Equatorial Countercurrent. Beginning in 1981, anomalously high T av propagated westward from the central North Pacific near 16°N and approached the maritime coast of Asia late in the year. During early 1982 it propagated rapidly down the coast toward the equator, then along the equator to the west coast of South America. This initial development was similar to the development of the 1968–69 and 1972–73 ENSO events in the western North Pacific observed by White and others. In mid-1982, this initial development was followed by a reduction in the Southeast Trade Winds over the western and central equatorial Pacific and a rapid intensification of the ENSO episode.

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Caroline C. Ummenhofer, Alexander Sen Gupta, Peter R. Briggs, Matthew H. England, Peter C. McIntosh, Gary A. Meyers, Michael J. Pook, Michael R. Raupach, and James S. Risbey

Abstract

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 Niño–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 Niño and a positive IOD event (La Niña 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 Niña and both positive and negative IOD events. In particular, La Niña 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.

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Caroline C. Ummenhofer, Alexander Sen Gupta, Peter R. Briggs, Matthew H. England, Peter C. McIntosh, Gary A. Meyers, Michael J. Pook, Michael R. Raupach, and James S. Risbey
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