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Wasyl Drosdowsky

Abstract

The variability of the Australian summer monsoon is reexamined using data covering 35 monsoon seasons. A new, easily applied objective definition of active and break phases of the monsoon, based solely on the zonal wind at Darwin, is proposed. Attempts to define “wet westerly” onsets are shown to be misleading, since no clear relationship is found between westerly winds and rainfall, on the timescales associated with the transition between active and break phases. The resulting dates of monsoon onset at Darwin differ from those reported in some recent studies, resulting in significantly different relationships with the Southern Oscillation. In particular, the date of monsoon onset is shown to be significantly related to, and hence predicted from, prior values of the Southern Oscillation index. Also in contrast to a number of recent studies that have highlighted the so-called 40–50-day oscillation in the Australian summer monsoon, no dominant timescales are found in the length of the active periods or in the recurrence time between active phases.

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Wasyl Drosdowsky

Abstract

A nonlinear time series forecasting scheme developed by Sugihara and May has been applied to the Southern Oscillation index. Although forecast skill is comparable only to persistence or linear (autoregressive) methods, the scheme has the advantage of identifying close analogs to the current situation, if these exist. The operational implementation of the scheme in the Seasonal Climate Outlook issued by the National Climate Centre of the Australian Bureau of Meteorology is described and its performance during the 1991/92 El Niño–Southern Oscillation event is examined.

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Wasyl Drosdowsky
and
Mark Williams

Abstract

The response of the tropospheric circulation over Australia and the southwest Pacific to extremes of the Southern Oscillation is documented using correlation, regression, and compositing techniques applied to seasonal means of rawinsonde station data. A strong and seasonally varying signal is found in the four fields of geopotential height, temperature, zonal, and meridional wind components. The composite analysis shows that while the response to extremes of the oscillation is in general linear, some significant local nonlinear departures are observed.

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Wasyl Drosdowsky
and
Lynda E. Chambers

Abstract

An operational system for the prediction of Australian seasonal rainfall variations using sea surface temperature anomaly (SSTA) patterns over the Indian and Pacific Oceans is described. The SSTA patterns are represented by rotated principal components, with individual monthly values at 1- and 3-month lead times used as predictors;for example, November and January SSTAs are used to forecast March–May seasonal rainfall. The historical seasonal rainfall is also represented by rotated principal components of a gridded 1° rainfall dataset, with the principal component loadings used as weights to project the forecasts back to the original 1° grid points.

Forecasts of seasonal rainfall in two (above/below median) or three categories (terciles) are produced using linear discriminant analysis. Hindcast skill, measured by the linear error in probability space (LEPS) skill score has been assessed using cross validation. Experiments were also performed using a double or nested cross-validation procedure to select the best model or combination of predictors. The model chosen for operational seasonal forecasts uses the first two rotated SSTA components lagged by 1 and 3 months as predictors for every season and location, to maintain continuity of forecast probabilities between the overlapping 3-month seasons.

Current values of the principal component amplitudes are calculated by projecting either the Bureau of Meteorology’s or the National Centers for Environmental Prediction’s SST analysis onto the set of SST principal components. The hindcasts and experimental real-time forecasts over the 5-yr period from January–March 1994 to December–February 1998/99 indicate improved skill over parts of southern Australia during the autumn period using the SST-based schemes when compared with forecasts using the Southern Oscillation index alone.

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Wasyl Drosdowsky
and
Matthew C. Wheeler

Abstract

A forecast product focusing on the onset of the north Australian wet season using a dynamical ocean–atmosphere model is developed and verified. Onset is defined to occur when a threshold rainfall accumulation of 50 mm is reached from 1 September. This amount has been shown to be useful for agricultural applications, as it is about what is required to generate new plant growth after the usually dry period of June–August. The normal (median) onset date occurs first around Darwin in the north and Cairns in the east in late October, and is progressively later for locations farther inland away from these locations. However, there is significant interannual variability in the onset, and skillful predictions of this can be valuable. The potential of the Predictive Ocean–Atmosphere Model for Australia (POAMA), version 2, for making probabilistic predictions of onset, derived from its multimember ensemble, is shown. Using 50 yr of hindcasts, POAMA is found to skillfully predict the variability of onset, despite a generally dry bias, with the “percent correct” exceeding 70% over about a third of the Northern Territory. In comparison to a previously developed statistical method based solely on El Niño–Southern Oscillation, the POAMA system shows improved skill scores, suggesting that it gains from additional sources of predictability. However, the POAMA hindcasts do not reproduce the observed long-term trend in onset dates over inland regions to an earlier date despite being initialized with the observed warming ocean temperatures. Understanding and modeling this trend should lead to further enhancements in skill.

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Wasyl Drosdowsky
,
Greg J. Holland
, and
Roger K. Smith

Abstract

North Australian Clouds Lines are distinctive, squall-line phenomena that occur in easterly flow across northern Australia. Three basic types have been identified, ranging from a long, narrow line of convective clouds (Type 1) to a severe squall line (Type 3). In this paper we examine a group of Type 1 lines, which occurred during the first phase of the Australian Monsoon Experiment (AMEX). The lines occurred in an ambient easterly flow with a distinct maximum near 850 hPa. Most of the lines developed on the western side of deep convective cells along the sea-breeze front in a manner that had substantial similarities to the African squall-line development described by Bolton. The resolvable structure resembled a shallow version of the Moncrieff–Miller squall line.

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Noel E. Davidson
,
Kevin J. Tory
,
Michael J. Reeder
, and
Wasyl L. Drosdowsky

Abstract

The onset of the Australian monsoon is examined using (i) reanalysis data for seasons when enhanced observational networks were available and (ii) a 15-yr onset composite. Similar to previous findings, onset is characterized by a sudden strengthening and deepening in tropical westerly winds, which are overlain with upper-tropospheric easterlies. All onsets are preceded by up to a 7-day preconditioning period of enhanced vertical motion and moistening. During the transition season, the 6 weeks prior to onset, a number of moist westerly events occur. Generally they are only sustained for short periods and overlain by upper-level westerly winds, suggesting an association with midlatitude troughs, which protrude into the deep Tropics.

For individual years and for a 15-yr composite, monsoon onset is associated with major cyclogenesis events over the southwest Indian Ocean in the presence of a subtropical jet over the eastern Indian Ocean. The proposed mechanism for extratropical–tropical interaction is northeastward Rossby wave propagation from the cyclogenesis region toward the Tropics at upper levels. At these levels, westerly winds extend to nearly 10°S and provide a favorable background flow for such propagation. The process eventually results in the amplification of an equatorward-extending midlatitude upper trough and tropical ridge, which appears to trigger the development of the underlying monsoon trough. To test the hypothesis, the influence of high-latitude cyclogenesis on the tropical circulation is investigated with the aid of an idealized, dry, three-dimensional, baroclinic wave channel model. The initial state consists of (i) a zonally constant baroclinic region centered on 40°S, from which the high-latitude cyclogenesis develops, (ii) a weak monsoon trough at 15°S, and (iii) a subtropical jet at 25°S.

The major findings from the simulations are as follows: 1) There is evidence of northeastward Rossby wave propagation from the cyclogenesis region toward low latitudes. 2) Consistent with theoretical studies, the subtropical jet plays a key role by providing a favorable westerly background flow for group propagation into the Tropics. 3) High-latitude cyclogenesis in the presence of a subtropical jet can influence the meridional location, zonal structure, vorticity, and divergence in the monsoon trough. 4) Vorticity and divergence changes are consistent with enhancement of the monsoon trough (increases in low-level cyclonic vorticity) and the potential for triggering a large-scale convective outbreak (changes in upper-level divergence).

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Carsten S. Frederiksen
,
Huqiang Zhang
,
Ramesh C. Balgovind
,
Neville Nicholls
,
Wasyl Drosdowsky
, and
Lynda Chambers

Abstract

An evaluation of trial seasonal forecasts during the 1997/98 El Niño, using an atmospheric GCM forced by persisted sea surface temperature and sea-ice anomalies, is presented. Generally, forecasts of seasonal anomalies of precipitation, surface air temperature, 200-hPa geopotential height, and mean sea level pressure (MSLP) are shown to have statistically significant skill in the Tropics and subtropics, but predominantly over the oceans. Surface air temperature and 200-hPa height anomalies are also skillfully forecast over land in the 30°S–30°N latitudinal band, and, in contrast to precipitation and MSLP, also show significant skill in the extratropics. The global pattern of significant skill seems not to be oversensitive to the use of a Kuo or a mass-flux convection scheme (Tiedtke), although the global root-mean-square errors are consistently larger, in the latter case.

Results from multidecadal simulations of the model, when forced by observed sea surface temperature and sea-ice, show that the model reproduces quite well the observed global Southern Oscillation index relationships and that these go some way to explaining the skill in the model forecasts. In addition, the global patterns of skill are consistent with those seen in the model forecasts. An estimate of the role of sea surface temperature and sea-ice in forcing interseasonal climate variations, suggests that the model displays forecasts skill in those areas where this forcing plays a large, if not dominant, role. In areas where internal, or chaotic, variability plays a dominant role, the model shows little statistically significant skill.

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