A Monsoon-like Southwest Australian Circulation And Its Relationship With Rainfall In The Southwest Western Australian

Along with the continued warming trend during the second half of the 20th century, drought is existed over most parts of terrestrial in the global. The great impacts of drought on natural environment and human society, such as desertification and ecological degradation have already attached increasing attentions of the international scientific community and the public. Studies of analyzing the statues, attributions and projecting the future trend of drought have been the focused and hot issues in many projects and researches. As the drought becomes severer, research for a typical region is not only helpful for further understanding the causes of the drought in the certain region, also would be useful for improving the understanding and providing case study to other similar regions.Southwest Western Australia (SWWA) locates in the southwest corner of the Western Australia. It is not only the political, economic, cultural, educational, and tourism center of the Western Australia, and also the wine region and wheat belt of the Western Australia. The observed rainy season precipitation of the SWWA has significantly declined since 1950s. The reduction in winter rainfall resulted in an even sharper fall in the stream flow in the southwestern Australia and heavily impacted the water resources in the state. The attribution of rainfall variation becomes the focus of many researches and has drawn much attention.Based on the observation data, reanalysis datasets, and model data in recent 6 decades, the variability and circulation features influencing the SWWA winter rainfall (SWR) are investigated. It is found that the climate of southwest Australia bears a strong seasonality in the annual cycle and exhibits a monsoon-like atmospheric circulation, which is termed as the southwest Australian circulation (SWAC). The seasonal march, interannual and interdecadal variability of the SWAC, and its relationship with the SWR is discussed. The external forcing and predictability of the SWAC are also explored. Based on the well-coupled linkage between the SWAC and SWR, empirical models are established to predict the SWWA seasonal rainfall by the SWAC. Finally, the comparison study between the SWAC and the classical subtropical monsoon system-East Asian summer monsoon (EASM) is carried out to further comprehend the SWAC. Major conclusion can be summarized as follows:(1) The relationship between the SWR and the known climatic systems that influencing the Australian rainfallThe impacts of the known modes (e.g. El Nino-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), ENSO Modoki (EM) and the Southern Hemisphere Annular Mode (SAM)) that influencing the Australian rainfall on the SWR are analyzed. It is found that they can not explain the rainfall variation of the SWWA neither in early (May to July, MJJ) or late winter. Particularly, the reported significant inverse relationship between the SAM and SWR is caused by an exceptional extreme year.(2) The advance of the SWAC conceptionBased on the investigation on rainfall distribution, seasonal march of the circulation features, dynamic and thermal structures of the atmospheric circulation, and thermal contrast in land and sea in the southwest Australia, we find that the circulation over SWWA shows strong seasonality, and exhibits monsoon-like characteristics. That is, alternate wet and dry seasons, seasonal reversal of winds, and an evident land-sea thermal contrast. Taking account of these monsoonal characteristics, we proposed the conception of monsoon-like SWAC by the first time.(3) The seasonal march, interannual, interdecadal variation of the SWAC and its relationship with the SWWA rainfallBased on Conclusion (2), the seasonal, interannual, and interdecadal variation of the SWAC is discussed, the onset and strength indices of the SWAC are defined, and the relationship between the SWAC and SWR is analyzed. Result indicates that the SWAC explains not only a large portion of the interannual variability of SWR in both early and late winter, but also the long-term drying trend over SWWA in early winter is attributed to the weakened of the SWAC. The well-coupled SWAC-SWR relationship seems to be largely independent of the known effects of ENSO, IOD, SAM and EM, nor the responds of them. The result offers qualified support for the argument that the SWAC may contribute to the rainfall decrease in the SWWA early winter rainfall.(4) The relationship between the SWAC, SAM and SWWA rainfall, and the external forcing of the SWACWe find that the SAM activity does not produce modulation to the SWAC signal which has a relation with the SWR. With regard to the external forcing of the SWAC, we suggest two possible drivers. The first may be the shift of the planetary-scale thermal convection that inducs the seasonal shift of the subtropical high and results the reversal winds over SWWA. Second, the Indian Ocean SSTs westward of SWWA may provide another external forcing to the SWAC. The result is further established by numerical experiments.(5) The establishment of the empirical model to reckon seasonal SWWA rainfallBased on the well-coupled linkage between the SWAC and SWR, the empirical models are built to hindcast the seasonal SWWA rainfall in both early and late winter by the SWAC, respectively. Hindcast is performed for the 1948-2006 period, which shows hindcast prediction skill based on the single SWAC is better than the combination of other climatic modes. The empirical models provide perspective tool to forecast the SWWA seasonal rainfall.(6) Assessment of the simulation skills of the SWAC by the IPCC AMIP modelsThe seasonal features, annual cycle, and corresponding circulation anomalies of the SWAC are evaluated using the IPCC AMIP models. Result shows that except the NCAR CCSM3.0, the else models can capture the basic seasonal features and corresponding circulation anomalous characteristics of the SWAC, but not for the seasonal march procession and interannual variability. Thus the relevant part of the models should be improved to well simulate the SWWA climate.(7) Analogy the SWAC and the East Asian Summer Monson (EASM)Since the SWAC and EASM both are located at subtropical regions, and in the same longitude scale, we analogy the similarlities and dissimilarities of the two monsoon system. The seasonal march and strength of the two monsoon systems are both closely attached with the position and shift of the subtropical high in each hemisphere. We find that the early winter rainfall over SWWA has a closely linkage with the midsummer rainfall of the north China in both interannual and interdecadal timescale, especially strong in the interdecadal timescale. Further, the early winter rainfall over SWWA and midsummer rainfall in the north China both are closely attached with the mean position of the subtropical high ridge. One important issue is that the early winter rainfall of the SWWA is two months advanced the north China midsummer rainfall, which provides a potential predictability source for the north China midsummer rainfall.