Dynamical Predictability of Leading Interannual Variability Modes of the Asian-Australian Monsoon in Climate Models

The dynamical prediction of the Asian-Australian monsoon(AAM)has been an important and long-standing issue in climate science.In this study,the predictability of the first two leading modes of the AAM is studied using retrospective prediction datasets from the seasonal forecasting models in four operational centers worldwide.Results show that the model predictability of the leading AAM modes is sensitive to how they are defined in different seasonal sequences,especially for the second mode.The first AAM mode,from various seasonal sequences,coincides with the El Niño phase transition in the eastern-central Pacific.The second mode,initialized from boreal summer and autumn,leads El Niño by about one year but can exist during the decay phase of El Niño when initialized from boreal winter and spring.Our findings hint that ENSO,as an early signal,is conducive to better performance of model predictions in capturing the spatiotemporal variations of the leading AAM modes.Still,the persistence barrier of ENSO in spring leads to poor forecasting skills of spatial features.The multimodel ensemble(MME)mean shows some advantage in capturing the spatiotemporal variations of the AAM modes but does not provide a significant improvement in predicting its temporal features compared to the best individual models in predicting its temporal features.The BCC_CSM1.1M shows promising skill in predicting the two AAM indices associated with two leading AAM modes.The predictability demonstrated in this study is potentially useful for AAM prediction in operational and climate services.

THE TEMPORAL AND SPATIAL CHARACTERISTICS OF MOISTURE BUDGETS OVER ASIAN AND AUSTRALIAN MONSOON REGIONS

Apparent moisture sink and water vapor transport flux are calculated by using NCAR/NCEP reanalyzed daily data for water vapor and wind fields at various levels from 1980 to 1989.With the aid of EOF analysis method,temporal and spatial characteristics of moisture budgets over Asian and Australian monsoon regions are studied.The results show that there is apparent seasonal transition of moistrue sink and water vapor transport between Asian monsoon region and Australian monsoon region.In winter,the Asian monsoon region is a moisture source,in which three cross-equatorial water vapor transport channels in the 'continent bridge',at 80°E and 40°E～50° transport water vapor to the Australian monsoon region and southern Indian Ocean which are moistrue sinks.In summer,Australian Monsoon region anmd southern Indian Ocean are moistrue sources and by the three cross-equatorial transport channels water vapor is transport to the Asian monsoon region which is a moisture sink.In spring and autumn,ITCZ is the main moisture sink and there is no apparent water vapor transport between Asian monsoon region and Australian monsoon region.

亚澳季风区水汽输送季节转换特征

利用NCEP/NCAR1957～2001年45年逐日的再分析资料,从地面开始积分计算整层的水汽输送通量,从气候平均的角度分析了亚澳季风区大尺度水汽输送的季节转换演变特征。分析发现,亚澳季风区水汽输送由冬季向夏季的季节转换的基本特征是:夏季大值输送带的建立及其自西向东伸展,伴随着斯里兰卡低涡活动、自南向北的越赤道输送和副高的东撤、南海夏季风建立等一系列天气气候事件;而冬季形势的建立则是副高南侧东风输送带的西伸,伴随夏季大值输送带的断裂、西撤,最后形成亚洲低纬东风输送带,进而形成由北向南的越赤道输送以及澳大利亚和南印度洋夏季风水汽输送。伴随着冬、夏季节转换,中南半岛以西和以东地区的西风水汽输送的经向移动表现出完全不一样的特征,表明印度季风和东亚-西太平洋季风的形成机制有很大不同。

THE MECHANISM OF THICKNESS OF GEOPOTENTIAL HEIGHT DIPOLE AND ITS RELATION WITH CLIMATE VARIABILITY IN ASIA-AUSTRALIA IN BOREAL WINTER

With the EOF of reanalysis data being analyzed, a northern-southern dipole is found in the upper troposphere geopotential height field of over the Asian-Australian monsoon region in the winter of the Northern Hemisphere(NH),which is defined as Asian-Australian dipole(AAD) in this study. Its intensity index is defined as AADI. Correlation and synthetic analysis illustrate that AADI is closely related to the weather and climate of Asian-Australian region in boreal winter. The index can reflect the simultaneous anomalies of temperature and precipitation on interannual and decadal scales in the boreal winter of Asian-Australian region. The superposition of the decadal and interannual signals is significant for the relationship between the AADI and climate change. The index can be used as an indicator of intensity of the Asian-Australian monsoon. In the years of strong AADI, the East Asia major deep trough is stronger,the Subtropical High is weaker and the Alaska ridge and the westerly jet are stronger than those in normal years.Enhanced meridional circulation between high and low latitudes exists in the years of strong AADI. These relationships reflect the intrinsic link between the anomalies in the upper troposphere geopotential height and climate in the Asian-Australian region.