Atmospheric rivers impacting mainland China and Australia: climatology and interannual variations

In this study we have built two atmospheric river (AR) databases for mainland China and Australia using Japanese 55-year Reanalysis data with manual detections. By manually checking the magnitude, shape and orientation of vertically integrated vapour transport fields calculated from the reanalysis data and analysing its embedded synoptic patterns and other meteorological information, we detected 625 AR events over mainland China during 1986–2016 and 576 AR events over the Australian continent during 1977–2016. This manuscript documents the mean climatology, spatial distributions, seasonality and interannual variations of ARs occurring in these two regions. We also assessed possible underlying drivers influencing AR activities. Our results showed that: (i) most ARs over mainland China occured in its lower latitudes, including southern, eastern and central China, but ARs also reached its far north and northeast regions. In Australia, most ARs occurred in the states of Western Australia, South Australia and part of New South Wales and Victoria. These regions of high AR frequencies also frequently experienced Northwest Cloud Bands during the cool season; (ii) ARs in China reached their peak during the East Asian summer monsoon season (May–September). This was also the period when AR frequency in the Australian region tended to be higher, but its seasonal variation was weaker than in China; (iii) ARs exhibited large interannual variations in both regions and a declining trend in central and eastern China; (iv) there was a notable influence of tropical sea surface temperatures (SSTs) on the AR activities in the region, with the ARs in Australia being particularly affected by Indian Ocean SSTs and El-Nino Southern Oscillation (ENSO) in the tropical Pacific. ARs in China appear to be affected by ENSO in its decaying phase, with more ARs likely occurring in boreal summer following a peak El Nino during its preceding winter; (v) the Western Pacific Subtropical High plays a dominant role in forming major moisture transport channels for ARs in China, and South China Sea appears to be a key moisture source. In the Australian region, warm and moist air from the eastern part of the tropical Indian Ocean plays a significant role for ARs in the western part of the continent. In addition, moisture transport from the Coral Sea region was an important moisture source for ARs in its east. Results from this study have demonstrated the value of using AR diagnosis to better understand processes governing climate variations in the A–A region.