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

Abstract

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-Niño 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.

Potential connections between atmospheric rivers in China and Australia

“…Firstly, during this period, a pair of ARs occurred in China and Australia and brought heavy precipitation in both regions. Secondly, as aforementioned in the Introduction section, results from Wu et al (2020) found that ARs in China tended to peak during the boreal summer season, whereas ARs in Australia tended to peak during the same time but in its cool season. Therefore, choosing the June 2016 boreal summer (austral winter) case further allowed us to investigate why ARs tend to share the same seasonality in Australia and China.…”

“…6c. It is also the peak season for the NCWB occurrence in the Australian region, as discussed in Ye et al (2020) and Wu et al (2020). From Fig.…”

“…Frederiksen and Balgovind (1994) and Frederiksen and Frederiksen (1996) further linked it with SST gradients in the Indian Ocean, which produce an enhanced influx of warm tropical air from the northwest of Australia towards the continent. Furthermore, from the AR dataset developed in this project (Ye et al 2020), Wu et al (2020) found ARs can occur simultaneously in both regions. All these results raise several important questions: (i) why do the ARs in China and Australia tend to occur during the same period but in different seasons?…”

“…As part of an Australian Bureau of Meteorology (BoM) and China Meteorological Administration (CMA) bilateral project on AR analyses in the A-A monsoon region (Ye et al 2020), Wu et al (2020) reported climatological characteristics of the ARs reaching China and Australia based on manual detections from a 30-year AR database. They documented AR mean climatologies, seasonality, interannual variations and potential drivers in East Asia and Australia.…”

Atmospheric rivers associated with summer heavy rainfall over the Yangtze Plain

“…To address this issue, we need to examine the year-toyear variations. Although in the current collaborative AR project Wu et al (2020) produced a manual detected AR dataset, there is a lack of in-depth analysis of ARs and rainfall interannual variations in the Yangtze River region. We plan to develop our own AR automatic detection scheme that is more applicable in the East Asian region and then revise our results obtained in this study based on the resulting AR dataset.…”