Atmospheric rivers in the Australia-Asian region

Abstract

In recent years,more andmore attention has been focused on the atmospheric river (AR) diagnosis and its application in characterising atmospheric moisture transport between the tropics and the extratropics. Significant research (see, for example, references in Ye et al. 2020) has been conducted over North America due to a close association between ARs and extreme rainfall events over thewest coast.More studies are beginning to emerge over other parts of the globe such as, for example, along the South American coasts and parts of Europe. There are hundredsofARstudiesbeingpublishedeachyear frommeteorologicalandhydrometeorologicalcommunitiesworldwide.However, there are only very limited AR studies in our region even though we have a significant number of synoptic events, such as Northwest Cloud Bands (NWCBs), which share some common features as described by the AR concept. Therefore, it is very pleasing to see this collaboration between scientists in the AustralianBureauofMeteorologyandtheChinaMeteorological Administration focused on this series of AR diagnostic studies over the Australia-Asian (A-A) region. I commend the strong scientific leadership and significant efforts of Dr. Huqiang Zhang and his collaborators in conducting this important researchandfurtherdeveloping their studies intosixmanuscripts in this Research Front of the Journal of Southern Hemisphere Earth Systems Science. I believe they will attract more interest from our research community and lead to further investigations of this important topic. I agree with the authors that the term ‘atmospheric river’ may create the wrong impression that the research is about ‘rivers’ in the sky,when in fact it refers to anarrowbandof stronghorizontal water vapour transport concentrated in the lower troposphere.As the authors point out, the use of the word ‘river’ comes from the fact that the amount of atmospheric vapour flux associated with such a structure is about the samevolume as for river flows on the ground. In this Research Front, the researchers have comprehensively documented their analysis ofARs inour region.Theyhave conducted detailed observational case studies of AR characteristics operating in the A-A region and their differences to ARs reported for the North American middle and high latitudes (Ye et al. 2020). They have applied backward trajectory analysis to explore the atmospheric moisture source for such ARs and highlighted tropical moisture as the primary contributor to the corresponding rainfall generated in the extratropics. They have investigatedthepotential linksbetweenARsinEastAsiaandover theAustralian continent andused suchconnections to explain the seasonality of NWCBs and ARs in our region (Xu et al. 2020a). They have further proposed a mechanism associated with teleconnections in the subtropical highs of both hemispheres to explain these connections. They have also assessed the potential ofusingARsto linkmodelskill (bothNWPandseasonal forecast) inforecastingthesestrongmoisturetransportswiththeir forecasts ofextremerainfall inbothcountries (Chenetal.2020;Liangetal. 2020). The authors also developedAR databases over China and Australia for a 30-year period usingmanual detections (Wu et al. 2020). These datasets were used to document the seasonality, interannual variations of ARs in both regions and identify some significant trends over Chinawhich are consistent with observed changes in its extreme rainfall characteristics. Finally, they reported on a study, using a suite of CMIP5 models, assessing theskillofcurrentclimatemodels insimulatingARsderivedfrom reanalysis data, and thepotential changes inARsunder awarmed climate (Xu et al. 2020b). Furthermore, they reported on some changeswhich are different from those reported for other regions due to the response of the Western Pacific Subtropical High to global warming. The results fromthiscollaborativeproject provideconvincing evidence of the value of applying AR diagnostics to gain a better understanding of atmospheric moisture transports affecting our weather and climate. The studies also show the potential of using ARs to improvemodel rainfall forecasts.Manystudies, including my own research on the impacts of tropical oceans such as the meridional IndianOceanDipoleonourweatherandclimate,have already shown the influence of the tropics on the dynamics and moisture conditions in our middle and high latitudes. The application of AR diagnostics provides an additional tool in understanding these relationships. I acknowledge the amount of work needed to conduct these studies which was made possible through this collaborative project. As the authors acknowledge, there is still more work that could be done using this analysis technique.TheARdatabase ofWu et al. (2020) is a very valuable resource and there is scope for developing it further by, for example, introducing automatic detections of ARs and incorporating the diagnosis as part of the routine operational products in CSIRO PUBLISHING