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RESEARCH FRONT
CSIRO PUBLISHING
Journal of Southern Hemisphere Earth Systems Science, 2020, 70, 3–16
https://doi.org/10.1071/ES19025
Atmospheric rivers in the Australia-Asian region:
a BoM–CMA collaborative study
Chengzhi Ye A, Huqiang Zhang B,E, Aurel Moise B,C and Ruping Mo D
A
Hunan Meteorological Service, Changsha, China.
B
Bureau of Meteorology, Melbourne, Vic., Australia.
C
Centre for Climate Research Singapore, Meteorological Service Singapore, Singapore.
D
Environment and Climate Change Canada, Vancouver, Canada.
E
Corresponding author. Email: Huqiang.Zhang@bom.gov.au
Abstract. The name ‘atmospheric river’ (AR) could easily be misinterpreted to mean rivers flowing in the sky. But, ARs
actually refer to narrow bands of strong horizontal water vapour transport that are concentrated in the lower troposphere.
These bands are called ‘atmospheric rivers’ because the water vapour flux they carry is close to the volume of water carried
by big river systems on the ground. ARs can cause heavy rainfall events if some physical mechanisms, such as orographic
enhancement, exist to set up the moisture convergence and vertical motions necessary to produce condensation. In recent
decades, these significant moisture plumes have attracted increasing attention from scientific communities, especially in
North America and western Europe, to further understand the connections between ARs and extreme precipitation events
which can trigger severe natural disasters such as floods, mudslides and avalanches. Yet very limited research has been
conducted in the Australia-Asian (A-A) region, where the important role of atmospheric moisture transport has long been
recognised for its rainfall generation and variations. In this paper, we introduce a collaborative project between the Australian
Bureau of Meteorology and China Meteorological Administration, which was set up to explore the detailed AR
characteristics of atmospheric moisture transport embedded in the A-A monsoon system. The project in China focused
on using AR analysis to explore connections between moisture transport and extreme rainfall mainly during the boreal
summer monsoon season. In Australia, AR analysis was used to understand the connections between the river-like Northwest
Cloud Band and rainfall in the region. Results from this project demonstrate the potential benefits of applying AR analysis to
better understand the role of tropical moisture transport in rainfall generation in the extratropics, thus achieve better rainfall
forecast skills at NWP (Numerical Weather Prediction), sub-seasonal and seasonal time scales. We also discuss future
directions of this collaborative research, including further assessing potential changes in ARs under global warming.
Keywords: atmospheric river, extreme rainfall, monsoon, narrow band of strong moisture transport, Northwest Cloud
Band, weather and seasonal forecasts.
Received 29 March 2019, accepted 28 August 2019, published online 28 August 2020
1 Introduction refers to narrow bands of strong horizontal water vapour trans-
Atmospheric rivers (ARs) have received increasing attention port concentrated in the lower troposphere, which can cause
from meteorological and hydrological communities in recent extreme rainfall when these moisture belts meet with some
decades due to the close association of ARs with extreme dynamical lifting mechanisms, such as orographic uplift. Nev-
rainfall events, mainly occurring on the western coasts of North ertheless, the terminology of AR, which was coined by Zhu and
America (Zhu and Newell 1994; Ralph et al. 2004, 2006; Newell (1994, 1998), has now been widely accepted in the
Neiman et al. 2008a, 2008b; Smith et al. 2010; Dettinger et al. scientific literature (Gimeno et al. 2014; Ralph et al. 2017).
2011; Warner et al. 2012, 2015; Gershunov et al. 2017, 2019; These bands are called ‘atmospheric rivers’ because if the water
Mo et al. 2019), South America (Garreaud 2013; Viale et al. vapour flux carried within them were to precipitate out, the
2018) and Europe (Lavers et al. 2011, 2012; Lavers and Vilarini volume of water would be equivalent to that transported by big
2013, 2015). As Ralph et al. (2017) pointed out, there has been a river systems on the ground (Zhu and Newell 1994).
rapid increase in scientific publications on ARs, from a few in Fig. 1 shows the characteristics of a typical AR making
the 1990s to over 200 per year in recent years. The analogy to landfall in western Canada at 0000 UTC 29 January 2018. It fits
‘river’ could be misleading, because it can be misinterpreted as well with the conceptual model for the ARs over the Northeast
suggesting there are rivers in the sky operating like these on the Pacific shown in Gimeno et al. (2014) based on an observational
ground, with fixed channels and tributaries. Instead, the term campaign over the west coast of North America (Ralph et al.
Journal compilation Ó BoM 2020 Open Access CC BY-NC-ND www.publish.csiro.au/journals/es
4 Journal of Southern Hemisphere Earth Systems Science C. Ye et al.
(a) IWV and frontal analysis (b) WVF (colours) and wind speed (lines, m s–1) (c) Sounding at Port Hardy, BC, Canada
60°N 100
g m–2 s–1
150 275 150
250
200 225 200
40°N
Pressure (hPa)
200 250
Pressure (hPa)
250
300 175 300
150
400
400
20°N 125
100 500
500
600 75
700
700 50
850
850 25
0 1000
160°W 140°W 120°W 1000
–30 –20 –10 0 10 20 30 40
mm 0 300 600 900 1200 1500 1800
10 20 30 40 50 60 70 Range (km) Temperature (°C)
Fig. 1. Conceptual model based on a typical AR making landfall on the west coast of North America at 0000 UTC 29 January 2018. (a) Vertically integrated
water vapour (IWV) associated with a polar cold front along with LLJ (low-level jet), with the AR being schematically depicted by the dashed arrow.
(b) Cross-section through the AR along the northwest-southeast red line in (a) for horizontal water vapour flux (WVF, coloured shading) and wind speed
(black lines with contour interval 5 m s1). Note that the core of the AR is below the 850-hPa level and is co-located with a pre-cold frontal low-level jet.
(c) Vertical structure of the AR shown as the Skew-T log-P diagram of the sounding profile observed at Port Hardy, BC, Canada (50.688N, 127.368W).
2004). This AR, bounded by the 20-mm contour of vertically 60°N 1000
integrated water vapour (IWV), was a long (expanding well
beyond 2000 km in length), narrow (on average with ,500–
800 km width) and shallow (1–2.5 km in depth) band of strong
horizontal water vapour transport in the lower troposphere. It
was associated with a strong pre-cold frontal low-level jet (LLJ)
as documented by many other studies (Ralph et al. 2004, 2005; 40°N
Neiman et al. 2008a, 2008b; Gimeno et al. 2014). Another
important feature in Fig. 1 is that, at the AR head (shown at Port
Hardy, BC, Canada), the sounding profiles of temperature and
dewpoint temperature suggest that its vertical distribution fol-
lowed closely to a near moist-neutral stratification. On the west
20°N
coast of North America, AR centres are usually located at a level
lower than the coastal mountains. Therefore, when they make
landfall on the coast, the orographic enhancement of rainfall can
lead to extreme precipitation events (Ralph et al. 2004, 2006;
Neiman et al. 2008a, 2008b).
Nevertheless, when the AR concept was first introduced in the
0
1990s, there were debates on the differences between ARs and
160°W 140°W 120°W
other concepts already used for describing moisture sources and
transport in extratropical rainfall generation, such as the warm kg m–1 s–1
0 250 500 750 1000 1250 1500
conveyor belts (WCBs) and tropical moisture exports (TMEs).
The distinct and complementary concepts used in exploring
Fig. 2. Schematic diagram showing the connection between tropical
atmospheric moisture transport outside the tropics were not moisture export (TME), a warm conveyor belt (WCB) associated with a
properly defined until an international AR workshop in 2015 mid-latitude cyclone, and a landfalling AR shown as the vertically integrated
(Dettinger et al. 2015), where consensus was reached. As shown vapour transport (IVT) valid at 0000 UTC 29 January 2018. The vectors
in Fig. 2 which follows another conceptual diagram in Dettinger shows the zonal and meridional components of IVT and the shaded colors
et al. (2015), it is now generally agreed that a WCB refers to a shows the magnitudes of IVT (kg m1 s1).
zone of dynamically uplifted heat and moisture transport close to
a midlatitude cyclone. The atmospheric moisture is often trans-
ported into the WCB by an AR. TMEs are zones of intense midlatitude sources and convergences of vapour along their
atmospheric moisture transport out of the tropics, and that paths (Sodemann and Stohl 2013; Dettinger et al. 2015).
tropical originated water vapour is frequently conducted by Numerous studies in the last decades have documented the
ARs toward cyclones and WCBs. Note that TMEs provide AR contribution to large-scale water vapour transport and
important vapour sources for ARs, but most ARs also incorporate extreme precipitation events in orographically favoured
Atmospheric rivers in the Australia-Asian region Journal of Southern Hemisphere Earth Systems Science 5
locations around the world (Smirnov and Moore 1999; Gyakum source and transport, and how these processes affect its rainfall
2000; Ralph et al. 2004, 2005, 2006; Neiman et al. 2008a, effectiveness.
2008b; Roberge et al. 2009; Lavers et al. 2011, 2012; Newman Given the lack of assessment of ARs and the importance
et al. 2012; Lavers and Villarini 2013; Guan and Waliser 2015; of their moisture transport in affecting the weather and climate
Mahoney et al. 2016; Ralph et al. 2017; Browning 2018; Zhang in the A-A region, a bilateral project on AR research was
et al. 2019). However, this relatively new meteorological conducted over 2017–2019 under a cooperation agreement
terminology has not been generally adopted in the Australia- between the China Meteorological Administration (CMA) and
Asian (A-A) region, except for a few recent studies (Hirota et al. the Australian Bureau of Meteorology (BoM). This collabora-
2016; Paltan et al. 2017; Yang et al. 2018). It is well known that tive project was designed to fill the gap in AR research and apply
weather and climate in the Asian region are dominated by this diagnosis for supporting weather and climate services in
monsoonal flows that bring warm and moist air from the tropics the region. Some preliminary results were presented at the
into the subtropical and extratropical areas (Ding 2004; Zhang 2018 Australian Meteorological and Oceanographic Society-
2010) through several channels (Chen et al. 1991; Chang et al. International Conference on Southern Hemisphere Meteorology
2004; Ding 2004; Wang 2006), but it is unclear to what extent and Oceanography (5–9 February, Sydney, Australia), and a
such transport is realised as ARs and/or other mechanisms. We special issue of the Journal of Southern Hemisphere Earth
note that several recent studies started to analyse ARs in the Systems Science has been arranged to publish these results to
Asian monsoon region (Hirota et al. 2016; Yang et al. 2018), but promote AR studies in the region. The main purpose of this
it is unclear whether the established AR conceptual model can paper is to provide a scientific background on AR research, list
be directly applied in the A-A monsoon system and to what key scientific questions addressed by this project, report on the
extent the monsoon rainfall intraseasonal variations are associ- project’s progress and discuss future directions.
ated with its AR activities. The remainder of this paper is organised as follows. Section 2
Another attraction of conducting AR analysis in the Austra- gives a brief overview of ARs, their detections and current
lian region is a significant weather phenomenon known as the applications in regions outside the A-A region. Section 3
Northwest Cloud Band (NWCB). As described by Tapp and reviews key scientific questions addressed by our BoM-CMA
Barrell (1984), in satellite weather images the NWCB is a river- collaborative project and highlights some important results from
like, long and narrow band (typically extending over 2500 km the project. In Section 4, the limits from current studies and
and existing for two to four days) that often originates in the future directions of this collaborative project are discussed.
warm and moist Indian Ocean, south and west of Indonesia. It
thus bears a strong similarity to the AR phenomenon. Wright
2 Atmospheric rivers: an overview
(1997) showed the NWCB as one of the synoptic patterns
that results from tropical–extratropical interactions. Studies 2.1 A brief historical perspective
from Frederiksen and Balgovind (1994) and Frederiksen and As shown in Fig. 1, an AR can be generally defined as a long,
Frederiksen (1996) further linked the formation of NWCB with narrow and transient corridor of strong horizontal water vapour
sea surface temperature (SST) gradients in the Indian Ocean. transport (American Meteorological Society 2019). The concept
Using a general circulation model, Frederiksen and Balgovind of a ‘river’ in the sky was introduced in the early 1990s to describe
(1994) simulated an increased frequency of NWCB with some filamentary structures of atmospheric water vapour trans-
enhanced SST gradients between the Indonesian Archipelago port. In a pilot study, Newell et al. (1992) showed the presence of
and the central Indian Ocean and with enhanced influx of warm these moisture filaments, which have lengths many times their
tropical air from northwest Australia. In a follow-up study, widths and persist for many days while being transported through
Frederiksen and Frederiksen (1996) used a two-level primitive- the lower atmosphere. They termed these features ‘tropospheric
equation model to demonstrate that an enhanced SST gradient rivers’, given that some of them may carry as much water as the
can produce instability modes resembling circulation changes Amazon River. The name ‘atmospheric river’ was used in a later
surrounding the onset of NWCB. article by Zhu and Newell (1994) (also see Newell and Zhu 1994).
Although NWCBs are likely to produce extensive rainfall The great importance of ARs in large-scale water vapour trans-
over Australia, their effectiveness in generating heavy precipi- port was further recognised by Zhu and Newell (1998), who
tation is still under debate (Ummenhofer et al. 2011). The showed that ARs may carry essentially the total meridional
complex relationship between NWCB and rainfall could partly moisture transport observed in the extratropical atmosphere,
be attributed to the different atmospheric moisture sources that despite occupying only about 10% of the total longitudinal length
support the formation of the cloud band. Early studies, such as at a given latitude.
Hill (1977), stressed the importance of tropical thunderstorm Before the AR concept was widely known and accepted, some
activity in injecting tropical warm and moist air into the studies used the term ‘Pineapple Express’ to describe the strong
midtroposphere, which is then advected south-eastwardly along LLJs blowing from the Hawaiian area (where pineapples are
the band. However, Tapp and Barrell (1984) showed that grown) towards the west coast of North America (e.g. Lackmann
tropical convection is not always evident near the origin of and Gyakum 1999; Mo et al. 2009). Other names used for
NWCB. Early studies of Hill (1964, 1969, 1977) also showed the describing the strong water vapour transport in the atmosphere
importance of a second NWCB-supporting moisture source off include ‘moisture conveyor belt’ (Bao et al. 2006), ‘Maya
eastern Australia. Therefore, AR analysis has the potential to Express’ (Dirmeyer and Kinter III 2009) and ‘tropical moisture
offer some insight into the formation of NWCB, its moisture export’ (Knippertz and Wernli 2010). Nevertheless, since the
6 Journal of Southern Hemisphere Earth Systems Science C. Ye et al.
influential paper by Ralph et al. (2004), the term ‘atmospheric Ralph et al. (2004) showed that the IWV derived from
river’ has been adapted and received great publicity in the meteorological satellite observations can be used as a proxy
meteorological and hydrological communities. Numerous recent for detecting the presence of ARs: an AR is defined as an
studies have demonstrated that ARs play a crucial role in elongated area with a minimum IWV threshold of 20 mm, a
determining the occurrence of floods, breaking of droughts and minimum length of 2000 km and a maximum width of 1000 km.
managing water resources in many parts of the world (Gimeno Their analysis found a close correlation between IWV and IVT
et al. 2014; Ralph et al. 2017 and references therein). As in the midlatitudes, and the identified ARs were typically
illustrated in Fig. 2, the general view now is that ARs provide associated with the pre-cold-frontal LLJs within oceanic extra-
the link between moisture sources in the tropics (TMEs) and the tropical cyclones (Ralph et al. 2005). This IWV-based AR
moisture embedded within the middle-latitude WCBs, and ARs definition has been successfully applied in many subsequent
are directly responsible for some heavy precipitation events studies (Ralph et al. 2006; Neiman et al. 2008a, 2008b; Guan
(Sodemann and Stohl 2013; Dettinger et al. 2015). et al. 2010; Dettinger et al. 2011; Ralph et al. 2011; Ralph and
In the A-A monsoon region, Zhu and Newell (1998) found Dettinger 2012). An automated objective algorithm for the AR
that persistent monsoon circulations are supported by transient identification based on IWV has also been developed by Wick
filamentary water vapour fluxes. These features may be consid- et al. (2013).
ered as semipermanent ARs. Joseph and Moustaoui (2000) used Because the IWV-based AR definition does not include wind
a simple, dynamically consistent model to demonstrate the information, it does not address the fundamental processes for
development of filamentary moisture transport in a monsoon- water vapour transport. When both wind and moisture data are
like flow. He et al. (2007) described some patterns of strong available, IVT is a more appropriate variable for the AR defini-
large-scale moisture transport in the A-A monsoon region as tion (Zhu and Newell 1998; Lavers et al. 2012; Lavers and
‘great moisture rivers’. They found that the precursory signals in Villarini 2013; Wick 2014; Guan and Waliser 2015; Mahoney
the seasonal transition from the winter to summer pattern could et al. 2016; Mo and Lin 2019). A range of definitions and
be revealed by the establishment of a strong southerly total thresholds are used in various studies, and currently there is an
moisture transport value exceeding 100 kg m1s1. The termi- intercomparison project to compare results from these studies
nology of AR has been adopted in some recent studies in the A-A (Shields et al. 2018). Rather than using a fixed threshold, Guan
monsoon region (e.g. Hirota et al. 2016; Paltan et al. 2017; Yang and Waliser (2015) developed a technique for objective AR
et al. 2018). However, there is still a lack of understanding about detection using varying thresholds based on the IVT distributions
whether ARs embedded in the large-scale monsoonal flow are calculated from reanalysis data covering a long period (normally
an important mechanism for moisture transport in the monsoon over 30 years). They used the 85th percentile of IVTs with
system and to what extent AR activities have contributed to geometry requirements of .2000 km length and a length/width
monsoon intraseasonal rainfall variations. ratio .2 as the AR-defining threshold. There is merit in such a
definition, as the magnitude of atmospheric moisture transport
2.2 Methods for atmospheric river detection
and its impacts on rainfall may vary with different climate
The two most frequently used approaches to detect and define backgrounds. For operational applications, a simpler definition
ARs are integrated water vapour (IWV) and integrated vapour can also be given as an elongated area with a minimum IVT
transport (IVT), which can be defined as: threshold of 250 kg m1 s1, a minimum length of 2000 km and a
ð ps ð ps length/width ratio .2 (e.g. Wick 2014; Mo and Lin 2019).
IWV ¼ g 1 qdp; IVT ¼ jQj; Q ¼ g1 qVh dp; ð1Þ Mahoney et al. (2016) pointed out that the IVT-based AR
0 0 definition is particularly useful for distinguishing the strong
moisture transport corridor where the climate background is
where g is the acceleration of gravity, q is the specific humidity, moist, such as in southeast USA. This is likely also true in the
Vh is the horizontal wind vector, Q is a two-dimensional IVT A-A monsoon region. As an example, Fig. 3 shows the IWV and
vector, p is the pressure taken as the vertical coordinate, and ps is IVT distributions at 1800 UTC January 2017, derived from the
the surface pressure. The vertical integrations in Eqn 1 are Japanese 55-year Reanalysis (JRA-55) (Kobayashi et al. 2015;
carried out from the Earth’s surface to the top of the atmosphere. Harada et al. 2016). Based on the IWV-based definition, we can
Therefore, the IWV represents the total mass of water vapour per readily identify a strong AR from the tropical western Pacific
unit area above the Earth’s surface. It is also called the towards the west coast of North America in Fig. 3a (Mo et al.
‘precipitable water’. The IVT is the magnitude of Q, represent- 2019). In addition, several ARs can also be identified in the
ing the total horizontal transport of water vapour per unit area extratropical areas of the South Indian Ocean, the South Pacific
above the Earth’s surface. Note that both IWV and IVT are Ocean and the Atlantic Ocean. There is an indication of a weak
dominated by conditions in the lower troposphere because of the AR along the south coast in Australia. On the other hand, the AR
rapid decrease of specific humidity with altitude. In practice, it signal over the southeast USA is poorly defined. By comparison,
suffices to carry the vertical integration up to the 300 hPa level in as corridors of strong water vapour transport, the ARs are better
the upper troposphere, but for the convenience of operational distinguished by the IVT-based definition in Fig. 3b. In particu-
application implementation, one can choose to simply take the lar, the ARs across the southeast USA and Australia can be
IWV value integrated from the surface to the top of the readily recognised in Fig. 3b.
atmosphere, as this variable is often saved in numerical model As pointed out by Shields et al. (2018), different methods
outputs. used to detect ARs on numerical model gridded datasets and/or
Atmospheric rivers in the Australia-Asian region Journal of Southern Hemisphere Earth Systems Science 7
(a) Integrated water vapour (IWV)
60°N
30°N
0
30°S
60°S
0 30°E 60°E 90°E 120°E 150°E 180° 150°W 120°W 90°W 60°W 30°W
mm
10 20 30 40 50 60 70
(b) Integrated vapour transport (IVT)
60°N
1000
30°N
0
30°S
60°S
0 30°E 60°E 90°E 120°E 150°E 180° 150°W 120°W 90°W 60°W 30°W
kg m–1 s–1
0 250 500 750 1000 1250 1500
Fig. 3. The (a) IWV and (b) IVT distributions based on the JRA-55, valid at 1800 UTC January 2017.
reanalysis data can vary substantially in the derived AR char- deceleration of air motion can lead to strong moisture conver-
acteristics (including frequency, duration and intensity) due to gence on the windward side. This, together with the orographic
the different chosen methods. It is hard to choose one against uplift, can lead to heavy precipitation in the mountains and
another without a reference dataset to compare. Therefore, in the lowlands alike (Lackmann and Gyakum 1999; Mo et al. 2019).
BoM-CMA project, we dedicated significant resources to con- In addition, horizontal convergence and uplift may also be
duct manual AR detections by combining IVT calculations from generated by other mechanisms, such as midlatitude frontal
six-hourly JRA-55 data (https://jra.kishou.go.jp/JRA-55/ systems and blockings (e.g. Mo and Lin 2019). Different dynam-
index_en.html) with manually checked daily synoptic charts ical mechanisms for AR-induced heavy precipitation in the A-A
and satellite images. This effort was to make sure that the region will be further explored in this BoM-CMA project through
detected ARs not only satisfy certain thresholds often used in several case studies (Chen et al. 2020; Xu et al. 2020a).
the automatic schemes but they also actually exist as recogni-
sable weather patterns in an operational weather forecasting 3 Key scientific questions in this research
environment. Detailed description is provided by Wu et al. Although several studies have used global reanalysis datasets to
(2020) in this special issue. Our AR dataset covers the AR events document ARs in the global context (e.g. Guan and Waliser
in Australia and China domains for 1977–2016. This will serve 2015; Gimeno et al. 2016; Waliser and Guan 2017), there is a
as the reference dataset in our future selection of a suitable lack of detailed analysis focusing on the ARs in the A-A region.
automatic detection algorithm that will allow us to implement In this section, we highlight some fundamental features of
the AR diagnosis in operational systems. moisture transport in the monsoon region and discuss how AR
In general, the strength of precipitation is directly associated diagnosis can be used to further improve our understanding of
with the convergence of water vapour transport, rather than the the monsoon system and the skills in forecasting monsoon
strength of moisture transport (Benton and Estoque 1954; Mo and rainfall at NWP, seasonal and climate time scales.
Lin 2019; Mo et al. 2019). Along the west coast of North As pointed out in a review paper by Gimeno et al. (2016),
America, when an AR approaches a mountain barrier, the there are two main moisture sources for precipitation, namely
8 Journal of Southern Hemisphere Earth Systems Science C. Ye et al.
(a) Cor(IWV, IVT) in June (d) Cor(Pr, IVT) in June
0.5 0.5
40°N 40°N
20°N 20°N 0.9
0.8
0 0
90°E 120°E 90°E 120°E
(b) Cor(IWV, IVT) in July (e) Cor(Pr, IVT) in July 0.7
0.5 0.5
40°N 40°N 0.6
0.5
20°N 20°N
0.4
0 0
90°E 120°E 90°E 120°E 0.3
(c) Cor(IWV, IVT) in August (f ) Cor(Pr, IVT) in August
0.5 0.5 0.2
40°N 40°N
0.1
20°N 20°N
0 0
90°E 120°E 90°E 120°E
Fig. 4. (a)–(c) Correlation vectors of monthly mean IVT vectors with the monthly mean IWV (precipitable water) averaged
over the selected areas in East Asia (marked by blue lines) in June, July and August. (d)–(f) Same as in (a)–(c) but with the
monthly precipitation averaged over the selected areas. The vector magnitudes, ranging from 0 to 21/2, are colour-shaded.
(i) continental evaporation/transpiration; and (ii) moisture trans- Associated with these monsoonal flows is a huge transport of
ported from adjacent oceanic regions. Van der Ent et al. (2010) warm and moist air out of the tropics, which produces heavy
assessed the contribution of water recycling through land summer monsoon rainfall in the areas where strong moisture
evapotranspiration to the generation of precipitation across the convergence occurs. The relationships of moisture transport
globe and estimated approximately 15–30% as the continental (represented by IVT vector) with water vapour content in the
water recycling rate over the A-A summer monsoon region. atmosphere (represented by IVW) and rainfall over East Asia in
Therefore, atmospheric moisture originating from tropical the summer season can be seen in Fig. 4. The left panel shows the
warm oceans plays a dominant role in supporting and sustaining correlation vectors of monthly mean (June, July and August)
rainfall over land. IVT vectors with the area-averaged monthly mean IWV from
Weather and climate in our region are dominated by the A-A the JRA-55 over three selected regions for the 30-year period of
monsoon, with a remarkable seasonal wind reversal. In the East 1979–2018. The right panel shows the same correlation vectors,
Asian subtropical monsoon, the prevailing winds change from but with the area-averaged monthly precipitation from the
north-easterly in boreal winter into south-westerly (Chen et al. Climate Prediction Center Global Unified Precipitation data
1991; Ding 2004; Chang et al. 2004; Wang 2006; Zhang 2010). (Chen et al. 2008; NOAA 2019). To highlight intraseasonal
Atmospheric rivers in the Australia-Asian region Journal of Southern Hemisphere Earth Systems Science 9
rainfall migration and the associated moisture transport in the project focus on detailed features of moisture transport
course of the East Asian monsoon development, the three embedded in the overall monsoon circulation, mainly in
selected regions used in Fig. 4 are: (i) the southern domain the subtropical area.
(110oE–125oE, 20oN–30oN) in June when early monsoon rain- (iii) How skilful are current numerical weather prediction
fall is mainly located in southern China; (ii) the central-northern (NWP) models in forecasting ARs in the region, and can
domain (110oE–125oE and 30oN–40oN) in July when heavy the AR forecast skill help us to indirectly extend our
monsoon rainfall occurs in the Yangtze River and North China forecasts of extreme rainfalls during the Asian summer
Plain area; and (iii) the far northeast domain (120oE–135oE and monsoon season?
35oN–45oN) in August when the monsoon rainfall advances (iv) What are the underlying drivers for the intraseasonal,
further into northeast China, the Korean Peninsula and Japan. seasonal and interannual variations of ARs in the A-A
We see that the summer atmospheric water content and heavy region, and what is the potential predictability of these ARs?
rainfall in the East Asian region are driven by strong moisture
transport from the tropics. There are clear correspondences to Although several studies, such as Waliser and Guan (2017)
the well-known three preferred moisture channels for tropical and DeFlorio et al. (2018), showed the AR occurrence in the
moisture affecting the East Asian summer monsoon rainfall and Australian region, how it impacts on the regional weather and
its northward migration (Chen et al. 1991; Ding 1994, 2004; climate is far from clear. In contrast to the significant orographic
Chang et al. 2004): (i) the south-westerly flow from the Bay of blocking and the massive WPSH dominating the west Pacific
Bengal; (ii) the southerly flow from the South China Sea region; which form the preferred channels for moisture transport into
and (iii) the anticyclonic south-easterly flow on the west flank of the East Asia, the topographic influence over the Australian
the Western Pacific Subtropical High (WPSH). continent is relatively weak. Therefore, ARs in Australia require
At the early stage of the East Asian summer monsoon, the stronger interactions between midlatitude cyclonic systems and
large amount of IWV and high precipitation over South China subtropical systems to produce these long and narrow bands of
(Fig. 4a and d) are supported mainly by the steady warm and moisture transport. Another key feature of our AR analysis in the
moist south-westerly flow across the Bay of Bengal, Indochina Australian region is that we try to link moisture transport
Peninsula. At the same time, warm and moist airflow from South through ARs to some prominent synoptic patterns. As discussed
China Sea region provides another important moisture source. in the Introduction, the river-like NWCB is an important
As the summer monsoon further develops and moves north- weather phenomenon affecting large-scale rainfall in the conti-
eastward in July and August, moisture carried by the anticy- nent. As an example, Fig. 5 shows a classic NWCB occurrence
clonic flow of the WPSH becomes the dominant source for during 28–30 August 2016. Associated with this NWCB was a
sustaining the heavy monsoon rainfall in subtropical and extra- band of northwest–southeast rainfall across the continent. In this
tropical regions of East Asia. special issue, Chen et al. (2020) will provide a detailed analysis
The critical importance of the WPSH (its position, size and of the AR and NWCB features during this event.
intensity) in influencing the summer rainfall in East Asia has In recent years, several studies identified the linkage between
been documented by many studies (e.g. Wang et al. 2013; Ying NWCB and SST conditions in the tropical Indian Ocean.
et al. 2017). To a large extent, it is the co-influence between the Modelling and theoretical work from Frederiksen and
Tibetan Plateau and a semistationary WPSH that gives rise to the Balgovind (1994) and Frederiksen and Frederiksen (1996)
preferred moisture channels across East Asia during the boreal demonstrated that the SST gradients in the tropical Indian Ocean
summer season. From the patterns displayed in Fig. 4, one can favoured the development of NWCB with an influx of tropical
see some AR-like features, i.e. an elongated band of strong warm and moist air into northwest Australia. Further studies
moisture transport. However, it is unclear if these bands reflect (e.g. Risbey et al. 2009a, 2009b; Ummenhofer et al. 2011;
the collective effect of ARs occurring in the region for convey- Frederiksen et al. 2014) showed that during the negative phase
ing moisture into the extratropics. It is also unclear whether there of the Indian Ocean Dipole (IOD) (warm over the eastern part of
are different features between these East Asian ARs and the ones the tropical Indian Ocean and cooler over its western part), a
in North/South America. Consequently, the main tasks for this region of above average rainfall tends to occur on the Australian
project in the East Asian region are defined around the following continent from the northwest to the southeast, which is sup-
scientific questions: ported by enhanced moisture transport from the warm Indian
Ocean into the Australian continent. All these suggest a potential
(i) What is the role of ARs in moisture transport in the Asian role of ARs in the formation of NWCB.
summer monsoon season, and are monsoon extreme rain- To illustrate the linkage between moisture transport and
fall events and their intraseasonal variations (active and rainfall variations in Australia, Fig. 6 shows correlation vectors
break phases) associated with AR activities embedded of the monthly mean IVT with the area-averaged monthly
within the large-scale monsoon flow? rainfall in north-western and south-eastern parts of the region
(ii) What are the main characteristics of ARs embedded in the for June, July and August. For the north-western part, one can
Asian monsoon system, which are mainly located inside clearly see that rainfall variation is significantly linked to the
continental East Asia, in contrast to the ones in North/ enhanced moisture transport from the eastern part of the tropical
South America? So far, most of the published AR studies Indian Ocean and nearby warm waters close to Indonesia. The
have focused on moisture transport from the subtropics elongated IVT patterns are of high similarity to moisture
into the midlatitudes. In contrast, the AR studies in this transport associated with NWCB.10 Journal of Southern Hemisphere Earth Systems Science C. Ye et al.
(a)
(b)
Rainfall (mm)
400 mm
300 mm
200 mm
150 mm
100 mm
50 mm
25 mm
15 mm
10 mm
5 mm
1 mm
0 mm
Australian Rainfall Analysis (mm)
30th August 2016
Australian Bureau of Meteorology
http://www.bom.gov.au
© Commonwealth of Australia 2016, Australian Bureau of Meteorology Issued: 30/06/2016
Fig. 5. (a) Satellite cloud image at 05:30 pm AEST on 29 August 2016. (b) Observed rainfall on 30 August 2016.
Another feature in Fig. 6a–c is that the northwest–southeast propagation pathway to link the influence of IOD warming
extension of enhanced moisture transport becomes more evident phases to the rainfall variations in southeast Australia. There-
in the late austral winter month (August) when midlatitude fore, the pathway of tropical-originated moisture affecting
cyclonic systems penetrate more equatorward and their interac- Australian rainfall is more variable compared to its East Asian
tions with the subtropical anticyclonic system over the north- counterpart. With these considerations in mind, in the Australian
eastern part of the continent become much stronger. This is region, our project focused on:
similar to the synoptic patterns associated with NWCB as
(v) Exploring the connections between ARs and NWCB. It is
discussed in Wright (1997). For the rainfall in the southeast,
unclear if the long and narrow river-like NWCB is associ-
patterns in Fig. 6d–f tend to suggest that there is limited moisture
transport directly from the northwest but the contribution of ated with an AR or the cloud band merely reflects the effect
tropical moisture from the northeast increases. Indeed, instead of midlatitude cyclonic dynamics, when such a system
of using moisture transport, Cai et al. (2011) used a wave passes by. In this project, Chen et al. (2020) and Wu et al.Atmospheric rivers in the Australia-Asian region Journal of Southern Hemisphere Earth Systems Science 11
(a) Cor(Pr, IVT) in June (d ) Cor(Pr, IVT) in June
10°S 10°S
0.5 0.5
20°S 20°S
30°S 30°S
0.9
40°S 40°S
0.8
120°E 140°E 120°E 140°E
0.7
(b) Cor(Pr, IVT) in July (e) Cor(Pr, IVT) in July
10°S 10°S
0.5 0.5
0.6
20°S 20°S
0.5
30°S 30°S
0.4
40°S 40°S
120°E 140°E 120°E 140°E 0.3
(c) Cor(Pr, IVT) in August (f ) Cor(Pr, IVT) in August
10°S
0.5
10°S
0.5
0.2
20°S 20°S 0.1
30°S 30°S
40°S 40°S
120°E 140°E 120°E 140°E
Fig. 6. (a)–(c) Correlation vectors of the monthly mean IVT vectors with the monthly precipitation averaged over
northwest Australia (110oE–130oE and 15oS–27oS, as marked by blue lines) in June, July and August. (d)–(f) The same as in
(a)–(c) but with the monthly precipitation averaged over southeast Australia (135oE–155oE and 30oS–40oS). The vector
magnitudes, which range from 0 to 21/2, are colour-shaded.
(2020) have used backward trajectory analysis to explore the moisture source for NWCB formation and whether a
the moisture source of ARs and found the moisture NWCB supported by an AR is more likely to produce
originating from the tropical Indian Ocean is indeed very heavy rainfall or not. Analyses from Chen et al. (2020) and
important for NWCB rainfall generation in the interior of Wu et al. (2020) clearly demonstrated that when NWCB is
Australia. For rainfall in southeast Australia, the moisture supported by an AR, it is more likely to produce heavy
sources from both the midlatitudes and the tropical waters rainfall.
in the northeast are important.
(vi) Exploring whether AR analysis can help us to better Another important question we addressed in this project is
understand the NWCB-rainfall connections, including why the occurrence of NWCBs and ARs in the Australian region12 Journal of Southern Hemisphere Earth Systems Science C. Ye et al.
tend to peak in the austral winter at the same time as the AR models and their connections to tropical SST warming and
events in the Asian monsoon season reach their peaks in the changes in monsoon circulations and moisture transport. In this
boreal summer. Are there any linkages between ARs in the two project, Xu et al. (2020b) further conducted an analysis of ARs
regions? The analysis of Xu et al. (2020a) showed potential simulated by a suite of CMIP5 models and this will be pursued in
connections between ARs in boreal summer in East Asia and future analyses of newly released CMIP6 model results.
ARs in the austral winter in Australia. They termed these ‘AR
bifurcation’, with one branch of the warm and moist air over the 4 Concluding remarks
tropical Indian Ocean and western Pacific Ocean being trans- In this paper, we have reviewed the scientific background of AR
ported into the East Asian region and another branch penetrating diagnosis and its current applications in understanding regional
into the Australian continent. Furthermore, they have analysed weather and climate. We have documented why AR analysis is
bifurcated AR events in the past two decades and found that they important for the study of the A-A monsoon climate, what are
are more likely to occur in the boreal summer months. Most of the new features in our BoM-CMA collaborative AR research,
the bifurcations occurred in the boreal summer following the and finally, what are the key scientific questions we wanted to
decaying phase of an El Niño in its preceding winter. They have address in this project. We have pointed out that there are several
attributed this to delayed ENSO influence on the WPSH and key features in this collaborative project that make our studies
subtropical high in the Australian region. different from existing AR analyses (Ralph et al. 2004, 2005,
As ARs are influenced by how much tropical warm and 2006; Neiman et al. 2008a, 2008b; Roberge et al. 2009; Lavers
moist air can be transported by large-scale circulations into the et al. 2011, 2012; Newman et al. 2012; Lavers and Villarini
extratropics, their activities can be modulated by two condi- 2013; Guan and Waliser 2015; Mahoney et al. 2016; Ralph et al.
tions: (i) how warm the tropical oceans are; and (ii) how strong 2017; Waliser and Guan 2017; Mo et al. 2019).
the large-scale tropical circulations are. Therefore, ARs are One key feature is that our AR analysis focused on the
more directly linked to SST conditions in the tropical oceans tropical–subtropical interactions within a monsoon climate, in
than the rainfall in the extratropics. One may expect some which ARs can be generally wider and less filamentary, as
higher predictability of ARs occurring in the A-A monsoon compared to the ARs in extratropical areas (Waliser and Guan
region at subseasonal to seasonal time scales. In fact, recent 2017). This is illustrated in a schematic diagram in Fig. 7, which
studies by DeFlorio et al. (2018) and Nardi et al. (2018) have shows ARs in the Asian monsoon context. Firstly, in the Asian
documented some forecast skills of ARs from several opera- monsoon system, the Iranian and Tibetan plateaus act as a
tional systems. Accordingly, assessing the potential seasonal dynamical blocking of the low-level Somali jet stream (Boos
forecast application of ARs was another important component and Kuang 2010). At the same time, the surface elevated heating
in this collaborative project; some preliminary results are favours the formation of large-scale monsoon vortex and
shown in Liang et al. (2020). How AR activities in the A-A cyclonic flows surrounding the Tibetan Plateau (Wu and Liu
region are influenced by dominant SST patterns in the tropics 2016). Both factors contribute to the establishment of south-
associated with El-Niño Southern Oscillation and IOD are westerly monsoon flow, which penetrates from the tropical
documented in Wu et al. (2020). South Asia into the subtropical East Asia.
Finally, several recent studies (e.g. Ramos et al. 2016; Another dominant process influencing the East Asian summer
Espinoza et al. 2018) have discussed the Intergovernmental monsoon and its moisture transport is the WPSH. ARs in the
Panel on Climate Change (IPCC) Fifth Assessment Report Asian monsoon system act as carriers for moisture along the
model-simulated ARs and their potential changes in a globally preferred channels in the Asian monsoon background. Within
warming climate. This topic is important to the study of ARs in these channels, there are pulses of ARs supplying tropical warm
the monsoon region because there are competing effects of and moist air to the extratropical monsoon region. This is
global warming on ARs: warmed tropical oceans against different from AR studies in other regions, which focused on
changes in monsoon circulation. It is widely accepted that global how moisture from the tropics is transported into the midlatitude
SST warming provides more abundant tropical moisture sources cyclonic systems to produce heavy rainfall events over moun-
through enhanced oceanic evaporation. Nevertheless, the tainous coastal areas (e.g. Ralph et al. 2006; Dettinger et al. 2011;
responses of monsoon circulation to global warming can be Lavers et al. 2012; Mahoney et al. 2016; Zhang et al. 2019; Mo
very complex, with some models predicting weakened monsoon et al. 2019). A recent case study by Mo and Lin (2019) showed a
whereas others predict an enhanced one. Indeed, our studies by similar influence of the subtropical high in the western Atlantic
Dong et al. (2016) and Zhang et al. (2016) showed that different on a winter AR across central and eastern North America.
SST warming patterns and different monsoon circulation Furthermore, our AR analysis in the Australian region
responses to the global warming are the key factors leading to focused on documenting what the role of AR is in the formation
large uncertainty in these model-projected changes in monsoon of the NWCB, a key synoptic feature influencing Australian
development. ARs are the combined effect of these two pro- rainfall variations (Tapp and Barrell 1984; Wright 1997). In this
cesses. Therefore, in this project we also explored potential project, we explored the connections between ARs and NWCBs
changes in ARs in the monsoon region and how these changes and used backward trajectory analysis to explore the moisture
can be linked to model-projected rainfall changes. As a first source of ARs and found that moisture originating from the
attempt, Xu (2018) documented the changes in extreme rainfall tropical Indian Ocean is very important for rainfall generation in
in the A-A monsoon region, simulated by a series of IPCC the interior of the Australian continent associated with NWCBs
Coupled Model Intercomparison Project Phase 5 (CMIP5) (Chen et al. 2020; Wu et al. 2020). Furthermore, the analysis byAtmospheric rivers in the Australia-Asian region Journal of Southern Hemisphere Earth Systems Science 13
Fig. 7. Schematic diagram illustrating ARs (shown as the grey arrow) in the context of the East Asian
monsoon system. The monsoon diagram was provided by Dr. Ping Liang of Shanghai Regional Climate
Center based on a figure produced by Dr. Zuqiang Zhang (pers. comm. August 2019). MJO ¼ Madden-
Julian Oscilliation. Black dotted arrow represents an eastward movement of Tibetan Plateau vortex.
Wu et al. (2020) has shown that when NWCB is supported by an region, more analyses are needed to assess such potential using
AR, it is more likely to produce heavy rainfall. This explains operational coupled seasonal forecasting products, such as in the
why NWCBs do not always produce heavy rainfall during their work by DeFlorio et al. (2018) and Nardi et al. (2018). Based on
passages, as some of the cloud bands are associated with the the dataset produced by manual AR detection, we also plan to
condensation of midlatitude air masses rather than the warm and develop or select automatic AR detection algorithms applicable
moist air transported by ARs from the tropics (which is more in the A-A region. This will allow us to conduct AR-based
rainfall efficient). diagnoses to support operational rainfall forecasts based on the
Our analysis also proposed a potential linkage between ARs NWP guidance and/or other seasonal forecasting models.
occurring almost simultaneously in the East Asian summer Finally, we will further expand our AR analysis in some
monsoon in Australia (austral winter). Xu et al. (2020a) termed climate change projection studies (e.g. Ramos et al. 2016;
these as ‘AR bifurcation’, with one branch of the warm and Espinoza et al. 2018). This is a very important research topic
moist air over the tropical oceans being transported into the East in the A-A region because there are competing effects of global
Asian region through an AR embedded within the summer Asian warming on ARs: warmed tropical oceans against changes in
monsoon and another branch penetrating into the Australian monsoon circulation. While global SST warming provides more
continent. They found that most of the AR bifurcations occurred abundant tropical moisture sources through enhanced oceanic
during the boreal summer following the decaying phase of an El evaporation, the monsoon circulation responses to global warm-
Niño in its preceding winter, and they attributed such connec- ing can be very complex (Zhang et al. 2012, 2013, 2016; Dong
tions to the influence of tropical SSTs on modulating the et al. 2016; Zhao and Zhang 2016). Following the work by Xu
positions and intensities of WPSH and SPCZ. Nevertheless, et al. (2020b), more detailed AR analysis will help to understand
much more research is needed to further consolidate the results and narrow down uncertainty in regional climate change projec-
from this project and uncover the underlying reasons. tions in the A-A region, and the BoM-CMA collaboration will
Overall, results from this collaborative project have demon- continue to make contributions in this regard.
strated the scientific benefits of applying AR analysis in the A-A
region to achieve better rainfall forecasts at NWP, subseasonal Conflicts of interest
and seasonal time scales. There are many important aspects of The authors declare no conflicts of interest.
the ARs occurring in the A-A region that warrant further
detailed studies. Although Dong (2018) and Liang et al. Acknowledgments
(2020) discussed the application of ARs in supporting seasonal The authors would like to thank the BoM and CMA for the agreement
and subseasonal precipitation forecasts in the Asian monsoon and support of this bilateral research project. The discussions with14 Journal of Southern Hemisphere Earth Systems Science C. Ye et al.
Drs. H. Hendon (BoM), B. Guan and D. Waliser of JPL, and G. Martin and Meteorological and Oceanographic Society-International Conference on
Peili Wu of UKMO, are highly acknowledged. Dr. Zuqiang Zhang of CMA Southern Hemisphere Meteorology and Oceanography, 5–9 February,
provided a draft version of Fig. 7. We also thank the constructive comments 2018, Sydney, Australia.
from Drs. Greg Roff, Vinod Kumar and Gabrielle Gascon during the internal Dong, G., Zhang, H., Moise, A., Hanson, L., Liang, P., and Ye, H. (2016).
review process. Comments and suggestions from two anonymous reviewers CMIP5 model-simulated onset, duration and intensity of the Asian
have considerably improved the manuscript. This research is supported by a summer monsoon in current and future climate. Climate Dyn. 46,
BoM-CMA collaborative project (JWG–16 4.1). The participation of CMA 355–382. doi:10.1007/S00382-015-2588-Z
scientists is supported by a National Key R & D Program of China Espinoza, V., Waliser, D. E., Guan, B., Lavers, D. A., and Ralph, F. M.
(2018YFC1507200) and a Key Scientific Research Project of Hunan (2018). Global analysis of climate change projection effects on atmo-
Meteorological Bureau (XQKJ16A001, XQKJ17D001). spheric rivers. Geophys. Res. Lett. 45, 4299–4308. doi:10.1029/
2017GL076968
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