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Francis et al., Sci. Adv. 2020; 6 : eabc2695 11 November 2020
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ATMOSPHERIC SCIENCE
On the crucial role of atmospheric rivers in the two
major Weddell Polynya events in 1973 and 2017
in Antarctica
Diana Francis1*, Kyle S. Mattingly2, Marouane Temimi3, Rob Massom4, Petra Heil4
This study reports the occurrence of intense atmospheric rivers (ARs) during the two large Weddell Polynya events
in November 1973 and September 2017 and investigates their role in the opening events via their enhancement
of sea ice melt. Few days before the polynya openings, persistent ARs maintained a sustained positive total energy
flux at the surface, resulting in sea ice thinning and a decline in sea ice concentration in the Maud Rise region. The
ARs were associated with anomalously high amounts of total precipitable water and cloud liquid water content
exceeding 3 SDs above the climatological mean. The above-normal integrated water vapor transport (IVT above
the 99th climatological percentile), as well as opaque cloud bands, warmed the surface (+10°C in skin and air tem-
perature) via substantial increases (+250 W m−2) in downward longwave radiation and advection of warm air
masses, resulting in sea ice melt and inhibited nighttime refreezing.
INTRODUCTION
Atmospheric rivers (ARs) are narrow corridors of strong horizontal
water vapor transport associated with a low-level jet stream ahead
of the cold front of an extratropical cyclone and located within the
cyclone’s warm conveyer belt (1). In both hemispheres, ARs are be-
lieved to account for more than 90% of the annual moisture trans-
port from the tropics into high latitudes, during a relatively small
number of transient events that cover up to only 10% of the globe’s
surface (2).
In polar regions, where AR activity has been increasing in recent
years (3,4), the ability of ARs to rapidly transport large amounts of
moisture and heat poleward has significant consequences for both
land and sea ice. Their role in short-duration but high-volume melt
events over the Arctic and Antarctic has been highlighted in recent
years (5). Research to date has shown that ARs can increase ice melt
by several physical mechanisms, including (i) enhancement of the
water-vapor greenhouse effect, (ii) the formation of extensive cloud
bands that retain outgoing longwave (LW) radiation and re-emit it
back to the surface, (iii) the release of condensational latent heat in
the advected air mass (6), (iv) increase in surface melt energy via
liquid precipitation (7), and (v) the generation of turbulent heat
fluxes into the ice (8). Moreover, ARs can indirectly foster ice melt
by enhancing the deepening of the cyclone ahead of which they de-
velop, with intense ARs strengthening the cyclone by providing
more water vapor for latent heat release (9). In addition, ARs are
closely related to the atmospheric fronts over the Southern Ocean
(10), which, in turn, reinforces subantarctic cyclone dynamics. The
highest frequency of these fronts is typically found in the latitude
belt 40°S to 60°S in both summer and winter, and they can extend
poleward to lengths exceeding 2000km (10).
Cyclones around Antarctica are known to significantly alter the
sea ice field both dynamically (e.g., through wave action and ice drift)
and thermodynamically (11,12). This is particularly important in
winter, when cyclones are thought to be the primary transporter of
heat and moisture into the polar regions (13,14) and therefore con-
tribute to sea ice melt in the absence of solar radiation (15,16).
Despite their rarity (i.e., around 12 major events per year in
West Antarctica), ARs are a key factor in driving both surface melt-
ing on the major ice shelves of West Antarctica (4) and mass loss on
the Greenland Ice Sheet because of enhanced downward LW radia-
tion and turbulent heat fluxes (17–19). On the other hand, ARs can
also contribute to local snow accumulation on the ice sheet (20)—
although sharp losses in surface mass balance caused by AR radia-
tive forcing can locally exceed the moderate gain from snow accu-
mulation during summer (3,4).
Regarding sea ice, a growing body of recent Arctic work (21,22)
shows that atmospheric moisture intrusions, along with associated
cloud liquid water and ice content, can sharply increase downwelling
LW radiation—to initiate surface melt and inhibit subsequent re-
freezing (23–25). This work further shows that warm moist air in-
trusions associated with blocking events can induce significant
decline in Arctic sea ice (26) as the efficiency of the atmosphere to
radiatively cool to space decreases, thereby increasing the amount
of energy retained in the atmosphere and reradiated back toward
the surface (25).
Furthermore, in situ measurements of winter sea ice in the Arctic
have shown that, when ARs occur over the ice, they generate dis-
tinct thermal waves that propagate into the ice and decrease the ice
basal growth rate. Pulses of warming induced by warm moist air
intrusions have been observed down to a depth of 150cm in Arctic
sea ice, with a 5-day time lag (8). This process could have an even
more significant impact on Antarctic sea ice, which is known to be
thinner than Arctic sea ice with an estimated mean thickness of less
than 2m (27). Despite the crucial role of ARs in altering ice condi-
tions and the atmosphere-ice-ocean energy balance, no attention
has been given to their impact on sea ice in Antarctica particularly
during the winter season when the albedo effect is negligible and the
LW radiation effect dominates.
1Khalifa University of Science and Technology, P. O. Box 54224, Abu Dhabi, United
Arab Emirates. 2Institute of Earth, Ocean, and Atmospheric Sciences, Rutgers
University, New Brunswick, NJ 08901-8554, USA. 3Department of Civil, Environ-
mental, and Ocean Engineering (CEOE), Stevens Institute of Technology, Hoboken,
NJ 07030, USA. 4Australian Antarctic Division and Australian Antarctic Program Partne r-
ship, Private Bag 80, c/o University of Tasmania, Hobart, Tasmania 7001, Australia.
*Corresponding author. Email: diana.francis@ku.ac.ae
Copyright © 2020
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
NonCommercial
License 4.0 (CC BY-NC).
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Within the sea ice zones of both hemispheres, persistent and re-
current regions of open water are knowns as polynyas. They are tens
of thousands of square kilometers in areal extent and usually occur
at specific locations preconditioned by the ocean circulation (28).
For marine ecosystems, they constitute recurrent “windows” in the
sea ice cover and entail ecologically important “oases” that enable
marine life to overwinter at high latitudes and encourage enhanced
primary production in the spring (29).
In the Southern Ocean, most of the polynyas occur near the coast
except a few, such as the Weddell Polynya, which occurs within the
midocean sea ice cover of the eastern Weddell Sea (28). The latter
overlies the Maud Rise seamount (centered on 66°S and 3°E and
reaching to within 1200m of the surface), which together with the
Weddell Gyre entrain relatively warm Weddell Deep Water to the
surface via deep convection (30). This provides an ideal location for
the initialization of a midocean polynya (31,32) where the sea ice
is generally thin and has low concentration even during the winter
months (33).
The Weddell Polynya was observed for the first time in 1974in
Nimbus 5 satellite sea ice concentration imagery, and the second
major opening occurred in September 2017 (32,34). In those years,
its maximum area varied between ~200,000 and 300,000km2. By
virtue of its vast size and location, the polynya makes a major con-
tribution to the wintertime transfer of heat and moisture between
the ocean and atmosphere and ventilation of the deep interior ocean.
In doing so, it strongly modulates regional and wider oceanic and
atmospheric properties and circulation (35).
Given these factors, there is strong motivation to better under-
stand and model the nature and drivers of Weddell Polynya openings,
to improve their representation in models, determine their wide-
ranging effects, and more accurately predict whether these rare
but important events will occur more frequently (or not) in a warm-
ing climate. Various processes have been proposed to explain the
polynya formation and maintenance, all involving the complex
atmosphere–ocean sea ice–seabed interaction system. These include
(i) changes in atmospheric circulation associated with La Niña and
negative Southern Annular Mode (SAM) (34), (ii) upper-ocean pre-
conditioning (36) concurrent with severe storms (37,38), and (iii)
intensification of westerly winds causing a spin-up of the Weddell
Gyre (36).
Recent studies on the Weddell Polynya 2017 event have high-
lighted the contributions of wind forcing from cyclones (37) and
the oceanic forcing from warm water upwelling (38). An additional
potentially important factor that has been neglected to date is the
presence and role of coincident AR events. That is the focus of this
study, which (i) connects these previously identified processes to
larger-scale poleward atmospheric heat and moisture transport by
ARs and (ii) quantifies how ARs contribute to sea ice surface melt
and inhibit sea ice formation through cloud, precipitation, and sur-
face energy balance effects.
In this work, we show the occurrence of intense ARs (Fig.1,AandB)
during the cold late-winter months over the Weddell Sea. We then
carry out a first investigation of their role in generating sea ice melt
and contributing to the major opening events of the Weddell Polynya
observed in the 1970s (Fig.1C) and in September 2017 (Fig.1D). In
Results and Discussion, we present and discuss our results for the
2017 and the 1973 polynya events. Because of the lack of high-
resolution data for the event in 1973, we mainly use it to corroborate
our findings for the 2017 event and to highlight a few differences
between these two extraordinary events. The “Summary and con-
clusions” section summarizes our findings, and Materials and
Methods details the data and methods used in this study.
RESULTS AND DISCUSSION
ARs during September 2017
The large-scale circulation in the Southern Hemisphere during austral
winter 2017 and, more particularly, in September 2017 was marked
by an amplified zonal wave three pattern [as described in detail in
(37) and the “ARs and polynya occurrence in November 1973” sec-
tion] during which poleward transport of warm and moist air was
considerably intensified. At that time, the existing below-average
deep low and above-average ridge in the South Atlantic sector (37)
steered intense and narrow moisture bands from South America
and the central Atlantic to the Weddell Sea over periods of several
days. In addition, the SAM was positive during this episode (33,37).
A positive correlation exists between a positive SAM index and both
cyclone and fronts frequency in the Weddell Sea and its upstream
environs (39). Moreover, a positive interannual correlation is par-
ticularly marked between a positive SAM index and the intensity of
fronts (39).
Figure2 and fig. S1 show the four most intense AR events
identified during this period to be associated with core midlati-
tude integrated water vapor transport (IVT) values exceeding
800kg m−1s−1 and stretching thousands of kilometers from 30°S to
70°S. The ARs covered the entire winter sea ice zone in the Weddell
Sea between 5°W and 10°E. The sea ice edge in the region at this
time was near 60°S (37). Besides these spectacular events—observed
on 31 August and 13, 16, and 28 September 2017—several ephem-
eral moisture plumes associated with lesser IVT values were also
identified and can be detected in the time series plot in Fig.3.
The spatiotemporal evolution of the ARs during the full period
can be examined in an animation of IVT maps provided as the
Supplementary Materials.
On 31 August 2017, an intense AR is seen in the IVT maps
(Fig.2A) to originate over the southeastern coast of South America
and then expand over the South Atlantic. From there, a deep 500-hPa
trough around 10°W and a blocking high-pressure ridge down-
stream around 10°E directed the influx of moisture along 0°E toward
the Antarctic coast. The IVT direction within the AR was observed
to drive the moisture poleward and roughly perpendicular to the
Antarctic coast (Fig.2A). The deep 500-hPa trough present around
60°S and 10°W and directly to the west of the AR (Fig.2,AandB)
developed into a very deep and large cyclone on the following day
(not shown), directing even more moisture into the Maud Rise re-
gion. The AR was associated with positive normalized anomalies of
precipitable water (PWAT), where PWAT exceeded 2 SDs from the
mean, implying highly anomalous water vapor content relative to
climatology (Fig.2B).
On 13 September 2017, i.e., 1 day before the initial large opening
of the Weddell Polynya in 2017 that was centered on 65°S to 5°E (37),
a strong AR with a core IVT exceeding 800kg m−1s−1 approached
the Antarctic coast in the Weddell Sea having emanated from the
South Atlantic (Fig.2C). The presence of a blocking high-pressure
ridge to the east of the AR and centered on 20°W to 40°S directed
anomalous poleward moisture transport (Fig.2D). The PWAT as-
sociated with the AR on 13 September was anomalously high, with
values exceeding 2.5 SDs above the climatological mean (Fig.2D).
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Near the coast, the high PWAT band associated with the AR spread
over several degrees in longitude between 20°W and 25°E (Fig.2D).
Wake turbulence in the form of counterrotating vortices trailing from
the central axis of the AR can be seen in the PWAT field and wind
vectors (Fig.2D); this may have been caused by cyclonically rotating
masses associated with the horizontal pressure difference and strong
lower level jet within the AR.
On 16 September 2017, the day when the Weddell Polynya dou-
bled in size (37), extremely high values of IVT were observed to be
concentrated in a long narrow band stretching from subtropical lati-
tudes (35°S) to the interior Antarctic Ice Sheet, with its core axis around
0°E, i.e., above the polynya (Fig.2E). The AR signature in the PWAT
appeared as a long arc-shaped strip of anomalously high water vapor
content over that part of the sea ice zone where the polynya had
opened on 14 September and extending toward the coast (Fig.2F).
The situation was similar for the AR on 28 September 2017 (fig. S1A),
when a plume with core IVT values of 400kg m−1s−1 propagated
poleward to cross the Antarctic coast around 0°E with IVT values of
around 100kg m−1s−1. This represents the extreme state of the
coastal Antarctic lower troposphere associated with ARs (40). At
this time, the entire Eastern Weddell Sea was characterized by above-
average precipitable water content (fig. S1B).
Because of the strongly decreased water vapor capacity of colder
air over the sea ice zone, the magnitude of IVT associated with the
AR decreased from over 800kg m−1s−1 equatorward of the sea ice
edge to ~200kg m−1s−1 over the Maud Rise region. However, the
associated PWAT anomalies were strongly positive in this area, demon-
strating the exceptional nature of these events and their ability to
rapidly inject anomalously large amounts of moisture into the other-
wise relatively dry atmosphere of Antarctica during winter. The
decrease in IVT values near the coast is also associated with precipi-
tation during the AR’s traverse of the sea ice zone. The AR event on
13 September 2017 was associated with a total hourly water-equivalent
precipitation rate higher than 2mm (fig. S2A), while the precipitation
rate during the event on 16 September 2017 exceeded 1.5 mm/hour
(fig. S2B). Most of the precipitation during the AR events was in the
form of warm snow, as the 2-m air temperature over the area affected
by the ARs did not fall below −5°C (fig. S2).
Fig. 1. ARs and the Weddell Polynya. Cloud bands associated with ARs on 13 September 2017 (A) and on 16 September 2017 (B) observed in natural colors by the
Spinning Enhanced Visible and InfraRed Imager (source, The European Organisation for the Exploitation of Meteorological Satellites). (C) The large Weddell Polynya on
23 September 1974, which initially opened on 22 November 1973 and remained open for the following three winters, and (D) the second large opening, which occurred
in September 2017. The location of the polynya is indicated by the letter P on (A) and (B).
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As shown in the satellite visible imagery (Fig.1,AandB),
each AR was characterized by an elongated band of strato-
cumulus clouds constituting both cloud liquid water and ice
(37), a composition known to have the most substantial impact on
the surface radiation budget in polar regions (3, 8). The clouds
within the ARs were associated with total column cloud liq-
uid water content larger than 200 g m−2, which resulted in
strongly positive anomalies (more than 4 SDs above the cli-
matological mean) over the eastern Weddell Sea (Fig.2,G
andH).
S
S
W
A
S
S
B
S
S
C
S
S
W
D
S
S
E
s (A, C, and E)
S
S
W
F
(B, D, and F)
to
MSLP (hPa) (B, D, and F)
(dam) (A, C, and E) (G and H)
S
S
G
S
S
W
H
ARs
SIC
IVT (kg m s )
SIC
PWAT
August
September
anoms (kgm )
CLW anoms(g m )
September
September
September
Fig. 2. The characteristics of the 2017 AR events. ERA5 reanalysis maps for 2017 of the following: (i) IVT magnitude (shaded) and direction (black vectors) and geopotential
heights at 500 hPa in black contours on (A) 31 August at 0000 UTC, (C) 13 September at 0000 UTC, and (E) 16 September at 0000 UTC. (ii) Standardized anomalies of pre-
cipitable water (shaded), 1000- to 700-hPa mean winds in vectors and mean sea level pressure (MSLP) in black contours on (B) 31 August at 0000 UTC, (D) 13 September
at 0000 UTC, and (F) 16 September at 0600 UTC. (iii) Standardized anomalies of total column cloud liquid water (shaded), MSLP in gray contours, and 10-m winds in black
vectors on (G) 13 September at 0000 UTC and (H) 16 September at 0000 UTC. On all panels, ARs are outlined in blue contours, specific values of satellite-derived sea ice
concentration (SIC) are superimposed in pink contours, and the green box corresponds to the area used to average the quantities shown in Fig. 3.
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Impact of ARs on sea ice during the 2017 polynya
The Weddell Polynya in 2017 opened on 14 September 2017 (37).
Before this date, the eastern Weddell Sea witnessed several AR events
of different intensity and duration. Figure3 shows the time series
from 20 August to 30 September 2017 of different variables averaged
over the area 2°W to 8°E, 63°S to 67°S, which encompasses the area
of the polynya at the end of September 2017. The AR events in late
August 2017 and the beginning of September 2017 were transient
and associated with area-averaged IVT values below 100kg m−1s−1
and total column water vapor values below 7kg m−2 (Fig.3B). The
area-averaged sea ice concentration decreased gradually after each
of these events and reached 85% by 10 September 2017 (Fig.3A).
During the same period, the area-averaged sea ice thickness de-
creased by 10 cm, reaching 40cm on 10 September 2017 (Fig.3A).
Both skin and 2-m air temperature are seen to increase gradually at
the beginning of each AR event and then decrease after the decay of
the ARs (Fig.3C).
On 11 September 2017, an intense AR event occurred over the
same region and persisted during the following 2 days. The 11 to
13 September AR was in fact an extreme event with hourly area-
averaged IVT values on 13 September peaking well above the 99th
percentile of both the August to September 1979–2017 climatology
(Fig.3) and the July to October 1979–2017 climatology (not shown).
The area-averaged skin temperature and 2-m air temperature in-
creased by +10°C (from −15.3° to −5.3°C) during this event (Fig.3C),
and localized spikes in 2-m temperature were reported during this event
with maximum surface temperature reaching −1°C (Fig.4,EandF).
During the same period, the area-averaged sensible heat flux (SHF)
SIC
APolynya opening
IVT
(kgm s)
B
C
C
temp.
Skin
temp
Wm
D
SHF
LHF
Wm
Dashed:
CERES
E
SWnet
LWnet
Wm
F
Rnet
Fnet
mm w.e. hour
G
Snow
accum.
SIT (m)
SIC SIT
ARs
TCWV
(kg m )
IVT TCWV
Fig. 3. The impact of the 2017 AR events. Time series from 20 August 2017 to 30 September 2017 of the following (from top to bottom): (A) Satellite-derived sea ice
concentration and sea ice thickness (SIT). (B) ERA5 IVT and TCWV. (C) ERA5 2-m surface temperature and skin temperature. (D) ERA5 SHF at the surface and latent heat flux (LHF)
at the surface. (E) Net longwave radiation (LWnet) and net shortwave radiation (SWnet) at the surface from ERA5 (solid lines) and from CERES (dashed lines). (F) ERA5 net radiation
(Rnet) at the surface (solid lines) and from CERES (dashed lines) and ERA5 total energy flux at the surface (Fnet); positive values are toward the surface. (G) ERA5 snow accumulati on
rates. All quantities are averaged over the domain 2°W to 8°E, 63°S to 67°S (green box in Figs. 2 and 4). Dotted/dashed lines on panels B (D) are the 95th/99th (5th/1st)
percentiles (relative to August to September climatology) showing times when IVT, TCWV, SHF, and LHF exceeded climatologically extreme values. The blue shading in
the background indicates times when an AR was present over the domain. Snow accumulation is expressed in millimeter water equivalent per hour (mm w.e. hour−1).
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at the surface switched from negative to positive (+20 Wm−2, Fig.3D).
The area-averaged net shortwave radiation (SWnet) at the surface
decreased because of the presence of clouds (Fig.3E), but the area-
averaged net longwave radiation (LWnet) at the surface increased by
more than +50 Wm−2, as observed in both ERA5 data and Clouds
and the Earth's Radiant Energy System (CERES) satellite data
(Fig.3E). Although there were no above-freezing area-averaged
temperatures during this event, there was a sustained positive
(+25 Wm−2) heat flux at the surface (Fnet) from 10 September to
13 September because of the anomalously warm and moist conditions
associated with the persistent AR (Fig.3F). Notably, positive or
near-zero melt energy values were sustained for four consecutive nights
during 10 September to 13 September, indicating inhibited ability of
sea ice to refreeze in the presence of enhanced atmospheric moisture
and cloud cover. This input of energy is more than twice the heat input at
the ice surface from the atmosphere by solar heating (less than 10 Wm−2)
at the time of maximum annual sea ice melt in December (41).
The area-averaged sea ice concentration started to decline gradually
at the beginning of this AR event (i.e., on 11 September 2017), reaching
a first low on 13 September and dropping further on 14 September
S
S
W
A
S
S
W
B
S
S
C
D
S
S
W
E
S
S
W
MSLP (hPa) (E and F)
Skin temp. ( C) (E and F)
ARs
SIC
(D−F)
F
S
S
W
SIC
TOALW
September
up (W m ) (A)
SFC LWdown (W m ) (B)
TOALWnet anoms
(W m) (C)
Fnet (W m ) (D)
Skin temp. anoms. (C)(E and F)
Septembe
r
September
September
Fig. 4. The impact of the ARs on 12-13 September 2017. ERA5 reanalysis maps for the 13 September 2017 AR event of the following: (A) All-sky upwelling LW radiation
at the TOA on 13 September at 0000 UTC. The green dashed contour represents the domain shown in (D) to (F). (B) All-sky downwelling LW radiation at the surface (SFC)
on 13 September at 0000 UTC. (C) Standardized anomalies of TOA LW (shaded) on 13 September 2017 at 0000 UTC. Positive values of LW fluxes are toward Earth’s surface,
and negative values are away from the surface. Positive anomalies of TOA LW flux represent less outgoing LW radiation. (D) Total energy flux at the surface (shaded) on
12 September 2017 at 1300 UTC over the domain marked by the green dashed contour on (A). (E) Skin temperature anomalies on 12 September 2017 at 1300 UTC (shaded).
(F) Same as (E) but on 13 September 2017 at 0000 UTC. On (D) to (F), MSLP is in gray contours and 10-m winds in gray vectors, and sea ice concentration, skin temperature,
and 2-m temperature are superimposed for particular values as annotated on the figures. The green box represents the area used to average the quantities shown in
Fig. 3. ARs are outlined in blue on all panels. All anomalies are calculated relative to the 1979–2017 climatological reference period.
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(Fig.3A). This extreme AR event was marked at its end by the largest
daily drop in sea ice concentration on record [i.e., all sea ice seasons
(July to October) since 1987], where sea ice concentration dropped
by ~12% on a single day (Fig.3A) resulting in a polynya event.
Additional AR events and associated radiative forcing occurred
after the opening of the polynya (Fig.3). For instance, an intense
AR event occurred on 16 and 17 September 2017 (Fig.2,EandF,
and fig. S3) and resulted in additional 10% decrease in the area-
averaged sea ice concentration (Fig.3A) and in a doubling in the
size of the Weddell Polynya (figs. S2 and S3). Toward the end of
September, a persistent AR event associated with high IVT (~95th per-
centile) and total column water vapor greater than the 99th percen-
tile occurred over the eastern Weddell Sea (fig. S1) and resulted in
an even larger polynya (50,000 km2), which then remained open
until the end of the ice season. The occurrence of synoptic-scale
intense ARs after the opening of the polynya (i.e., the sustained
event on 27 to 30 September) delivered additional energy input to
both the upper ocean (Fig.3F) and the ice cover around the polynya
(fig. S3). This, in turn, prevented the formation of new sea ice in the
polynya area and decreased sea ice concentration around the polynya
via surface, bottom, and lateral melt by both AR-induced atmo-
spheric (as shown here) and oceanic heat (42).
The change in ocean-atmosphere interactions before and after
the polynya opening was evident in the time series. Before 14 Sep-
tember, SHF and latent heat flux (LHF) oscillated above and below
0, but after the loss of sea ice, SHF was persistently negative until the
AR event at the end of September (Fig.3D). The Fnet switched to a
negative regime because of heat loss from the ocean after the polyn-
ya opening (Fig.3F). According to ERA5, the amount of evapora-
tion that occurred from the newly open ocean was unprecedented,
with the daily mean LHF on 16 September 2017 being the lowest on
record for any July to October day during 1979–2017 (Fig.3D).
The AR events were accompanied by large amounts of precipita-
tion (Fig.3G). On the basis of ERA5 analysis, the precipitation type
during these events was snow. However, it was probably very warm
snow as surface temperatures were in the −10° to−5°C range. The
amount of snowfall on 13 and 16 September was extreme, being
considerably greater than the 99th percentile of July to October
1979–2017 climatology (fig. S2).
The exceptional amount of snowfall over the Eastern Weddell
Sea during the AR events that occurred before the opening of the
polynya (i.e., 11 to 13 September 2017), likely enhanced the melting
via insulation effects (43). The thermal conductivity of the snow be-
ing an order of magnitude lower than that of sea ice (44), a deep
snowpack over thin ice in the winter can effectively decouple the sea
ice from the atmosphere and insulate it from frigid polar air, which
prevents its growth (43,45). In addition, the sea ice surrounding the
freshly opened polynya continued to be affected by the subsequent
synoptic-scale ARs and associated snowfall and radiative forcing.
These ARs affected a larger area than that of the polynya (Fig.4 and
fig. S3). This may have prevented sea ice growth around the polynya
and contributed to maintaining it open even after the decay of the
ARs/cyclones.
The AR-induced alteration of the energy balance seen in the
time series, averaged over the polynya area, was also observed over
the whole area affected by the ARs (i.e., Fig.4). For instance, the
exceptional AR event during 11 to 13 September 2017, which im-
mediately preceded the opening of the polynya, significantly altered
the radiative fluxes at the surface and at the top of atmosphere
(TOA). During this period, all-sky upwelling LW radiation maps
show that less LW radiation by 100 Wm−2 was able to leave the
TOA in the AR area compared to surrounding areas (Fig.4A), with
a minimum cooling efficiency around 0°E that corresponded to the
area of maximum PWAT and total column cloud liquid water con-
tent (Fig.2,DandH). This reservoir of energy was retained in the
atmosphere and radiated back to the surface. The maps of all-sky
downwelling LW radiation at the surface (Fig.4B) show that the sea
ice surface below the AR footprint received 250 Wm−2 more energy
than surrounding areas in the same sector.
The areas of elevated surface LW fluxes on 13 September 2017
(Fig. 4B) highlight a similar spatiotemporal coincidence with the
large precipitable water amount and cloud liquid water content
(Fig.2,D andH) brought by the ARs (Fig.2,B andE), i.e., both
were colocated within the area in which the polynya subsequently
opened (Fig.1D).
The signature of ARs is clearly apparent in the LW radiation
anomalies (Fig.4C) where the LW radiation retained in the atmo-
sphere and within the AR was higher than the climatological mean
by fully 3 SDs. The increase in LW radiation at the surface re-
sulted in a positive total energy flux (Fig.4D). For instance, and on
12 September 2017 at 1300 UTC, the area affected by the AR received
more than +25 Wm−2 instantaneous energy flux (Fig.4C). The in-
crease in input of energy at the surface by LW radiation contributed
to the increase of surface skin temperature, which was anomalously
positive by more than 10°C within the AR footprint (Fig.4,EandF).
The sea ice concentration in the polynya area decreased during
the 11 to 13 September 2017 event from 85% to below 50% by the
end of the day on 13 September 2017 (Fig.4E). It is worth noting
that this reduction in sea ice concentration occurred before the ar-
rival of the cyclone above the polynya area (Fig.4F). This implies
that the sea ice cover was primarily reduced by the AR-induced
warming during the 4 days preceding the opening and then was fol-
lowed by additional decrease via sea ice drift by strong cyclonic
winds i.e., divergence in the ice motion field (37). Without the re-
duction in the sea ice cover by the ARs, the cyclones may have
not been able to open alone the compact winter sea ice cover in
this region.
The radiative forcing induced by ARs increased the vulnerability
of the polynya-region sea ice to dynamical forcing by the cyclones
that developed behind the ARs in the subsequent hours. ARs are
also known to favor more intense cyclones by providing additional
latent heat (9), which may have increased the impact of ARs on the
sea ice both thermodynamically (by sea ice melt) and dynamically
(by strengthening the cyclones).
All AR events were followed by the development of synoptic-
scale deep cyclones situated to the west of the ARs [Figs.2 (B, D,
and F) and 4F and fig. S1A]. These cyclones were studied in detail in
(37) and were found to cause wind-stress forcing on the ice cover,
already weakened by the ARs, to trigger a polynya event. In this
sense, ARs and cyclones seem to work as a coupled system wherein
intense ARs weaken the sea ice cover and strengthen the cyclones by
providing more water vapor for latent heat release (9). Moreover,
the cyclones enhance the poleward transport of moisture and heat,
which, in turn, deteriorates the sea ice cover (as demonstrated here).
Therefore, we conclude that the Weddell Polynya event in 2017 re-
sulted from ice melt and inhibited nighttime refreezing initiated by
the ARs and immediately followed by wind-driven ice divergence
caused by the cyclones.
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The ERA5-derived surface wind speeds during these events in the
order of 30 ms−1 persisted for the entire duration of the combined
AR/cyclone events (37). From the histogram analysis (Fig.5), the surface
winds associated with the AR/cyclone on 13 and 16 September 2017
were exceptional, with the daily mean 10-m wind speed being greater
than the 99th percentile relative to August to September climatology
(Fig.5F).
The observed temperature increase at the surface, and ice loss
during the AR events preceding the polynya opening, were also
evident during the events after the opening i.e., on 16, 17, and
28 September 2017. This, in turn, resulted in a significant enlarge-
ment of the already opened polynya (figs. S1 to S3).
The exceptional nature of the ARs in 2017
Histograms of several daily-averaged ERA5 variables (Fig.5) provide
a climatological context for the AR events in 2017 that provoked the
Weddell Polynya event. The distribution of daily mean IVT for all
August to September 1979–2017 days reveals that the persistent AR
event on 12 and 13 September was among the most intense on re-
cord with the daily mean IVT values above the 95th percentile of all
August to September climatology (Fig.5A) and the hourly IVT values
peaking well above the 99th percentile (Fig.3). The net LW radiation
at the surface generated by this event was above the 95th percentile
of the full-year climatology for 1979–2017 (fig. S2D), which led to a
positive daily mean net radiation at the surface, and the associated
values were greater than the 95th percentile of all August to September
1979–2017 days (Fig.5B). The extreme AR events resulted in excep-
tional warmth at the surface with daily mean skin temperature and
2-m air temperature above the 95th percentile of all August to
September climatological values (Fig.5,CandD).
The distribution of daily mean snow accumulation during the
AR events on 13 and 16 September was exceptionally unusual compared
to the record, being well above the 99th percentile (Fig.5E). More
particularly, snow accumulation during the AR on 16 September,
2 days after the initial opening of the polynya, was unique relative to
the record (Fig.5E), which may have prevented the bottom growth
of new sea ice because of insulation effects at the sea ice in the sur-
roundings of the newly opened polynya (43).
The synoptic-scale cyclones that immediately followed the AR
events were associated with exceptionally high 10-m wind speeds
compared to the August to September climatology (Fig.5F). The daily
mean 10-m wind speeds during the cyclones on 13 and 16 September
were greater than the 99th percentile, whereas those associated with
the cyclone on 1 September were above the 95th percentile (Fig.5F).
The exceptional nature of the AR/cyclone events in 2017 highlights
the crucial role of the heat and moisture transported by the ARs in
Count
IVT (kg m s )
A
Netradiation (W m )
B
Count
Skintemp. (C)
C
(C)
D
Count
Mean hourly snowaccumulation (mmw.e.)
E
(m s )
Min&max
F
Fig. 5. The exceptional nature of the 2017 AR events. Histograms showing the distribution of daily mean values for all August to September climatology during 1979–2 017,
spatially averaged over 2°W to 8E°E and 63°S to 67°S of the following: (A) IVT, (B) net radiation at the surface, (C) skin temperature, (D) 2-m surface air temperature, (E) mean hour-
ly snow accumulation in millimeter water equivalent, and (F) 10-m wind speed. The colored vertical lines correspond to daily mean values during the 28 August–17 Sept ember 2017
period. The gray lines correspond to the 1st and 99th percentiles, the black lines correspond to the minimum and maximum values, and the dashed gray lines correspond
to the 5th and 95th percentiles.
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initiating a polynya event and the role of the exceptionally strong
winds carried by the cyclones in accentuating a decrease in sea ice
concentration via sea ice drift.
ARs and polynya occurrence in November 1973
Although the Weddell Polynya was spotted in satellite observations
for the first time in winter 1974 (46), this was in fact 1 year after its
actual initial opening in November 1973 (identified here by inspect-
ing the satellite data). Because the polynya opened in the late-spring
season, it did not refreeze during that sea ice year, and in fact, it re-
mained open for three consecutive winters probably because of the
altered ocean circulation (28). It was not until September 2017 that
the second large opening occurred (as discussed in the previous
section), after a short-lived and smaller Weddell Polynya event in
August 2016 that was also associated with an intense AR and severe
storm activity (38). The first opening in 1973 occurred at 63°S and
5°E to 12°E, which is 2° to the north and 3° to the east of the initial
opening in 2017. This position was coincident with the most south-
ern extremity of the AR on 20 November 1973, which was further
north compared to the ARs in 2017 (Fig.6A), as was the polynya
opening. The simultaneous nature of the openings and the AR foot-
prints in each polynya event implies that ARs, by virtue of their
effect on LW radiation, were a key contributing factor to this phe-
nomenon, and one that has been overlooked to date.
The AR maps shown in Fig.6 reveal a band of high IVT emanat-
ing on 18 November 1973 from the tropics (25°S, i.e., a lower
latitude than in 2017) and propagating poleward over the South
Atlantic toward Antarctica with its core axis around 0°E (Fig.6A).
The PWAT associated with the AR was anomalously high, with
positive values exceeding 2.5 SDs from the climatological mean
(Fig.6B). Total column of cloud liquid water in the AR sector
was higher than 100 gm−2 (Fig.6C). Highly positive anomalies of
LW radiation, of fully 3 SDs above the climatological mean, were
seen in the AR sector (Fig.6D). This demonstrated a high retention
of LW radiation in the atmosphere within the AR, coincident with
the high content of water vapor (Fig.6B) and cloud liquid water
(Fig.6C).
The AR and an associated large deep cyclone to the west and
centered at 20°W (Fig.6A) approached the Weddell Sea on 18 No-
vember 1973 at 0000 UTC and remained until 20 November at 0600
UTC. During this 2-day period, the sea ice concentration decreased
significantly (Fig.6E) and the polynya opened first on 22 November
1973 at 63°S (Fig.6F).
Given these factors, we propose that the loss of sea ice in the
polynya area at the time of polynya opening was associated with the
downwelling LW flux anomalies. ARs, elevated water vapor and
cloud liquid water content, increased LW fluxes, and decreased sea
ice concentration are concomitant and common features of each of
the polynya opening events examined, i.e., 1973, 2016 (not shown),
and 2017.
SUMMARY AND CONCLUSIONS
This study reveals the occurrence of intense ARs in the late-winter
season in the Weddell Sea and demonstrates their role in initiating
major opening events of the Weddell Polynya. By virtue of their ef-
fect in significantly increasing downwelling LW fluxes (because of
strongly increased high water vapor amount and cloud liquid water
content), warm air temperatures and precipitation in the form of
heavy warm snow, ARs are identified as an important factor in these
phenomena—and one which has been overlooked to date.
Combining satellite observations and reanalysis data, we identi-
fied the occurrence of intense ARs that preceded the two major
Weddell Polynya opening events in November 1973 and September
2017 by 1 to 2 days. By investigating the effects of ARs on sea ice
conditions during these two events, we found that the resulting
warm air advection and increase in LW fluxes at the surface, due to
the presence of opaque clouds and increased water vapor content in
the atmosphere within the AR area, significantly reduced the sea ice
thickness and concentration.
The exceptional atmospheric conditions during the AR events
under scrutiny increased the sea ice vulnerability to the wind-driven
ice divergence associated with the cyclones that formed shortly
behind the ARs [see (37)]. The role of the synoptic-scale ARs was
crucial in initiating and maintaining the polynya by melting ice and
preventing refreezing. Cyclones, a ubiquitous feature around Ant-
arctica, may have not been able to trigger a polynya event without
the presence of the ARs, which preconditioned the ice cover over a
large area. The role of the ARs was in twofold, persistently fragilizing
the ice cover through radiative effects (before and after the opening
of the polynya) and likely strengthening the cyclones by providing
additional water vapor for latent heat release (9,10).
Here, we propose that while the special atmospheric conditions
induced by the ARs and the accompanying cyclones play the role of
triggers in the two major Weddell Polynya events in 1973 and 2017,
ocean processes are essential to the polynya longer-term precondi-
tioning, enlargement, and maintenance (32,36,38). As reported in (38),
warm near-surface ocean temperatures of unknown origin were ob-
served in oceanic float data during the polynya formation in September
2017 [see also (34)]. The study also concluded that while the for-
mation of Weddell Polynya events may be explained by upward salt
transfer from convective mixing, what controls the initial openings
is still unknown. On the basis of our analysis, ARs and associated
warm conditions are likely at the origin of this warming and, together
with the subsequent cyclones, control the initial openings.
Francis et al. (37) have shown that winter of 2017 were exceptional
in terms of heat and moisture transport from midlatitudes toward
Antarctica. In this study, we have presented evidence that the most
significant transport occurred in the form of ARs, supported by atmo-
spheric blocking ahead of the ARs associated with a pronounced zonal
wave number three pattern during this particular winter (i.e., 33 and 37).
The new results presented here highlight the need for focused
research into the potentially important wider effects of ARs on the
Antarctic sea ice environment and ecosystem (as well as other
polynyas). This relates (among other things) to (i) their modulation
of LW and SW radiation fluxes, surface temperature, and snowfall;
(ii) their possible contribution to observed patterns of change/vari-
ability in Antarctic sea ice coverage (47); and (iii) their possible role
in recent Antarctic ice-shelf disintegration events, e.g., Larsen B in
2002 (48).
Developing improved understanding of ARs, their effect on the
coupled sea ice–ocean-atmosphere–ice sheet biological system and
the possible regional and seasonal dependencies involved, is of
increasing importance. Under projected future climate change, the
frequency of AR events is predicted to increase by ~50% globally
and 60% in the southern midlatitudes (49), as well as a general poleward
shift in AR landfall location (5). It is also anticipated that ARs will
become longer and wider and will entail stronger and more effective
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mechanisms for the atmospheric transport of high levels of integrated
water vapor between the low-/mid-southern latitudes and the Antarctic
Ocean and continent. Because of the increased atmospheric moisture
in a warmer climate, the intensity of AR-related precipitation is pre-
dicted to increase as well (5).
As stated in Introduction, research into the regional role of ARs
in the coupled Antarctic sea ice–ocean-atmosphere–ice sheet system
lags behind that in the Arctic. There, recent studies (3,22) have con-
firmed that AR events can cause sea ice melt or inhibit sea ice
growth due to the increased cloud cover–downward LW radiation
mechanism, especially during winter when there is no shortwave
radiation to offset the positive LW cloud radiative forcing. No similar
long-term study of the impacts of moisture transport events on the
entire Antarctic sea ice cover has been conducted to determine
whether a similar mechanism is at work there, which constitutes a
further avenue for future research. This would also improve the
prediction of likely sea ice change and variability under future
climate change.
S
S
W
A
(dam) (A)
MSLP (hPa) (B)
S
S
W
B
(B)to
kg m s (A)
S
S
C
S
S
W
D
E
ARs
F
IVT (kg m s ) (A)
Cloud liquidwater
(g m ) (C)
(E and F)
PWAT anoms
(kg m )(B)
TOALWnetanoms
(W m)(D)
SIC
November
November
November November
Fig. 6. The 1973 polynya event. JRA55 reanalysis maps on 18 November 1973 at 1800 UTC of the following: (A) IVT magnitude (shaded) and direction (black vectors) and
geopotential heights at 500 hPa in black contours. Red contours represent areas with climatological IVT PR above the 85th percentile. (B) Standardized anomalies of
precipitable water (shaded), 1000- to 700-hPa mean winds in vectors, and MSLP in black contours. (C) Total column cloud liquid water (shaded) on 19 November 1973 at
0600 UTC. (D) Standardized anomalies of TOA LW (shaded) on 19 November 1973 at 0600 UTC relative to the 1970–2000 climatological reference period. Positive anom-
alies of TOA LW flux represent less outgoing LW radiation. (E) Daily sea ice concentration in the small domain represented in the green box in (D) from Nimbus 5 on
19 November 1973. (F) Same as (E) but on 22 November 1973. The solid yellow line is the 15% contour, and the dotted yellow line is the 50% contour. The 15% contour
of sea ice concentration was used to delineate the polynya. (A to E) The ARs are outlined in blue contours.
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Last, the possibility that winter AR events in the Antarctic may
also affect the seasonal evolution of the sea ice environment through
the subsequent summer months (and beyond) deserves consideration.
In the Arctic, it has been observed that ARs not only affect the sur-
face temperature over the sea ice but also induce warming waves
that propagate downward through the sea ice interior to affect its
properties and also reduce ice basal growth (8). This process has
been shown to influence the Arctic sea ice thickness at the onset of
seasonal melt in spring, and the probability of similar processes in
the relatively thin Antarctic sea ice pack merits investigation. An
additional major unknown relates to the possible effects of ARs in
increasing regional snowfall/accumulation over the Antarctic sea
ice zone—to potentially affect regional sea ice melt/persistence giv-
en its insulative properties and its contribution to snow-ice forma-
tion (15). We hope that this study will motivate such investigations.
MATERIALS AND METHODS
The atmospheric analysis is based on data from the Japanese 55-year
Reanalysis (JRA55) (50) for the 1973 period and from the ERA5
reanalysis (51) for the 2017 period. The ERA5 radiative fluxes are
provided as mean rates over the hour before the given time step (e.g.,
the output at 1200 UTC is the mean rate during 1100 to 1200 UTC),
whereas the JRA55 fluxes are three hourly averages (e.g., the output
at 1200 UTC is the mean rate during 0900 to 1200 UTC). Because of
the strong diurnal signal in LW radiation over land, we used the values
for the specific time of day to calculate anomalies relative to the
1970–2000 distribution for JRA55 and 2008–2017 for ERA5. For
example, for LW anomalies mapped at 1200 UTC on 13 September
2017, the distribution of all September 1200 UTC values during the
climatological reference period was used to calculate anomalies.
Furthermore, to investigate the anomalous character of the at-
mospheric conditions during the 2017 Polynya event, we calculated
for several atmospheric variables, hourly standardized anomalies,
and percentile ranks relative to all hourly ERA5 August to September
values during the full record (1979–2017). In addition, a climato-
logical histogram analysis has been performed for the 2017 Polynya
event where daily averaged variables during September 2017 are
compared to the climatology with August to September 1979–2017
as climatological reference period. The histograms represent the
distribution of daily mean values spatially averaged over −2°W to
8°E and−63°S to −67°S, for all August to September months during
1979–2017.
ARs are detected and identified through analysis of IVT at six-
hourly time steps from JRA55 and ERA5 reanalysis data, using a
modified version of the detection algorithm used in (3). The IVT
values within the 1000- to 200-hPa layer are first calculated over the
Southern Hemisphere poleward of 10°S, according to the formula
IVT =
1
─
g ∫
1000hPa
200hPa qVdp (1)
where g=9.80665 ms−2 is gravitational acceleration, q and V are
specific humidity (kg kg−1) and vector wind (m s−1) at the given
pressure level, respectively, and dp is the difference between adja-
cent pressure levels. Pressure levels are incremented by 50 hPa be-
tween 1000 and 500 hPa and by 100 hPa between 500 and 200 hPa,
while IVT units are kg m−1s−1. The climatological percentile rank of
IVT (IVT PR) at each time step is then calculated by comparing the
IVT value at each grid cell to the distribution of all IVT values at
that grid cell for the given month during the climatological refer-
ence period for each reanalysis. The climatological reference peri-
ods used here are 1970–2000 for JRA55 and 2008–2017 for ERA5.
Following (3), the AR detection procedure begins by finding
contiguous regions wherein actual or “raw” IVT values are above a
certain threshold and the IVT PR value is ≥85%. The minimum
IVT threshold is set to 50kg m−1s−1 [as opposed to 150kg m−1s−1
in (3)] because IVT values near Antarctica are generally less than in
the Northern Hemisphere high latitudes (40). Potential ARs are
then filtered by applying size, location, length, length-to-width ra-
tio, and mean transport direction criteria. These requirements en-
sure that the identified features are long, narrow, coherent bands of
poleward moisture transport in (and connecting) the middle and
polar latitudes of the Southern Hemisphere, i.e., they bear the char-
acteristics of ARs. See (3) for additional details on the AR detection
and mapping algorithm.
Sea ice extent and concentration data were taken from the Nimbus 5
satellite observations (52) for the 1973 period, whereas for the 2017
period, they were obtained from the National Oceanic and Atmo-
spheric Administration/National Snow and Ice Data Center (NSIDC)
Climate Data Record of Passive Microwave Sea Ice Concentration,
version 3 and its near-real-time version (53). These data are mapped
on the NSIDC polar stereographic grid with a nominal 25km by
25km grid cell area at both daily and monthly temporal resolution.
Sea ice thickness was derived from the satellite Soil Moisture and
Ocean Salinity mission (54).
For surface and TOA radiative flux quantities during the 2017
event, we use the satellite-derived CERES Synoptic (CERES-SYN)
version 3 dataset (55). CERES-SYN provides daily LW surface and
TOA flux quantities over a 1°×1° resolution grid in both clear-sky
and all-sky conditions and covering March 2000 to present.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/6/46/eabc2695/DC1
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Acknowledgments: We would like to thank the editors and the anonymous reviewers for
contribution to the peer review of this work. C. Eayrs is acknowledged for help in Nimbus 5
satellite data processing. Funding: This work was supported by Masdar Abu Dhabi Future
Energy Company, United Arab Emirates, grant 8434000221. K.S.M. was supported by a NASA
Earth and Space Science Fellowship (NASA grant number NNX16A022H). The contribution of
R.M. and P.H. was supported by the Australian Antarctic Division and by the Australian
Government’s Australian Antarctic Partnership Program and contributes to AAS Project 4116.
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Francis et al., Sci. Adv. 2020; 6 : eabc2695 11 November 2020
SCIENCE ADVANCES | RESEARCH ARTICLE
13 of 13
Author contributions: D.F. conceived the study and wrote the initial and the revised
manuscript. K.S.M. analyzed the satellite and reanalysis data. M.T., R.M., and P.H. provided
input on result analysis. All authors interpreted results and provided input to the final
manuscript. Competing interests: The authors declare that they have no competing interests.
Data and materials availability: All data needed to evaluate the conclusions in the paper are
present in the paper and/or the Supplementary Materials. Additional data related to this paper
may be requested from the authors. Correspondence and requests for materials should be
addressed to D.F.
Submitted 15 April 2020
Accepted 25 September 2020
Published 11 November 2020
10.1126/sciadv.abc2695
Citation: D. Francis, K. S. Mattingly, M. Temimi, R. Massom, P. Heil, On the crucial role of
atmospheric rivers in the two major Weddell Polynya events in 1973 and 2017 in Antarctica.
Sci. Adv. 6, eabc2695 (2020).
on November 12, 2020http://advances.sciencemag.org/Downloaded from
2017 in Antarctica
On the crucial role of atmospheric rivers in the two major Weddell Polynya events in 1973 and
Diana Francis, Kyle S. Mattingly, Marouane Temimi, Rob Massom and Petra Heil
DOI: 10.1126/sciadv.abc2695
(46), eabc2695.6Sci Adv
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REFERENCES http://advances.sciencemag.org/content/6/46/eabc2695#BIBL
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