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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 temperature) 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.
Francis et al., Sci. Adv. 2020; 6 : eabc2695 11 November 2020
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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.
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 2000km (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 (1719). 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 (2325). 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 150cm 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 2m (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:
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
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 1200m 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 1974in
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 300,000km2. 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,AandB)
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.
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).
Figure2 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
800kg m−1s−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,AandB)
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 800kg m−1s−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 400kg m−1s−1 propagated
poleward to cross the Antarctic coast around 0°E with IVT values of
around 100kg m−1s−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 800kg m−1s−1 equatorward of the sea ice
edge to ~200kg m−1s−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 2mm (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,AandB),
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
s (A, C, and E)
(B, D, and F)
MSLP (hPa) (B, D, and F)
(dam) (A, C, and E) (G and H)
IVT (kg m s )
anoms (kgm )
CLW anoms(g m )
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. Figure3 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 100kg m−1s−1
and total column water vapor values below 7kg 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 40cm 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,EandF).
During the same period, the area-averaged sensible heat flux (SHF)
APolynya opening
(kgm s)
mm w.e. hour
SIT (m)
(kg m )
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 Wm−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 Wm−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 Wm−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 Wm−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
MSLP (hPa) (E and F)
Skin temp. ( C) (E and F)
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)
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,EandF,
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
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 Wm−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,DandH). 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 Wm−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 andH) brought by the ARs (Fig.2,B andE), 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 Wm−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,EandF).
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 ms−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
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,CandD).
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
IVT (kg m s )
Netradiation (W m )
Skintemp. (C)
Mean hourly snowaccumulation (mmw.e.)
(m s )
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 gm−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
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.
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.
(dam) (A)
MSLP (hPa) (B)
kg m s (A)
IVT (kg m s ) (A)
Cloud liquidwater
(g m ) (C)
(E and F)
PWAT anoms
(kg m )(B)
(W m)(D)
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.
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
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
200hPa qVdp (1)
where g=9.80665 ms−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−1s−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 50kg m−1s−1 [as opposed to 150kg m−1s−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 25km by
25km 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.
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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.
on November 12, 2020 from
Francis et al., Sci. Adv. 2020; 6 : eabc2695 11 November 2020
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
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, 2020 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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... On top of the ocean and the glaciological factors which precondition ice shelves for calving over long periods of time, the atmospheric forcing may also play a role during the calving events. For instance, atmospheric forcing can trigger ice shelf calving events (e.g., and can be reinforced by large-scale circulation anomalies such as the zonal wavenumber 3 pattern which regularly occurs in the Southern Hemisphere midlatitudes (Francis et al., , 2020. reports on the crucial role that atmospheric conditions played in the September 2019 calving of the Amery Ice Shelf. ...
... In fact, the SAM index for February 2021 was 2.19, which is more than 1 standard deviation above the 1979-2020 mean (0.04 ± 1.97). A combination of La Niña and a positive SAM has also been found to have promoted the September 1974 and 2017 polynya events in the Weddell Sea (Francis et al., 2020). It is important to note that this combination of ENSO and SAM phases is expected as La Niña events typically co-occur with positive SAM events (Fogt & Marshall, 2020). ...
... A few days prior to the A-74 calving event (20-26 February 2021, Figure 2b), the La Niña signal noted above dominates the circulation (cf. Figure 2a), as evidenced by the anomalous low pressure over the Bellinghausen/ Amundsen Seas, although there are some differences to the monthly mean anomaly (in particular in the South Atlantic sector): The low to the north of the BIS extends to 35°S just south of Uruguay, whereas in the monthly mean plot (Figure 2a) there is a ridge over the northern part of this region. The circulation associated with the sea-level pressure pattern ( Figure 2b) draws in the moist and warm air from lower latitudes toward Antarctica, promoting intense cyclones and associated dynamical forcing (wind stress and turbulence) at the mouth of Antarctic ice shelves (Francis et al., 2020). This can be clearly seen in Figure 2d, which shows the meridional heat flux for the same period (negative values indicate southward/poleward transport of heat), with the anomalies being negative around the target region at 1 standard deviation below the mean. ...
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The calving of Antarctic ice shelves remains unpredictable to date due to a lack of understanding of the role of the different climatic components in such events. In this study, the role of atmospheric forcing in the calving of the Brunt Ice Shelf (BIS) in February 2021 is investigated using a combination of observational and reanalysis data. The occurrence of a series of extreme cyclones around the time of the calving induced an oceanward sea surface slope of more than 0.08º leading to the calving along a pre-existing rift. The severe storms were sustained by the development of a pressure dipole on both sides of the BIS associated with a La Niña event and the positive phase of the Southern Annular Mode. Poleward advection of warm and moist low-latitude air over the BIS area just before the calving was also observed in association with atmospheric rivers accompanying the cyclones. Immediately after the calving, strong offshore winds continued and promoted the drift of the iceberg A-74 in the Weddell Sea at a speed up to 700 m day-1. This study highlights the contribution of local atmospheric conditions to ice-shelf dynamics. The link to the larger scale circulation patterns indicates that both need to be accounted for in the projections of Antarctic ice shelf evolution.
... The 2-day mean surface salinity before and after the precipitation event presented in the case study saw a decrease of 0.012 g kg −1 , increasing the buoyancy of the surface waters. Francis et al. (2020) provide further evidence of the role of ARs in the Weddell Sea by relating the effects of ARs on sea ice melt, partly due to oceanic heat gain from the associated moist, warm advected air masses 1,320 km southward of this location. They observed similar precipitation rates to this study (∼2 mm hr −1 ), which are thought to have assisted in the formation of the Weddell polynya in 2017. ...
... They observed similar precipitation rates to this study (∼2 mm hr −1 ), which are thought to have assisted in the formation of the Weddell polynya in 2017. While Francis et al. (2020) investigate these processes during late winter, this study was conducted during austral summer, when solar radiation is at a maximum ( Figure S2 in Supporting Information S1). Thus, the thermal contribution to buoyancy also is at a maximum (Equation 1). ...
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Atmospheric rivers (ARs) dominate moisture transport globally; however, it is unknown what impact ARs have on surface ocean buoyancy. This study explores the surface buoyancy gained by ARs using high‐resolution surface observations from a Wave Glider deployed in the subpolar Southern Ocean (54°S, 0°E) between 19 December 2018 and 12 February 2019 (55 days). When ARs combine with storms, the associated precipitation is significantly enhanced (189%). In addition, the daily accumulation of AR‐induced precipitation provides a buoyancy gain to the surface ocean equivalent to warming by surface heat fluxes. Over the 55 days, ARs accounted for 47% of the total precipitation equating to 10% of the summer surface ocean buoyancy gain. This study indicates that ARs play an important role in the summer precipitation over the subpolar Southern Ocean and that they can alter the upper‐ocean buoyancy budget from synoptic to seasonal timescales.
... ARs are narrow long bands of enhanced moisture fluxes originating from the mid-latitudes and sub-tropics 16 . They have been associated with particular instances of high temperature, moisture, and wind speed in the lower troposphere [17][18][19] along with sometimes co-occurring with foehn winds while causing major melt events 20,21 and being linked with instances of sea-ice decay 22 . However, a systematic long-term analysis of extreme events co-occurring with ARs still needs to be conducted along the AP to show their role in ice-shelf weakening and the eventual initiation of ice-shelf calving and disintegration events. ...
... The tendency of ARs to create widespread melt pond formation makes them possible precursors of ice-shelf collapse via hydrofracturing cascades like what was observed on the Larsen B in 2002 13 . This combined with wind stress and radiative forcing leading to sea-ice clearing thus allowing swells to apply strain along the ice-shelf fronts 14,22 makes ARs a unique forcing of ice-shelf weakening. These conditions occur during periods of enhanced poleward heat advection from anticyclonic activity just east of the northern AP 36 ( Supplementary Fig. 1). Figure 4 provides a visualization summarizing all the processes during AR landfalls that are linked to ice-shelf weakening. ...
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The disintegration of the ice shelves along the Antarctic Peninsula have spurred much discussion on the various processes leading to their eventual dramatic collapse, but without a consensus on an atmospheric forcing that could connect these processes. Here, using an atmospheric river detection algorithm along with a regional climate model and satellite observations, we show that the most intense atmospheric rivers induce extremes in temperature, surface melt, sea-ice disintegration, or large swells that destabilize the ice shelves with 40% probability. This was observed during the collapses of the Larsen A and B ice shelves during the summers of 1995 and 2002 respectively. Overall, 60% of calving events from 2000–2020 were triggered by atmospheric rivers. The loss of the buttressing effect from these ice shelves leads to further continental ice loss and subsequent sea-level rise. Under future warming projections, the Larsen C ice shelf will be at-risk from the same processes. The most intense atmospheric rivers to hit the Antarctic Peninsula induce extremes in temperature, surface melt, sea ice disintegration or swell that destabilize the ice shelves with 40% probability, suggest analyses of observations and regional climate model simulations.
... The physical processes relevant to AR-induced ice melt or impeded ice growth include (1) enhanced downward longwave radiation (DLW) due to the greenhouse effect of water vapour, the cloud radiative effect (CRE) and condensational heating release, (2) reduction or even sign change in turbulent heat fluxes from the ice surface, (3) the insulating capacity of snow and (4) melt energy carried by rainfall (for example, refs. 22,[25][26][27][28][29][30][31]. ...
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In recent decades, Arctic sea-ice coverage underwent a drastic decline in winter, when sea ice is expected to recover following the melting season. It is unclear to what extent atmospheric processes such as atmospheric rivers (ARs), intense corridors of moisture transport, contribute to this reduced recovery of sea ice. Here, using observations and climate model simulations, we find a robust frequency increase in ARs in early winter over the Barents–Kara Seas and the central Arctic for 1979–2021. The moisture carried by more frequent ARs has intensified surface downward longwave radiation and rainfall, caused stronger melting of thin, fragile ice cover and slowed the seasonal recovery of sea ice, accounting for 34% of the sea-ice cover decline in the Barents–Kara Seas and central Arctic. A series of model ensemble experiments suggests that, in addition to a uniform AR increase in response to anthropogenic warming, tropical Pacific variability also contributes to the observed Arctic AR changes.
... Fig. 2c and d shows an anomalous narrow band of moisture from tropical Africa, known as atmospheric river (AR, Massoud et al., 2020;Dezfuli et al., 2021;Bozkurt et al., 2021), being advected into the Middle East along the eastern flank of the cut-off lows. This significant supply of moisture via the AR helps in fueling the cut-off low and maintaining it over time (e.g., Francis et al., 2020b). As the cut-off low reached the mountainous regions over southeastern Turkey, deep convection developed (Figs. ...
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Large amounts of dust in the air can disrupt daily activities and pose a threat to human health. In May 2022, consecutive major dust storms occurred over the Middle East resulting in severe environmental, social and health impacts. In this study, we investigate the exceptional factors driving these storms and the effects of the dust clouds. Using a combination of satellite, in-situ and reanalysis datasets, we identify the atmospheric triggers for the occurrence of these severe dust storms, characterize their three-dimensional structure and evaluate the dust radiative impact. The dust emission was promoted by density currents emanating from deep convection over Turkey. The convective systems were triggered by cut-off lows from mid-latitudes fed by moisture from African atmospheric rivers. The dust clouds were transported southward at 4 km in altitudes but sunk to ground levels when they reached the southern Arabian Peninsula due to strong subsidence. At a station in coastal UAE, the dust caused a 350 W m-2 drop in the surface downward shortwave flux and a 70W m-2 increase in the longwave one during the dust episodes. This contributed to a 9ºC increase in nighttime temperatures which exacerbated the effects of the heat for the population. The newly highlighted mechanism for dust emission in the Middle East, in which a cut-off low interacts with an atmospheric river, as well as direct observations of the dust impact on the radiative budget can contribute to reducing associated uncertainties in climate models.
... The occurrence of localscale circulations (land-sea-breeze), calm conditions at night and a stable nocturnal boundary layer are the main ingredients for the development of fog in the region. Besides the sea-breeze, atmospheric rivers, which are narrow and elongated regions of high water vapour content emanating from lower latitudes (e.g., Francis et al., 2020bFrancis et al., , 2021b, can also act as the moisture source for fog events (Verma et al., 2022). Fog can occur year-round in the UAE although it is more common between October and February. ...
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In this study, the link between the occurrence of consecutive fog days in the United Arab Emirates (UAE) and the associated synoptic-scale circulation is investigated. This is particularly pertinent, as such a link may provide an important predictive skill for a phenomenon that has a pronounced impact on road and air traffic but is still poorly simulated by numerical models. A cluster analysis of all consecutive fog days from January 1983 to December 2021 indicated that the positive phase of the East Atlantic/Western Russia teleconnection pattern, Eastern Pacific La Nina events, and the circumglobal wavenumber 5 pattern promote the occurrence of multiple fog days in the UAE. The fog’s radiative impacts, as estimated from satellite data, revealed that the fog in the UAE is more optically thick than that observed elsewhere. A trend analysis over the period 1983 to 2021 revealed that consecutive fog events have become more frequent and longer-lasting but less intense (i.e., associated with higher values of visibility). The fog’s spatial extent over the UAE at its mature stage has also decreased over time. An analysis of the trends in the surface and top of atmosphere (TOA) radiative fluxes indicated that over the period 2000-2021, the fog clouds have likely become less reflective, with a statistically significant decrease in the surface downward shortwave and TOA upward longwave radiation fluxes. Long-term measurements of fog microphysics in the region are needed to better understand the variability in the properties of the fog cloud droplets.
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Sea ice production within polynyas, an outcome of the atmosphere‐ice‐ocean interaction, is a major source of dense water and hence key to the global overturning circulation, but is poorly quantified over open‐ocean polynyas. Using the two recent extensive open‐ocean polynyas within the wider Maud Rise region of the Weddell Sea in 2016 and 2017, we here explore the sea ice energy budget and estimate their sea ice production based on satellite retrievals, in‐situ hydrographic observations and the Japanese 55‐year Reanalysis. We find that the oceanic heat flux amounts to 36.1 and 30.7 W m⁻² within the 2016 and 2017 polynyas, respectively. Especially the 2017 open‐ocean polynya produced nearly 200 km³ of new sea ice, which is comparable to the production in the largest Antarctic coastal polynyas. Finally, we determine that ice production is highly correlated with and sensitive to skin temperature and wind speed, which affect the turbulent fluxes. It is also strongly sensitive to uncertainties in the sea ice concentration and 1,000 hPa temperature, which all urgently need to be better monitored at high latitudes. Lastly, more process‐oriented campaigns are required to further elucidate the role of open‐ocean polynya on the local and global ocean circulations.
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Plain Language Summary Antarctica is the driest continent on Earth. The rare snowfall events on the cold Antarctic desert usually come from so‐called atmospheric rivers (ARs), the same type of systems that bring winter precipitation along the western coasts of the American continents, such as the Pacific Northwest in the United States. Here we estimate how much precipitation on Antarctica is associated with these ARs. Even though they only occur a few days per year, ARs explain around 13% of the total Antarctic precipitation. Even more importantly, we find a strong link between year‐to‐year variations in Antarctic precipitation and ARs, underlining the importance of these systems for understanding current and future changes of the Antarctic ice sheet contribution to global sea level rise.
Interannual variability of the winter AR activities over the Northern hemisphere is investigated. The leading modes of AR variability over the North Pacific and North Atlantic are first identified and characterized. Over the Pacific, the first mode is characterized by a dipole structure with enhanced AR frequency along the AR peak region at about 30° N and reduced AR frequency further north. The second mode exhibits a tri-pole structure with a narrow band of positive AR anomalies at about 30° N and sandwiched by negative anomalies. Over the Atlantic, the first mode exhibits an equatorward shift of the ARs with positive anomalies and negative anomalies located on the equatorward and poleward side of the AR peak region at about 40° N , respectively. The second mode is associated with the strengthening and eastward extension of the AR peak region which is sandwiched by negative anomalies. A large ensemble of atmospheric global climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6), which shows high skills in simulating these modes, is then used to quantify the roles of sea surface temperature (SST) forcing versus internal atmospheric variability in driving the formation of these modes. Results show that SST forcing explains about half of the variance for the Pacific leading modes, while that number drops to about a quarter for the Atlantic leading modes, suggesting higher predictability for the Pacific AR variability. Additional ensemble driven only by observed tropical SST is further utilized to demonstrate the more important role that tropical SST plays in controlling the Pacific AR variability while both tropical and extratropical SST exert comparable influences on the Atlantic AR variability.
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A 500‐year‐long high‐resolution Community Earth System Model simulation under preindustrial radiative forcing produces Maud Rise Polynyas (MRPs) and Weddell Sea Polynya (WSP) events intermittently from decadal (MRP) to multidecadal (WSP) timescales. This periodicity is correlated with the variation of a regional Southern Annular Mode (SAM) index (evaluated regionally instead of circum‐Antarctic) with corresponding variations of precipitation and winds over the Weddell Sea. A negative index causes the upper‐ocean salinity to increase over multiple decades. Ultimately, brine rejection during seasonal sea‐ice formation superimposed on the multidecadal increase raises the upper‐ocean salinity beyond the tipping point for triggering deep convection that leads to polynyas. The initiation of polynya events is thus controlled by surface properties while the location of initiation is determined by bathymetric features. The persistent Taylor column effect, which is well represented by the high‐resolution model topography of the Maud Rise seamount, preconditions this region for MRP initiation. Therefore, MRPs form more frequently than WSPs in the simulation. When the upper‐ocean salinity is high, deep convection in the MRP region tends to be stronger, in which case MRPs are also more likely to grow into WSPs. Once WSPs emerge, they affect the regional atmospheric circulation and associated variables. We propose a regional coupled ocean‐atmosphere mechanism to explain both the periodic emergence of polynyas and the periodic variation of the regional SAM index. Although the temperature and salinity of Weddell Deep Water show upward trends due to model drift, these density‐compensating changes do not affect the frequency of polynya formation.
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Within the Copernicus Climate Change Service (C3S), ECMWF is producing the ERA5 reanalysis, which embodies a detailed record of the global atmosphere, land surface and ocean waves from 1950 onwards once completed. This new reanalysis replaces the ERA‐Interim reanalysis that was started in 2006 (spanning 1979 onwards). ERA5 is based on the Integrated Forecasting System (IFS) Cy41r2 which was operational in 2016. ERA5 thus benefits from a decade of developments in model physics, core dynamics and data assimilation. In addition to a significantly enhanced horizontal resolution of 31 km, compared to 80 km for ERA‐Interim, ERA5 has hourly output throughout, and an uncertainty estimate from an ensemble (3‐hourly at half the horizontal resolution). This paper describes the general setup of ERA5, as well as a basic evaluation of characteristics and performance. Focus is on the dataset from 1979 onwards that is currently publicly available. Re‐forecasts from ERA5 analyses show a gain of up to one day in skill with respect to ERA‐Interim. Comparison with radiosonde and PILOT data prior to assimilation shows an improved fit for temperature, wind and humidity in the troposphere, but not the stratosphere. A comparison with independent buoy data shows a much improved fit for ocean wave height. The uncertainty estimate reflects the evolution of the observing systems used in ERA5. The enhanced temporal and spatial resolution allows for a detailed evolution of weather systems. For precipitation, global‐mean correlation with monthly‐mean GPCP data is increased from 67% to 77%. In general low‐frequency variability is found to be well‐represented and from 10 hPa downwards general patterns of anomalies in temperature match those from the ERA‐Interim, MERRA‐2 and JRA‐55 reanalyses. This article is protected by copyright. All rights reserved.
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Atmospheric rivers (ARs) are an important component of the hydrological cycle linking moisture sources in lower latitudes to the Antarctic surface mass balance. We investigate AR signatures in the atmospheric vertical profiles at the Dronning Maud Land coast, East Antarctica, using regular and extra radiosonde measurements conducted during the Year of Polar Prediction Special Observing Period November 2018 to February 2019. Prominent AR events affecting the locations of Neumayer and Syowa cause a strong increase in specific humidity extending through the mid-troposphere and a strong low-level jet (LLJ). At Neumayer, the peak in the moisture inversion (up to 4 g kg−1) is observed between 800 and 900 hPa, while the LLJ (up to 32 m s−1) is concentrated below 900 hPa. At Syowa the increase in humidity is less pronounced and peaks near the surface, while there is a substantial increase in wind speed (up to 40 m s−1) between 825 and 925 hPa. Moisture transport (MT) within the vertical profile during the ARs attains a maximum of 100 g kg−1 m s−1 at both locations, and is captured by both ERA-Interim and ERA5 reanalysis data at Neumayer, but is strongly underestimated at Syowa. Composites of the enhanced MT events during 2009-19 show that these events represent an extreme state of the lower-tropospheric profile compared to its median values with respect to temperature, humidity, wind speed and, consequently, MT. High temporal- and vertical-resolution radiosonde observations are important for understanding the contribution of these rare events to the total MT towards Antarctica and improving their representation in models.
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Plain Language Summary During the Antarctic winter, small pancake ice floes, which form rapidly in wavy conditions, dominate new ice growth and create a dynamic environment. However, there are only a handful of local observations of pancake ice drift, particularly during the intense polar cyclones that frequently reshape the ice cover. More observations are needed to generate better understanding and modeling of pancake ice response to winds, waves, and currents. We describe a set of pancake ice drift and wave‐in‐ice measurements over 9 days in which four polar cyclones affected the region, from buoys deployed on pancake floes 100 km from the ice edge. We also develop an ice drift model.The data show how the cyclones affect ice drift and contain the fastest reported ice speed in the Southern Ocean (0.75 m s⁻¹). The instantaneous drift speed closely correlates with the wind speed, and the ice also displays a 13 hr period rotational motion that we reproduce in the model with forcing from ocean currents. We show that pancake ice is in free drift, despite sea ice covering the entire ocean surface in the measurement region and that the model predicts drift accurately over 2 days with calibration of only two parameters.
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Atmospheric rivers (ARs) are characterized by intense moisture transport, which, on landfall, produce precipitation which can be both beneficial and destructive. ARs in California, for example, are known to have ended drought conditions but also to have caused substantial socio-economic damage from landslides and flooding linked to extreme precipitation. Understanding how AR characteristics will respond to a warming climate is, therefore, vital to the resilience of communities affected by them, such as the western USA, Europe, East Asia and South Africa. In this Review, we use a theoretical framework to synthesize understanding of the dynamic and thermodynamic responses of ARs to anthropogenic warming and connect them to observed and projected changes and impacts revealed by observations and complex models. Evidence suggests that increased atmospheric moisture (governed by Clausius–Clapeyron scaling) will enhance the intensity of AR-related precipitation — and related hydrological extremes — but with changes that are ultimately linked to topographic barriers. However, due to their dependency on both weather and climate-scale processes, which themselves are often poorly constrained, projections are uncertain. To build confidence and improve resilience, future work must focus efforts on characterizing the multiscale development of ARs and in obtaining observations from understudied regions, including the West Pacific, South Pacific and South Atlantic.
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Recent major melting events in West Antarctica have raised concerns about a potential hydrofracturing and ice shelf instability. These events often share common forcings of surface melt-like anomalous radiative fluxes, turbulent heat fluxes and föhn winds. Using an atmospheric river detection algorithm developed for Antarctica together with surface melt datasets, we produced a climatology of atmospheric river-related surface melting around Antarctica and show that atmospheric rivers are associated with a large percentage of these surface melt events. Despite their rarity (around 12 events per year in West Antarctica), atmospheric rivers are associated with around 40% of the total summer meltwater generated across the Ross Ice Shelf to nearly 100% in the higher elevation Marie Byrd Land and 40–80% of the total winter meltwater generated on the Wilkins, Bach, George IV and Larsen B and C ice shelves. These events were all related to high-pressure blocking ridges that directed anomalous poleward moisture transport towards the continent. Major melt events in the West Antarctic Ice Sheet only occur about a couple times per decade, but a 1–2 °C warming and continued increase in atmospheric river activity could increase the melt frequency with consequences for ice shelf stability.
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Over the 40‐year satellite record, there has been a slight increasing trend in total annual mean Antarctic sea ice extent of approximately 1.5% per decade that is made up of the sum of significantly larger opposing regional trends. However, record increases in total Antarctic sea ice extent were observed during 2012‑2014, followed by record lows (for the satellite era) through 2018. There is still no consensus on the main drivers of these trends, but it is generally believed that the atmosphere plays a significant role and that seasonal timescales and regional scale processes are important. Despite considerable yearly and regional variability, the mean seasonal cycle of growth and melt of Antarctic sea ice is strikingly consistent, with a slow growth but fast melt season. If we are to project trends in Antarctic sea ice and understand changes on longer timescales, we need to understand the mechanisms related to the seasonal cycle separately from those that drive variability. Twice‐yearly changes in the position and intensity of the zonal winds circling Antarctica are thought to drive the system by working with or against the evolving sea ice edge to slow the autumn advance and hasten the spring melt. Open water regions, created by divergence associated with the zonal winds, amplify the spring melt through increased warming of the upper ocean. Climate models fail to accurately reproduce mean Antarctic sea ice extent and overestimate its year‐to‐year variability, but they tend to capture the pattern and timing of the Antarctic seasonal cycle.
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Significance A newly completed 40-y record of satellite observations is used to quantify changes in Antarctic sea ice coverage since the late 1970s. Sea ice spreads over vast areas and has major impacts on the rest of the climate system, reflecting solar radiation and restricting ocean/atmosphere exchanges. The satellite record reveals that a gradual, decades-long overall increase in Antarctic sea ice extents reversed in 2014, with subsequent rates of decrease in 2014–2017 far exceeding the more widely publicized decay rates experienced in the Arctic. The rapid decreases reduced the Antarctic sea ice extents to their lowest values in the 40-y record, both on a yearly average basis (record low in 2017) and on a monthly basis (record low in February 2017).
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A large retreat of sea-ice in the ‘stormy’ Atlantic Sector of the Arctic Ocean has become evident through a series of record minima for the winter maximum sea-ice extent since 2015. Results from the Norwegian young sea ICE (N-ICE2015) expedition, a five-month-long (Jan-Jun) drifting ice station in first and second year pack-ice north of Svalbard, showcase how sea-ice in this region is frequently affected by passing winter storms. Here we synthesise the interdisciplinary N-ICE2015 dataset, including independent observations of the atmosphere, snow, sea-ice, ocean, and ecosystem. We build upon recent results and illustrate the different mechanisms through which winter storms impact the coupled Arctic sea-ice system. These short-lived and episodic synoptic-scale events transport pulses of heat and moisture into the Arctic, which temporarily reduce radiative cooling and henceforth ice growth. Cumulative snowfall from each sequential storm deepens the snow pack and insulates the sea-ice, further inhibiting ice growth throughout the remaining winter season. Strong winds fracture the ice cover, enhance ocean-ice-atmosphere heat fluxes, and make the ice more susceptible to lateral melt. In conclusion, the legacy of Arctic winter storms for sea-ice and the ice-associated ecosystem in the Atlantic Sector lasts far beyond their short lifespan.
Mass loss from the Greenland Ice Sheet (GrIS) has accelerated over the past two decades, coincident with rapid Arctic warming and increasing moisture transport over Greenland by atmospheric rivers (ARs). Summer ARs affecting western Greenland trigger GrIS melt events, but the physical mechanisms through which ARs induce melt are not well understood. This study elucidates the coupled surface-atmosphere processes by which ARs force GrIS melt through analysis of the surface energy balance (SEB), cloud properties, and local- to synoptic-scale atmospheric conditions during strong summer AR events affecting western Greenland. ARs are identified in MERRA-2 reanalysis (1980–2017) and classified by integrated water vapor transport (IVT) intensity. SEB, cloud, and atmospheric data from regional climate model, observational, reanalysis, and satellite-based datasets are used to analyze melt-inducing physical processes during strong, > 90th percentile “AR 90+ ” events. Near AR “landfall”, AR 90+ days feature increased cloud cover that reduces net shortwave radiation and increases net longwave radiation. As these oppositely-signed radiative anomalies partly cancel during AR 90+ events, increased melt energy in the ablation zone is primarily provided by turbulent heat fluxes, particularly sensible heat flux. These turbulent heat fluxes are driven by enhanced barrier winds generated by a stronger synoptic pressure gradient combined with an enhanced local temperature contrast between cool over-ice air and the anomalously warm surrounding atmosphere. During AR 90+ events in northwest Greenland, anomalous melt is forced remotely through a clear-sky foehn regime produced by down-slope flow in eastern Greenland.
A spectacular resurgence of interest in the topic of ocean wave/sea ice interactions has unfolded over the last two decades, fueled primarily by the deleterious ramifications of global climate change on the polar seas. The Arctic is particularly affected, with a widespread reduction of the extent, thickness, and compactness of its sea ice during the summer, creating an ice cover that is analogous to that in the Southern Ocean surrounding Antarctica. With the additional fetches over which waves can form and mature within more open ice fields, there has also been a documented global uptrend of winds and wave height, which is most severe at high latitudes. Bigger ocean waves affect the way sea ice forms, contribute to how the ice edge moves, penetrate farther into the sea ice, have more destructive power to break up the ice and to change the distribution of floe sizes because the ice is weaker, and assist in lateral melting. These feedbacks collectively identify a parametrization currently absent from Earth system models, as well as shortcomings in wave forecasts arising from limited understanding of the impact of sea ice on ocean waves. Expected final online publication date for the Annual Review of Fluid Mechanics Volume 52 is January 7, 2020. Please see for revised estimates.