On 25 February 2022 Antarctic sea ice extent dropped to a satellite‐era record low level of 1.92 × 106 km2, 0.92 × 106 km2 below the long‐term mean. The area of sea ice was also at a record low level ...of 1.24 × 106 km2. Although no individual sector was at a record low, at the minimum there were negative sea ice anomalies in all sectors of the Southern Ocean, with the largest in the Ross (contributing 46%) and Weddell Seas (26%). The Amundsen Sea Low had a record low depth in October/November 2021, with a series of very deep depressions giving strong offshore winds. These accelerated ice loss during the melt season, creating a 1.00 × 106 km2 coastal polynya in the Ross Sea. In the northern Weddell Sea, westerly winds of record strength led to ice export from the region.
Plain Language Summary
Sea ice is a critical part of the Antarctic climate system that is an important breeding habitat for seals and other marine biota, as well as playing an important part in the global ocean circulation. Unlike the Arctic, where sea ice extent has decreased over recent decades, sea ice around the Antarctic actually increased in extent for a large part of the record starting in the late 1970s. However, on 25 February 2022 the extent dropped to a record low level of 1.924 million square kilometers, 1 million square kilometers below the long‐term average. The origins of the sea ice loss can be traced back to extremely deep storms in the Ross Sea in October and November 2021, and the very strong southerly winds that the storms had on their western flank. These moved sea ice away from the Antarctic coast, exposing the ocean and allowing solar heating to warm the ocean and give further sea ice melt. An additional factor was the very strong westerly winds north of the Weddell Sea that moved sea ice out of this basin and toward the east.
Key Points
Antarctic sea ice extent dropped to a record low level of 1.92 × 106 km2 on 25 February 2022
There were negative sea ice anomalies in all sectors of the Southern Ocean, with the largest in the Ross and Weddell Seas
Deep storms in October/November 2021 led to low sea ice concentration and a large coastal polynya that accelerated sea ice loss
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The presence of the Kerguelen Plateau and surrounding bathymetric features has a strong influence on the persistently eastward flowing Antarctic Circumpolar Current （ACC）, resulting in enhancement of ...surface chlorophyll-a （Chl-α） in the downstream section of the plateau along the polar front （PF）. The phenomenon is reported in this paper as the island mass effect （IME）. Analysis of climatological Chl-a datasets from Aqua- Moderate Resolution Imaging Spectroradiometer （Aqua- MODIS） and Sea-viewing Wide Field-of-view Sensor （SeaWiFS） shows distinct bloomy plumes （Chl-α 〉 0.5 mg/m3） during austral spring-summer spreading as far as -1800 km offshore up to 98°E along the downstream of the north Kerguelen Plateau （NKP）. Similar IME phenomena is apparent over the south Kerguelen Plateau （SKP） with the phytoplankton bloom extending up to 96.7°E, along the southern boundary of ACC. The IME phenomena are pronounced only during austral spring-summer period with the availability of light and sedimentary source of iron from shallow plateau to sea surface that fertilizes the mixed layer. The NKP bloom peaks with a maximum areal extent of 1.315 million km2 during December, and the SKP bloom peaks during January with a time lag of one month. The blooms exist for at least 4 months of a year and are significant both as the base of regional food web and for regulating the biogeochemical cycle in the Southern Ocean. Even though the surface water above the Kerguelen Plateau is rich in Chl-a, an exception of an oligotrophic condition dominated between NKP and SKP due to apparent intrusion of iron limited low phytoplankton regime waters from the Enderby basin through the north- eastward Fawn Trough Current.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
In Austral summer 2016/2017, the sea ice extent (SIE) in the Weddell Sea dropped to a near‐record value in the satellite era (1.88 × 106 km2), a large negative seasonal anomaly that persisted in an ...unprecedented fashion for the following three summers. Various atmospheric and oceanic factors played a part in the change. Ice loss started in September 2016 when the northern Weddell Sea experienced westerly winds of record strength, advecting multiyear sea ice from the region. In late 2016, a polynya over Maud Rise contributed to low SIE over the eastern Weddell Sea. With extensive areas of open water early in the summer, upper ocean temperatures increased by ~0.5°C, with the anomalies persisting in subsequent years. The reappearance of the Maud Rise polynya in 2017, high ocean temperatures, and storms of record depth kept the summer SIE low.
Plain Language Summary
Sea ice is an extremely important part of the Antarctic environment, providing an essential habitat for seals, seabirds, and Antarctic krill. It also caps the ocean, strongly influencing the transfer of heat from the relatively warm ocean into the frigid Antarctic atmosphere. In addition, it affects the ocean circulation through the release of salt when ice grows. In 2016/2017, the extent of summer sea ice in the Weddell Sea, Antarctica, dropped to a near‐record level in the satellite era, which starts in late 1978. The ice loss was in part caused by the reappearance of the Maud Rise polynya, an area of open water within the main area of pack ice. The ice‐free conditions allowed more energy from the Sun to be absorbed by the ocean, with the higher ocean temperatures persisting in subsequent years and slowing the formation of new ice. In parallel, westerly winds of record strength carried sea ice out of the region, so that the summer sea ice extent in the Weddell Sea is now 1,000,000 km2 less than in 2013/2014.
Key Points
Summer sea ice in the Weddell Sea, Antarctica decreased to a near‐record low extent in 2016/2017 and has subsequently remained low
Westerly winds of record strength and the reappearance of the Maud Rise polynya were significant factors in the change in ice conditions
The Weddell Sea summer ice extent is now 1,000,000 km2 less than in 2013/2014
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The 2023 Antarctic sea ice extent (SIE) maximum on 7 September was the lowest annual maximum in the satellite era (16.98 × 106 km2), with the largest contributions to the anomaly coming from the Ross ...(37.7%, −0.57 × 106 km2) and Weddell (32.9%, −0.49 × 106 km2) Seas. The SIE was low due to anomalously warm (>0.3°C) upper‐ocean temperatures combined with anomalously strong northerly winds impeding the ice advance during the fall and winter. Northerly winds of >12 ms−1 in the Weddell Sea occurred because of negative pressure anomalies over the Antarctic Peninsula, while those in the Ross Sea were associated with extreme blocking episodes off the Ross Ice Shelf. The Ross Sea experienced an unprecedented SIE decrease of −1.08 × 103 km2 d−1 from 1 June till the annual maximum. The passage of quasi‐stationary and explosive polar cyclones contributed to periods of southward ice‐edge shift in both sectors.
Plain Language Summary
Sea ice provides a vital habitat for life in the Southern Ocean, and plays an important role in the ocean circulation, the dynamics of the Earth's climate, the biogeochemical cycle, and the regional ecosystem. Climatologically, Antarctic sea ice expands northwards from the continent each autumn and winter. However, in 2023 an unprecedented slow ice expansion occurred in the Southern Ocean ahead of the annual maximum on 7 September of 16.98 × 106 km2, which was 1.46 × 106 km2 below the long‐term average. In fact, the area covered by ice remained at a record low level every day from 21 April 2023 until 11 November 2023. Our findings suggest that an impact of upper‐ocean warming and changes in winds, combined with heat and moisture fluxes, extreme winds and high ocean waves associated with polar cyclones (storms), contributed to these record low ice conditions. In particular, cyclones caused episodes of exceptional slow ice expansion or even retreat, leading to negative ice anomalies. For instance, the ice‐edge in the Weddell Sea was moved southwards quickly in a few days (up to 256 km southward) with an ice area loss of ∼2.3 × 105 km2, equivalent to the size of United Kingdom.
Key Points
The 2023 Antarctic sea ice extent maximum on 7 September (16.98 × 106 km2) was the lowest annual maximum in the satellite era
Anomalous upper‐ocean warming and strong northerly winds contributed to impeding the ice expansion in the Ross and Weddell Seas
Quasi‐stationary and explosive polar cyclones contributed to periods of southward ice‐edge shift in both sectors
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The appearance of phytoplankton blooms within sea ice cover is of high importance considering the upper ocean primary production that
controls the biological pump, with implications for atmospheric ...CO2
and global climate. Satellite-derived chlorophyll a concentration showed
unprecedented phytoplankton blooms in the Maud Rise polynya, Southern Ocean,
with chlorophyll a reaching up to 4.67 mg m−3 during 2017.
Multi-satellite data indicated that the bloom appeared for the first time since
the entire mission records started in 1978. An Argo float located in the
polynya edge provided evidence of bloom conditions in austral spring 2017
(chlorophyll a up to 5.47 mg m−3) compared to the preceding years with
prevailing low chlorophyll a. The occurrence of bloom was associated with the
supply of nutrients into the upper ocean through Ekman upwelling (driven
by wind stress curl and cyclonic ocean eddies) and improved light conditions of
up to 61.9 einstein m−2 d−1. The net primary production from the
Aqua Moderate Resolution Imaging Spectroradiometer chlorophyll-based
algorithm showed that the Maud Rise polynya was as productive as the
Antarctic coastal polynyas, with carbon fixation rates reaching up to
415.08 mg C m−2 d−1. The study demonstrates how the
phytoplankton in the Southern Ocean (specifically over the shallow
bathymetric region) would likely respond in the future under warming
climate conditions and continued melting of Antarctic sea ice.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Abstract
Severe rainfall during the recent northeast monsoon season (October, November and December) in 2021 caused unexpected disasters (triggered flooding and landslides) in the pilgrim city of ...Tirupati and the metropolitan city of Chennai over the peninsular of India (PI). The remote and regional influence of several ocean‐atmospheric climate indices on northeast monsoon rainfall (NEMR) during the OND season is investigated to understand the contributory mechanisms of extreme rainfall events over the PI during 2021. The interannual variability of NEMR and its relationship with the OND sea surface temperature (SST) anomaly over the Niño3.4 and Indian Ocean Dipole (IOD) regions are considered. In addition, SST anomalies over the Bay of Bengal (BoB), such as east BoB (EBoB), east equatorial Indian Ocean (EEIO) and their gradients (GSST) connections with NEMR, are also examined. The impact of Siberian High (SH), Western Pacific High (WPH), Arabian Sea Surface Pressure (ASSP), easterly waves (EW) and land surface temperature contrast (LSTC) is also analysed and incorporated to find a specific linkage with NEM 2021. The results show that excess NEMR during OND 2021 is suggestively associated with favourable large‐scale conditions like negative IOD and the La Niña phase over the Indo‐Pacific region. In November 2021, two deep depressions made landfall over Tamil Nadu (within the latitude of 11°–13°N) and over adjoining areas, which caused copious amounts of rainfall (maximum ~90 mm·day
−1
), leading to flooding conditions over Tirupati and Chennai. During the NEM season, abundant warming of BoB and weak vertical wind shear, along with negative anomalies of ASSP, modulated the cyclonic activities that enhanced the sufficient moisture transports from BoB to the PI. The MSLP anomalies over the ASSP region depict a significant correlation (−0.47) with the interannual variability of NEM as compared to SH and WPH during the OND season of 2021.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In the backdrop of global warming, Antarctic sea-ice variability showed an overall expansion with the regional heterogeneity of increasing and decreasing patterns. Analysis of satellite derived ...sea-ice extent, during 1979 to 2015, in the Indian Ocean sector of Antarctica (IOA) revealed expansion of 2.4±1.2% decade-1. We find strengthening of westerly wind during the austral summer (between 50°S to 62°S) facilitated northward advection of a cool and fresh layer. Also, the strong westerly wind cools the upper ocean due to net heat loss from the ocean surface. The combined effect of northward advection of cold fresh layer and net heat loss from the surface, favours sea-ice expansion in the subsequent seasons, in the IOA region, north of 62°S. However, sea-ice retreat was observed near the Kerguelen Plateau, due to upper ocean warming, and hence a non-annular pattern of sea-ice extent in the IOA was observed.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Antarctic waters are known to be optically unique and the standard empirical ocean colour algorithms applied to these waters may not address the regional bio-optical characteristics. This article ...sheds light on the performance of current empirical algorithms and a regionally optimized algorithm (ROA) for the retrieval of chlorophyll-a (chl-a) concentration from Aqua-Moderate Resolution Imaging Spectroradiometer (Aqua-MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) in the Indian Ocean Sector of Southern Ocean (IOSO). Analysis indicated that empirical algorithms used for the retrieval of chl-a concentration from Aqua-MODIS and SeaWiFS underestimate by a factor varying from 2 to 2.9, resulting in underestimation when in situ chl-a exceeds about 0.3 mg m
−3
. To explain these uncertainties, a study was carried out to understand the effect of phytoplankton pigment composition and pigment packaging on remote-sensing reflectance (R
rs,λ
), from the analysis of phytoplankton-specific absorption coefficient (a
ph,
*
λ
). The spatial variation of phytoplankton groups analysed using diagnostics pigments (DP) indicated shifting of the phytoplankton community structure from offshore to coastal Antarctic, with a significant increasing trend for diatoms and a decreasing trend for haptophytes population. The diatom-dominated population exhibits lower a
ph,
*
λ
in the 405-510 nm region (with relative flattening in 443-489 nm) compared with the a
ph,
*
λ
spectra of the haptophytes-dominated population that peaks near 443 nm. The flattening of a
ph,
*
λ
spectra for the diatom-dominated population was attributed to its larger cell size, which leads to pigment packaging (intracellular shading) and in turn results in higher R
rs,λ
. The relationship between pigment composition (normalized by chl-a) and blue:green absorption band ratios (a
ph,
*
443
:a
ph,
*
555
and a
ph,
*
489
:a
ph,
*
555
) corresponding to the Aqua-MODIS and SeaWiFS bands showed in-phase associations with most of the pigments such as 19ʹ-hexanoyloxyfucoxanthin, 19ʹ-butanoyloxyfucoxanthin, peridinin, and zeaxanthin. In contrast, the out-of-phase association observed between the blue:green absorption ratios and fucoxanthin indicated apparent deviations from the general pigment retrieval algorithms, which assumes that blue:green ratios vary in a systematic form with chl-a. The out-of-phase correspondence suggests that the increasing trend of fucoxanthin pigments towards the Antarctic coast was associated with the decreasing trend of blue:green absorption ratios and in turn results in higher R
rs,λ
. Therefore, an increase in R
rs,λ
leads to underestimation of chl-a from Aqua-MODIS and SeaWiFS in the IOSO region.
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BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
In the background of global warming, the necessity for finding a new predictor of Indian summer monsoon rainfall (ISMR) is realised recently as the relationship between the ISMR and the predictive ...physical parameters keeps changing both in space and time. We developed a linkage between the satellite-derived sea ice over the Southern Ocean and ISMR through the interaction of physical processes occurring between the cryosphere, hydrosphere, and atmospheric environment. The statistical and possible physical linkage were explored using consistent and reliable 38-year time series observations. Lead-lag cross correlation indicated a significant relationship between Southern Ocean sea ice (SOSI) extent and ISMR over the Bellingshausen–Amundsen Sea (BAS). The most significant relationship was obtained in austral autumn (r = −0.5, p < 0.05) with no time lag. We analysed atmospheric circulation pattern over the BAS and Ross Sea corresponding to extreme rainfall events. Excess (deficit) rainfall years showed anomalous cyclonic (anticyclonic) wind patterns that led to anomalous positive (negative) air temperature that facilitated anomalous negative (positive) sea ice conditions in BAS. Hence, the excess rainfall (deficit) events were observed corresponding to a large negative (positive) anomalous sea ice condition in the BAS. This linkage likely occurred through El Niño–Southern Oscillation (ENSO) associated sea surface temperature (SST) variability. To find out the dominant mode of variability and common frequency distribution in the SOSI extent, Niño3.4 SST and ISMR, we have performed the wavelet analysis for exploring the coherence between the respective variables. The spectral analysis showed that both the SOSI extent and ISMR have dominant oscillations in 2–8 years frequency bands similar to the ENSO variability. This observed relationship can be used as the input for future works for physically linking the BAS sea ice condition with the ISMR through the dynamic and thermodynamic ocean-atmospheric components.
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•Developed linkage between Antarctic sea ice and Indian monsoon rainfall extremes.•Most significant relationship was obtained over the BAS in austral autumn.•Excess (deficit) rainfall corresponding to negative (positive) anomalous BAS SIC.•This linkage likely occurred through ENSO related SST variability.•SIE, ISMR, and ENSO have dominant oscillations in 2–8 years frequency bands.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The seasonality of carbon dioxide partial pressure （pC02）, air-sea CO2 fluxes and associated environ- mental parameters were investigated in the Antarctic coastal waters. The in-situ survey was ...carried out from the austral summer till the onset of winter （January 2012, February 2010 and March 2009） in the Enderby Basin. Rapid decrease in pCO2 was evident under the sea-ice cover in January, when both water column and sea-ice algal activity resulted in the removal of nutrients and dissolved inorganic carbon （DIC） and increase in pH. The major highlight of this study is the shift in the dominant biogeochemica/ factors from summer to early winter. Nutrient limitation （low Si/N）, sea-ice cover, low photosynthetically active radiation （PAR）, deep mixed layer and high upwelling velocity contributed towards higher pCO2 during March （early winter）. CO2 fluxes suggest that the Enderby Basin acts as a strong CO2 sink during January （-81 mmol m-2 d 1）, however it acts as a weak sink of CO2 with -2.4 and -1.7 mmol m 2 d-1 during February and March, respectively. The present work, concludes that sea ice plays a dual role towards climate change, by decreasing sea surface pCO2 in summer and enhancing in early winter. Our observations emphasize the need to address seasonal sea-ice driven CO2 flux dynamics in assessing Antarctic contributions to the global oceanic CO2 budget.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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