Antarctic Sea Ice Variability in the Southern Ocean-Climate System Proceedings of a Workshop (2017) / Chapter Skim
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Antarctic Sea Ice Variability in the Southern Ocean-Climate System: Proceedings of a Workshop
Antarctic Sea Ice Variability in the Southern Ocean-Climate System: Proceedings of a Workshop
Pages 1-36
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Some participants highlighted how negative feedbacks between the ocean and sea ice keep Antarctic sea ice thin. Other studies suggest that there may be a modest decrease in Antarctic sea ice extent from anthropogenic warming, but that the trend is overwhelmed by a recent increase associated with high internal variability, resulting in the recent slight expansion of Antarctic sea ice extent.
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Many participants said that process-based understanding is critical for improving understanding of the mechanisms of Antarctic sea ice variability. Process studies also provide validation of global coupled models.
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Antarctic sea ice variability, • Evaluating evidence for key hypotheses about the processes controlling recent Antarctic sea ice variability and change, and • Highlighting knowledge gaps and important areas for new research that could clarify the mechanisms of past sea ice variability and help constrain projections of future Antarctic sea ice variability. The workshop was planned and moderated by a seven-member ad hoc committee appointed by the National Academies (the committee member biographical sketches are in Appendix F)
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The next section provides more detail on a range of proposed processes and mechanisms that control observed sea ice variability and change. The final section summarizes individual workshop participants' views on future needs and opportunities associated with observing, modeling, and understanding Antarctic sea ice variability.
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A OBSER RVATIONAL RECORD OF ANTARC L O CTIC SEA ICE VARIABILIT E TY Ba ackground Th Antarctic experienced modest increases in ma he d aximum and minimum se ice extent and ea t sea ice concentration since late 1978, when ro n outine satelliite measurem ments started until 2014 d, (Comiso et al., 2011; Meier et al., 2013; Parkinso and Cava M 2 on alieri, 2012; T Turner et al., 2015; Zwally et y al., 2002)
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. r Antarctic sea ice form c mation and persistence are sensitive to air and se surface te p a ea emperatures,, and considering that Anta arctic air tem mperatures an Southern Ocean SSTs have increa nd s ased slightly since 1950 (Arm mour and Bitz, 2015; Turn et al., 201 the trend in Antarctic sea ice are not necess ner 15)
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. The highly dynamic Antarctic sea ice edge region/MIZ is a particular challenge to observe with passive microwave sensors due to the predominance of new/thin ice and ice surface wetness (caused by wave-ice interaction)
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. Including these data changes the trend in overall net Antarctic sea ice extent to negative, as there is evidence that sea ice extent was larger in 1973-1975.
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Dr. Nathan Kurtz, NASA Goddard Space Flight Center, discussed the contribution of Antarctic sea ice thickness observations to understanding of sea ice volume change.
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In Situ Observations Dr. Michael Meredith, British Antarctic Survey, said that ocean observations are critical for understanding Antarctic sea ice, especially given that the ocean is an integral component of the high-latitude air-sea-ice system (see Box 2 which describes the role of the Southern Ocean in Antarctic sea ice)
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fr rom 2008 to 2014 in the wworld ocean. Much of the Soouthern Ocean sea ice zon has only 25 of target c ne 5% coverage (red The target total d)
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Dr. Stephen Ackley, The University of Texas at San Antonio, noted that proxy data on ice extent may be difficult to compare with passive microwave extent estimates based on the 15% contour.
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. • Coraline algae which can be found on underwate rocks thro C e, n o er oughout the world's ocea ans, because it is correlated with sea ice cover (presen c w c nce/absence of light; Ha e alfar et al., 20 013)
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REGIONAL AND SEASONAL VARIABILITY IN ANTARCTIC SEA ICE Observations of Antarctic sea ice show that there is significant temporal and spatial variability in the extent, which may be one reason why a simple explanation for the slight overall positive trend in the passive microwave satellite record of Antarctic sea ice extent has proven to 8 See http://www.ecmwf.int/en/research/climate-reanalysis/era-interim. 9 See http://www.ecmwf.int/en/research/climate-reanalysis/era-20c.
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n ohn niversity of C Colorado Bo oulder said th the oft-ciited hat weakly positive trend in circumpo sea ice extent is actually the result of large a strongly p d olar e and contrastin regional trends, some with magnit ng t e tudes equiva alent to those observed iin the Arctic. In e particular there have been strong negative monthly ext r, gly tent trends in the Bellings n shausen and Amundse Seas and strongly pos en sitive trends in the Ross S sector (S i Sea Stammerjohn et al., 2008 see n 8; Figure 6)
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Stamme A erjohn, the sppatial and temmporal varia ability points to the enorm mous analytical challenges associated with understa w anding Antar rctic sea ice variability, an to the nd importance of finer-g grained analyyses to understand sea ic and its fat in the Sou ce te uthern Ocean n.
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re zed ng a Fuurthermore, different reg gions in differ rent seasons contributed to the Anta s d arctic-wide 10 anomalie during the seasons of both sea ice advance an retreat, a in many cases there es b nd and were con ntrasting seaasonal anoma in the different regio For exam alies ons. mple, in 2014 conditions in 4, the weste Weddelll Sea contrib ern buted heavily to (austral)
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According to several participants, much of the research to date has focused on understanding the notable discrepancy between observations and models, which generally exhibit a decline in Antarctic sea ice extent over the last 30-50 years. If the models are improved to the point that they can reliably reproduce past sea ice conditions, then they could also be used to disentangle the roles of internal variability and human-caused drivers of changes in sea ice, as well as to project how sea ice might change in the future.
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, provided an introduction on the many different types of models and experiments that can be used to study the processes impacting Antarctic sea ice extent. One type is free-running coupled climate models.
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Dr. Lorenzo Polvani, Colu D P umbia Univerrsity, said it is important to recognize that the mo s odels simulate the sea ice extent seaso cycle qu well (Zun et al., 2013 Furthermo many t e onal uite nz 3)
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F Alexand Haumann, ETH Züric said that t differenc in observ and simu D der ch, the ces ved ulated Antarctic sea ice variability may be caused by different at c b y tmospheric c circulation paatterns and ocean sta ability (Haummann, 2011; Haumann et al., 2014)
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Although models are useful for exploring mechanisms, several participants said that models are not ready to be used for attribution. Separating Internal Variability from Anthropogenic Forcing During the discussion, many participants noted that the discussion of the drivers of Antarctic sea ice extent variability has been framed as either anthropogenic forcing or internal variability.
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Dr. Polvani also noted that observations of Antarctic sea ice extent, SSTs, and zonal winds appear to be largely uncorrelated with the increase in carbon dioxide and the decrease in ozone.
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Dr. Doug Martinson, Lamont-Doherty Earth Observatory, emphasized that the ocean–sea ice negative feedback keeps Antarctic sea ice thin (and thus seasonal)
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. a This is not a comprehensive list of all possible mechanisms of Antarctic sea ice variability.
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(2013) conducted a modeling study where they simulated processes associated with increased ice sheet melt in a coupled climate model to explore how those processes impacted sea ice extent.
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He concluded that an externally forced westerly wind impact on Antarctic sea ice coverage is plausible, but solid evidence has yet to emerge. Further progress has been hindered by the lack of long-term observations, the wide range of reanalysis estimates, the influence of internal variability, and the systematic bias across models.
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Influence of Stratospheric Ozone Depletion There was additional workshop discussion on the influence of stratospheric ozone on Antarctic sea ice variability through its connection to local winds. Since the 1970s, westerly winds have strengthened and moved poleward, exhibiting a positive SAM-like pattern.
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Ferreira believes that scientists should refrain from making statements about ozone depletion contributing or not to recent Antarctic sea ice trends. The implications of the two time scales goes beyond the SAM/ozone context: The complex ocean dynamics observed in the ozone case is likely to be triggered by other sources of SAM variability (internal variability, tropical teleconnection, and CO2 forcing)
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Tropical Atlantic Teleconnections Tropical Atlantic SSTs are another potential mechanism for Antarctic sea ice variability. According to Dr.
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Despite the fact that these processes are known to influence sea ice variability, Stammerjohn emphasized that the state of SAM and ENSO cannot explain variability in all seasons and regions and thus Antarctic sea ice variability more generally (Liu et al., 2004; Yuan and Li, 2008)
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Oceanographic Measurements Oceanographic measurements were also identified by many participants as important for understanding Antarctic sea ice variability. For example: • In situ ocean observations to assess vertical and horizontal distributions of heat and freshwater anomalies.
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, • Recalibration of the present data (post-2000) with scatterometer data against passive microwave data to better define sea ice extent (problems with 15% contour)
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Modeling Studies and Intercomparisons Workshop participants also discussed modeling studies and intercomparisons to advance understanding of Antarctic sea ice variability. Some participants said that there might be a value in doing experiments with a high-resolution atmospheric model, especially in the Ross Sea.
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The lack of a ready explanation of Antarctic sea ice variability and its drivers, as well as discrepancies between models and observations, contributes to a lingering public misconception
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Key observations, model impovements, and new research that were discussed during the workshop might clarify the processes and mechanisms that control observed sea ice variability and improve prediction of Antarctic sea ice.
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