The climate of the Western Antarctic Peninsula (WAP) is the most rapidly changing in the Southern Hemisphere, with a rise in atmospheric temperature of nearly 3°C since 1951 and associated cryospheric impacts. We demonstrate here, for the first time, that the adjacent ocean showed profound coincident changes, with surface summer temperatures rising more than 1°C and a strong upper-layer salinification. Initially driven by atmospheric warming and reduced rates of sea ice production, these changes constitute positive feedbacks that will contribute significantly to the continued climate change. Marine species in this region have extreme sensitivities to their environment, with population and species removal predicted in response to very small increases in ocean temperature. The WAP region is an important breeding and nursery ground for Antarctic krill, a key species in the Southern Ocean foodweb with a known dependence on the physical environment. The changes observed thus have significant ecological implications.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005GL024042

Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20th century

The Antarctic climate system varies on timescales from orbital, through millennial to sub-annual, and is closely coupled to other parts of the global climate system. We review these variations from the perspective of the geological and glaciological records and the recent historical period from which we have instrumental data (the last 50 years). We consider their consequences for the biosphere, and show how the latest numerical models project changes into the future, taking into account human actions in the form of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, we provide an essential Southern Hemisphere companion to the Arctic Climate Impact Assessment.

/pdf/antarctic-climate-change-and-the-environment-3mdc1ymkfh.pdf

Antarctic climate change and the environment

The Antarctic Peninsula is experiencing one of the fastest rates of regional climate change on Earth, resulting in the collapse of ice shelves, the retreat of glaciers and the exposure of new terrestrial habitat. In the nearby oceanic system, winter sea ice in the Bellingshausen and Amundsen seas has decreased in extent by 10% per decade, and shortened in seasonal duration. Surface waters have warmed by more than 1 K since the 1950s, and the Circumpolar Deep Water (CDW) of the Antarctic Circumpolar Current has also warmed. Of the changes observed in the marine ecosystem of the western Antarctic Peninsula (WAP) region to date, alterations in winter sea ice dynamics are the most likely to have had a direct impact on the marine fauna, principally through shifts in the extent and timing of habitat for ice-associated biota. Warming of seawater at depths below ca 100 m has yet to reach the levels that are biologically significant. Continued warming, or a change in the frequency of the flooding of CDW onto the WAP continental shelf may, however, induce sublethal effects that influence ecological interactions and hence food-web operation. The best evidence for recent changes in the ecosystem may come from organisms which record aspects of their population dynamics in their skeleton (such as molluscs or brachiopods) or where ecological interactions are preserved (such as in encrusting biota of hard substrata). In addition, a southwards shift of marine isotherms may induce a parallel migration of some taxa similar to that observed on land. The complexity of the Southern Ocean food web and the nonlinear nature of many interactions mean that predictions based on short-term studies of a small number of species are likely to be misleading.

/pdf/climate-change-and-the-marine-ecosystem-of-the-western-4suxhw91pw.pdf

Climate change and the marine ecosystem of the western Antarctic Peninsula

The Antarctic Peninsula region is undergoing rapid change: a warming in winter of almost 6 °C since 1950, the loss of six ice shelves, the retreat of 87% of the marine glaciers, and decreases in winter sea-ice duration. Concurrently, there is evidence of ecosystem change along the western Antarctic Peninsula (wAP). Since the life histories of most polar marine species are synchronized with the seasonal cycle of sea ice, we assess how the seasonal sea-ice cycle is changing in the wAP region. Four new metrics of seasonal sea-ice variability were extracted from spatial maps of satellite derived daily sea-ice concentration: (a) day of advance, (b) day of retreat, (c) the total number of sea-ice days (between day of advance and retreat), and (d) the percent time sea-ice was present (or sea-ice persistence). The spatio-temporal variability describes distinct on-to-offshore and alongshore differences in ice–ocean marine habitats, characterized overall by a longer sea-ice season in coastal regions (6.8–7.9 months) versus a shorter sea-ice season over the shelf (4.1–5.3 months), with on-to-offshore differences increasing south-to-north. Large perturbations in the seasonality of the marine habitat occur in association with ENSO and Southern Annular Mode (SAM) variability. The local atmospheric response to these climate modes is largely a strengthening of the meridional winds during spring-to-autumn, which in turn affect the timing of the sea-ice retreat and subsequent advance. These perturbations are embedded in overall trends towards a later sea-ice advance, earlier retreat and consequently shorter sea-ice season, the impacts of which are expected to affect ecosystem functionality in the wAP region. A suite of ocean–atmosphere–ice interactions are described that are consistent with the amplified warming in late autumn, early winter.

Sea ice in the western Antarctic Peninsula region : Spatio-temporal variability from ecological and climate change perspectives

The Scotia Sea ecosystem is a major component of the circumpolar Southern Ocean system, where productivity and predator demand for prey are high. The eastward-flowing Antarctic Circumpolar Current (ACC) and waters from the Weddell–Scotia Confluence dominate the physics of the Scotia Sea, leading to a strong advective flow, intense eddy activity and mixing. There is also strong seasonality, manifest by the changing irradiance and sea ice cover, which leads to shorter summers in the south. Summer phytoplankton blooms, which at times can cover an area of more than 0.5 million km2, probably result from the mixing of micronutrients into surface waters through the flow of the ACC over the Scotia Arc. This production is consumed by a range of species including Antarctic krill, which are the major prey item of large seabird and marine mammal populations. The flow of the ACC is steered north by the Scotia Arc, pushing polar water to lower latitudes, carrying with it krill during spring and summer, which subsidize food webs around South Georgia and the northern Scotia Arc. There is also marked interannual variability in winter sea ice distribution and sea surface temperatures that is linked to southern hemisphere-scale climate processes such as the El Nino–Southern Oscillation. This variation affects regional primary and secondary production and influences biogeochemical cycles. It also affects krill population dynamics and dispersal, which in turn impacts higher trophic level predator foraging, breeding performance and population dynamics. The ecosystem has also been highly perturbed as a result of harvesting over the last two centuries and significant ecological changes have also occurred in response to rapid regional warming during the second half of the twentieth century. This combination of historical perturbation and rapid regional change highlights that the Scotia Sea ecosystem is likely to show significant change over the next two to three decades, which may result in major ecological shifts.

/pdf/spatial-and-temporal-operation-of-the-scotia-sea-ecosystem-a-exei95wj4g.pdf

Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web.

[ 1] A year-round sequence of hydrographic casts is used to trace the evolution of the upper ocean waters in Marguerite Bay, western Antarctic Peninsula (wAP), between 1998 and 2002. Winter 1998 was anomalous, showing an unusually deep mixed layer that became progressively more saline until spring, reaching salinities as high as 34.0. The remnant of this mixed layer ( the Winter Water, WW) was the deepest and most saline observed. Atmospheric and cryospheric conditions were anomalous during winter 1998 at both local and regional scales. Locally, we observed low sea ice concentrations, high air temperatures, and a high frequency of northerly winds. These are the local manifestations of the strong ENSO event of 1997/1998 that was then rapidly weakening. At the regional scale, this ENSO produced significant anomalies in the sea ice distribution throughout the Amundsen-Bellingshausen Sea area, and a large-scale low-pressure anomaly over the southeast Pacific was seen to be responsible for the warm, northerly winds. We use a coupled mixed-layer/ice production model to investigate the ENSO-driven forcings for the anomalous ocean conditions observed in winter 1998. This reveals that ice production is the main control on upper ocean stratification, and that the deep, saline mixed layer in 1998 was forced by anomalous sea ice conditions on spatial scales larger than purely local. We conclude that the near-coastal hydrography along the Peninsula shows a profound response to ENSO, with atmospheric and cryospheric forcings both implicated.

/pdf/impact-of-the-1997-98-enso-on-upper-ocean-characteristics-in-lklcpqk4fs.pdf

Impact of the 1997/98 ENSO on upper ocean characteristics in Marguerite Bay, western Antarctic Peninsula

The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea–ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

Southern ocean carbon and heat impact on climate

Variability Of The Shelf Circulation Around South Georgia, Southern Ocean

Abstract Southern Ocean phytoplankton are especially subjected to pronounced seasonal and interannual changes in light availability. Although previous studies have examined the role of light in these environments, very few combined pigment-based taxonomy with flow cytometry to better discriminate the light response of various phytoplankton groups. In particular the different populations within the diverse and important taxonomic group of diatoms require further investigation. Six incubation experiments (9–10 days) were performed during the main productive period with natural seawater collected at the Western Antarctic Peninsula. Standing stock of Phaeocystis spp. cells displayed relatively fast accumulation under all levels of light (low, medium, high; 4–7, 30–50 and 150–200 µmol quanta m −2 s −1 ), whilst the small- and larger-sized diatom populations (4.5 and 20 µm diameter) exhibited faster accumulation in medium and high light. In contrast, intermediate-sized diatoms (11.5 µm diameter) displayed fastest net growth under low light, subsequently dominating the phytoplankton community. Low light was a key factor limiting accumulation and peak phytoplankton biomass, except one incubation displaying relatively high accumulation rates under low light. The 3-week low-light period prior to experimentation likely allowed adaptation to maximize achievable growth and seems a strong determinant of whether the different natural Antarctic phytoplankton populations sustain, thrive or decline. Our study provides improved insight into how light intensity modulates the net response of key Antarctic phytoplankton, both between and within taxonomic groups. 

/pdf/control-of-antarctic-phytoplankton-community-composition-and-3fh87nae.pdf

Meredith P. Meredith

Papers

Impact of the 1997/98 ENSO on upper ocean characteristics in Marguerite Bay, western Antarctic Peninsula

Southern ocean carbon and heat impact on climate

Variability Of The Shelf Circulation Around South Georgia, Southern Ocean

Control of Antarctic phytoplankton community composition and standing stock by light availability