Defining Southern Ocean fronts and their influence on biological and physical processes in a changing climate

Abstract

The Southern Ocean is a critical component of the global climate system and an important ecoregion that contains a diverse range of interdependent flora and fauna. The Southern Ocean also hosts numerous fronts: sharp boundaries between waters with different characteristics. As they strongly influence exchanges between the ocean, atmosphere and cryosphere, fronts are of fundamental importance to the climate system. However, rapid advances in physical oceanography over the past 20 years have challenged previous definitions of fronts and their response to anthropogenic climate change. Here, we review the implications of this recent research for the study of climate, ecology and biology in the Southern Ocean. We include a frontal definition "user guide" to clarify the current debate and facilitate future research. The Southern Ocean, generally defined as the global ocean south of about 35 • S that encircles the Antarctic continent, is unique oceanographic environment due to the lack of continental barriers blocking its flow and the strong winds that blow over its surface 1. At large scales, the Southern Ocean is characterised by both the intense eastward flowing Antarctic Circumpolar Current (ACC), one of the most powerful current systems on Earth, and strongly tilted isopycnals (lines of constant density) that shallow to the south. Observations of the Southern Ocean dating back to the Discovery expedition in the 1920s revealed that the transition from warmer subtropical waters to colder Antarctic waters does not occur smoothly, but is concentrated into a series of sharp transition zones, aligned generally east-west, that have come to be called fronts 2. Further observations revealed that salinity, oxygen, nutrients and various other tracers showed similar behaviour, and that between the fronts, water properties are relatively homogeneous. As such, fronts delimit the boundaries between different water-masses with distinct environmental characteristics 3. These fronts also tend to coincide with the location of narrow yet very intense currents known as "jets" 4 that dominate the ACC's flow 5. The Southern Ocean is divided by fronts into a number of distinct biophysical zones, and hence a number of distinct habitats, which in turn support distinct biota 6, 7. Numerous studies have shown that seabirds and marine mammals tend to congregate and forage in and around fronts 7. As the Earth continues to warm due to anthropogenic climate change, it is vital that we understand how these fronts and jets will respond to changes in the global climate system, and what influence that might have on associated ecosystems 8-10. Due to its remoteness and harsh climate, undertaking field studies in the Southern Ocean is both difficult and expensive. As a result, the Southern Ocean is amongst the most data-sparse of all major ocean basins, which has hindered progress on key questions regarding its dynamics and ecological communities 10. In recent decades however, a deluge of new data from satellites and Argo profiling floats, along with e...

Global Drivers on Southern Ocean Ecosystems: Changing Physical Environments and Anthropogenic Pressures in an Earth System

“…However, this approach was challenged because these parameters are affected by the large-scale thermal expansion and steric sea-level rise occurring as a result of the warming of this circumpolar region. Recent studies found no significant long-term trend in either the annual mean latitude of zonal wind jets between 1979 and 2009 (Swart et al, 2015) or the zonally averaged latitude of ACC transport, which seem insensitive to changes in the SO's broad-scale structure (Gille, 2014;Chapman et al, 2020). The location of the ACC may be constrained by sea floor and land mass topography, particularly close to narrow gaps between land masses, such as Drake's Passage (high confidence; Moore et al, 1999).…”

“…Ice shelf disintegration could have complex effects on upper ocean mixing and vertical carbon and nutrient fluxes depending on the relative forcing of increased exposure to winds and altered strength and position of the buoyant meltwater plume (e.g., Randall-Goodwin et al, 2015). Projected increases in eddy activity within the ACC and the anticipated changes in ocean currents and frontal positions will influence the transport and distribution of nutrients and carbon around the Southern Ocean, as well as air-sea exchange of CO 2 (Sallée et al, 2012;Rintoul, 2018;Chapman et al, 2020).…”

Changing Biogeochemistry of the Southern Ocean and Its Ecosystem Implications

“…The solubility pump, whereby atmospheric CO 2 is taken up by dissolution into surface waters and subsequently subducted into the subsurface, exporting CO 2 into the ocean interior, is particularly strong in the high southern latitudes due to cold surface waters and deep and intermediate water mass formation (e.g., Sabine et al, 2004;Van Heuven et al, 2014;Gruber et al, 2019b). The Southern Ocean also has regions of strong upwelling linked to oceanographic fronts, which brings CO 2 -rich deep waters to the surface, increasing pCO 2 in surface waters, altering the carbonate system equilibrium and driving CO 2 release to the atmosphere (e.g., Pardo et al, 2017;Chapman et al, 2020). The biological pump is also important in the Southern Ocean, particularly during spring and summer (e.g., Ducklow et al, 2001;DeVries et al, 2012;Cavan et al, 2019a).…”

“…The Southern Ocean CO 2 sink has taken up approximately 40% of the total oceanic uptake of anthropogenic CO 2 (Orr et al, 2001;Fletcher et al, 2006;DeVries, 2014), increasing surface water pCO 2 and causing ocean acidification (Section "Ocean Acidification and Its Effects on the Ecosystem"). Export of CO 2 to the deep Southern Ocean occurs in specific locations and depends on the interactions between physical properties, such as mixed layer depth, ocean currents, fronts, eddies and winds, all of which are potentially sensitive to climate variability and change, and with bathymetric features (Sallee et al, 2012;Chapman et al, 2020). In addition to intense seasonality in sea-air CO 2 fluxes driven by the solubility and biological pumps, a high degree of interannual variability has been reported for particular regions, such as the WAP (Ito et al, 2018;Brown et al, 2019) and the Ross Sea (Dejong and Dunbar, 2017).…”

Foraging in a dynamic environment: Response of four sympatric sub‐Antarctic albatross species to interannual environmental variability

“…Nel et al (2001) showed that once a GHA egg hatches, adult GHA breeding at Marion Island no longer travel to the STCZ and all foraging activity is restricted around prey aggregating mesoscale eddies in the lee of the ABFZ. As a result of climate change, southward shifts of the major frontal zones within the Southern Ocean has been predicted (Kim & Orsi, 2014;Wilson et al, 2016 but see Chapman et al, 2020;Meijers et al, 2012). Asdar (2018) recently showed that if there is a 1° latitudinal shift, either north or south, in the position of the PF, the PF will no longer interact with the ABFZ, producing fewer eddies.…”