Southern Ocean Region Research Articles

Simulation Framework and Case Studies for the Design of Sea Surface Salinity Remote Sensing Missions

<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> -band microwave radiometers have now been used to measure sea surface salinity (SSS) from space for over a decade with the SMOS, Aquarius, and SMAP missions, and it is expected that the launch of the CIMR mission in the later half of this decade will ensure measurement continuity in the near future. Beyond these missions, it is useful to consider how future missions can be designed to meet different scientific objectives and performance requirements as well as to fit within different cost spaces. In this article, we present a software simulator for remote sensing measurements of ocean state capable of generating L1- and L2- equivalent data products for an arbitrary spacecraft mission including multifrequency fixed-pointing or scanning microwave radiometers.This simulator is then applied to case studies of SSS measurement over selected areas of interest, including the Gulf Stream, Southern Ocean, and Pacific tropical instability wave regions. These simulations illustrate how different design choices concerning receiver bandwidth and revisit time can improve the detection of SSS features in these regions from the mesoscale to the seasonal scale.

Customized sea‐surface temperature indicators linking to streamflow at different timescales

AbstractInvestigation of SST–streamflow connectivity unravels the large‐scale climate influences that may have a potential role in modulating local hydrological components. Most studies exploring this relationship only focus on a single timescale; however, various atmospheric and oceanic phenomena occur at different temporal scales, which must be considered. This study examines the association of sea surface temperature (SST) and streamflow in Germany, divided into three regions, viz. Alpine, Atlantic and Continental, at timescales ranging from seasonal to interannual by integrating wavelet transform and complex network techniques. Wavelet transform is used to decompose the time series into multiple frequency signals. The network theory identifies the spatial connections for the 99 percentile correlation coefficient value based on these decomposed signals. The degree centrality metric is used to evaluate the characteristics of the spatially embedded networks. Our results re‐establish known SST regions that have a potential connection with the various streamflow regions of Germany. Spatial patterns that resemble the North Atlantic SST tripole‐like pattern is predominant for Alpine streamflow regions at finer timescale. Equatorial Atlantic Mode regions observed for Atlantic streamflow at interannual timescale and Vb weather system connected regions in the Mediterranean Sea have appeared for all the streamflow regions of Germany. Besides, continental streamflow regions exhibited combined characteristics of the Alpine and Atlantic streamflow spatial patterns. In addition to the above regions, we also identify the scale‐specific patterns in the Pacific, Indian and Southern Ocean regions at different timescales.

Climate drivers of Southern Ocean phytoplankton community composition and potential impacts on higher trophic levels

Southern Ocean phytoplankton production supports rich Antarctic marine ecosystems comprising copepods, krill, fish, seals, penguins, and whales. Anthropogenic climate change, however, is likely to drive rearrangements in phytoplankton community composition with potential ramifications for the whole ecosystem. In general, phytoplankton communities dominated by large phytoplankton, i.e., diatoms, yield shorter, more efficient food chains than ecosystems supported by small phytoplankton. Guided by a large ensemble of Earth system model simulations run under a high emission scenario (RCP8.5), we present hypotheses for how anthropogenic climate change may drive shifts in phytoplankton community structure in two regions of the Southern Ocean: the Antarctic Circumpolar Current (ACC) region and the sea ice zone (SIZ). Though both Southern Ocean regions experience warmer ocean temperatures and increased advective iron flux under 21st century climate warming, the model simulates a proliferation of diatoms at the expense of small phytoplankton in the ACC, while the opposite patterns are evident in the SIZ. The primary drivers of simulated diatom increases in the ACC region include warming, increased iron supply, and reduced light from increased cloudiness. In contrast, simulated reductions in ice cover yield greater light penetration in the SIZ, generating a phenological advance in the bloom accompanied by a shift to more small phytoplankton that effectively consume available iron; the result is an overall increase in net primary production, but a decreasing proportion of diatoms. Changes of this nature may promote more efficient trophic energy transferviacopepods or krill in the ACC region, while ecosystem transfer efficiency in the SIZ may decline as small phytoplankton grow in dominance, possibly impacting marine food webs sustaining Antarctic marine predators. Despite the simplistic ecosystem representation in our model, our results point to a potential shift in the relative success of contrasting phytoplankton ecological strategies in different regions of the Southern Ocean, with ramifications for higher trophic levels.

Circum-Antarctic abundance and properties of CCN and INPs

Abstract. Aerosol particles acting as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs) play a major role in the formation and glaciation of clouds. Thereby they exert a strong impact on the radiation budget of the Earth. Data on abundance and properties of both types of particles are sparse, especially for remote areas of the world, such as the Southern Ocean (SO). In this work, we present unique results from ship-borne aerosol-particle-related in situ measurements and filter sampling in the SO region, carried out during the Antarctic Circumnavigation Expedition (ACE) in the austral summer of 2016–2017. An overview of CCN and INP concentrations over the Southern Ocean is provided and, using additional quantities, insights regarding possible CCN and INP sources and origins are presented. CCN number concentrations spanned 2 orders of magnitude, e.g. for a supersaturation of 0.3 % values ranged roughly from 3 to 590 cm−3. CCN showed variable contributions of organic and inorganic material (inter-quartile range of hygroscopicity parameter κ from 0.2 to 0.9). No distinct size dependence of κ was apparent, indicating homogeneous composition across sizes (critical dry diameter on average between 30 and 110 nm). The contribution of sea spray aerosol (SSA) to the CCN number concentration was on average small. Ambient INP number concentrations were measured in the temperature range from −5 to −27 ∘C using an immersion freezing method. Concentrations spanned up to 3 orders of magnitude, e.g. at −16 ∘C from 0.2 to 100 m−3. Elevated values (above 10 m−3 at −16 ∘C) were measured when the research vessel was in the vicinity of land (excluding Antarctica), with lower and more constant concentrations when at sea. This, along with results of backward-trajectory analyses, hints towards terrestrial and/or coastal INP sources being dominant close to ice-free (non-Antarctic) land. In pristine marine areas INPs may originate from both oceanic sources and/or long-range transport. Sampled aerosol particles (PM10) were analysed for sodium and methanesulfonic acid (MSA). Resulting mass concentrations were used as tracers for primary marine and secondary aerosol particles, respectively. Sodium, with an average mass concentration around 2.8 µg m−3, was found to dominate the sampled, identified particle mass. MSA was highly variable over the SO, with mass concentrations up to 0.5 µg m−3 near the sea ice edge. A correlation analysis yielded strong correlations between sodium mass concentration and particle number concentration in the coarse mode, unsurprisingly indicating a significant contribution of SSA to that mode. CCN number concentration was highly correlated with the number concentration of Aitken and accumulation mode particles. This, together with a lack of correlation between sodium mass and Aitken and accumulation mode number concentrations, underlines the important contribution of non-SSA, probably secondarily formed particles, to the CCN population. INP number concentrations did not significantly correlate with any other measured aerosol physico-chemical parameter.

The Daily‐Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations

AbstractWater mass transformation in the Southern Ocean is vital for driving the large‐scale overturning circulation, which transports heat from the surface to the ocean interior. Using profiling gliders, this study investigates the role of summertime buoyancy forcing and wind‐driven processes on the intraseasonal (1–10 days) mixed layer thermohaline variability in three Southern Ocean regions southwest of Africa important for water mass transformation—the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), and Marginal Ice Zone (MIZ). At intraseasonal time scales, heat flux was shown as the main driver of buoyancy gain in all regions. In the SAZ and MIZ, shallow mixed layers and strong stratification enhanced mixed layer buoyancy gain by trapping incoming heat, while buoyancy loss resulted primarily from the entrainment of cold, salty water from below. In the PFZ, rapid mixing linked to Southern Ocean storms set persistently deep mixed layers and suppressed mixed layer intraseasonal thermohaline variability. In the polar regions, lateral stirring of meltwater from seasonal sea‐ice melt dominated daily mixed layer salinity variability. We propose that these meltwater fronts are advected to the PFZ during late summer, indicating the potential for seasonal sea‐ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of how the mixed layer thermohaline properties respond to small spatiotemporal processes, emphasizing the importance of surface forcing occurring between 1 and 10 days on the mixed layer water mass transformation in the Southern Ocean.

MIROC4を基にした化学輸送モデルと逆解法とを用いた大気中一酸化二窒素の動態解析

Atmospheric nitrous oxide (N2O) contributes to global warming and stratospheric ozone depletion, so reducing uncertainty in estimates of emissions from different sources is important for climate policy. In this study, we simulate atmospheric N2O using an atmospheric chemistry-transport model (ACTM), and the results are first compared with the in situ measurements. Five combinations of known (a priori) N2O emissions due to natural soil, agricultural land, other human activities, and sea–air exchange are used. The N2O lifetime is 127.6 ± 4.0 yr in the control ACTM simulation (range indicates interannual variability). Regional N2O emissions are optimized using Bayesian inverse modeling for 84 partitions of the globe at monthly intervals, using measurements at 42 sites around the world covering 1997–2019. The best estimated global land and ocean emissions are 12.99 ± 0.22 TgN yr−1 and 2.74 ± 0.27 TgN yr−1, respectively, for 2000–2009, and 14.30 ± 0.20 TgN yr−1 and 2.91 ± 0.27 TgN yr−1, respectively, for 2010–2019. On regional scales, we find that the most recent ocean emission estimation, with lower emissions in the Southern Ocean regions, fits better with that predicted by the inversions. Marginally higher (lower) emissions than the inventory/model for the tropical (extratropical) land regions are estimated and validated using independent aircraft observations. Global land and ocean emission variabilities show a statistically significant correlation with El Niño Southern Oscillation (ENSO). Analysis of regional land emissions shows increases over America (Temperate North, Central, and Tropical), Central Africa, and Asia (South, East, and Southeast) between the 2000s and 2010s. Only Europe as a whole recorded a slight decrease in N2O emissions due to the chemical industry. Our inversions suggest revisions to seasonal emission variations for three of the 15 land regions (East Asia, Temperate North America, and Central Africa), and the Southern Ocean region. The terrestrial ecosystem model (Vegetation Integrative SImulator for Trace Gases) can simulate annual total emissions in agreement with the observed N2O growth rate since 1978, but the lag-time scales of N2O emissions from nitrogen fertilizer application may need to be revised.

Surface chlorophyll concentration as a mesoplankton biomass assessment tool in the Southern Ocean region

AbstractAimMesoplankton of the Southern Ocean is the key trophic link between phytoplankton and other consumers and an important part of the biological carbon pump, a critical component of climate regulation. Although general patterns of mesoplankton distribution and life cycles are known, we still cannot estimate mesoplankton standing stocks at specified ocean‐basin scales.LocationSouthern Ocean, 34.4−56.9°S, 0–14.4°E.Time periodDecember 2009.Major taxa studiedAll mesoplankton 0.2–20.0 mm.MethodsWe assessed biomass of dominant taxa and total wet mesoplankton biomass (B, mg/m3) using surface chlorophyll a concentration (Chl) as a proxy of phytoplankton biomass and surface temperature (ST), a basic hydrological parameter. We sampled three strata within the layer 0–300 m at 43 stations with a Judey net (0.1 m2, 180 µm).ResultsWe identified 163 taxa and calculated their biomass, biomass of major trophic groups, and total mesoplankton biomass. Monthly Chl and ST derived from satellite imaging were averaged at various temporal and spatial scales. B in the upper layer and in the integrated layer 0–300 m was significantly linked to antecedent Chl signals; the most significant regressions were found between B and monthly Chl and ST averaged two months prior to the survey and 10° upstream of the Antarctic Circumpolar Current. Larger mesoplankton fractions showed longer response periods to Chl signal than smaller fractions.Main conclusionsOur results suggest the integral mesoplankton stock is 2.47–3.69 Gt wet weight wtot (ww), or 0.12–0.18 Gt C, in the whole Southern Ocean. Mesoplankton was dominated by omnivores (69%–74%) followed by carnivores (25%) and herbivores (20%), which suggests the leading role of omnivory in the Southern Ocean pelagic during the austral summer. We anticipate that these results will help to provide a deeper insight into the Southern Ocean ecosystems and will benefit models predicting changes in the carbon–climate Earth system.

Observations and Modeling of Rime Splintering in Southern Ocean Cumuli

AbstractRecent studies have suggested a correct representation of cloud phase in the Southern Ocean region is important in climate models for an accurate representation of the energy balance. Satellite retrievals indicate many of the clouds are predominantly liquid, despite their low temperatures. However, clouds containing high numbers of ice crystals have sometimes been observed in this region and implicated the secondary ice production process called rime splintering. This study re‐examines rime splintering in Southern Ocean cumuli using both a new data set and high‐resolution numerical modeling. Measurements acquired during the Southern Ocean Clouds Radiation Aerosol Transport Experimental Study (SOCRATES) provide an evaluation of the amount of ice in shallow cumuli sampled over two days in this region. The measurements sometimes exhibit seven orders of magnitude or more ice particles compared to amounts expected from measurements of ice‐nucleating particles (INP) on the same days. Cumuli containing multiple updrafts had the greatest tendency to contain high ice concentrations and meet the expected conditions for rime splintering. Idealized numerical modeling, constrained by the observations, suggests that the multiple updrafts produce more frozen raindrops/graupel, and allow them to travel through the rime‐splintering zone over an extended period of time, increasing the number of ice particles by many orders of magnitude. The extremely low number of INP in the Southern Ocean thus appears to require special conditions like multiple updrafts to help glaciate the cumuli in this region, potentially explaining the predominance of supercooled cumuli observed there.

South polar skua (Catharacta maccormicki) as biovectors for long-range transport of persistent organic pollutants to Antarctica

Migratory bird species may serve as vectors of contaminants to Antarctica through the local deposition of guano, egg abandonment, or mortality. To further investigate this chemical input pathway, we examined the contaminant burdens and profiles of the migratory South polar skua (Catharacta maccormicki) and compared them to the endemic Adélie penguin (Pygoscelis adeliae). A range of persistent organic pollutants were targeted in muscle and guano to facilitate differentiation of likely exposure pathways. A total of 56 of 65 targeted analytes were detected in both species, but there were clear profile and magnitude differences between the species. The South polar skua and Adélie penguin muscle tissue burdens were dominated by p,p′-dichlorodiphenyldichloroethylene (mean 5600 ng g−1 lw and 330 ng g−1 lw respectively) and hexachlorobenzene (mean 2500 ng g−1 lw and 570 ng g−1 lw respectively), a chemical profile characteristic of the Antarctic and Southern Ocean region. Species profile differences, indicative of exposure at different latitudes, were observed for polychlorinated biphenyls (PCBs), with lower chlorinated congeners and deca-chlorinated PCB-209 detected in South polar Skua, but not in Adélie penguins. Notably, the more recently used perfluoroalkyl substances and the brominated flame retardants, hexabromocyclododecane and tetrabromobisphenol A, were detected in both species. This finding suggests local exposure, given the predicted slow and limited long-range environmental transport capacity of these compounds to the eastern Antarctic sector.

Southern Ocean latitudinal gradients of cloud condensation nuclei

Abstract. The Southern Ocean region is one of the most pristine in the world and serves as an important proxy for the pre-industrial atmosphere. Improving our understanding of the natural processes in this region is likely to result in the largest reductions in the uncertainty of climate and earth system models. While remoteness from anthropogenic and continental sources is responsible for its clean atmosphere, this also results in the dearth of atmospheric observations in the region. Here we present a statistical summary of the latitudinal gradient of aerosol (condensation nuclei larger than 10 nm, CN10) and cloud condensation nuclei (CCN at various supersaturations) concentrations obtained from five voyages spanning the Southern Ocean between Australia and Antarctica from late spring to early autumn (October to March) of the 2017/18 austral seasons. Three main regions of influence were identified: the northern sector (40–45∘ S), where continental and anthropogenic sources coexisted with background marine aerosol populations; the mid-latitude sector (45–65∘ S), where the aerosol populations reflected a mixture of biogenic and sea-salt aerosol; and the southern sector (65–70∘ S), south of the atmospheric polar front, where sea-salt aerosol concentrations were greatly reduced and aerosol populations were primarily biologically derived sulfur species with a significant history in the Antarctic free troposphere. The northern sector showed the highest number concentrations with median (25th to 75th percentiles) CN10 and CCN0.5 concentrations of 681 (388–839) cm−3 and 322 (105–443) cm−3, respectively. Concentrations in the mid-latitudes were typically around 350 cm−3 and 160 cm−3 for CN10 and CCN0.5, respectively. In the southern sector, concentrations rose markedly, reaching 447 (298–446) cm−3 and 232 (186–271) cm−3 for CN10 and CCN0.5, respectively. The aerosol composition in this sector was marked by a distinct drop in sea salt and increase in both sulfate fraction and absolute concentrations, resulting in a substantially higher CCN0.5/CN10 activation ratio of 0.8 compared to around 0.4 for mid-latitudes. Long-term measurements at land-based research stations surrounding the Southern Ocean were found to be good representations at their respective latitudes; however this study highlighted the need for more long-term measurements in the region. CCN observations at Cape Grim (40∘39′ S) corresponded with CCN measurements from northern and mid-latitude sectors, while CN10 observations only corresponded with observations from the northern sector. Measurements from a simultaneous 2-year campaign at Macquarie Island (54∘30′ S) were found to represent all aerosol species well. The southernmost latitudes differed significantly from both of these stations, and previous work suggests that Antarctic stations on the East Antarctic coastline do not represent the East Antarctic sea-ice latitudes well. Further measurements are needed to capture the long-term, seasonal and longitudinal variability in aerosol processes across the Southern Ocean.

Untangling local and remote influences in two major petrel habitats in the oligotrophic Southern Ocean.

Ocean circulation connects geographically distinct ecosystems across a wide range of spatial and temporal scales via exchanges of physical and biogeochemical properties. Remote oceanographic processes can be especially important for ecosystems in the Southern Ocean, where the Antarctic Circumpolar Current transports properties across ocean basins through both advection and mixing. Recent tracking studies have indicated the existence of two large-scale, open ocean habitats in the Southern Ocean used by grey petrels (Procellaria cinerea) from two populations (i.e., Kerguelen and Antipodes islands) during their nonbreeding season for extended periods during austral summer (i.e., October to February). In this work, we use a novel combination of large-scale oceanographic observations, surface drifter data, satellite-derived primary productivity, numerical adjoint sensitivity experiments, and output from a biogeochemical state estimate to examine local and remote influences on these grey petrel habitats. Our aim is to understand the oceanographic features that control these isolated foraging areas and to evaluate their ecological value as oligotrophic open ocean habitats. We estimate the minimum local primary productivity required to support these populations to be much <1% of the estimated local primary productivity. The region in the southeast Indian Ocean used by the birds from Kerguelen is connected by circulation to the productive Kerguelen shelf. In contrast, the region in the south-central Pacific Ocean used by seabirds from the Antipodes is relatively isolated suggesting it is more influenced by local factors or the cumulative effects of many seasonal cycles. This work exemplifies the potential use of predator distributions and oceanographic data to highlight areas of the open ocean that may be more dynamic and productive than previously thought. Our results highlight the need to consider advective connections between ecosystems in the Southern Ocean and to re-evaluate the ecological relevance of oligotrophic Southern Ocean regions from a conservation perspective.

Global Connectivity of Southern Ocean Ecosystems

Southern Ocean ecosystems are globally important. Processes in the Antarctic atmosphere, cryosphere, and the Southern Ocean directly influence global atmospheric and oceanic systems. Southern Ocean biogeochemistry has also been shown to have global importance. In contrast, ocean ecological processes are often seen as largely separate from the rest of the global system. In this paper, we consider the degree of ecological connectivity at different trophic levels, linking Southern Ocean ecosystems with the global ocean, and their importance not only for the regional ecosystem but also the wider Earth system. We also consider the human system connections, including the role of Southern Ocean ecosystems in supporting society, culture, and economy in many nations, influencing public and political views and hence policy. Rather than Southern Ocean ecosystems being defined by barriers at particular oceanic fronts, ecological changes are gradual due to cross-front exchanges involving oceanographic processes and organism movement. Millions of seabirds and hundreds of thousands of cetaceans move north out of polar waters in the austral autumn interacting in food webs across the Southern Hemisphere, and a few species cross the equator. A number of species migrate into the east and west ocean-basin boundary current and continental shelf regions of the major southern continents. Human travel in and out of the Southern Ocean region includes fisheries, tourism, and scientific vessels in all ocean sectors. These operations arise from many nations, particularly in the Northern Hemisphere, and are important in local communities as well as national economic, scientific, and political activities. As a result of the extensive connectivity, future changes in Southern Ocean ecosystems will have consequences throughout the Earth system, affecting ecosystem services with socio-economic impacts throughout the world. The high level of connectivity also means that changes and policy decisions in marine ecosystems outside the Southern Ocean have consequences for ecosystems south of the Antarctic Polar Front. Knowledge of Southern Ocean ecosystems and their global connectivity is critical for interpreting current change, projecting future change impacts, and identifying integrated strategies for conserving and managing both the Southern Ocean and the broader Earth system.

Brominated Flame Retardants in Antarctic Air in the Vicinity of Two All-Year Research Stations

Continuous atmospheric sampling was conducted between 2010–2015 at Casey station in Wilkes Land, Antarctica, and throughout 2013 at Troll Station in Dronning Maud Land, Antarctica. Sample extracts were analyzed for polybrominated diphenyl ethers (PBDEs), and the naturally converted brominated compound, 2,4,6-Tribromoanisole, to explore regional profiles. This represents the first report of seasonal resolution of PBDEs in the Antarctic atmosphere, and we describe conspicuous differences in the ambient atmospheric concentrations of brominated compounds observed between the two stations. Notably, levels of BDE-47 detected at Troll station were higher than those previously detected in the Antarctic or Southern Ocean region, with a maximum concentration of 7800 fg/m3. Elevated levels of penta-formulation PBDE congeners at Troll coincided with local building activities and subsided in the months following completion of activities. The latter provides important information for managers of National Antarctic Programs for preventing the release of persistent, bioaccumulative, and toxic substances in Antarctica.

Dispersion of Droplet Size Distributions in Supercooled Non‐precipitating Stratocumulus from Aircraft Observations Obtained during the Southern Ocean Cloud Radiation Aerosol Transport Experimental Study

AbstractThe characteristics of cloud droplet size distributions and statistical relations of the relative dispersion (ε) with the vertical velocity (w) and with the interstitial aerosol concentration (Nia) are investigated for ubiquitous supercooled shallow stratocumulus observed over the Southern Ocean (SO) using aircraft measurements obtained during the Southern Ocean Cloud Radiation Aerosol Transport Experimental Study. Distinct vertical variations have been found using 36 non‐precipitating cloud profiles. The cloud droplet effective radius (re) increases nearly monotonically from 5.3 ± 1.9 μm at cloud base to 9.4 ± 2.2 μm at cloud top. The ε decreases rapidly from cloud base (0.42 ± 0.13) and then remains relatively constant in the upper cloud layer (0.27 ± 0.09). This study also shows robust dependence of ε on both Nia and w. The ε increases (decreases) with increasing Nia (w) at a 95% confidence level when values of w (low Nia) are restricted to a small range. The important roles of aerosols and dynamics on ε are demonstrated and are crucial to estimating aerosol indirect radiative forcing, especially for pristine SO regions where models almost universally underestimate reflected radiation.

Triple Water Vapour–Isotopologues Record from Chhota Shigri, Western Himalaya, India: A Unified Interpretation based on δ17O, δ18O, δD and Comparison to Meteorological Parameters

The objective of this study is to investigate and understand the source and transportation of water vapour in the western Himalayan region—that is still missing—using water vapour stable isotopologues and air mass trajectory diagnostics. We report the first-time triple oxygen isotopic compositions of water vapour from high altitude western Himalaya (Chhota Shigri, India) and compare them with meteorological conditions at the site of investigation as well as tracked backwards through the Lagrangian air mass trajectory diagnostics. A total of 21 water vapour samples were collected using a quantitative cryogenic method. δ17O and δ18O values show a significant correlation coefficient of 0.999 (p &amp;lt;0.01). The temporal variations of δ17O, δ18O, δD, D-excess and 17O-excess are 1.2, 2.3, 17.3, 11.6 and 39‰ permeg, respectively. δ17O and δ18O exhibit significant (p &amp;lt; 0.05) diurnal variations along with meteorological parameters. Chhota Shigri vapour isotopic results show a clear difference in the 17O-excess value compared to near the south Indian Ocean and the Southern Ocean regions, reflecting the influence of local moisture recycling at the continental site. NCEP/NCAR reanalyses show lower Specific Humidity during the sampling period (September, ending month of the Indian summer monsoon) favouring evaporative conditions which are further corroborated through the Lagrangian moisture diagnostics suggesting frequent moisture uptake and moisture loss in specific regions.

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

The manuscript assesses the current and expected future global drivers of Southern Ocean (SO) ecosystems. Atmospheric ozone depletion over the Antarctic since the 1970s, has been a key driver, resulting in springtime cooling of the stratosphere and intensification of the polar vortex, increasing the frequency of positive phases of the Southern Annular Mode (SAM). This increases warm air-flow over the East Pacific sector (Western Antarctic Peninsula) and cold air flow over the West Pacific sector. SAM as well as El Niño Southern Oscillation events also affect the Amundsen Sea Low leading to either positive or negative sea ice anomalies in the west and east Pacific sectors, respectively. The strengthening of westerly winds is also linked to shoaling of deep warmer water onto the continental shelves, particularly in the East Pacific and Atlantic sectors. Air and ocean warming has led to changes in the cryosphere, with glacial and ice sheet melting in both sectors, opening up new ice free areas to biological productivity, but increasing seafloor disturbance by icebergs. The increased melting is correlated with a salinity decrease particularly in the surface 100 m. Such processes could increase the availability of iron, which is currently limiting primary production over much of the SO. Increasing CO2is one of the most important SO anthropogenic drivers and is likely to affect marine ecosystems in the coming decades. While levels of many pollutants are lower than elsewhere, persistent organic pollutants (POPs) and plastics have been detected in the SO, with concentrations likely enhanced by migratory species. With increased marine traffic and weakening of ocean barriers the risk of the establishment of non-indigenous species is increased. The continued recovery of the ozone hole creates uncertainty over the reversal in sea ice trends, especially in the light of the abrupt transition from record high to record low Antarctic sea ice extent since spring 2016. The current rate of change in physical and anthropogenic drivers is certain to impact the Marine Ecosystem Assessment of the Southern Ocean (MEASO) region in the near future and will have a wide range of impacts across the marine ecosystem.

Temporal patterns of iron limitation in the Ross Sea as determined from chlorophyll fluorescence

The Ross Sea is one of the most productive regions in the Southern Ocean, with a significant role in carbon cycling as well as the massive abundance of higher trophic levels. The seasonal cycle is well established with an early summer Phaeocystis antarctica bloom that declines followed by a diatom bloom in late summer. This seasonal progression of the phytoplankton has been linked to the availability of iron in the mixed layer. Investigating the temporal progression of iron limitation is often limited by both the decreased sampling resolution from traditional platforms, such as ships, and the lack of regular deployments of specific sensors that can measure phytoplankton physiology. Through the use of a novel technique that uses the degree of quenching (NPQGlider), determined from a standard fluorometer deployed on a buoyancy glider, a proxy for iron limitation, αNPQ, was calculated for a glider time series in the Ross Sea from December 2011 to February 2012. Surface chlorophyll concentrations indicated that there were four stages: the first being a pre-bloom phase, the second in which phytoplankton growth was rapid, resulting in the accumulation of biomass; the third in which biomass in the surface layer decreased, and the fourth in which chlorophyll concentrations remained low but the POC:Chl ratio increased. The levels of NPQGlider in this region were much higher compared to other Southern Ocean regions, with the highest levels in the third phase. Similarly, αNPQ remained low throughout most of the time series except for the transition from the second to third phase when the surface biomass decreases. The increase in POC:Chl ratios in the final phase combined with the low values of αNPQ suggest the switch from a Phaeocystis antarctica bloom to a potentially non-iron limited diatom bloom. These results confirm that the application of novel methodologies to proven and reliable sensors will provide a greater understanding of biogeochemical cycles and their controls throughout the ocean.

Changes in Local and Global Climate Feedbacks in the Absence of Interactive Clouds: Southern Ocean–Climate Interactions in Two Intermediate-Complexity Models

AbstractThe global-mean climate feedback quantifies how much the climate system will warm in response to a forcing such as increased CO2 concentration. Under a constant forcing, this feedback becomes less negative (increasing) over time in comprehensive climate models, which has been attributed to increases in cloud and lapse-rate feedbacks. However, out of eight Earth system models of intermediate complexity (EMICs) not featuring interactive clouds, two also simulate such a feedback increase: Bern3D-LPX and LOVECLIM. Using these two models, we investigate the causes of the global-mean feedback increase in the absence of cloud feedbacks. In both models, the increase is predominantly driven by processes in the Southern Ocean region. In LOVECLIM, the global-mean increase is mainly due to a local longwave feedback increase in that region, which can be attributed to lapse-rate changes. It is enhanced by the slow atmospheric warming above the Southern Ocean, which is delayed due to regional ocean heat uptake. In Bern3D-LPX, this delayed regional warming is the main driver of the global-mean feedback increase. It acts on a near-constant local feedback pattern mainly determined by the sea ice–albedo feedback. The global-mean feedback increase is limited by the availability of sea ice: faster Southern Ocean sea ice melting due to either stronger forcing or higher equilibrium climate sensitivity (ECS) reduces the increase of the global mean feedback in Bern3D-LPX. In the highest-ECS simulation with 4 × CO2 forcing, the feedback even becomes more negative (decreasing) over time. This reduced ice–albedo feedback due to sea ice depletion is a plausible mechanism for a decreasing feedback also in high-forcing simulations of other models.

Estimation of Physiological Factors Controlling Carbon Isotope Fractionation in Coccolithophores in Photic Zone and Core‐Top Samples

AbstractEstimates of past pCO2 prior to ice‐core records have often been sought by measuring carbon isotopic fractionation during photosynthesis (Ɛp) in alkenone‐producing algae present in the fossil record. Photic zone and core‐top Ɛp show strong latitudinal gradients, which have been interpreted as an effect of growth rate. The factor b, used in previous studies to account for the changes in growth rate, shows high dependence on [CO2]aq in culture experiments, making difficult the deconvolution of nutrients versus other effects in the field. We combine coccolithophore growth rate output from a global model, limited by sea surface temperature (SST) and [PO43−] with a statistical model that include [CO2]aq, cell size and light to predict Ɛp, and examine the influence of different environmental and physiological variables on the observed expression of Ɛp in field samples. Back calculations of the growth rate necessary to match the observed Ɛp in photic zone and core‐top samples using the statistical model show high correlation with modeled growth rates using [PO43−] and SST in temperate regions. For Southern Ocean and tropical upwelling regions, with colder and warmer SST, respectively, there might be additional factors controlling growth rate not accounted for in the model, such as grazing or nutrient recycling. We evaluate the potential to reconstruct past growth rates using planktonic foraminifera‐ and coccolithophore‐based proxies related to water column structure. We found best application of these proxies as potential indicators of past [PO43−], which can subsequently be used in combination with proxy temperature, to estimate past coccolithophore growth rates.

High Abundances of Microplastic Pollution in Deep-Sea Sediments: Evidence from Antarctica and the Southern Ocean.

Plastic pollution in Antarctica and the Southern Ocean has been recorded in scientific literature since the 1980s; however, the presence of microplastic particles (<5 mm) is less understood. Here, we aimed to determine whether microplastic accumulation would vary among Antarctic and Southern Ocean regions through studying 30 deep-sea sediment cores. Additionally, we aimed to highlight whether microplastic accumulation was related to sample depth or the sediment characteristics within each core. Sediment cores were digested and separated using a high-density sodium polytungstate solution (SPT) and microplastic particles were identified using micro-Fourier-transform infrared spectroscopy (μFTIR). Microplastic pollution was found in 93% of the sediment cores (28/30). The mean (±SE) microplastics per gram of sediment was 1.30 ± 0.51, 1.09 ± 0.22, and 1.04 ± 0.39 MP/g, for the Antarctic Peninsula, South Sandwich Islands, and South Georgia, respectively. Microplastic fragment accumulation correlated significantly with the percentage of clay within cores, suggesting that microplastics have similar dispersion behavior to low density sediments. Although no difference in microplastic abundance was found among regions, the values were much higher in comparison to less remote ecosystems, suggesting that the Antarctic and Southern Ocean deep-sea accumulates higher numbers of microplastic pollution than previously expected.