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In high-nutrient low-chlorophyll waters of the western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia, most likely due to an enhanced supply of the limiting micronutrient iron. Shallow sediments and atmospheric dust deposition are believed to be the main iron sources. However, their relative importance is still unclear and in the South Georgia region have yet not been ascertained because iron measurements are very few. In this study, we use austral summer dissolved iron (dFe) data around South Georgia (January and February 2008) with a coupled regional hydrodynamic and biogeochemical model to investigate natural iron fertilization around the island. The biogeochemical component of the model includes an iron cycle, where sediments and dust deposition are the sources of iron to the ocean. The model captures the characteristic flow patterns around South Georgia, hence simulating a large phytoplankton bloom to the north, i.e., downstream, of the island. Modelled dFe concentrations agree well with observations (mean difference and root mean square errors of ~ 0.02 nM and ~ 0.81 nM) and form a large plume to the north of the island that extends eastwards for more than 800 km. In agreement with observations, highest dFe concentrations are located along the coast and decrease with distance from the island. Sensitivity tests indicate that most of the iron measured in the main bloom area originates from the coast and the very shallow shelf-sediments (depths < 20 m) while dust deposition plays a minor role, with almost no effects on surface chlorophyll a concentrations. Iron sources such as run-off not represented explicitly in the model, but that likely contribute to the iron plumes observed around South Georgia, are also discussed together with the potential effects their temporal variability may have on the system. [ABSTRACT FROM AUTHOR]

Various human activities, including fossil fuel combustion and forest clearing, emit about eight petagrams (or billion tons) of carbon in the form of CO2 into the atmosphere annually. The global ocean absorbs about two petagrams of CO2, and about a half of that amount is absorbed by the Southern Ocean south of 30°S, thus slowing the rapid accumulation of CO2 in the atmosphere. Partial pressure of CO2 (pCO2) is a measure of the chemical driving force for the CO2 exchange between the ocean and the atmosphere. This paper discusses its space and time distribution over the Southern Ocean. The major sink zone for atmospheric CO2 is located in a latitude belt between 30°S and 50°S, where the biological utilization of CO2 and cooling of warm subtropical waters flowing southward produce low seawater pCO2. Strong winds in this zone also enhance the ocean's uptake. Although the source-sink conditions vary over a wide range through the seasons in the areas south of 50°S, this zone is a small sink on an annual average. Winter observations show that surface water pCO2 values in the source region for Antarctic Intermediate Water have increased at a rate faster than the atmospheric increase rate, suggesting that the ocean CO2 sink intensity has been weakening for several decades and has changed from a net sink to a net source since 2005. The results of ocean general circulation-biogeochemistry model studies are found to be consistent with the observations. [ABSTRACT FROM AUTHOR]

An atmospheric river affecting Australia and the Southern Ocean on 28–29 January 2018 during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) is analyzed using nadir‐pointing W‐band cloud radar measurements and in situ microphysical measurements from a Gulfstream‐V aircraft. The AR had a two‐band structure, with the westernmost band associated with a cold frontal boundary. The bands were primarily stratiform with distinct radar bright banding. The microphysical evolution of precipitation is described in the context of the tropical‐ and midlatitude‐sourced moisture zones above and below the 0°C isotherm, respectively, identified in Part I. In the tropical‐sourced moisture zone, ice particles at temperatures less than −8°C had concentrations on the order of 10 L−1, with habits characteristic of lower temperatures, while between −8°C and −4°C, an order of magnitude increase in ice particle concentrations was observed, with columnar habits consistent with Hallett‐Mossop secondary ice formation. Ice particles falling though the 0°C level into the midlatitude‐sourced moisture region and melting provided "seed" droplets from which subsequent growth by collision‐coalescence occurred. In this region, raindrops grew to sizes of 3 mm and precipitation rates averaged 16 mm hr−1. Plain Language Summary: Atmospheric rivers (ARs) are long, narrow zones of enhanced horizontal poleward water vapor transport that have profound effects on the global hydrological cycle. This and a companion study are the first to analyze an AR affecting Australia and the Southern Ocean through an observational and modeling approach, respectively. There were two precipitation bands associated with the AR, which were primarily stratiform in character with an enhancement of radar reflectivity near the melting level. Part I showed that moisture in the upper part of the AR (above the 0°C level) was primarily sourced from the tropics, while moisture in the lower part (below the 0°C level) was primarily sourced from the middle latitudes. The microphysical evolution of precipitation through these two zones from cloud top to the ground within this AR is investigated. Ice particles forming and growing at altitudes above the 0°C level served as the "seeds" that allowed for further growth below the 0°C level as they melted and collided with droplets. The concentrations, shapes, and size distributions of the ice particles falling through each zone are presented and related to microphysical growth processes occurring within the bands. Key Points: The physical structure and microphysical properties of a summer austral atmospheric river are describedIce particle concentrations increased to >100 L−1 in the zone from −8°C to −4°C, consistent with secondary ice formationIce particles falling from the tropical‐sourced moisture stream aloft trigger coalescence below the 0°C isotherm [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) are a geochemically relevant family of semivolatile compounds originating from fossil fuels, biomass burning, and their incomplete combustion, as well as biogenic sources. Even though PAHs are ubiquitous in the environment, there are no previous studies of their occurrence in the Southern Ocean and Antarctic atmosphere. Here we show the gas and aerosol phase PAHs concentrations obtained from three sampling cruises in the Southern Ocean (Weddell, Bellingshausen, and South Scotia Seas), and two sampling campaigns at Livingston Island (Southern Shetlands). This study shows an important variability of the atmospheric concentrations with higher concentrations in the South Scotia and northern Weddell Seas than in the Bellingshausen Sea. The assessment of the gas-particle partitioning of PAHs suggests that aerosol elemental carbon contribution is modest due to its low concentrations. Over the ocean, the atmospheric concentrations do not show a temperature dependence, which is consistent with an important role of long-range atmospheric transport of PAHs. Conversely, over land at Livingston Island, the PAHs gas phase concentrations increase when the temperature increases, consistently with the presence of local diffusive sources. The use of fugacity samplers allowed the determination of the air-soil and air-snow fugacity ratios of PAHs showing that there is a significant volatilization of lighter molecular weight PAHs from soil and snow during the austral summer. The higher volatilization, observed in correspondence of sites where the organic matter content in soil is higher, suggests that there may be a biogenic source of some PAHs. The volatilization of PAHs from soil and snow is sufficient to support the atmospheric occurrence of PAHs over land but may have a modest regional influence on the atmospheric occurrence of PAHs over the Southern Ocean. [ABSTRACT FROM AUTHOR]

A scientific plan is presented that proposes the construction of carbon dioxide (CO2) deposition plants in the Antarctic for removing CO2 gas from Earth's atmosphere. The Antarctic continent offers the best environment on Earth for CO2 deposition at 1 bar of pressure and temperatures closest to that required for terrestrial air CO2 'snow' deposition-133 K. This plan consists of several components, including 1) air chemistry and CO2 snow deposition, 2) the deposition plant and a closed-loop liquid nitrogen refrigeration cycle, 3) the mass storage landfill, 4) power plant requirements, 5) prevention of dry ice sublimation, and 6) disposal (or use) of thermal waste. Calculations demonstrate that this project is worthy of consideration, whereby 446 deposition plants supported by sixteen 1200-MW wind farms can remove 1 billion tons (1012 kg) of carbon (1 GtC) annually (a reduction of 0.5 ppmv), which can be stored in an equivalent 'landfill' volume of 2 km × 2 km × 160 m (insulated to prevent dry ice sublimation). The individual deposition plant, with a 100 m × 100 m × 100 m refrigeration chamber, would produce approximately 0.4 m of CO2 snow per day. The solid CO2 would be excavated into a 380 m × 380 m × 10 m insulated landfill, which would allow 1 yr of storage amounting to 2.24 × 10−3 GtC. Demonstrated success of a prototype system in the Antarctic would be followed by a complete installation of all 446 plants for CO2 snow deposition and storage (amounting to 1 billion tons annually), with wind farms positioned in favorable coastal regions with katabatic wind currents. [ABSTRACT FROM AUTHOR]

In this study, we defined an index of the Antarctic Circumploar Wave (ACW) and analyzed its variability for the period 1951-2010. A regime shift of the circumpolar westerly in the Southern Ocean and an interdecadal change of the ACW, which occurred around the mid-1970s, were identified. Associated with these changes, the variations of the ACW show three distinct sub-periods: 1951-1973, 1974-1980, and 1981-2010. They are characterized by different speeds, amplitudes, and wave structures. We briefly investigated possible mechanisms responsible for the different behaviors of the ACW during the three periods. [ABSTRACT FROM AUTHOR]

The article offers the author's insights on the Southern Ocean Seasonal Cycle Experiment (SOSCEx), which is the main source of the global carbon dioxide (CO2) mitigation plans. The author says that the Southern Ocean plays an important role in the uptake of anthropogenic (CO2) and in the natural (CO2) exchange between the atmosphere and oceans. He mentions the main themes of SOSCEx including stratification dynamics, (CO2), and carbon export.

An eddy-resolving quasigeostrophic model of the Southern Ocean coupled to a dynamic atmospheric mixed layer is used to compare the performance of two different wind stress parameterization schemes. The first is the standard quadratic drag law, based on atmospheric velocity alone, whereas the second (more exact) formulation is based on the difference between ocean and atmosphere velocities. The two different schemes give very similar magnitudes of mean stress; however, the relative velocity scheme has substantially lower power input, resulting in a weaker eddy field, and consequently, greater circumpolar transport. These results are explored in terms of the existing theories of the Antarctic Circumpolar Current (including eddy saturation and eddy damping) and the implications for modeling the Southern Ocean are discussed. [ABSTRACT FROM AUTHOR]

We investigate the interannual variability in the flux of CO2 between the atmosphere and the Southern Ocean on the basis of hindcast simulations with a coupled physical-biogeochemical-ecological model with particular emphasis on the role of the Southern Annular Mode (SAM). The simulations are run under either pre-industrial or historical CO2 concentrations, permitting us to separately investigate natural, anthropogenic, and contemporary CO2 flux variability. We find large interannual variability (±0.19 PgC yr-1) in the contemporary air-sea CO2 flux from the Southern Ocean (<35°S). Forty-three percent of the contemporary air-sea CO2 flux variance is coherent with SAM, mostly driven by variations in the flux of natural CO2, for which SAM explains 48%. Positive phases of the SAM are associated with anomalous outgassing of natural CO2 at a rate of 0.1 PgC yr-1 per standard deviation of the SAM. In contrast, we find. an anomalous uptake of anthropogenic CO2 at a rate of 0.01 PgC yr-1 during positive phases of the SAM. This uptake of anthropogenic CO2 only slightly mitigates the outgassing of natural CO2, so that a positive SAM is associated with anomalous outgassing in contemporaneous times. The primary cause of the natural CO2 outgassing is anomalously high oceanic partial pressures of CO2 caused by elevated dissolved inorganic carbon (DIC) concentrations. These anomalies in DIC are primarily a result of the circulation changes associated with the southward shift and strengthening of the zonal winds during positive phases of the SAM. The secular, positive trend in the SAM has led to a reduction in the rate of increase of the uptake of CO2 by the Southern Ocean over the past 50 years. [ABSTRACT FROM AUTHOR]