Surface in situ measurements of atmospheric CO, CH4, and CO2 combined with selected meteorological measurements at the University of Wollongong, Australia

Wollongong, Australia is an urban site at the intersection of anthropogenic, biomass burning, biogenic and marine sources of atmospheric trace gases. The location offers a valuable opportunity to study drivers of atmospheric composition in the Southern Hemisphere. Here, a record of surface carbon monoxide (CO), methane (CH4) and carbon dioxide (CO2) was measured with an in situ Fourier transform infrared trace gas analyser between April 2011 and August 2014. Clean air was found to arrive at Wollongong in approximately 10% of air masses. Biomass burning influence was evident in the average annual cycle of clean air CO during austral spring. A significant negative short-term trend was found in clean air CO (-1.5 nmol/mol/a), driven by a reduction in northern Australian biomass burning. Significant short-term positive trends in clean air CH4 (5.4 nmol/mol/a) and CO2 (1.9 ?mol/mol/a) were consistent with the long-term global average trends. Polluted Wollongong air was investigated using wind-direction/wind-speed clustering, which revealed major influence from local urban and industrial sources from the south. High values of CH4, with anthropogenic DCH4/DCO2 enhancement ratio signatures, originated from the northwest, in the direction of local coal mining. A pollution climatology was developed for the region using back trajectory analysis and DO3/DCO enhancement ratios. Ozone production environments in austral spring and summer were associated with anticyclonic meteorology on the east coast of Australia, while ozone depletion environments in autumn and winter were associated with continental transport, or fast moving trajectories from southern latitudes. This implies the need to consider meteorological conditions when developing policies for controlling air quality.

Dust flux in the Arabian Sea

Eolian grain size and flux were measured on samples from 11 Arabian Sea sediment traps deployed 200-1250 km offshore. The timing of increased grain size is coincident with the onset of strong summer monsoon winds and dust storm activity over the Arabian Peninsula and Middle East. Data spanning a full annual cycle show that eolian grain size is highly correlated with barometric pressure (r=-0.91) and wind speed (r=0.84), enabling calibration of the downcore record in terms of these primary meteorological variables. Eolian flux is highly correlated with organic carbon flux (r=0.80); both increase 6-8 weeks after the grain size increase and summer monsoon onset. This lag, and the low correlation between eolian grain size and eolian flux (r=0.36), likely result from the differential sinking rates of large and small dust particles in the surface waters as well as biological scavenging associated with monsoon-induced productivity.

Working on a baseline for the Kongsfjorden food web: production and properties of macroalgal particulate organic matter (POM), supplementary material

Macroalgae, in particular kelps, produce a large amount of biomass in Kongsfjorden, which is to a great extent released into the water in an annual cycle. As an example, the brown alga Alaria esculenta loses its blade gradually, 3 ± 0.8 % of the blade area per day (August 2012), thereby adding to the pool of particulate organic matter (POM) in the fjord. Upon release small thallus pieces are "aging" in that they are prone to leaching and serving as substrate for microorganisms, thus turning into palatable food for suspension and bottom feeders. In order to define a macroalgal baseline for the Kongsfjorden food web, stable isotopes d14C and d15N were measured in individuals of A. esculenta, Saccharina latissima and Laminaria digitata directly sampled after collection and in artificially produced POM (aPOM) of A. esculenta that was allowed to age under experimental conditions. In aPOM from this species sampled in August 2012 the C/N ratios decreased between d1 and d8 of a 14-day culture period in parallel to the fading photosynthetic activity of the algal fragments as demonstrated by use of an Imaging-PAM. Microscopic observations of the aPOM in August 2012 and 2013 revealed the frequent occurrence of small brown algal endo- and epiphytes. First feeding experiments with Mysis oculata (Mysids) and Hiatella arctica (Bivalves) showed that these species can ingest macroalgal POM. The importance of kelp-derived POM for the food web is subject of the current research.

Global Ocean Particulate Organic Carbon flux merged with satellite parameters

The efficiency of the biological pump of carbon to the deep ocean depends largely on the biologically mediated export of carbon from the surface ocean and its remineralization with depth. Global satellite studies have primarily focused on chlorophyll concentration and net primary production (NPP) to understand the role of phytoplankton in these processes. Recent satellite retrievals of phytoplankton composition now allow for the size of phytoplankton cells to be considered. Here, we improve understanding of phytoplankton size structure impacts on particle export, remineralization and transfer. Particulate organic carbon (POC) flux observations from sediment traps and 234Th are compiled across the global ocean. Annual climatologies of NPP, percent microplankton, and POC flux at four time series locations and within biogeochemical provinces are constructed, and sinking velocities are calculated to align surface variables with POC flux at depth. Parameters that characterize POC flux vs. depth (export flux ratio, labile fraction, remineralization length scale) are then fit to the aligned dataset. Times of the year dominated by different size compositions are identified and fit separately in regions of the ocean where phytoplankton cell size showed enough dynamic range over the annual cycle. Considering all data together, our findings support the paradigm of high export flux but low transfer efficiency in more productive regions and vice versa for oligotrophic regions. However, when parsing by dominant size class, we find periods dominated by small cells to have both greater export flux and lower transfer efficiency than periods when large cells comprise a greater proportion of the phytoplankton community.

Optical depth of Atlantic dust as netCDF files

African dust outbreaks are the result of complex interactions between the land, atmosphere, and oceans, and only recently has a large body of work begun to emerge that aims to understand the controls on-and impacts of-African dust. At the same time, long-term records of dust outbreaks are either inferred from visibility data from weather stations or confined to a few in situ observational sites. Satellites provide the best opportunity for studying the large-scale characteristics of dust storms, but reliable records of dust are generally on the scale of a decade or less. Here the authors develop a simple model for using modern and historical data from meteorological satellites, in conjunction with a proxy record for atmospheric dust, to extend satellite-retrieved dust optical depth over the northern tropical Atlantic Ocean from 1955 to 2008. The resultant 54-yr record of dust has a spatial resolution of 1° and a monthly temporal resolution. From analysis of the historical dust data, monthly tropical northern Atlantic dust cover is bimodal, has a strong annual cycle, peaked in the early 1980s, and shows minimums in dustiness during the beginning and end of the record. These dust optical depth estimates are used to calculate radiative forcing and heating rates from the surface through the top of the atmosphere over the last half century. Radiative transfer simulations show a large net negative dust forcing from the surface through the top of the atmosphere, also with a distinct annual cycle, and mean tropical Atlantic monthly values of the surface forcing range from -3 to -9 W/m**2. Since the surface forcing is roughly a factor of 3 larger in magnitude than the top-of-the-atmosphere forcing, there is also a positive heating rate of the midtroposphere by dust.

Temperature time series and physical properties of snow samples of a high-arctic permafrost site on Svalbard, Norway

Independent measurements of radiation, sensible and latent heat fluxes and the ground heat flux are used to describe the annual cycle of the surface energy budget at a high-arctic permafrost site on Svalbard. During summer, the net short-wave radiation is the dominant energy source, while well developed turbulent processes and the heat flux in the ground lead to a cooling of the surface. About 15% of the net radiation is consumed by the seasonal thawing of the active layer in July and August. The Bowen ratio is found to vary between 0.25 and 2, depending on water content of the uppermost soil layer. During the polar night in winter, the net long-wave radiation is the dominant energy loss channel for the surface, which is mainly compensated by the sensible heat flux and, to a lesser extent, by the ground heat flux, which originates from the refreezing of the active layer. The average annual sensible heat flux of -6.9 W/m**2 is composed of strong positive fluxes in July and August, while negative fluxes dominate during the rest of the year. With 6.8 W/m**2, the latent heat flux more or less compensates the sensible heat flux in the annual average. Strong evaporation occurs during the snow melt period and particularly during the snow-free period in summer and fall. When the ground is covered by snow, latent heat fluxes through sublimation of snow are recorded, but are insignificant for the average surface energy budget. The near-surface atmospheric stratification is found to be predominantly unstable to neutral, when the ground is snow-free, and stable to neutral for snow-covered ground. Due to long-lasting near-surface inversions in winter, an average temperature difference of approximately 3 K exists between the air temperature at 10 m height and the surface temperature of the snow.

Ice and snow characteristics in Prydz Bay in 2006

Under the Chinese National Antarctic Research Expedition program in 2006, the annual thermal mass balance of landfast ice in the vicinity of Zhongshan Station, Prydz Bay, east Antarctica, was investigated. Sea ice formed from mid-February onward, and maximum ice thickness occurred in late November. Snow cover remained thin, and blowing snow caused frequent redistribution of the snow. The vertical ice salinity showed a 'question-mark-shaped' profile for most of the ice growth season, which only turned into an 'I-shaped' profile after the onset of ice melt. The oceanic heat flux as estimated from a flux balance at ice-ocean interface using internal ice temperatures decreased from 11.8 (±3.5) W/m**2 in April to an annual minimum of 1.9 (±2.4) W/m**2 in September. It remained low through late November, in mid-December it increased sharply to about 20.0 W/m**2. Simulations applying the modified versions of Stefan's law, taking account the oceanic heat flux and ice-atmosphere coupling, compare well with observed ice growth. There was no obvious seasonal cycle for the thermal conductivity of snow cover, which was also derived from internal ice temperatures. Its annual mean was 0.20 (±0.04) W/m/°C.

Incubation experiments to examine the production and loss of CH3I in surface seawater

In order to investigate production pathways of methyl iodide and controls on emissions from the surface ocean, a set of repeated in-vitro incubation experiments were performed over an annual cycle in the context of a time-series of in-situ measurements in Kiel Fjord (54.3 N, 10.1E). The incubation experiments revealed a diurnal variation of methyl iodide in samples exposed to natural light, with maxima during day time and losses during night hours. The amplitude of the daily accumulation varied seasonally and was not affected by filtration (0.2µm), consistent with a photochemical pathway for CH3I production. The methyl iodide loss rate during night time correlated with the concentration accumulated during daytime. Daily (24 hour) net production (Pnet) was similar in magnitude between in vitro and in situ mass balances. However, the estimated gross production (Pgross) of methyl iodide ranged from -0.07 to 2.24 pmol/day and were 5 times higher in summer than Pnet calculated from the in-situ study [Shi et al., 2014]. The large excess of Pgross over Pnet revealed by the in-vitro (incubation) experiments in summer is a consequence of large losses of CH3I by as-yet uncharacterized processes (e.g. biological degradation or chemical pathways other than Cl- substitution).

Sediment color variation in laminated Holocene sediments of ODP Site 178-1098

Continuous sediment color records with a resolution of one measurement per millimeter were generated for Site 1098 (Palmer Deep, Antarctic Peninsula) from digital images of the core surfaces to test if the laminated intervals at this site will allow for analysis of high-frequency climate variability in the Circum-Antarctic. Long-term variation in color values correlates with gamma-ray attenuation bulk density. Darker colors are found in laminated intervals with lower bulk density, high biogenic silica, and high total organic carbon content. Darker color values result from the addition of dark laminae to background sediments that show little variation in color. The thicknesses of dark and light laminae were measured in the top 25 meters composite depth to determine the temporal resolution of the laminae. The alternation between dark, biogenic-rich laminae and background sediment essentially represents an annual cycle, but the sediment is not consistently varved. The modal thickness of light laminae is close to the long-term average annual accumulation rate, and results indicate that approximately half of the dark/light couplets in distinctly laminated intervals represent a single year. Missing biogenic laminae are interpreted to represent reduced primary productivity during cold years with delayed breakup of the sea-ice cover.

Raw data of SCADCP (self-contained Acoustic Doppler Current Profiler) from three moorings in the Eastern Weddell Sea, 2005-2008

The success of any efforts to determine the effects of climate change on marine ecosystems depends on understanding in the first instance the natural variations, which contemporarily occur on the interannual and shorter time scales. Here we present results on the environmental controls of zooplankton distribution patterns and behaviour in the eastern Weddell Sea, Southern Ocean. Zooplankton abundance and vertical migration are derived from the mean volume backscattering strength (MVBS) and the vertical velocity measured by moored acoustic Doppler current profilers (ADCPs), which were deployed simultaneously at 64°S, 66.5°S and 69°S along the Greenwich Meridian from February, 2005, until March, 2008. While these time series span a period of full three years they resolve hourly changes. A highly persistent behavioural pattern found at all three mooring locations is the synchronous diel vertical migration (DVM) of two distinct groups of zooplankton that migrate between a deep residence depth during daytime and a shallow depth during nighttime. The DVM was closely coupled to the astronomical daylight cycles. However, while the DVM was symmetric around local noon, the annual modulation of the DVM was clearly asymmetric around winter solstice or summer solstice, respectively, at all three mooring sites. DVM at our observation sites persisted throughout winter, even at the highest latitude exposed to the polar night. Since the magnitude as well as the relative rate of change of illumination is minimal at this time, we propose that the ultimate causes of DVM separated from the light-mediated proximal cue that coordinates it. In all three years, a marked change in the migration behaviour occurred in late spring (late October/early November), when DVM ceased. The complete suspension of DVM after early November is possibly caused by the combination of two factors: (1) increased availability of food in the surface mixed layer provided by the phytoplankton spring bloom, and (2) vanishing diurnal enhancement of the threat from visually oriented predators when the illumination is quasi-continuous during the polar and subpolar summer. Zooplankton abundance in the water column, estimated as the mean MVBS in the depth range 50-300 m, was highest end of summer and lowest mid to end winter on the average annual cycle. However, zooplankton abundance varied several-fold between years and between locations. Based on satellite and in situ data of chlorophyll and sea ice as well as on hydrographic measurements, the interannual and spatial variations of zooplankton mean abundance can be explained by differences in the magnitude of the phytoplankton spring bloom, which develops during the seasonal sea ice retreat. Whereas the vernal ice melt appears necessary to stimulate the blooming of phytoplankton, it is not the determinator of the blooms magnitude, its areal extent and duration. A possible explanation for the limitation of the phytoplankton bloom in some years is top-down control. We hypothesise that the phytoplankton spring development can be curbed by grazing when the zooplankton had attained high abundance by growth during the preceding summer.

On the Southern Ocean CO2 uptake and the role of the biological carbon pump in the 21st century, links to supplementary material

We use a suite of eight ocean biogeochemical/ecological general circulation models from the MAREMIP and CMIP5 archives to explore the relative roles of changes in winds (positive trend of Southern Annular Mode, SAM) and in warming- and freshening-driven trends of upper ocean stratification in altering export production and CO2 uptake in the Southern Ocean at the end of the 21st century. The investigated models simulate a broad range of responses to climate change, with no agreement ona dominance of either the SAM or the warming signal south of 44° S. In the southernmost zone, i.e., south of 58° S, they concur on an increase of biological export production, while between 44 and 58° S the models lack consensus on the sign of change in export. Yet, in both regions, the models show an enhanced CO2 uptake during spring and summer. This is due to a larger CO 2 (aq) drawdown by the same amount of summer export production at a higher Revelle factor at the end of the 21st century. This strongly increases the importance of the biological carbon pump in the entire Southern Ocean. In the temperate zone, between 30 and 44° S all models show a predominance of the warming signal and a nutrient-driven reduction of export production. As a consequence, the share of the regions south of 44° S to the total uptake of the Southern Ocean south of 30° S is projected to increase at the end of the 21st century from 47 to 66% with a commensurable decrease to the north. Despite this major reorganization of the meridional distribution of the major regions of uptake, the total uptake increases largely in line with the rising atmospheric CO2. Simulations with the MITgcm-REcoM2 model show that this is mostly driven by the strong increase of atmospheric CO2, with the climate-driven changes of natural CO2 exchange offsetting that trend only to a limited degree (~10%) and with negligible impact of climate effects on anthropogenic CO2 uptake when integrated over a full annual cycle south of 30° S.

DEP-derived density from two snow trenches from Kohnen Station, Dronning Maud Land, Antarctica from the 2012/13 field season

The density of firn is an important property for monitoring and modeling the ice sheet as well as to model the pore close-off and thus to interpret ice core-based greenhouse gas records. One feature, which is still in debate, is the potential existence of an annual cycle of firn density in low-accumulation regions. Several studies describe or assume seasonally successive density layers, horizontally evenly distributed, as seen in radar data. On the other hand, high-resolution density measurements on firn cores in Antarctica and Greenland showed no clear seasonal cycle in the top few meters. A major caveat of most existing snow-pit and firn-core based studies is that they represent one vertical profile from a laterally heterogeneous density field. To overcome this, we created an extensive dataset of horizontal and vertical density data at Kohnen Station, Dronning Maud Land on the East Antarctic Plateau. We drilled and analyzed three 90 m long firn cores as well as 160 one meter long vertical profiles from two elongated snow trenches to obtain a two dimensional view of the density variations. The analysis of the 45 m wide and 1 m deep density fields reveals a seasonal cycle in density. However, the seasonality is overprinted by strong stratigraphic noise, making it invisible when analyzing single firn cores. Our density dataset extends the view from the local ice-core perspective to a hundred meter scale and thus supports linking spatially integrating methods such as radar and seismic studies to ice and firn cores.