Ingunn Skjelvan

Direct measurements of the CO2 flux over the ocean: Development of a novel method

Over the ocean, eddy correlation measurements of the air-sea CO2 flux obtained with open-path sensors have typically been an order of magnitude larger than those estimated by other techniques or sensors. It is shown here that this discrepancy is due to cross sensitivity to water vapor fluctuations: a novel correction procedure is demonstrated, tested against an independent data set and proved to be robust. After correction, the observed gas transfer velocities are in reasonable agreement with published values obtained using closed-path sensors or by tracer techniques. Data from open-path sensors may now be used for air-sea CO2 flux estimation, greatly increasing the information available on air-sea gas transfer velocity.

PHYSICAL EXCHANGES AT THE AIR-SEA INTERFACE UK-SOLAS Field Measurements

As part of the U.K. contribution to the international Surface Ocean–Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosol-size spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.

Annual carbon fluxes in the upper Greenland Sea based on measurements and a box-model approach

Annual carbon flux in the upper Greenland Sea based on measurements and a box model approach

Open ocean gas transfer velocity derived from long‐term direct measurements of the CO2 flux

Air-sea open ocean CO2 flux measurements have been made using the Eddy Covariance (EC) technique onboard the weathership Polarfront in the North Atlantic between September 2006 and December 2009. Flux measurements were made using an autonomous system ‘AutoFlux’. CO2 mass density was measured with an open-path infrared gas analyzer. Following quality control procedures, 3938 20-minute flux measurements were made at mean wind speeds up to 19.6 m/s, significantly higher wind speeds than previously published results. The uncertainty in the determination of gas transfer velocities is large, but the mean relationship to wind speed allows a new parameterisation of the gas transfer velocity to be determined. A cubic dependence of gas transfer on wind speed is found, suggesting a significant influence of bubble-mediated exchange on gas transfer.

The Nordic Seas carbon budget: Sources, sinks, and uncertainties

[1] A carbon budget for the Nordic Seas is derived by combining recent inorganic carbon data from the CARINA database with relevant volume transports. Values of organic carbon in the Nordic Seas’ water masses, the amount of carbon input from river runoff, and the removal through sediment burial are taken from the literature. The largest source of carbon to the Nordic Seas is the Atlantic Water that enters the area across the Greenland-Scotland Ridge; this is in particular true for the anthropogenic CO2. The dense overflows into the deep North Atlantic are the main sinks of carbon from the Nordic Seas. The budget show that presently 12.3 ± 1.4 Gt C yr −1 is transported into the Nordic Seas and that 12.5 ± 0.9 Gt C yr −1 is transported out, resulting in a net advective carbon transport out of the Nordic Seas of 0.17 ± 0.06 Gt C yr −1 . Taking storage into account, this implies a net air-to-sea CO2 transfer of 0.19 ± 0.06 Gt C yr −1 into the Nordic Seas. The horizontal transport of carbon through the Nordic Seas is thus approximately two orders of magnitude larger than the CO2 uptake from the atmosphere. No difference in CO2 uptake was found between 2002 and the preindustrial period, but the net advective export of carbon from the Nordic Seas is smaller at present due to the accumulation of anthropogenic CO2.

Oxygen fluxes in the Norwegian Atlantic Current

Oxygen and phosphate measurements from two sections across the Norwegian Atlantic Current, the Gimsoy-NW section from 67.5°N 9°E to 71.5°N 1°E and the Bjornoya-W section along 74.5°N from 7 to 15°E, are used to estimate oxygen fluxes in the surface layer and between the atmosphere and the ocean. Vertical entrainment velocities of 0.9 m day−1 for the winter season and 0.1 m day−1 for the summer season are found and applied to the upper 300 m. The resulting oxygen fluxes to the surface layer driven by this vertical mixing are 0.58±0.05 and 0.27±0.02 mol O2 m−2 year−1 at the Gimsoy-NW and Bjornoya-W sections, respectively. Oxygen fluxes to the surface layer due to phytoplankton production are 2.6 and 3.4 mol O2 m−2 year−1, which represent the net community production at the two sections. Estimated uncertainties in these numbers are ±15%. The surface water is a sink for atmospheric oxygen during fall and winter and a source during the productive season for both sections. On an annual basis there is a net uptake of oxygen from the atmosphere, 3.4±0.4 mol O2 m−2 year−1 at the Gimsoy-NW section and 4.9±0.5 mol O2 m−2 year−1 at the Bjornoya-W. A decrease in temperature of 1°C to 1.5°C seen between the Gimsoy-NW section and the Bjornoya-W section is the main reason for the increased atmospheric flux of oxygen at the latter section. An oxygen budget made for the area bounded by the two sections gives a net advective flux of oxygen out of the area of approximately 10 mol O2 m−2 year−1. The increased concentration of oxygen corresponding to the decrease in surface layer temperatures going northwards in the Norwegian Atlantic Current is mainly attributed to the air–sea oxygen exchange and phytoplankton production in this area.

A Review of the Inorganic Carbon Cycle of the Nordic Seas and Barents Sea

Studies of the inorganic carbon cycle have been performed in the Nordic Seas and the Barents Sea since the 1980s. Here we present a review over current knowledge on carbon transport between the different reservoirs in the area, and the transformation of inorganic carbon to organic matter. The carbon transport and transformation are closely related to other biogeochemical processes in the region, for instance air-sea gas exchange and primary production. Horizontal transport of carbon through the Nordic Seas (about 9 Pg C yr -1 ) is orders of magnitude larger than transports due to air-sea gas exchange and deep mixing. The Nordic Seas and the Barents Sea are sinks for atmospheric carbon dioxide throughout the year (in the range of 20-85 g C m -2 yr -1 , for the different regions). However, the strength of the sink appears to be decreasing, at least for the Nordic Seas. The carbon transport due to deep-water formation has been estimated to 0.12 Pg C for the year 1994 in the Greenland Sea; however, this will vary with location, duration, and magnitude of deep mixing, showing large fluctuations. Carbon is also sequestered in the Nordic Seas and the Barents Sea due to biological activity, and the export production out of the surface layers in this area is in the range of 15-75 g C m -2 yr -1 . Rising atmospheric carbon dioxide levels and changes in climate will to a varying degree affect the different processes, and this is briefly addressed in the paper.

/The Nordic Seas : An overview

The aim of this overview paper is to provide a brief synthesis of the five review papers contained in the monograph. Prevailing south-westerly winds, oceanic flow patterns, and oceanic summer heat storage make the Nordic Seas region having temperatures 10 to 20 °C above the mean temperature at similar latitudes. The combination of the large heat import from south and the polar location implies that the region is prone to natural climate variations and particularly vulnerable for external forcings. Proxy data for the Holocene epoch indeed reveal large high-frequency climate fluctuations, as well as long-term variations spanning the ‘medieval warm period’ and the ‘little ice age’. In phase with a strengthening of the westerly winds since the 1960s, several oceanic key variables show trends unprecedented in available instrumental records, some of which extends back 50-100 years. State of the art climate models indicate that several of the changes may be linked to increased greenhouse gas forcing, and are therefore likely to be sustained or even amplified in the future. Furthermore, the marine cycling of carbon, and by that the major greenhouse gas carbon dioxide, is closely linked to the climate state of the region. The Nordic Seas region is, as one of few ocean locations, a sink for atmospheric carbon dioxide throughout the year. With the rapid developments in data acquisition, computational resources, and societal concerns for climate change and environmental issues, the review papers give an updated account of the present knowledge of the complex climate states of the Nordic Seas, and how the Nordic Seas influence the climate outside the region.

Constraints on Carbon Drawdown and Export in the Greenland Sea

Data on the inorganic carbon system, the distribution of oxygen, nitrate, and phosphate, as well as particle sedimentation and plankton biomass collected from winter 1993 to summer 1996 in the central Greenland Sea show that although this area is a sink for atmospheric carbon throughout the year, relatively little of the carbon fixed by photosynthesis into organic compounds in the surface waters is eventually sequestered in deep waters. Rather, due to intensive biological remineralization of organic matter within the winter mixed layer, the bulk of carbon is retained in the upper few hundred meters of the water column. The sequestration of biogenic carbon is constrained by the depth of the winter mixed layer, in that deep winter mixing effectively increases the depth below which true export can occur. There is potential for increased export with increased rates of deep convection. Likewise, a reduction in heterotrophic recycling in near-surface waters may enhance the effectiveness of the biological pump. However, because of our still limited understanding of the interactions between the biological and solubility pumps in this region, the extent to which export may be enhanced is unclear.

Effects of sea‐ice and biogeochemical processes and storms on under‐ice water fCO2 during the winter‐spring transition in the high Arctic Ocean: Implications for sea‐air CO2 fluxes

We performed measurements of carbon dioxide fugacity (fCO2) in the surface water under Arctic sea ice from January to June 2015 during the Norwegian young sea ICE (N-ICE2015) expedition. Over this period, the ship drifted with four different ice floes and covered the deep Nansen Basin, the slopes north of Svalbard and the Yermak Plateau. This unique winter-to-spring dataset includes the first winter-time under-ice water fCO2 observations in this region. The observed under-ice fCO2 ranged between 315 µatm in winter and 153 µatm in spring, hence was undersaturated relative to the atmospheric fCO2. Although the sea ice partly prevented direct CO2 exchange between ocean and atmosphere, frequently occurring leads and breakup of the ice sheet promoted sea-air CO2 fluxes. The CO2 sink varied between 0.3 and 86 mmol C m−2 d−1, depending strongly on the open-water fractions (OW) and storm events. The maximum sea-air CO2 fluxes occurred during storm events in February and June. In winter, the main drivers of the change in under-ice water fCO2 were dissolution of CaCO3 (ikaite) and vertical mixing. In June, in addition to these processes, primary production and sea-air CO2 fluxes were important. The cumulative loss due to CaCO3 dissolution of 0.7 mol C m−2 in the upper 10 m played a major role in sustaining the undersaturation of fCO2 during the entire study. The relative effects of the total fCO2 change due to CaCO3 dissolution was 38%, primary production 26%, vertical mixing 16%, sea-air CO2 fluxes 16%, and temperature and salinity insignificant. This article is protected by copyright. All rights reserved.