Determination of air‐sea ice transfer coefficient for CO2: Significant contribution of gas bubble transport during sea ice growth

Air‐ice CO2 fluxes were measured continuously from the freezing of a young sea‐ice cover until its decay. Cooling seawater was as a sink for atmospheric CO2 but asthe ice crystalsformed,sea ice shifted to a source releasing CO2 to the atmosphere throughout the whole ice growth. Atmospheric warming initiated the decay, re‐shifting sea‐ice to a CO2 sink. Combining these CO2 fluxes with the partial pressure of CO2 within sea ice, we determined gas transfer coefficients for CO2 at air‐ice interface for growth and decay. We hypothesize that this difference originates from the transport of gas bubbles during ice growth, while only diffusion occurs during ice melt. In parallel, we used a 1D biogeochemical model to mimic the observed CO2 fluxes. The formation of gas bubbles was crucial to reproduce fluxes during ice growth where gas bubbles may account for up to 92 % of the upward CO2 fluxes

Antarctic sea ice trophic status

This study focuses on analyses and validation of 1 month forecasts (OMFs) of weak Indian monsoons based on 10 member ensemble hindcasts (retrospective forecasts) of the NCEP Climate Forecast System (CFS) model for the period 1981–2008. The weak monsoon episodes chosen for the analysis correspond to summer monsoon months which were characterized by significant deficits in the All-India monthly rainfall of − 20% of the climatological normal. Examination of the CFS-OMFs shows poor skill of the model in capturing the observed rainfall and circulation anomalies during weak monsoons. The present analysis suggests that deficiencies in realistically capturing the ocean-atmosphere coupling in the tropical Indian Ocean (IO) introduces biases in simulating sea surface temperature and rainfall anomalies in the equatorial region, which in turn affects the monsoon precipitation forecasts over the sub-continent. In particular, the mean thermocline in the near-equatorial IO is found to be practically flat in the CFS model, so that the near-equatorial anomalies in the model are not strong enough to weaken the summer monsoon circulation and reduce the monsoon precipitation over India. By examining a 100 year free run of the CFS model, it is seen that moderate monsoon-droughts simulated by the model have weak teleconnections with the equatorial IO dynamics. On the other hand, intense monsoon-droughts in the CFS-model are found be remarkably linked with the equatorial IO anomalies. It is suggested that improving the slope of the equatorial IO thermocline and allowing for more realistic IO-monsoon coupling in the CFS-model would be an important step for improving the skill of extended-range monsoon forecasts

Assessing the O2 budget under sea ice: An experimental and modelling approach

The objective of this study was to assess the O2 budget in the water under sea ice combining observations and modelling. Modelling was used to discriminate between physical processes, gas-specific transport (i.e., ice-atmosphere gas fluxes and gas bubble buoyancy) and bacterial respiration (BR) and to constrain bacterial growth efficiency (BGE). A module describing the changes of the under-ice water properties, due to brine rejection and temperature-dependent BR, was implemented in the one-dimensional halo-thermodynamic sea ice model LIM1D. Our results show that BR was the dominant biogeochemical driver of O2 concentration in the water under ice (in a system without primary producers), followed by gas specific transport. The model suggests that the actual contribution of BR and gas specific transport to the change in seawater O2 concentration was 37% during ice growth and 48% during melt. BGE in the water under sea ice, as retrieved from the simulated O2 budget, was found to be between 0.4 and 0.5, which is in line with published BGE values for cold marine waters. Given the importance of BR to seawater O2 in the present study, it can be assumed that bacteria contribute substantially to organic matter consumption and gas fluxes in ice-covered polar oceans. In addition, we propose a parameterization of polar marine bacterial respiration, based on the strong temperature dependence of bacterial respiration and the high growth efficiency observed here, for further biogeochemical ocean modelling applications, such as regional or large-scale Earth System model

Assessing the O2 budget under sea ice: An experimental and modelling approach

The objective of this study was to assess the O2 budget in the water under sea ice combining observations and modelling. Modelling was used to discriminate between physical processes, gas-specific transport (i.e., ice-atmosphere gas fluxes and gas bubble buoyancy) and bacterial respiration (BR) and to constrain bacterial growth efficiency (BGE). A module describing the changes of the under-ice water properties, due to brine rejection and temperature-dependent BR, was implemented in the one-dimensional halo-thermodynamic sea ice model LIM1D. Our results show that BR was the dominant biogeochemical driver of O2 concentra- tion in the water under ice (in a system without primary producers), followed by gas specific transport. The model suggests that the actual contribution of BR and gas specific transport to the change in seawater O2 concentration was 37% during ice growth and 48% during melt. BGE in the water under sea ice, as retrieved from the simulated O2 budget, was found to be between 0.4 and 0.5, which is in line with published BGE values for cold marine waters. Given the importance of BR to seawater O2 in the present study, it can be assumed that bacteria contribute substantially to organic matter consumption and gas fluxes in ice-covered polar oceans. In addition, we propose a parameterization of polar marine bacterial respiration, based on the strong temperature dependence of bacterial respiration and the high growth efficiency observed here, for further biogeochemical ocean modelling applications, such as regional or large-scale Earth System model

Methane Chemistry in the Ice Covered Arctic Ocean from Winter to Summer Time

Methane is a powerful greenhouse gas. In the ocean, it originates from gas bearing sediments, can be produced by microorganisms in aerobic water column or released during sea ice formation in polar regions. Sea ice in the Arctic Ocean can act as a barrier for the oceanic methane to be released to the atmosphere. Thus, beneath the sea ice cover, methane can accumulate during winter time and emit in large volumes to the atmosphere when openings occur in the ice cover or when the ice starts melting. In this study we show unique data on dissolved methane concentrations in seawater and sea ice, collected during N-ICE2015 campaign that lasted from January to June 2015, with sampling from ice floes drifting in the Nansen basin, between 80 and 83°N. We found seawater methane concentrations generally 3 times higher than previously reported for the central Arctic Ocean. Elevated methane concentrations in bottom waters were only found in the area of the shallower Yermak plateau margin, where methane can potentially originate from sediments. Highest methane concentrations were found in surface waters in January north of 83° N beneath sea ice cover, showing that methane accumulates under sea ice cover in winter time

Drivers of inorganic carbon dynamics in first-year sea ice: a model study

Sea ice is an active source or a sink for carbon dioxide (CO2), although to what extent is not clear. Here, we analyze CO2 dynamics within sea ice using a one-dimensional halothermodynamic sea ice model including gas physics and carbon biogeochemistry. The ice-ocean fluxes, and vertical transport, of total dissolved inorganic carbon (DIC) and total alkalinity (TA) are represented using fluid transport equations. Carbonate chemistry, the consumption, and release of CO2 by primary production and respiration, the precipitation and dissolution of ikaite (CaCO3·6H2O) and ice-air CO2 fluxes, are also included. The model is evaluated using observations from a 6 month field study at Point Barrow, Alaska, and an ice-tank experiment. At Barrow, results show that the DIC budget is mainly driven by physical processes, wheras brine-air CO2 fluxes, ikaite formation, and net primary production, are secondary factors. In terms of ice-atmosphere CO2 exchanges, sea ice is a net CO2 source and sink in winter and summer, respectively. The formulation of the ice-atmosphere CO2 flux impacts the simulated near-surface CO2 partial pressure (pCO2), but not the DIC budget. Because the simulated ice-atmosphere CO2 fluxes are limited by DIC stocks, and therefore -2 d-1, we argue that the observed much larger CO2 fluxes from eddy covariance retrievals cannot be explained by a sea ice direct source and must involve other processes or other sources of CO2. Finally, the simulations suggest that near-surface TA/DIC ratios of ~2, sometimes used as an indicator of calcification, would rather suggest outgassing

Gases in sea ice: Update of recent findings, caveats and open questions

Sea ice exchanges gases with the atmosphere including climate and ozone-depleting gases We will rapidly present a state of the art of current large scale assessment of spring and summer uptake of atmospheric CO2. We will challenge these assessments with 1) new evidence of significant winter CO2 release for winter experiments 2) role of bubbles formation and transport within sea ice 3) impact of biofilm. Finally, comparison of air-ice fluxes derived from automated chamber and micrometeorological method and, mechanistic and box models show significant discrepancies that suggest that the contribution of sea ice to the air-ocean fluxes of CO2 remain an open question. We will also sea ice contribution to the fluxes of other gases as CH4, N2O, DMS and VOC

The role of sea ice in the carbon cycle of Polar Seas: 1D to 3D modelling

Sea ice participates actively in the biogeochemical cycle of carbon of Polar Oceans, yet to which extent is not clear. We investigated the processes that govern sea ice carbon dy- namics in Polar Regions through 1D to 3D modelling developments. First, we constrained all major physical and biogeochemical processes with respect to CO2 dynamics (carbon- ate chemistry, biological activity, ikaite (CaCO3•6H2O) precipitation and dissolution and ocean-ice-air CO2 fluxes) in a one-dimensional sea ice model. According to our model, the CO2 budget is driven by the CO2 uptake during ice growth and release by brine drainage, whereas other processes such as brine-air CO2 fluxes, despite significant, are secondary. Subsequently, based on these preliminary conclusions, we evaluated the role of sea ice in the carbon dynamics of Polar Oceans by using an ocean-ice coupled Global Earth System Model. Carbon dynamics (e.g. ocean-atmosphere CO2 fluxes) are driven by the contribution of sea ice growth regions in the Arctic Ocean (mainly the Siberian coast) and sea ice melt regions in the Southern Ocean (off the coast of Antarctica). In addition, the production of deep waters is low in the Arctic Ocean but significant in the Southern Ocean. Therefore, sea ice only contributes to the deep water export of carbon in the Southern Ocean. The role of sea ice in the biogeochemical cycle of carbon is significant and its representation by Global Earth System Models should be improved