Sea ice and its effect on CO₂ flux between the atmosphere and the Southern Ocean interior

Abstract

[1] The advance and retreat of sea ice produces seasonal convection and stratification, dampens surface waves and creates a separation between the ocean and atmosphere. These are all phenomena that can affect the air-sea gas transfer velocity (k 660 ), and therefore it is not straightforward to determine how sea ice cover modulates air-sea flux. In this study we use field estimates k 660 to examine how sea ice affects the net gas flux between the ocean and atmosphere. An inventory of salinity, 3 He, and CFC-11 in the mixed layer is used to infer k 660 during the drift of Ice Station Weddell in 1992. The average of k 660 is 0.11 m d −1 across nearly 100% ice cover. In comparison, the only prior field estimates of k 660 are disproportionately larger, with average values of 2.4 m d −1 across 90% sea ice cover, and 3.2 m d −1 across approximately 70% sea ice cover. We use these values to formulate two scenarios for the modulation of k 660 by the fraction of sea ice cover in a 1-D transport model for the Southern Ocean seasonal ice zone. Results show the net CO 2 flux through sea ice cover represents 14-46% of the net annual air-sea flux, depending on the relationship between sea ice cover and k 660 . The model also indicates that as much as 68% of net annual CO 2 flux in the sea ice zone occurs in the springtime marginal ice zone, which demonstrates the need for accurate parameterizations of gas flux and primary productivity under partially ice-covered conditions. Citation: Loose, B., and P. Schlosser (2011), Sea ice and its effect on CO 2 flux between the atmosphere and the Southern Ocean interior,

Currents and convection cause enhanced gas exchange in the ice–water boundary layer

“…At this stage, there are only a handful of estimates of k or k eff in the sea ice zone, and even fewer estimates of k ice . These studies indicate that gas exchange can still occur near 100 % ice cover (Loose and Schlosser, 2011) and k eff for ice covers !50 % is either highly variable or still poorly constrained (Fanning and Torres, 1991;Rutgers van der Loeff et al, 2014). Recently, Butterworth and Miller (2016) used the eddy covariance method to find a linear relationship between k and ice cover, but the study does not distinguish between airÁsea CO 2 flux and airÁice CO 2 flux, and airÁice fluxes of CO 2 are not negligible (Else et al, 2011;Miller et al, 2011;Delille et al, 2014).…”

“…with a 2-D model of the shallow water flow equations with variable density (Loose et al, 2005). The model produces a horizontal flow field that represents the vertically averaged velocity at each position in the tank (Fig.…”

Role of sea ice in global biogeochemical cycles: emerging views and challenges

“…However, lab experiments [Loose et al, 2009] indicate that such a relation is non-linear, although the underlying principles remain unclear. The processes that may contribute to non-linearities include dynamical fluxes through the sea ice cover [e.g., Nomura et al, 2010;Papakyriakou and Miller, 2011], but also changes in convective activity or stratification in the water column [Loose and Schlosser, 2011]. In addition, the sign of FCO 2 changes with location and time of the season, whereas the absolute value of FCO 2 may span several orders of magnitude.…”

Sea–air CO<sub>2</sub> fluxes in the Southern Ocean for the period 1990–2009

“…Sea-ice cover inhibits sea-air gas exchange in winter, although there is considerable uncertainty as to how the ice cover impacts sea-air fluxes at high latitudes in winter months (e.g. Rysgaard et al, 2011 andLoose andSchlosser, 2011). In the permanently ice free region of the AZ, pCO 2 tends to be lower than the atmosphere in summer owing to net biological production Metzl et al, 2006;Sokolov, 2008) and higher than the atmosphere in winter due to deep winter mixing.…”