Abstract
Abstract. In ice-covered regions it is challenging to determine constituent budgets – for heat and momentum, but also for biologically and climatically active gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we sought to evaluate if numerical model output helps us to better estimate the physical forcing that drives the air–sea gas exchange rate (k) in sea ice zones. We used the budget of radioactive 222Rn in the mixed layer to illustrate the effect that sea ice forcing has on gas budgets and air–sea gas exchange. Appropriate constraint of the 222Rn budget requires estimates of sea ice velocity, concentration, mixed-layer depth, and water velocities, as well as their evolution in time and space along the Lagrangian drift track of a mixed-layer water parcel. We used 36, 9 and 2 km horizontal resolution of regional Massachusetts Institute of Technology general circulation model (MITgcm) configuration with fine vertical spacing to evaluate the capability of the model to reproduce these parameters. We then compared the model results to existing field data including satellite, moorings and ice-tethered profilers. We found that mode sea ice coverage agrees with satellite-derived observation 88 to 98 % of the time when averaged over the Beaufort Gyre, and model sea ice speeds have 82 % correlation with observations. The model demonstrated the capacity to capture the broad trends in the mixed layer, although with a significant bias. Model water velocities showed only 29 % correlation with point-wise in situ data. This correlation remained low in all three model resolution simulations and we argued that is largely due to the quality of the input atmospheric forcing. Overall, we found that even the coarse-resolution model can make a modest contribution to gas exchange parameterization, by resolving the time variation of parameters that drive the 222Rn budget, including rate of mixed-layer change and sea ice forcings.
Highlights
The ocean surface is a dynamic region where momentum, heat and salt, as well as biogeochemical compounds, are exchanged with the atmosphere and with the deep ocean
We investigated mixed-layer depth (MLD) in A3 run compared to A2, and confirmed that the average MLD is the same between these two runs
We have used 36, 9 and 2 km versions of the ECCO ocean– sea ice coupled models based on the Massachusetts Institute of Technology general circulation model (MITgcm) to investigate whether numerical model outputs can be used to compensate for lack of data in constraining air–sea gas exchange rate in the Arctic
Summary
The ocean surface is a dynamic region where momentum, heat and salt, as well as biogeochemical compounds, are exchanged with the atmosphere and with the deep ocean. At the sea–air interface, gases of biogenic origin and geochemical significance are exchanged with the atmosphere. Theory indicates that the aqueous viscous sublayer, which has a length scale of 20 to 200 μm (Jähne and Haubecker, 1998), is the primary bottleneck for air–water exchange. Limitations in measurement at this critical scale have led to approximations of sea–air gas exchange based on indirect measurements. Four approaches involving data are typically used (Bender et al, 2011): (1) parametrization of the turbulent kinetic energy (TKE) at the base of the viscous sublayer, (2) tracing purposefully injected gases (Ho et al, 2006; Nightingale et al, 2000), (3) micro-meteorological methods (Zemmelink et al, 2006, 2008; Blomquist et al, 2010; Salter et al, 2011), and (4) radon deficit method. We examine the radon deficit method (4), together with a parameterization of the TKE forcing (1) that theoretically leads to the observed deficit in mixed-layer radon
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