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Beaufort Gyre: mechanisms of fresh water accumulation and release

A. Proshutinsky (ocean), W. Hibler (ice), R. Forsberg, A. Jahn, E. Watanabe, S. Hakkinen

Hydrographic climatology shows that due to a salinity minimum which extends from the surface to approximately 400m depth, the Canada Basin contains about 45,000 km3 of fresh water. This value is calculated relative to a reference mean salinity (34.8) of the Arctic Ocean and specifies how much fresh water is accumulated in this region from different sources (ice melting and freezing, rivers, atmospheric precipitation and water transport from the Pacific and Atlantic Oceans via straits). Proshutinsky et al. (2002) hypothesized that in winter, the wind in the Canada Basin drives sea ice and ocean  in a clockwise sense, accumulating freshwater in the Beaufort Gyre (BG) through Ekman convergence and subsequent downwelling. In summer, winds are weaker and the BG releases fresh water. At the same time, thermodynamic processes may also be important - in winter, ice growth and subsequent salt release reduce the FWC of the BG, and in summer, ice melt increases the FWC. The interplay between dynamic- and thermodynamic forcing is undoubtedly complex. This problem can be solved by AOMIP coordinated experiments specifically designed to understand the major mechanisms of fresh water accumulation and release in the BG Region. Appendix C.6 describes conditions of these experiments. See below.

Beaufort Gyre fresh water transformations: accumulation and release mechanisms

a.      Questions:

  • How well do models reproduce the variability of the FW storage in the Arctic, compared to observed FW content and SSH variability (interannual and seasonal time scale)?
  • What % of freshwater in the Beaufort Gyre is due to ice melt,  precipitation, river runoff from different rivers, and Pacific inflow?

b.      Experiments (following the experiment Andrey designed last year):

  • Compare model simulated FW content and SSH variability with observations and among each other to understand FW content variability from seasonal to decadal time scales
  • Analyze the contribution of FW from these different sources to the FW in the Arctic Ocean using Pacific water, river runoff, ice melt, and precipitation tracers. Compare with observational data on this subject.
  • Compare model outputs from 1948-2008, as well as shorter recent sub periods which can be extended if forcing data is available

c.      Which fields:

  • Monthly mean model output of the following fields for the region north of 60N including 60N (in original model grid):
  • Freshwater content in m. Please calculate freshwater content (FWC) as: FWC=Integral[(34.8-S)/34.8]dz, where dz is thickness of water layer from surface to bottom (including SSH elevation in case of free-surface models and also partial bottom cells’ depth, if it is used) with salinity S. If salinity is greater than 34.8 you will have negative freshwater content and in this case do not take these negative numbers into account and do not sum them up in you total FWC (no negative FW allowed)
  • SSH field
  • sea ice thickness and concentration, snow depth, sea ice, snow and reference water densities, sea ice salinity
  • precipitation minus evaporation fields
  • passive tracer fields, if available
  • Monthly sea ice and snow melt
  • Information about SSH in model: rigid lid or free surface
  • Information about river runoff: sources, method of introduction into model (reference salinity if virtual salt flux is used), seasonal and interannual variability, if any

Role of Ekman pumping for FW storage changes in the Beaufort Gyre

a.      Questions:

  • Has Ekman pumping played a major role in the recent freshening of the Beaufort Gyre?
  • To what degree has weakening ice interaction played a significant role in this increased Ekman convergence over the last several decades?

b.      Experiments:

  • Comparative experiments with ~30-40 year runs (e.g. 1970-2008)
  • It is also possible to do some of these comparisons with only a barotropic ocean model
  • We need to agree on an average region roughly defining the Beaufort Gyre.  Then intetrate the stress curl delivered to the ocean system from the various models as a time series over this region over the last several decades.  How has this changed and are they comparable?

c.      Which fields:

  • For 'imbedded' models whereby the convergence of ice is included in the region as well as water, the relevant term is the curl of the wind stress less the gradient of the ice stress.
  • For levitated models, you will have to just take the drag on the bottom of the ice.
  • Fields of sea ice thickness and area coverage and of freshwater storage for the agreed Beaufort Gyre region