 |
| Titel |
Contrasting effects of sea ice thinning and retreat on Arctic Ocean momentum influx |
| VerfasserIn |
Torge Martin, Michael Steele, Jinlun Zhang |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090525
|
| Publikation (Nr.) |
 EGU/EGU2014-4769.pdf |
|
|
|
|
|
| Zusammenfassung |
| The current thinning and retreat of the Arctic sea ice cover has a non-trivial effect on the
momentum transfer into the ocean as decreases of ice concentration and thickness have
different consequences. On the one hand, a thinner and thus weaker ice cover is more easily
forced by the wind to drift faster, which increases the momentum flux into the ocean. On
the other hand, sea ice retreat reduces momentum transfer because the ice surface
provides greater drag than the open water surface for common wind speeds. We
quantify these effects by analyzing the ocean surface stress from an Arctic-wide
coupled sea-ice ocean model simulation covering the period 1979–2012. Over this
time span the basin-wide annual mean ocean surface stress has been increasing
by 0.004N/m2/decade with an even steeper trend of 0.006N/m2/decade since
2000. The positive trend is associated with the winter months and caused by the
weakening of the ice cover. In summer, however, the ocean surface stress has been
decreasing at -0.002N/m2/decade. The latter can be attributed to the complete melt
of sea ice in vast areas of the Arctic Ocean during recent summers, exposing a
generally smoother ocean surface. Note, these trends are not a consequence of a
change in available momentum (wind forcing) but rather due to changing momentum
transfer.
Further, we introduce the concept of optimal ice concentration, which helps to understand the
effect of sea ice retreat: as sea ice provides greater drag than open water, the momentum
transfer increases with increasing ice concentration up to a point, the optimal ice
concentration, at which further gain of ice compactness is accompanied by exponentially
growing frictional losses due to flow interaction. For a common formulation of the ice
internal stress a concentration of 80% to 90% yields optimal amplification of the momentum
transfer into the ocean. While in the 1980s most of the Arctic Ocean featured ice
concentrations of more than 85% even in summer, summer sea ice concentrations
are predominantly below this optimal ice concentration during the recent decade. |
|
|
|
|
|
|