A numerical model for interdecadal variability of sea ice cover in the Greenland-Iceland-Norwegian Sea |
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Authors: | M A Morales-Maqueda A J Willmott M S Darby |
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Institution: | (1) Department of Mathematics, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom E-mail: maa21@cc.keele.ac.uk, a.j.willmott@keele.ac.uk, GB;(2) Department of Mathematics, University of Exeter, North Park Road, Exeter, Devon EX4 4QE, United Kingdom, GB |
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Abstract: | A coupled ocean-sea ice-atmosphere model is used to study interdecadal variability (∼40 years) of sea ice depth and concentration
in the Greenland-Iceland-Norwegian Sea. This oceanic region is represented by a meridionally aligned channel on a β-plane
with open zonal boundaries at 60 °N and 80 °N. The model consists of a one and a half layer reduced gravity ocean model, a
thermodynamic/dynamic sea ice model and an energy balance model of the atmosphere. The coupled model is driven by prescribed
surface wind stress, fluxes of heat, salt and ice at inflow points on the northern and southern open zonal boundaries and
annual distribution of solar radiation. It is shown that the coupled model supports unforced modes of interdecadal oscillation
resulting from a form of hydraulic control which regulates the total fluid volume in the oceanic active layer. The mechanism
for the oscillations relies on the presence of three key features: (1) a region of intense oceanic entrainment located in
the eastern part of the domain, (2) a vigorous southward flowing western boundary current, representing the East Greenland
Current (EGC), which supports most of the meridional transport across the domain, and (3) a marked buoyancy contrast between
the relatively salty domain interior and the much fresher western boundary region. During an oscillation excess water is pumped
into the domain via entrainment, thereby creating an active layer depth anomaly, which then propagates westward via long baroclinic
Rossby waves until it reaches the EGC where it is subsequently drained out of the domain across the southern open zonal boundary.
As the depth anomaly traverses the basin, an anomalous geostrophic circulation is established in which cold fresh Arctic water
enters the domain interior, and this eventually promotes enhanced thermodynamic sea ice growth. Consequently, the interdecadal
oscillations of the coupled model are characterised by pulse periods, typically spanning 20 years, during which there is an
abnormally large winter sea ice cover, separated by interpulse periods, lasting another 20 years, during which the winter
sea ice extent is nearly uniform and significantly smaller than in a pulse maximum. The duration of the interpulse periods
is dictated by the time it takes for the Rossby waves to traverse the basin. In addition to the interdecadal oscillation solution,
the coupled ocean-sea ice-atmosphere model is found to also have a stable cyclostationary state, with no interannual variability.
Stochastic forcing, in the form of randomly specified interannual anomalies of salinity (of maximum amplitude 0.1 ppt) or
ice inflow (of maximum amplitude 0.1 Sv) at the northern open zonal boundary, in both cases is capable of driving the model
from the cyclostationary state solution to the interdecadal variability one.
Received: 16 August 1996 / Accepted: 27 July 1998 |
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