In the present study the mechanisms for Bjerknes compensation of heat
transport variations through the atmosphere and ocean on decadal timescales
are investigated, using data output from a pre-industrial control run
of the Hadley Center Climate model (HadCM3). Using statistical analysis we
observe Bjerknes compensation on decadal timescales at latitudes in between
50N and 80N. A maximum compensation rate of 55% occurs at 70N. At this
latitude the correlation rate peaks when the ocean is leading the atmosphere
by one year. The mechanisms by which Bjerknes compensation occurs at this
latitude are investigated. Anomalies in oceanic heat transport appear to be
associated with variations in the strength of the Atlantic meridional
overturning circulation (MOC). The associated sea surface temperature (SST)
anomalies are in general too weak to assert a significant impact on the
atmosphere. At 60N however, such SST anomalies prelude the transition from
sea-ice coverage to open water after which the associated changes in heat
exchange with the atmosphere are strong enough to force an atmospheric
response. Due to the presence of a strong MOC component in the Atlantic ocean,
this interaction is confined to the region where the Northeast Atlantic and
Arctic Ocean connect. The atmospheric response to increased (decreased)
heating from below is a decreased (increased) poleward temperature gradient,
leading to a decreased (increased) heat transport by baroclinic eddies.
The direct impact of MOC-variations on sea-ice coverage in the Labrador Sea
is too weak to induce an atmospheric response there, which initially stays
confined to the Northeast Atlantic. The anomalous thermal low (high) that is
set-up by heating (cooling) from the ocean is associated with anomalous
advection of cold air from the Greenland land-mass, which eventually may
trigger(halt) the intermittent convection in the Irminger Sea. The associated
SST-anomalies are quickly advected by the Greenland Current into the Labrador
Sea where they have a similar impact on sea-ice coverage as the more direct
response of the Northeast Atlantic.

Full Article: http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F2007JCLI1562.1

Last Updated: 2008-04-21