Convection in the Deep Ocean
Attribute
Weddell Sea
Greenland Sea
Labrador Sea
Gulf of Lions
Importance for
deep water
renewal
Potentially large
Believed to be
large, but recently
questioned
Large
Small
Importance for
ice cover
Potentially large
Relatively small
Small
None
Ice export followed
by thermal
convection
Mechanisms
Role of
thermobaricity
Controls on
depths and rates
Trigger catastrophic
overturning ?
Reduces Scr, makes
plume penetration
more effective
Ice divergence, cold
air outbreaks,
doming of
isopycnals in gyre
Thermal
Small ?
Negligible
Wind speed and air
temperature
(surface cooling),
preconditioning
Summary of Thorkildsen & Haugan DSR 1999
• Steady state plume model including nonlinear equation of state,
dynamic pressure and both components of earth rotation.
• New scaling of plume radius.
• To achieve deep convection: Small elevation in density O (10-4
kgm-3) is sufficient; ambient must be only weakly stratified;
thermobaricity is the most important driving force.
• Inserting a warm and saline layer 600-1400m with same dr/dz
gives increased penetration for plumes which otherwise would
have stopped in the layer, and decreased penetration for plumes
which would have traveled through.
Summary of Løyning & Weber JGR 1997
• Infinitesimal perturbations can trigger thermobaric instability.
This is shown by classical linear stability analysis applied to
Boussinesq type equations with expansion in d/Ha where d is
layer thickness, as well as from nonlinear 2D equations.
• Thermobaric convection, i.e. when thermobaricity is the sole
cause of convection, asDT < bDS < (as + a1d) DT, gives
asymmetric cells with stronger circulation in the lower part. This
is shown from linear approximation and from numerical
solution of nonlinear equations.
• (Weber & Løyning 2000 Techn. Rept. Univ. Oslo: ”Thermobaric
effect on symmetric instability” show that thermobaricity
destabilises stratified, geostrophic flows. Small-amplitude rolls
have centers of circulation which are shifted towards the lower
part of the fluid layer).