Abstract:
A detailed examination of the development of a deep convection event observed
in the Greenland Sea in 1988-89 is carried out through a combination of modeling, scale
estimates, and data analysis. We develop a prognostic one-dimensional mixed layer model
which is coupled to a thermodynamic ice model. Our model contains a representation
of the lowest order boundary layer dynamics and adjustable coupling strengths between
the mixed layer, ice, and atmosphere. We find that the model evolution is not very
sensitive to the strength of the coupling between the ice and the mixed layer sufficiently
far away from the limits of zero and infinite coupling; we interpret this result in physical
terms. Further, we derive an analytical expression which provides a scale estimate of
the rate of salinification of the mixed layer during the ice-covered preconditioning period
as a function of the rate of ice advection. We also derive an estimate for the rate of the
mixed layer deepening which includes ice effects. Based on these scale estimates and model
simulations, we confirm that brine rejection and advection of ice out of the convection area
were essential ingredients during the preconditioning process. We also demonstrate that
an observed rise in the air temperature starting in late December 1988 followed by a period
of moderately cold ≈-10°C temperatures was key to the development of the observed
convection event. Finally, we show that haline driven deep convection underneath an
ice cover is possible, but unlikely to occur in the Greenland Sea. On the basis of these
results, we develop a coherent picture of the evolution of the convection process which
is more detailed than that presented in any previous work. We also comment on the
likelihood that deep convection occurred in the Greenland Sea in the past two decades
from an examination of historical data, and relate these findings to what is known about
the inter-annual variability of convective activity in the Greenland Sea.
