Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1994The thermal evolution of the Greenland Sea Gyre is investigated using both historical
data and tomographic results from the 1988-89 Greenland Sea Tomography Experiment.
Thermal evolution of the gyre center divides naturally into three periods: a preconditioning
phase (November-January), during which surface salinity is increased by brine rejection
from ice formation and by entrainment but in which the mixed-layer deepens only slowly to
a depth of some 150-200m, a deep mixing phase (February-March) during which the surface
mixed-layer deepens rapidly to approximately 1500m in the gyre center purely under the
influence of local surface cooling, and a restratification phase during which the products
of deep mixing are replaced by inflowing Arctic Intermediate Water (AIW). The onset of
the deep mixing phase occurs after ice formation in the gyre center stops, resulting in an
area of open water where large heat fluxes can occur. In surrounding regions, including the
odden region to the south, ice is still being formed, and the mixed layer does not deepen
significantly. To the north and west, closer to the steep topography of the continental shelf,
the inverse results show significant variability due to advection, and large temperature
and heat content fluctuations with a period of about 50 days are seen. The effects of
advection are deduced from heat and salt budgets, and appear to be important only during
the restratification phase for intermediate depths, and only during the summer for the
surface waters. Comparison of the tomographic results with point measurements indicates
that deep mixing occurs in a field of small plumes in which dense water sinks downwards,
surrounded by larger regions of upwelling. The plume geometry is consistent with that
predicted by numerical and laboratory models. Dynamical processes for bringing the AIW
to the surface in order to form deep water are not needed in this scenario, rather the surface
waters are modified until they match the density of the AIW after which surface cooling
drives convection
