Laboratory Experiments of Freshwater Discharge into the Ocean



A popular hypothesis in paleoclimatology posits that episodic discharges of melt water from northern hemisphere ice sheets caused abrupt changes in large-scale ocean circulation and climate during the last glacial and deglacial periods.  In this context, modern observations such as the accelerated melting of the Greenland Ice Sheet and the freshening of subpolar waters in the North Atlantic Ocean appear as causes of concern.  However, the mechanisms responsible for the offshore transport of freshwater discharged from the continents are poorly understood, implying that the ability to test the above hypothesis is limited.  Climate models, for example, cannot properly simulate this transport owing to various limitations, such as coarse spatial resolution and missing physics.

We propose a series of laboratory experiments with a rotating table to investigate the evolution of freshwater discharged along the western boundary of a northern hemisphere ocean.  Owing to the Earth rotation, freshwater discharged along the boundary of an ocean basin is expected to turn to the right (in the northern hemisphere) and form a narrow buoyant current along the boundary.  In our project the conditions conducive to the offshore transport of freshwater, such as a large freshwater source or the separation of the buoyant current from the coast near a coastline irregularity, will be explored.  The ability of a numerical model, which does not suffer the above limitations, to reproduce the observations made with a real fluid in the laboratory will also be examined.

Our work plan will include three tasks.  First, a pie-shaped basin representing the North Atlantic Ocean will be fixed on a rotating table in the Geophysical Fluid Dynamics Laboratory at WHOI.  It will include a continent with a rounded cape to represent the Grand Bank – a major coastline irregularity along northeastern North America where offshore export of modern shelf water is thought to occur. Second, laboratory experiments of freshwater discharge will be conducted.  The basin will be filled with salt water which will then be brought to solid body rotation (so the salt water will be at rest in the rotating frame).  Once solid body rotation is established, freshwater will be introduced into the basin along the left side “north” of the cape.  The evolution of freshwater in the basin will be observed from a camera co-rotating with the table.  Finally, a numerical model of the laboratory experiments, which has high spatial resolution and relatively complete physics, will be applied.  Emphasis will be placed on the ability of the model to simulate the offshore transport of freshwater as observed on the turntable. 

Merits (1) Our project will be relevant to the first research theme of the OCCI, which is related to “the role of the Atlantic Ocean in the climate system”, including “its remote forcings” (in our project, the forcing will be a large discharge of freshwater from the continent).  (2) It will bring together two PIs from different departments and will result in an original approach to a paleoceanographic problem.  (3) It will help the PIs solidify future research proposals to submit to external funding sources.  It will complement a project to submit to NSF in August 2009 (e.g., it is proposed there to compare laboratory data also with marine sediment records).