VERTICAL STRUCTURE OF THE UPPER OCEAN DURING THE MARINE LIGHT-MIXED LAYERS EXPERIMENT

The Marine Light-Mixed Layers (MLML) experiments took place in the sub arctic North Atlantic Ocean, approximately 275 miles south of Reykjavi k, Iceland, during 1989 and 1991. The 1991 field program took place fr om April 30 to September 6 and included a central surface mooring to d ocument the temporal evolution of physical, biological, and optical pr operties. In this paper we describe the physical variability observed at the 1991 mooring site, concentrating on the vertical structure of t emperature and velocity in the upper 300 m of the water column and the ir changes in response to heat and momentum fluxes at the sea surface. The deployment period included the spring transition, when upper ocea n restratification was initiated after deep winter mixing, and the fal l transition, when mixed layer deepening began again. The dominant sig nal in temperature was seasonal variation, with a 6 degrees C increase observed at the sea surface from May to August. Prior to development of the seasonal stratification, a period dominated by near-surface tem perature variability was observed in association with a 15-day mean fl ux of only 20 W m(-2) into the ocean. Pronounced day/night oscillation s of heat flux during this period resulted in alternating development and destruction of stratification and intense diurnal cycling of the m ixed layer depth. A qualitative comparison of the observed temperature structure to the prediction of a one-dimensional mixed layer model sh owed that local processes dominated during the initiation of restratif ication and during most of the summer warming period. Nonlocal process es were important after the fall transition.