The Marine Light-Mixed Layers (MLML) experiments took place in the subarctic North Atlantic Ocean, approximately 275 miles south of Reykjavik, Iceland, during 1989 and 1991. The 1991 field program took place from April 30 to September 6 and included a central surface mooring to document the temporal evolution of physical, biological, and optical properties. In this paper we describe the physical variability observed at the 1991 mooring site, concentrating on the vertical structure of temperature and velocity in the upper 300 m of the water column and their changes in response to heat and momentum fluxes at the sea surface. The deployment period included the spring transition, when upper ocean restratification was initiated after deep winter mixing, and the fall transition, when mixed layer deepening began again. The dominant signal 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 temperature variability was observed in association with a 15-day mean flux of only 20 W m(-2) into the ocean. Pronounced day/night oscillations of heat flux during this period resulted in alternating development and destruction of stratification and intense diurnal cycling of the mixed layer depth. A qualitative comparison of the observed temperature structure to the prediction of a one-dimensional mixed layer model showed that local processes dominated during the initiation of restratification and during most of the summer warming period. Nonlocal processes were important after the fall transition.
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