MICROSTRUCTURE ACTIVITY WITHIN A MINIFILAMENT IN THE COASTAL TRANSITION ZONE

This paper presents observations of a dynamic minifilament an summarizes the findings of three microstructure surveys across and within the meandering current structures of the Coastal Transition Zone (CTZ). An actively turbulent minifilament is discussed and possible consequences of the small-scale processes with regard to frontal dynamics are examined. Four different frontal structures were observed as part of the CTZ microstructtue program, two resembling meanders and two characterized as filaments. Microstructure measurements included continuous rapid sampling vertical profiler (RSVP) (Caldwell et al., 1985) and acoustic Doppler current profiles (ADCP) profiling along meridional lines bisecting the frontal features. Common features of the two meanders observed in 1986: (Moum et al., 1988) and 1987: (Dewey and Moum, 1990) include offshore and onshore flow patterns (consistent with meridional sections through meanders), rapid temporal evolution of the frontal structures between consecutive (10-24 hour) transects, no elevated turbulent activity below the pycnocline, despite the enhanced current shears (quiescent core within central region where pycnocline is near surface), and approximate geostrophy of the dominant currents at scales > 10 km. A large filament, or jetlike feature observed in 1988 (Dewey et al., 1991) was characterized by off-shore geostrophic flow, no enhanced turbulent activity, and significant asymmetry in the temperature, salinity and current fields, with stronger vorticity on the south (upwelled) side of the velocity maximum. An actively turbulent minifilament was also observed in 1988, and exhibited some characteristics not common to the larger meanders or filaments. A strongly divergent velocity signature was confined to the near-surface layer (< 60 m), with a cool, uplifted core, possibly a result of localized upwelling and subsurface turbulent mixing. Below the surface mixed layer, elevated turbulent dissipation rates were coincident with regions of high shear and correspondingly low Ri. However, the buoyancy flux from turbulent mixing was estimated to be too low to have formed the cool surface signature of the minifilament. The estimated Rossby number (R(o) = 1.3) for the minifilament suggests that significant departures from geostrophy are likely. However, due to the limited temporal and zonal resolution of the minifilament, no definitive ageostrophic analysis can be performed. Corresponding satellite images indicate that the observed structure, although narrow (10 km), may be as long as 100 km, be evolving over periods of days, and a common submesoscale feature in the swirling patterns revealed by advenced very hi resolution radiometer (AVHRR).