Chapter 4
FINE STRUCTURE AND MICROSTRUCTURE
Under light winds or heavy rainfall, upper ocean turbulence interacts
strongly with stratification and large vertical gradients can develop in the
upper few meters of the ocean.
4. FINE STRUCTURE AND MICROSTRUCTURE
In the oceanographic literature, the term fine structure is traditionally
reserved for inhomogeneities relating to stratification, while the term,
microstructure has often been applied to inhomogeneities associated with
small-scale turbulence (Gregg, 1975).
Air-sea momentum, heat, and freshwater fluxes are crucial factors
determining the thermohaline structure of the near-surface layer of the
ocean. Under moderate-to-high wind speed conditions, the upper ocean is
usually well mixed due to strong turbulence. When the wind drops below
about 5 m sP
-1
P,
and with solar warming, the turbulent regime dramatically
changes. The stabilizing buoyancy flux suppresses turbulent mixing and the
air-sea exchange is effectively trapped within a thin near-surface layer.
Under these conditions, the diurnal thermocline can be found close to the
ocean surface contributing to the fine structure and microstructure of the
near-surface layer.
In addition to solar heating, the freshwater cycle may substantially
modify the structure, dynamics, and thermodynamics of the near-surface
layer of the ocean. The main components of the hydrologic cycle in the
ocean are precipitation, evaporation, river discharge, and sea ice
melting/formation.
Precipitation effects are pronounced in low latitudes, especially within
intertropical convergence zones, warm pools, and monsoon regions. Lateral
advection of freshwater may contribute to the fine structure of the near-