THE DIURNAL MIXED LAYER IN LAKES AND OCEANS

SUMMARY

In this review the current ideas on diurnal mixed layers are discussed with reference to several observations. Essentially, solar heating sets up a shallow thermocline which traps and concentrates further influx of heat and momentum in the top few metres of the water. These shallow structures usually deepen during the afternoon and evening and are erased by convective cooling overnight. This trapping of momentum results in significant wind-driven diurnal currents which take a different form in bounded or unbounded water bodies. Although the net gain or loss of heat is negligible, the cumulative effect of these diel cycles results in a seasonal heating and cooling cycle. This stratification inhibits vertical exchange and may be a significant factor in plankton dynamics and pollution dispersion.

KEYWORDS:

• Diurnal mixed layer
• diel cycle
• neat flux
• wind-mixing
• South African lakes
• South African ocean