Abstract
Water column conditions in a lagoon near Jeddah are investigated on the basis of changes in potential energy. Three major factors including balance of surface heat at the air-sea interface, wind and tidal mixing are considered. A negative potential energy change dv/dt will develop stratification, whereas positive dv/dt will tend to mix the water column The tidal effect is greater in summer with wind mixing showing no great variations. The buoyancy effect of the heat balance at the surface is negative from April to October. This negative buoyancy effect will tend to develop stratification but the positive contributions of wind and tide counteract this and the water column remains mixed except in September and October, when a weak stratification may develop. Generally, the water column remains practically mixed throughout the year. The change in heat content of the water column from mid-April to mid-September is about 3.3 x 10(8) J. During this period the net heat input at the air interface is about 2.0 x 10(8) J. which is about 40% less than the heat content of the water column, showing that the heat is advected towards the central area from the shallower periphery of the lagoon.