Figure 2 shows the surface temperature over the shelf. There is less than 1° C variation in the sea surface temperature off Guatemala. Figure 3 a shows the hydrographic profile off Champerico. Except for a slight increase in surface temperature, the hydrographic situation is similar to that observed in March. The transition layer is located from 50 to 100 m depth, and has a temperature drop of 8°C and a reduction in dissolved oxygen from 4 to 1 ml/I. The cool oxygen-depleted bottom water seems thus to cover the edge of the shelf which is relatively steep below 100 m depth in this part of the region.
The sea surface temperature in the Tehuantepec Gulf shows a slight increase shorewards in the north but otherwise no gradient along the coast was observed. In the previous survey a distinct northward decrease from abt. 28°C to 25 - 26°C was observed together with some shoreward decrease in temperature. The hydrographic profile over the shelf to the east of the San Marcos bar is shown in Figure 3 a. The profile shows a similar hydro-graphic situation to that observed in March. The transition layer as indicated by the oxygen content lifts from inshore to offshore and drops again further seawards. This dome or ridge is known from previous hydrographic surveys of the Gulf and is explained as an effect of the existing current - and wind systems. The new profile run off Salina Cruz (Figure 3 b) shows a different hydrographic situation, the transition layer showing a tendency to lift shorewards. The layer has a drop in temperature of some 10°C from 50 to 100 m and has a reduction in dissolved oxygen from 4 to 1 ml/I. The lower part of the layer lifts sligthly shorewards, but this sign of upwelling is however not recognized in the sea surface temperature.
