From May through September 1983, surface buoys deployed as a component of the FOCAL experiment (Programme Fran¿ais Oc¿an et Climat dans l'Atlantique Equatorial) drifted in the cold waters of the South Equatorial Current in the equatorial Atlantic Ocean. The surface temperature (SST) is analyzed along the buoy tracks. In late May, the strong temperature drop near 4¿W and 3¿S is associated with vertical mixing as cold patches appear near the equator. Horizontal advection is important in June and July, as the cold water tongue expands southwards and westwards. It is also important later in the western Atlantic south of the equator, where the cooling by advection is balanced primarily by heating from the atmosphere.