Evidence of a Barrier Layer in the Sulu and Celebes Seas

Abstract

Variability in the surface isothermal and mixed layers of the Sulu and Celebes Seas is examined using the conductivity–temperature–depth data from the Navy's Master Oceanographic Observational Data Set (MOODS). Vertical gradient is calculated to determine isothermal layer depth with a criterion of 0.05°C m−1 for temperature profiles and mixed layer depth with a criterion of 0.015 kg m−4 for density profiles. When the isothermal layer depth is larger than the mixed layer depth, the barrier layer occurs. This study shows that the barrier layer occurs often in the Sulu and Celebes Seas. In the Sulu Sea, the barrier layer has seasonal variability with a minimum occurrence (38%) and a minimum thickness (3 m) in May and a maximum occurrence (94%) and a maximum thickness (36.5 m) in September. In the Celebes Sea, the barrier layer thickness changes from a maximum (49.7–62.0 m) in March–April to a minimum (9.6 m) in June. Possible mechanisms responsible for the barrier layer formation are discussed. In the Sulu Sea, the barrier layer may be formed by both rainfall and stratification; in the Celebes Sea, a rain-formed mechanism seems a major factor.