Thermohaline intrusions in the thermocline of the western tropical Pacific Ocean

Abstract

The existing high-resolution hydrographic data in the western tropical Pacific Ocean are used to explore the spatial distribution and primary characteristics of thermohaline intrusions in the thermocline. Statistics show that the vertical scales of intrusions are 20-40 m in the upper thermocline (22.0-26.0 sigma(0)) and 40-80 m in the lower thermocline (26.0-27.2 sigma(0)). In the upper thermocline, the most intensive intrusions exist at the equatorial front (EF) where north/south Pacific water masses converge, and the westward spreading of the north Pacific tropical water (NPTW) in the Philippines Sea also produces patches of intrusions surrounding its high-salinity tongue. In the lower thermocline, intrusions are also strong at the tropical front (TF) which is the boundary between the north Pacific subtropical/tropical waters. At the bottom of the thermocline (at about 27.0 sigma(0)), intrusions mainly exist near the western boundary, which are produced by intermediate water convergence through the advection of subthermocline western boundary flows. Most strikingly a "C"-shape distribution of intrusions at around 26.4 sigma(0) is revealed, covering the vicinity of the EF, the TF, and the Mindanao Current (MC), i.e., the western boundary pathway of the north Pacific subtropical cell (STC). Synoptic section analysis reveals that intrusions are more prominent on the warm/salty flank of the fronts, implying more cross-front tongues of cold/fresh water. Among the intrusions, those at the EF are of best lateral coherence which implies a unique driving mechanism involving near-inertial velocity perturbations near the equator.