In homogeneous and density-stratified inviscid shear flows, the mechanism for instability that is most commonly invoked and discussed is Kelvin–Helmholtz instability, as it occurs for a simple velocity discontinuity. There is a second mechanism, the wave-interaction mechanism, which is much more general, and is the subject of this paper. This mechanism depends on two free waves that propagate in opposite directions in a stratified shear flow, and which may become stationary relative to each other because of the shear. If this occurs, and their relative phase is suitably chosen, the velocity field of each wave increases the displacement of the other, and so the disturbance grows.We show that this mechanism is responsible for instability in a general class of symmetric but otherwise arbitrary velocity and density profiles, provided that the Richardson number R_i < ¼ in a central region of arbitrarily small thickness. A critical layer exists in this central region for the growing disturbance, but its role in the instability process is incidental. When R_i > ¼ everywhere, the flow is stable because the free waves described above are absorbed by the critical layer, and hence are heavily damped. The necessary criteria of Rayleigh and Fjortoft for instability in homogeneous fluid are seen to provide a suitable geometry for two interacting waves. Some specific examples are given, including a succinct explanation of Holmboe waves.

This study provides a climatology of the circulation and water mass distribution by using historical data combined with observations from dozens of recent cruises near the Philippine coast. The most striking results are related to the poleward contraction of the subtropical gyre on denser surfaces, with the bifurcation of the North Equatorial Current moving from about 15°N in the upper thermocline to about 20°N at intermediate depths. Though time variability and the possible errors in the data are rather large, the Halmahera eddy (HE) is clearly seen in the climatic mean fields, lying at about 3°N, 130°E near the surface and reaching the Mindanao coast on density surfaces around 27.2σθ. It seems that the previously observed Mindanao Undercurrent is merely a component of the recirculation associated with the HE. North Pacific Tropical Water (NPTW) and Intermediate Water (NPIW) enter the western ocean with their extreme properties centered at 15° and 20°N, respectively, and continue southward as fa... Abstract This study provides a climatology of the circulation and water mass distribution by using historical data combined with observations from dozens of recent cruises near the Philippine coast. The most striking results are related to the poleward contraction of the subtropical gyre on denser surfaces, with the bifurcation of the North Equatorial Current moving from about 15°N in the upper thermocline to about 20°N at intermediate depths. Though time variability and the possible errors in the data are rather large, the Halmahera eddy (HE) is clearly seen in the climatic mean fields, lying at about 3°N, 130°E near the surface and reaching the Mindanao coast on density surfaces around 27.2σθ. It seems that the previously observed Mindanao Undercurrent is merely a component of the recirculation associated with the HE. North Pacific Tropical Water (NPTW) and Intermediate Water (NPIW) enter the western ocean with their extreme properties centered at 15° and 20°N, respectively, and continue southward as fa...