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Deep ventilation in the Okinawa Trough induced by Kerama Gap overflow
Author(s) -
Nishina Ayako,
Nakamura Hirohiko,
Park JaeHun,
Hasegawa Daisuke,
Tanaka Yuki,
Seo Seongbong,
Hibiya Toshiyuki
Publication year - 2016
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2016jc011822
Subject(s) - thermocline , geology , trough (economics) , upwelling , sill , entrainment (biomusicology) , oceanography , buoyancy , hydraulic jump , water mass , bottom water , stratification (seeds) , salinity , deep ocean water , deep water , flow (mathematics) , petrology , biology , geometry , germination , quantum mechanics , rhythm , philosophy , mathematics , macroeconomics , seed dormancy , physics , botany , dormancy , economics , aesthetics
Near‐bottom water flowing over the Kerama Gap's sills is thought to ventilate the deep water below ∼1100 m depth in the Okinawa Trough and then upwell with 5–10 years residence time. The present study follows up on this phenomenon, using comprehensive profile data of temperature, salinity, dissolved oxygen, currents and turbulence obtained by intensive shipboard observations performed in June 2013 and June 2014 in the region. Strong near‐bottom subtidal flow with speeds exceeding 0.5 m s −1 was observed within a layer of about 100 m thickness over the western side of the peak of the main sill. Temperature and salinity sections along the Kerama Gap indicated some depressions and overturns of the deep water downstream of the strong overflow, suggesting the existence of breaking internal gravity waves and hydraulic jumps. Associated vertical diffusivities, estimated using the Thorpe scale and the buoyancy frequency, were three to four orders of magnitude larger than typical values observed in the thermocline of the open ocean (∼10 −5 m 2 s −1 ). The dissolved oxygen section also indicated strong vertical mixing and associated upwelling with the entrainment of the near‐bottom overflow water into the lower thermocline beneath the Kuroshio in the Okinawa Trough. The present study not only supports the previous conceptual model but also provides new evidence that the Okinawa Trough is an upwelling location where nutrient rich Philippine Sea intermediate water is sucked up into the lower thermocline below the Kuroshio.