Observation and Modelling of Turbulent Mixing in a Northwestern Pacific Ocean Trench

Turbulent mixing plays important role for maintaining global ocean circulation and substance exchange between deep and upper oceans, especially for deep trench with > 6000 m depth. In this study, we use historic observed data and a numerical model to investigate the vertical distribution of turbulent mixing coefficients (Kv) in the deepest trench of the World, i.e. southwestern Mariana Trench (SMT) of the Northwestern Pacific Ocean (NPO). It is found that Kv has large scale with 10^-3 m^2/s at 4000 m-5000 m depths above the SMT. In addition, same magnitude of Kv appears at above bottom of the Trench, even at the deepest place with 10000 m depth. Results suggest salty and cold Antarctic Bottom Water (AABW) from the Southern Ocean intrude into the SMT through the NPO at 4000 m depth and mix with warm and fresh water above the Trench sufficiently that cause large scale of Kv at this depth. Different with the upper trench, the large Kv above bottom of the SMT is generated by internal tide breaking in which barotropic tides (principally include M2 tide) are propagated into the SMT, then trapped by the topography of the SMT and transformed into internal tides. The internal tides interact with topography and motivate strong vertical mixing near bottom of the SMT.

Keywords: Turbulent mixing coefficient, Antarctic Bottom Water (AABW), Internal tide, Mariana Trench