Modelling of the bottom water flow through the Romanche Fracture Zone with a primitive equation model––Part 2: Comparison of vertical mixing parameterisations with observations

• Published in: Ocean Modelling
• Main Authors: Ferron, Bruno, Tréguier, Anne-Marie, Mercier, Herlé
• Other Authors: Laboratoire de physique des océans (LPO), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2003
• Subjects: Mixing; Parameterisation; Topography; Hydraulic control; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Antarctic; The Antarctic; Antarc*
• Online Access: https://hal.science/hal-00267648; https://doi.org/10.1016/S1463-5003(03)00024-6

International audience Three parameterisations of the vertical mixing of a primitive equation model that uses a geopotential coordinate system are tested against observations in the context of the Antarctic Bottom Water flow through the Romanche Fracture Zone. On an increasing complexity scale, the three parameterisations are a convection algorithm, a vertical diffusivity depending on a Richarson number, and a vertical diffusivity calculated from a turbulent kinetic energy (TKE) equation. The model uses a high vertical and horizontal resolution. It is shown that the convection algorithm does not produce enough vertical mixing compared to observations. This results in the propagation of a too dense water mass in the downstream basin. On the opposite, the Richardson number-dependent algorithm as well as the TKE algorithm produce a mixing comparable to the observations. With these two parameterisations, the main region of intensification of the vertical mixing is located downstream of the main sill that was also described as the major region of turbulence from observations. The mixing length of the TKE parameterisation is consistent with calculation of Thorpe scales from the fine structure of CTD data. Hence, the two successful parameterisations initially designed for the surface mixed layer also give satisfying results for overflows as soon as the hydraulic control and its associated downstream intensified vertical shear are correctly represented in the model.