VEX: Exploritory Moored Measurements in the Vema Fracture Zone

Principal Investigator

Michael McCartney
Senior Scientist, Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543 USA

Project Description

This project has the primary goal of measuring the magnitude and vertical and lateral structure of the eastward flow of bottom and deep waters through the Vema Fracture Zone (VFZ). This will establish mean values and intra-annual variabilities for a year long record, including transport distribution in temperature classes, at a location upstream of the primary sill, before the expected intense mixing at the sill blurs the distinction between bottom and deep waters. A secondary goal VEX is to estimate the intensity of the mixing that occurs between deep and bottom waters as they flow through the VFZ from heat and mass budgets of the measured bottom water flow and the disappearance of the coldest classes downstream along the VFZ. Exploratory measurements of the statistics of time series profiles of velocity shear and hydrography at the primary sill will independently provide estimates of the intense mixing regime through fine structure parameterizations, collaborative work with Kurt Polzin, WHOI.

Abstracts of Project Grants

A moored array will be deployed in the Vema Fracture Zone, a deep channel between the Guiana Basin and the eastern N. Atlantic through which bottom water from the Antarctic eneters the eastern N. Atlantic. The array of instruments will directly measure the transport and lateral structure of deep and bottom water flowing through the restricted channel and indirectly measure the degree of mixing and its variability. The instruments will stay in the water for roughly one year. The moored array will consist of four newly developed profiling moorings together with two more traditional curren meter moorings. Apart from the science questions that are the focus of the experiment, a by-product will be a comparison of new and traditional approaches to measuring abyssal circulation.

This project will analyze observations collected during the field phase of the Vema Fracture Zone Exploratory (VEX) Measurement program carried out in 2001-2003. The Vema Fracture Zone (VFZ) is an abyssal passage through the Mid-Atlantic Ridge near 11 degrees N, connecting the Guiana and Gambia Basins and permitting eastward flow of Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW). The VEX measurements were directed towards determination of the intensity and structure of the flow in the VFZ, the source pathways delivering waters to the VFZ, and the strength of the vertical mixing within the fracture zone. In the field phase of the project, 95 hydrographic stations were sampled and six moorings with a total of 18 current meters and three moored profilers were deployed. The completed analysis and synthesis of the VEX data will provide a greatly improved picture of the abyssal flow through a low-latitude fracture zone linking two of the North Atlantic abyssal basins. This will advance our understanding of the deep component of the meridional overturning circulation, particularly through the recognition that much of the total diapycnal flow occurs in the restricted areas of fracture zones and steep topography. The description of the VFZ flow will also aid in validating the representation of flow through narrow passages in large-scale numerical simulations. Broader Impacts: The moored profilers deployed during VEX for the first time outside their test and developmental phase, are novel instruments profiling current velocities, temperature, salinity, and pressure. Demonstrating the utility of simultaneous time series of velocity and CTD profiles for improved estimation of transports and mixing will benefit future moored profiler deployments. The analysis of the VEX data forms one component of the joint GAGE/VEX program. The funds requested will provide the opportunity for a young scientist to advance her skills in analyzing oceanographic processes using data from a variety of instrument sources.