This project is funded as an EArly-concept Grant For Exploratory Research (EAGER).
The Rapid Climate Change-Meridional Overturning Circulation and Heat Flux Array (RAPID-MOCHA) began monitoring meridional mass transports in the North Atlantic Ocean along a transatlantic section from North America to Africa in 2004. It estimates the climatically critical meridional overturning circulation (MOC) by differencing dynamic height profiles gathered from small clusters of moorings on either side of the Atlantic basin, measuring boundary current flows with current meters, measuring transport in the Florida Strait electrically, and using satellite winds to estimate Ekman transport. While bottom pressure gauges are used to estimate time-varying barotropic contributions, RAPID-MOCHA relies on an assumed spatially uniform temporally constant barotropic flow to estimate mean transport.
The first scientific use of the newly developed full-ocean-depth (surface to 6 km) autonomous underwater glider, Deepglider will complement the RAPID-MOCHA array. Deepgliders will be used to estimate absolute transports independently of RAPID-MOCHA by collecting repeat hydrographic sections of the extended western boundary region off Abaco, Bahamas. A pair of vehicles will repeatedly transit across 100 and 500 km wide overlapping sections between end members of the RAPID-MOCHA dynamic height moorings. These sections will be repeated about weekly and monthly, respectively, by Deepgliders, providing substantial spatial resolution compared to that provided by the moorings, although at considerably coarser temporal resolution. Each Deepglider is expected to last well over 1 year, possibly up to about 18 months. Integrated geostrophic shear inferred from horizontal density gradients resolved in the sections will be referenced to depth-averaged current inferred from each glider dive cycle. The difference between dead-reckoned glider displacement through the water and GPS displacement over the ground is used to estimate depth-averaged current. The Deepglider estimates will include the likely possibility of horizontally varying time-mean barotropic contributions to transport. The independent Deepglider estimates of transports will be compared to those from the RAPIDMOCHA array.
In addition, Deepgliders temporarily will be used in 'virtual mooring' mode to check the adequacy of the moorings in measuring dynamic height. Together, the complement of repeat section and moored time series will be used to assess errors and improve estimates of meridional transports in the extended western boundary region.
Intellectual Merit: The intellectual merit of this work lies in its connections to basic issues of global climate dynamics. The variability of the MOC is not well observed, let alone understood. The same can be said for the deep flow. Comparison of techniques by which the MOC is monitored is essential to establish their credibility and effectiveness. Deepglider repeat hydrography will provide independent measures of climatically critical ocean circulation transports, the western boundary contributions to MOC. Resolution of the temporal/spatial structure of western boundary currents is prerequisite to understanding how this portion of the climate system operates.
Broader Impact: This project will serve as a demonstration of efficacy and economy of full-depth gliders in monitoring ocean circulation not only along the RAPID-MOCHA line, but also along other transects. It will pioneer the use of autonomous gliders to monitor not only the upper ocean, but its deep regions as well. Currently Argo floats monitor the upper ocean globally, but the deep ocean is severely under-observed for climate change, a situation Deepgliders could alter. By making deep ocean access affordable, the Deepglider technology opens the possibility that the complete extent of global ocean climate change may be observed.
This project aimed to use 6000m depth-capable long-range autonomous underwater glider vehicles in their first scientific application. These vehicles, called Deepgliders, are designed to repeatedly dive from the sea surface to within a few meters of the deep ocean sea floor while collecting profiles of temperature, salinity, and dissolved oxygen for use in observing ocean circulation features. They transmit the data they collect via satellite ashore and accept commands from the shore station to control sampling and vehicle behavior. By being fully autonomous, having projected range long enough to cross an entire ocean basin, and small enough to be launched and recovered by a team of two persons from a small boat, Deepgliders are a considerably less costly means of measuring properties of the deep sea than are conventional shipboard measurements. The intention of this project was to demonstrate the use of Deepgliders in monitoring the Deep Western Boundary Current in the tropical North Atlantic Ocean, a current that is an integral part of the oceanic overturning circulation. We launched Deepgliders just offshore San Juan PR in a sequence of test deployments. The site was chosen for logisitic convenience as well as proximity to the deepest portion of the Atlantic Ocean, the Puerto Rico Trough, and to the Deep Western Boundary Current in order to demonstrate Deepglider performance. Initial deployments of the first vehicle were limited to a few days, as various unanticipated problems were exposed. After repair and suitable modification, this vehicle dove twice to 5920 m depth, 80 m shy of the design goal. Unfortunately, this vehicle was lost while it was drifting at the sea surface, due to intermittent, then permanent, failure to establish satellite communications. The second Deepglider launched faired much better. It managed to swim itself across the Puerto Rico Trench on a series of dives to 5250 m depth. On its last dive cycle, it surfaced 39 km away from where it had left the sea surface some 40 hours earlier. During its 275 km transect north from Puerto Rico, it detected the Deep Western Boundary Current as a slug of eastward flow snugged against the steep continental slope forming the south side of the Trench. Its sensors were able to detect small but robust differences in temperature and salinity profiles across the Trench, indicative of horizontal current even in the deep abyss. Engineering data confirmed that Deepglider endurance could be as long as 18 months making full ocean depth dives and its range could be 10,000 km, a quarter of the earth's circumference. Unfortunately, the second Deepglider also ultimately was lost, failing to communicate after about 3 weeks at sea, too far offshore to find and rescue even it it were at the sea surface. A few possible scenarios for its demise have been surmised from its behavior prior to loss and modifications made in current Deepglider design to prevent recurrence. Unfortunately, without vehicle recovery, positive identification of the failure mode is impossible. Following work, supported by a subsequent grant, has focused on increasing Deepglider reliability through extensive laboratory and field testing. When this work is complete, Deepgliders once again will be ready to probe the Deep Western Boundary Current as well as many other remote deep ocean features.
