Early in 1990 we learned that the Bermuda Container line was building a new ship, to be named MV Oleander, to operate on its Bermuda ? New Jersey route. That summer we approached the builders about installing an acoustic Doppler current profiler in its hull so we could monitor ocean currents along its route with focus on the Gulf Stream; an idea they enthusiastically endorsed. Thanks to their support we have been monitoring ocean currents from the New England slope to Bermuda almost without interruption since the fall of 1992. It has been quite a journey that continues today thanks to a new operating grant to Dr. Magdalena Andres at WHOI. 

The most recent grant focused on upgrading the operation to include two ADCPs on the new MV Oleander which arrived in early 2019. The ADCPs (38 kHz and 150 kHz) were installed during vessel construction in China. For reasons unknown the 38 was installed underneath the engine room making for serious service problems. The 150 was installed as planned but it is so far forward it was damaged twice most likely due to breaching and slamming at the surface in heavy seas. These issues are being addressed under the new grant. 

We published a comprehensive account of the first 25 years of operation in the September 2019 issue of the magazine Oceanography. In it we conclude that we cannot detect any significant trend in upper ocean transport over that time. This is not to say there isn?t any trend, but any trend is so small it is masked by the modest interannual variations in transport during that period. 

Another important finding was that the average velocity normal to the Bermuda ? NJ route goes to zero at about 1000 m depth: poleward at shallower depths, equatorward at greater depths. This is the depth of the maximum in the Atlantic meridional overturning circulation (AMOC) overturning stream function. This depth is also below the main thermocline and the subtropical wind-driven circulation. This finding allowed us to use archived hydrographic data to determine trends in the AMOC and the subtropical gyre transport over the last 70-90 years. We found that transport between the continental slope and Bermuda likely decreased about 2?0.8 Sv (1 Sv = 106 m3s-1) with most of this due to changes in the subtropical gyre transport, and an insignificant decrease in the AMOC (-0.4?0.6 Sv). While the uncertainties preclude tighter estimates of change, these geostrophic estimates are almost certainly the best ones to date of AMOC and wind-driven transport and recent trends at this latitude.  These results were reported in detail in the December 2022 issue of Geophysical Research Letters. 

The Oleander project has stimulated numerous spin-off activities. Notably, studies of the long-term warming on the northeast shelf and linkages to accelerated sea-level rise and marine heatwaves as well as seminal observationally-based studies of high-horizontal resolution dynamics.

On the technical side, two developments underway which we hope will greatly improve ADCP science on merchant marine vessels broadly. The first is the development of programmable, magnetically-attached hull camera that will be used to study the presence and character of bubble entrainment. Bubbles block the ADCP acoustics, so we need to better understand the factors that control their presence. High seas is obvious, but other factors as draft of the vessel, shape of the bow, and location of the ADCP in the hull come into play. It is high time we understand these factors better. The other development is to use computational fluid dynamics (CFD) to better understand how we can divert bubbles from under the ADCP, either by placing it in a fairing that extends below the hull or using chines to create a local upwelling of clear water. We want to evaluate the merits of fairings and chines from the point of view of maximizing clear water and minimum drag-cost to the ship. Both initiatives are underway and should help us optimize the relocation of the ADCPs in the Oleander when she is in dry dock this coming January. Developing a know-how for creating bubble-free spots or windows in the hull is an engineering effort that should have been done long ago; it has the potential to open up all kinds of possibilities for future acoustic and optical remote sensing on merchant marine vessels. 

Last Modified: 08/11/2023
Modified by: H. Thomas Rossby