Mapping Subsurface Ocean Currents
For two decades, Teledyne RDI’s Acoustic Doppler Current Profilers (ADCP) have been an industry standard. They measure the velocity of water currents in oceans and rivers. While fixed at one site, an ADCP offers excellent time coverage of closely-spaced measurements versus depth. Furthermore, ADCPs can be operated from a moving vessel or vehicle to explore the along-track distribution of currents versus depth. This versatility greatly expands the ADCP’s scope of applications.
ADCPs mounted on the seabed and moorings are well-known oceanographic tools. They record the history of local fluctuations of currents throughout much of the water column. In this article, we review some oceanographic applications of shipboard ADCPs.
From a data point of view, the ADCP combines what had been two complementary views of the ocean. From the early 1960’s, time series measurements were recorded at several depths. Current meters were deployed on deep-sea moorings and left unattended for long periods. A decade later, vertical profilers provided close measurements of water current velocity versus depth.
Each of these technologies revealed new, exciting, and different facets of the ocean. Yet their resolution was restricted to one dimension (1-d): time (current meters) or space (profilers). In contrast, the ADCP measured time series of vertical profiles, an inherently 2-d view of water motions.
Ocean currents move things around, transporting them for large distances. And the things being carried affect us. The heat and momentum conveyed by ocean currents affect our living environment. Examples include global climate change (e.g., ice ages) and weather events (e.g., El Nino, hurricanes). Strong currents also affect the choice of routes taken by shipping. Reducing travel time for a voyage can save large amounts of money in the competitive commercial arena.
Water currents carry biological and chemical constituents that affect life in and out of the sea. Currents can cause a global transfer of organisms, ranging from fish larvae to crocodiles. And currents can provide a virtual pipeline for nutrients, pollutants, and sediments.
The speed and direction of ocean currents can change in several dimensions: with depth, position, and time. ADCPs use sound to measure these changes. There are key advantages to using sound. It can measure remotely (even at great distances), there are no rotating parts to foul, and sound changes predictably with ocean conditions. Like the radar gun used to catch speeding cars, ADCPs use the Doppler Effect to measure motion.
This is the change in pitch of returning echoes compared to the transmitted sound. These echoes tell how fast particles carried by the water currents are moving and in what direction. And the measurements are made at many depths, all at the same time, creating an ocean current profile.
When mounted on ships and boats, ADCPs provide real-time data to aid decision-making at sea and to adapt field operations. But what sets the ADCP apart is the surveying of ocean and coastal currents from moving vessels. Seeing the 2-d distribution of currents -- along-track and through depth -- reveals the details of ocean circulation patterns.
This section shows why the ancients likened the Gulf Stream to a river flowing in the ocean.
In the adjacent graphic, you can see ADCP data from a long transect that crossed the Gulf Stream. At the left hand side, the *hot spot* is the Gulf Stream. The upper panel shows the near-surface region (200 m) of the lower panel. Warm colors signify strong northward currents while cool colors represent moderate flows headed south. The black region shows the sea bed; you can see significant variation in water depth across the panel. At the right hand side, the ADCP’s current measurements reach as deep as 1200 m. Current speed and direction change quite a bit from top to bottom and left to right. As well as the spatial variation seen in this snapshot, the flow changes from week to week and over longer time scales.
A compelling example of scientific surveys that use ADCPs is the California Cooperative Oceanic Fisheries Investigations (CalCOFI). Quarterly cruises are conducted off southern & central California. Researchers collect a suite of hydrographic and biological data on station and underway. Started in 1949, CalCOFI originally studied ecological aspects of the collapse of the sardine population off California.
Now the scope of the study/work is broader. It includes studying the marine environment off California, managing its living resources, and monitoring indicators of El Nino and climate change. The ADCP data are used together with objective mapping routines to create snapshots of the circulation patterns. Later processing yields impressive color-coded maps of sea-surface height. Eddies, which transport biological and chemical properties, are clearly seen. (http://adcp.ucsd.edu/calcofi).
Strong tidal flows around sharp headlands can generate pronounced eddies that span a kilometer or more. Due to their notable lateral shear, these eddies can stir and redistribute local water properties. Moreover, the lifespan and long-term role of these eddies can vary considerably depending on the bottom friction they experience.
An intriguing example of using ADCP-based surveys in a process study was conducted in Puget Sound, Washington. In conjunction with surface drifters, ADCPs were used to describe the spatial pattern of a tidally-generated eddy and to examine its spin down. Different survey plans were used to study the three-dimensional structure of the currents. For creating a map of the eddy field, the vessel traveled a rectangular circuit about once an hour through much of a tidal cycle. For examining the effects of strong lateral shears, the vessel traveled a bow-tie circuit centered on a drifter.
Results showed a two-layer structure with the surface layer lagging the deep flow by 1-2 hours. Data from the cross-channel transits showed similar time lags with the flow near the headland turning ahead of the mid-channel flow. Regions of strong lateral shear were seen to migrate upward from the bed as the flood tide strengthened.
Due to its strong currents carrying much warm water, the Gulf Stream is a major contributor to the global redistribution of heat from low to high latitudes. Yet measuring accurately the mean transport had been difficult. The Gulf Stream meanders around more than it is wide. Scientists needed a method for repeatedly sampling the horizontal structure of the current in a long-term program. To this end, researchers installed a ship-board ADCP on a container vessel that makes weekly trips between New York and Bermuda. Operating at 75 kHz, the ADCP provided a data set that had close measurements along track and deep into the ocean. As a result, the view showed a large section of ocean in limited amount of time. The red *hot spot* is the Gulf Stream. Warm colors describe the speed of currents headed north whereas cool colors describe currents headed south. (http://www.po.gso.uri.edu/rafos/research/ole/index.html)
These researchers later initiated a similar program to observe the Nordic Seas. For climate monitoring, the goal was to observe the over-turning circulation of the north-south flow. Operating the ADCP on a high-speed ferry proved a persistent challenge for the researchers due to bubble clouds under the hull. Not dissuaded, they repeatedly sampled upper-ocean currents from Denmark to Greenland. Individual transects showed highly variable currents that were dominated by eddy energy. Yet averaging these transects revealed organized mean currents along a dominant topographic ridge.
Accurately mapping the velocity of deep ocean currents from a ship moving faster than 3 m/s is challenging. It demands that shipboard ADCPs operate at a low frequency yet keep narrow acoustic beams. To achieve this balance, Teledyne RDI developed a phased array ADCP that emits four narrow beams of sound from a single transducer face.
This permitted longer range current profiling (800-1000 m) at a lower frequency (38 kHz). As a result, detailed spatial surveys of deep currents were accomplished, impossible to achieve any other way.
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