Shipboard Acoustic Doppler Current Profiler (ADCP) measurements are now a part of Marinexplore’s growing repository of ocean data. ADCP sensors are traditionally used to measure vertical profiles of ocean currents, providing a continuous in-depth view of circulation features.
ADCP is an amazing sensor that allows oceanographers to remotely measure currents in the oceans, providing a profile of the 3-dimensional circulation of the water column. The sensor works by emitting acoustic signals through several transducers, each one pointing in a different direction. These signals are reflected by the water at different levels, and by measuring the change in frequency of the reflected signal — the Doppler shift — we can determine the zonal, meridional and vertical components of the water speed. The ADCP can even be attached to the hull of a moving ship, thanks to advanced post-processing techniques that take into account the orientation, inclination and speed of the vessel in order to calculate the water speed referenced to the ocean bottom.
Our initial ADCP data comes from the Joint Archive for Shipboard ADCP (JASADCP), which provides hourly measurements for almost 2000 oceanographic cruises since 1992, as well as shipboard data collected during the Integrated Acoustic and Trawl Surveys of Pacific Hake. Our plan is to continuously grow this list, offering higher resolution data, as well as measurements from moored ADCPs.
The ADCP data introduced a new challenge for Marinexplore: visualizing gridded time × depth measurements in our data studio. To properly represent the data we built a new visualization widget for the sidebar, representing the ADCP measurements in an interactive 2-dimensional plot. Hovering the mouse over the plot will highlight where the measurement was made on the map, allowing you to quickly identify circulation features at that location. The interface will also display the values and the period when they were collected, providing a valuable tool to explore the available data before downloading it.
Let’s look at some of the data. I would like to show you what has been called “the mightiest current in the oceans” by Pickard and Emery (1990): the Antarctic Circumpolar Current (ACC). The ACC is unique as it is the only current that flows around the entire globe, flowing east around the Antarctic continent and connecting the Atlantic, Indian and Pacific oceans. Despite its relatively slow eastward flow of less than 20 cm/s, the ACC transports more water than any other current, since it extends from the sea surface to levels as deep as 4000 m.
The best location to study the ACC is in the Drake Passage, where the current is restricted to an 800-km-wide passage between the southern tip of South America and Antarctica. On the right side we can see this dataset, a cruise of the R/V N. B. Palmer, which recorded the flow of the ACC through the Passage using shipboard ADCP between 2010 and 2011. The data covers 10 m high bins from 50 m down to 1030 m.
Can we use this data to determine the transport of the ACC? Recent studies have revealed a mean ACC transport of 100-150 Sv, with a variation of 50 Sv within time scales of a month or two (Knauss, 1996). For comparison, the flow of the Amazon river, the strongest in the world, is on average only 0.2 Sv. (A Sverdrup, Sv, is defined as 1,000,000 cubic meters per second.) The figure below show the zonal velocity of the ACC down to 1030 m, for the first part of the cruise when the ship was travelling southward. We can obtain the total transport by multiplying the water speed by the area it represents, which gives us a total 40.39 Sv.
This number underestimates the transport of the ACC because we are looking only at the upper 1000 m of the water column. But the mean flow of the ACC is generated by barotropic processes due to the piling of the sea surface by strong westerly winds. This means that the flow is approximately constant vertically throughout the whole water column. We can obtain a better approximation of the current transport by calculating the average zonal velocity at each latitude and multiplying by the total water column depth (using data from ETOPO1) and by the distance between measurements.
Given these assumptions we obtain a transport for the ACC of 149.19 Sv, inside the expected range. If you are interested in the details of this analysis, I have published an iPython notebook. Copying the notebook it is possible to test the analysis on different periods, since many other ships have crossed the Drake Passage while measuring currents with ADCP.
I hope you are as excited as I am to have all this ADCP data easily available. How will it be useful for you? Leave us a comment and tell us about your plans to use this data, and which other ADCP datasets you would like to see on Marinexplore.
- Knauss, J. A., 1996: Introduction to Physical Oceanography. Prentice-Hall, Inc., 2nd Edition, 152-56.
- Pickard, G. L., and W. J. Emery, 1990: Descriptive Physical Oceanography, An Introduction. Permagon Press, 5th Edition, 173-76.