Inverted Echo Sounders (IES)
An inverted echo sounder is a fairly simple instrument consisting mainly of a transducer, which can produce sound waves and hear sound waves, and a precise clock. The inverted echo sounders used here at AOML send out a series of 24 10kHz or 12kHz sound pulses each hour. These pulses reflect when they hit the ocean surface, and 1-8 seconds later the IES records the precise amount of time between when each pulse is sent out and when the pulse is heard returning to the IES. The median value of the 24 pulses is then taken as the travel time for that hour (multiple pulses are needed to average out the changes in travel time due to waves at the ocean surface and other sources of noise). Because the speed of sound in seawater is dependent on temperature (and weakly upon salinity), as the water temperatures above the IES change over time the travel time measurement of the IES changes. These changes in travel time are small, measured in milleseconds, but with a precise clock these time changes can be accurately determined. Because they are small and relatively inexpensive (see the static picture at left), and because the data is so useful, the IES is becoming a popular oceanographic measuring tool.

The cartoon animation at right illustrates the measurement of the IES. Consider first the upper panel. Two IESs are illustrated on the sloping ocean bottom; the red lines indicate the isotherms (lines of constant temperature) of the thermocline (layer of rapid temperature change) in the ocean. The thermocline is illustrated as dipping to deeper depths as you move from left to right; this is intended to illustrate the change in thermocline depth across an oceanic front (such as the Gulf Stream). The movement of this "front" in the animation shows three types of motion observed in fronts in the real ocean: the front can move horizontally (typically called "meandering"); the whole thermocline can move up and down; and finally the slope of the front itself can weaken or strengthen. The vertical moving bars indicate the sound pulses sent out and received by the IESs. Note that in the real ocean the time it takes for these sound pulses to go to the surface and return to the IES (1-8 seconds depending on the water depth) is much shorter than the time scales of frontal movement such as that illustrated in the animation (fronts move like this on time scales of weeks or longer). The two lower panels illustrate the travel time that would be measured by these two fictional IESs; because sound moves faster through warm water than through cold water, you will see the travel times decreasing when the thermocline is deeper at a site (indicating there is more warm water over the site), and the travel times increase when the thermocline gets shallower (indicating less warm water over the site).

The travel time measurement of the IES is combined with other ocean measurements of temperature and salinity in order to estimate full-water-column profiles of temperature, salinity, and density. The result is a time series of profiles of these quantities at each IES site. More details of how IESs work and how they are combined with other data to obtain estimates of temperature and salinity profiles can be found in the references listed below.

If you have questions regarding the IES data available here on the AOML web page please contact Silvia L. Garzoli or Christopher S. Meinen.

Bottom pressure measurements

Useful IES and bottom pressure references:
The earliest IES work was done by H. Thomas Rossby and D. Randolph Watts at Yale University and later at the University of Rhode Island.

Rossby, T., 1969, On monitoring depth variations of the main thermocline acoustically. J. Geophys. Res., 74, 5542-5546.
Watts, D. R., and H. T. Rossby, 1977, Measuring dynamic heights with inverted echo sounders: results from MODE, J. Phys. Oceanog., 7, 345-358.

Here is a list of some of the papers published since 1995 which utilized IES data or bottom pressure measurements (please note this list is not complete).

Bianchi, A., and S. L. Garzoli, 1995, Variability and motion of the Brazil-Malvinas Front, GeoActa, 22, 74-90.

Chiswell, S. M., K. A. Donohue, and M. Wimbush, 1995, Variability in the Central Equatorial Pacific, 1985-1989, J. Geophys. Res., 100, 15849-15863.

Duncombe Rae, C. M., S. L. Garzoli, and A. L. Gordon, 1996, The eddy field of the south-east Atlantic Ocean: At statistical census from the BEST Project, J. Geophys. Res., 101, 11949-11964.

Garzoli, S. L., and A. L. Gordon, 1996, Origins and variability of the Benguela Current. J. Geophys. Res., 101, 897-906.

Garzoli, S. L., A. L. Gordon, V. M. Kamenkovich, D. Pillsbury, and C. M. Duncombe Rae, 1996, Variability and sources of the southeastern Atlantic circulation. J. Mar. Res., 54, 1039-1971.

Garzoli, S. L., G. J. Goni, A. Mariano, and D. Olson, 1997, Monitoring the upper southeastern Atlantic transport using altimeter data, J. Mar. Res., 55, 453-481.

Garzoli, S. L., and G. J. Goni, 2000, Combining altimeter observations and oceanographic data for ocean circulation and climate studies. IN: Satellites, Oceanography, and Society, edited by D. Halpern, Elsevier Science B. V., 79-97.

Garzoli, S. L., A. Ffield, and Q. Yao, 2003, North Brazil Current rings and the variability in the latitude of the retroflection, IN: Interhemispheric Water Exchange in the Atlantic Ocean, edited by G. Goni and P. Malanotte-Rizzoli, Elsevier Oceanography Series, 357-374.

Garzoli, S. L., A. Ffield, W. E. Johns, and Q. Yao, 2004, North Brazil Current retroflection and transports, J. Geophys. Res., 109, doi:10.1029/2003JC001775.

Goni, G., S. Kamholz, S. L. Garzoli, and D. B. Olson, 1996, Dynamics of the Brazil/Malvinas Confluence based on inverted echo sounders and altimetry, J. Geophys. Res., 101, 16273-16289.

Gordon, A. L., and R. D. Susanto, 1998, Makassar Strait transport: Initial estimate based on Arlindo results. Mar. Technol. Soc. J., 32, 34-45.

Hallock, Z. R., and W. J. Teague, 1995, On the meridional surface profile of the Gulf Stream at 55W, J. Geophys. Res., 100, 13615-13624.

Hallock, Z. R., and W. J. Teague, 1996, Evidence for a North Pacific deep western boundary current. J. Geophys. Res., 101, 6617-6624.

He, Y., D. R. Watts, and K. L. Tracey, 1998, Determining geostrophic velocity shear profiles with inverted echo sounders, J. Geophys. Res., 103, 5607-5622.

Hendry, R. M., D. R. Watts, and C. S. Meinen, 2002, Newfoundland Basin sea level variability from TOPEX/POSEIDON altimetry and inverted echo sounder/bottom pressure measurements, Canadian J. Remote Sensing, 28(4), 544-555.

Howden, S. D., and D. R. Watts, 1999. Jet streaks in the Gulf Stream. J. Phys. Oceanogr., 29, 1910-1924.

James, C. E., and M. Wimbush, 1995, Inferring dynamic height variations from acoustic travel time in the Pacific Ocean, J. Oceanogr., 51, 553-569.

Johns, W. E., T. J. Shay, J. M. Bane, and D. R. Watts, 1995, Gulf Stream structure, transport, and recirculation near 68W, J. Geophys. Res., 100, 817-838.

Katz, E. J., 1997, Waves along the Equator in the Atlantic, J. Phys. Oceanogr., 27, 2536-2544.

Katz, E. J., A. Busalacchi, M. Bushnell, F. Gonzalez, L. Gourdeau, M. McPhaden, and J. Picaut, 1995, A comparison of coincidental time series of the ocean surface height by satellite altimeter, mooring, and inverted echo sounder. J. Geophys. Res., 100, 25101-25108.

Kinoshita, H., Y. Micida, H. Nishida, and H. Yoritaka, 1996, Improvement in the geoid under TOPEX/POSEIDON passes in the region south of Japan. J. Adv. Mar. Sci. Tech. Soc., 2, 31-38.

Lindstrom, S. S., X. Qian, and D. R. Watts, 1997, Vertical motion in the Gulf Stream and its relation to meanders, J. Geophys. Res., 102, 8485-8503.

Meinen, C. S. and D. R. Watts, 1997, Further evidence that the sound speed algorithm of Del Grosso is more accurate than that of Chen and Millero, J. Acoust. Soc. Am., 102, 2058-2062.

Meinen, C. S. and D. R. Watts, 1998, Calibrating inverted echo sounders equipped with pressure sensors, J. Atmos. Ocean. Technol., 15(6), 1339-1345.

Meinen, C. S. and D. R. Watts, 2000, Vertical structure and transport on a transect across the North Atlantic Current near 42N: Time series and mean, J. Geophys. Res., 105(C9), 21869-21891.

Meinen, C. S. , 2001, Structure of the North Atlantic Current in stream-coordinates and the circulation in the Newfoundland Basin", Deep Sea Res., 48(7), 1553-1580.

Meinen, C. S., D. S. Luther, D. R. Watts, K. L. Tracey, A. D. Chave, and J. Richman, 2002, Combining inverted echo sounder and horizontal electric field recorder measurements to obtain absolute velocity profiles, J. Atmos. Ocean. Technol., 19(10), 1653-1664.

Meinen, C. S., and D. S. Luther, 2002, Mooring motion when the pressure sensors fail: A method employing inverted echo sounders, J. Atmos. Ocean. Technol., 19(9), 1451-1460.

Meinen, C. S. and D. S. Luther, 2003, Comparison of methods of estimating mean synoptic current structure in "stream coordinates" reference frames with an example from the Antarctic Circumpolar Current", Deep Sea Res. I, 50(2), 201-220.

Meinen, C. S., D. S. Luther, D. R. Watts, A. D. Chave, and K. L. Tracey, 2003, Mean stream-coordinates structure of the Subantarctic Front: Temperature, salinity, and absolute velocity, J. Geophys. Res., 108(C8), 3263, doi:10.1029/2002JC001545.

Meredith, M. P., J. M. Vassie, R. Spencer, and K. J. Heywood, 1997, The processing and application of inverted echo sounder data from the Drake Passage, J. Atmos. Oceanic Technol., 14, 871-882.

Mitchum, G. T., 1996, On using satellite altimetric heights to provide a spatial context for the Hawaii Ocean Time-series measurements. Deep Sea Res. II, 43, 257-280.

Pickart, R. S., Gulf Stream-generated topographic Rossby waves. J. Phys. Oceanogr., 25, 574-586.

Shay, T. J., J. M. Bane, D. R. Watts, and K. L. Tracey, 1995, Gulf Stream flow field and events near 68W, J. Geophys. Res., 100, 22565-22589.

Sun, C. and D.R. Watts. 2002, A pulsation mode in the Antarctic Circumpolar Current south of Australia, J. Phys. Oceanogr., 32, 1479-1495.

Teague, W. J., Z. R. Hallock, G. A. Jacobs, and J. L. Mitchell, 1995, Kuroshio sea surface height fluctuations observed simultaneously with inverted echo sounders and TOPEX/POSEIDON, J. Geophys. Res., 100, 24987-24994.

Waworuntu, J. M., Waworuntu, S. L. Garzoli, and D. B. Olson, 2001, Dynamics of the Makassar Strait, J. Mar. Res., 59, 313-325.

Tracey, K. L., S. D. Howden, and D. R. Watts, 1997, IES calibration and mapping procedures, J. Atmos. Oceanic Technol., 14, 1483-1493.

Watts, D. R., K. L. Tracey, J. M. Bane, and T. J. Shay, 1995, Gulf Stream path and thermocline structure near 74W and 68W, J. Geophys. Res., 100, 18291-18312.

Watts, D.R., C. Sun, and S. Rintoul, 2001. A two-dimensional gravest empirical mode determined from hydrographic observations in the Subantarctic Front. J. Phys. Oceanogr., 31, 2186-2209.

Watts, D.R., X. Qian, and K. L. Tracey. 2001. On mapping abyssal current and pressure fields under the meandering Gulf Stream. J. Atmos. Oceanic Technol., 18, 1052-1067.

Woodworth, P. L., J. M. Vassie, C. W. Hughes, and M. P. Meredith, 1996, A test of the ability of TOPEX/POSEIDON to monitor flows through the Drake Passage. J. Geophys. Res., 101, 11935-11947.