4.1: Bottles, Drifters, and Floats

If you’ve ever seen a plastic bottle adrift on the surface of a lake or the ocean, then you have a pretty good understanding of one of the earliest tools for tracking ocean currents. The drift bottle method relies on a return-to-sender card sealed inside a buoyant container—traditionally a glass bottle—that is tossed from a ship or shore. The card requests that the finder return the card with information on the date and location where the bottle was found. By comparing the starting point to the final destination, the path of the drift bottle may be inferred. Drift bottles have served as indicators of surface current speed and direction since at least 1802 and remain in use today (e.g., Stommel 1965; Storch et al. 2020). At Woods Hole Oceanographic Institute (WHOI), oceanographer Dean “Bump” Bumpus (1912–2002) oversaw the release of hundreds of thousands of drift bottles during his 40-year career (Lipsett 2014; Woods Hole 2021).

While drift bottles are not autonomous, their use inspired two types of passive float technologies in use today: drifters and floats. Drifters (a.k.a. surface drifting buoys) provide information on surface currents, and floats track currents at deeper depths. As WHOI physical oceanographer Bruce Warren (1937–2010) puts it, “Drifters float and floats sink” (e.g., Lumpkin et al. 2017). Drifters and floats represent what are known as Lagrangian platforms—devices that move with the currents—named after Italian mathematician Joseph-Louis Lagrange (1736–1813), who developed the mathematics of fluid flows (e.g., Knauss and Garfield 2017).

In the 1950s English oceanographer John Swallow (1923–1994) developed a float that could maintain a constant depth and transmit an acoustic signal so that it could be tracked by a ship or a receiving station (Swallow 1955). These acoustically transmitting floats—known as Swallow floats—provided some of the first hints at the complex, eddy-like nature of the ocean (e.g., Crease 1962).

In 1979 the National Oceanic and Atmospheric Administration (NOAA), along with the World Meteorological Organization and Intergovernmental Oceanographic Commission, established the Global Drifter Program (GDP; Niiler 2001). The most common drifter in use today, the Surface Velocity Program (SVP) drifter, consists of a small, spherical surface float attached to a nearly 23-foot-long (7 m) cloth cylinder with holes in it (to reduce drag), the so-called holey sock drogue. As of August 2022, some 1,244 drifters have been deployed by 26 countries. Each transmits position and temperature data to a satellite for upload to NOAA’s GDP database and website.

Swallow floats and SVP drifters set the stage for development of autonomous profiling floats, underwater robots that collect, store, and report data via satellite. The crown jewel of autonomous profiling floats is the Array for Real-Time Geostrophic Oceanography, known simply as Argo.

Put into service in 1999, the Argo floats take their name from the legendary ship Argo. Commanded by the Greek hero Jason, Argo, with his heroic crew, the Argonauts, set sail to find the Golden Fleece, the fur of a winged golden ram, as told in Greek mythology (Colavito 2014). The Argo floats have been hailed as “one of the scientific triumphs of our age” (Gillis 2014). Fittingly, the Argo floats serve under the watchful eye of the ocean-observing Jason satellites, named for the Greek mythological hero first mentioned by Homer in 800 BCE (Colavito 2014). Together they provide simultaneous ocean measurements above and below the surface.

An individual Argo float resembles a tall and skinny version of R2-D2, the feisty robot in the Star Wars movies. They move up and down in the water column by regulating their buoyancy, the balance between rising or sinking in the water column (see Chapter 13). As anyone who has used a floatie knows, your ability to float on the surface depends on the volume of air inside the floatie. Let the air out and you sink. Fill the float with air and you float. An Argo float works in a similar fashion, except instead of air, the float uses oil in an internal reservoir. When pumped into a flexible bladder at the bottom of the float, the oil occupies a greater volume and makes the float more buoyant. When the oil is removed, the bladder shrinks and the float sinks. By carefully controlling the bladder, the float can sink, rise, or stay at a constant depth.

Argo floats can measure, store, and transmit observations on a number of physical, chemical, and biological properties, including temperature, salinity, pH, oxygen, and nitrate. As of August 2022, a flotilla of 3,930 floats report data from the surface to depths of 6,562 feet (2,000 m) across the entire world ocean, including the Southern and Arctic Oceans. Over the span of their more-than-two-decade lifetime, the Argo floats have generated more than two million profiles of the upper ocean, nearly four times the number of profiles available prior to their implementation. More than 2,000 scientific papers have used Argo data to support their findings (e.g., Jayne et al. 2017).

In recent years a new generation of Argo floats—Deep Argo—are being deployed to obtain data over the entirety of the world ocean. Argo floats with nicknames such as Deep SOLO and Deep APEX developed in the United States will reach depths up to 19,685 feet (6,000 m). Deep NINJA and Deep ARVOR, under development by Japan and France, respectively, will operate at depths to 13,123 feet (4,000 m; Jayne et al. 2017). Oceanographers have also developed a new suite of sensors for measurements of optical and biogeochemical properties, a fleet of floats called Biogeochemical Argo (e.g., D’Ortenzio et al. 2020). These new Argo floats promise to shed light on the role of the deep ocean in climate change and improve our understanding of its physics, chemistry, and biology (Roemmich et al. 2019).

While all the above drifters and floats are traditionally deployed from ships, aircraft can launch floats too. The Air-Launched Autonomous Micro-Observer (ALAMO) can profile the ocean to depths of more than 3,900 feet (1,200 m). ALAMO floats were developed in response to a need for obtaining observations in places difficult to reach, such as ice-covered waters and the interior of hurricanes (e.g., Jayne and Bogue 2017; Goni et al. 2017). Deployments of ALAMO floats during the 2017 and 2018 Atlantic hurricane seasons yielded new insights into the response of the upper ocean to the passage of a hurricane and provided ground-truth data for hurricane forecast models (Sanabia and Jayne 2020). Ultimately, the better our understanding of factors that influence hurricane intensity, the more likely hurricane forecasts can provide accurate warnings and save property and human lives.