NOAA'S Adopt A Drifter Program

Interview with Dr. Luca Centurioni

Drifting Buoys: Disasters and Developments
June 27th, 2014
By Sonja Hartmann

Q: Can you define your role in the development and deployment of drifting buoys?

A: The Global Drifter Program is a scientific project in which I work with colleagues at NOAA (National Oceanic and Atmospheric Administration) to define the scientific direction and goals of the project. We determine which scientific questions to ask about ocean currents for climate science and understanding the interactions between the ocean surface and the atmosphere.

Within the Global Drifter Program, my general focus is on what the observations [gathered by GDP instruments] teach us [about climate science]. From Scripps [Institution of Oceanography], I oversee the technical development [of drifting buoys]. I work in labs with engineers to improve and expand the capabilities of drifting buoys. We create new sensors and find new ways to collect measurements.

Drifters are deployed by whoever is first scheduled to travel to the area where drifters are needed, sometimes by cargo vessels or research ships.

Q: In an interview with CNN, you demonstrated the ways that a drifter may potentially be able to aid the process of finding the missing Malaysia Airlines Flight 370 by explaining the structure and function of a drifting buoy. Were NOAA drifters used in the investigation for Malaysia Airlines Flight 370?

A: Drifter data are available worldwide and in real time to anyone with access to the GTS [Global Telecommunication  System] of the World Weather Watch, i.e. operational marine and meteorological forecast centers. Data transmitted include location and time (from which surface velocity can be computed), SST [Sea Surface Temperature] and air pressure.

Information gathered from drifters and other instruments can be used to constrain and/or validate numerical models. I am not directly involved in the search [for Malaysia Airlines Flight 370], but I think it is highly likely that drifter data were used directly or indirectly through assimilation into numerical models that simulate and forecast ocean currents. If objects floating on the ocean’s surface were spotted their path could be reconstructed or modelled with information from drifters and satellites, if a drifter was in the same area as debris at the same time, but that is very unlikely. It is much harder to retrace the path of an object and find its origin, which is what must be done in this plane search, than it is to follow it in real time with a drifting buoy. This incident was such a messy process with the search areas changing rapidly that it is hard to target a drifter’s dataset. When objects are found in the ocean, it is not uncommon to mark them so that they can later be relocated, or to deploy a drifter near the debris to track the object’s path. Investigators use all possible information from drifter data and measurements from other equipment to make an educated guess at the answer to their question, or develop numerical models to do so. Drifter information helps scientists make estimates, but doesn’t directly locate debris.

Problems such as this are why it is so important to maintain an adequate array of drifters, so that as many parts of the world as possible are covered, because even if debris from the plane is located, it is unlikely that there will be a drifter in that specific spot to inform others of the material’s path, even with the currently existing array of more than 1,250 drifters.

Q: How were drifters useful in the process of predicting effects of the Fukushima Nuclear Crisis? Were predictions made by drifter data accurate?

A: Drifters were used to give a ballpark estimate of when tsunami debris could be expected to wash ashore on the US west coast. The issue with nuclear meltdowns and the fate of radioactive water that leaked into the ocean is of different nature.

I took drifters near Fukushima and tracked their progress towards the North American West Coast to estimate the travel of  debris from the tsunami. My prediction that the drifting debris would take between one-and-a-half and two-and-a-half years to reach North American coast ended up being correct, as debris reached the coasts of Oregon and Alaska. Debris from this tsunami would not reach the coast of California because very often it is protected by upwelling currents that tend to move floating debris offshore. Although this information couldn’t contain the spread of the debris, the scientific interest helps agencies plan for their reactions and solutions to such problems. The government understands the need to have these instruments in place and to fund programs such as the GDP to develop infrastructure so that by planning ahead they can act quickly and deploy drifters or other equipment when situations occur.

Radiation is a different issue and the information from the drifters are of limited application there. Radioactive water can also mix vertically, not only horizontally along the surface as drifters track. Understanding mixing water over the entire North Pacific ocean requires more knowledge about dispersion and vertical movement than the drifters provide.

Q: Were drifters deployed in anticipation of Hurricane Sandy? What useful information did they supply and how was that information applied?

A: No, the problem there was that we stage the drifters at Keesler Air Force Base [in Biloxi, Mississippi] and it was not possible for us to deploy drifters from there using the C130-J [because the storm was located too far from this base]. This is way we are now developing a more compact drifter that can also be stored at multiple locations and deployed from a P-3 (i.e., a smaller plane).

To deploy hurricane drifters, “Hurricane Hunters” airdrop boxes containing drifters from a C130-J aircraft approximately 24 hours head of a storm, the trck of which is determined and forecasted by the National Hurricane Center . The planes that deploy these drifters are safe if they fly high inside of the storm, but not down low in front of the storm because. That is just is too dangerous. In deployment, hurricane drifters are set up along a 300 to 500 km-long line intercepting the forecasted track of the hurricane. These drifters are unique because they have a 150-meter tether [that extends down into the ocean] with temperature sensors along the cable. They measure vertical changes in temperature of the ocean’s surface, which helps the forecaster to better predict the intensification of the storm  based on how much heat is near the surface. The air pressure and wind speed data collected often come from near the center of the storm, and this information is also very useful understand the nature of the torm and contrain the numerical models. Although a variety of instruments are used to collect as much information as possible about hurricanes, drifters are unique in their ability to measure subsurface ocean temperature and atmospheric components as in-situ instruments. Additional measurements come from other floats and aircraft. The more observations that can be made the better, because hurricanes change rapidly and move very fast.

Q: How useful were drifters in tracking Hurricane Isaac in 2012?

A: I can’t quantify the value of drifters in Hurricane Isaac because that is a very technical question, but the National Hurricane Center seemed to be very pleased with the data, as they were able to successfully receive wind data in real time.

Q: How important were drifter data in tracking the spread of oil after the Deepwater Horizon oil spill?

A: In my mind that was very important because it allows the validation of numerical simulation of dispersion. Private companies also deployed drifters to track the oil.

Q: What do you see as the greatest need in terms of drifter development?

A: The GDP is the only global array that collects in-situ observations of surface velocity, SST (temperature) and SLP (pressure). We need to improve the temporal lag between data collection and data availability, especially for air pressure data for forecast purposes. We are uniquely positioned to measure upper ocean processes and air-sea interaction physics. We need to expand the sensor suite. Coming along in the short term are wave and pH sensors.

Q: Would these be used for disaster relief, other research, or both?

A: New sensor developments would have a variety of climate science and practical applications. PH sensors could ground truth important information about CO2 concentration in the ocean, which would have numerous implications for the understanding of climate change. The goal is to come up with sensors that can be used on a drifting buoy to better understand interactions between the ocean’s surface and the atmosphere. Wave sensors are new, inexpensive developments that have practical applications, to inform traveling ships of rough waters by measuring the energy and direction of surface gravitywaves. Another area of interest is lowering the cost of salinity sensors, as they are especially important to climate science. Also, more compact, air-deployable hurricane drifters.

Q: Could new developments affect the number of drifters or the location of drifter deployments?

A: Absolutely yes. The tendency is for drifters to last longer and to carry more sensors. This may require some repositioning of the array, for example at high latitudes where the melting ice is exposing unexplored stretches of the ocean.

The size of the global drifter array   is dictated by the need to to constrain the bias of of SST measurements from space with in-situ measurements from drifters.. If the scope of the GDP  is expanded to get more drifters to cover other aspects [of of oceanography and climate science], then there may be a need for more drifters. I like this question because the number of drifters in the array, now 1250, may change with developments, like the wave sensor, but currently, SST measurements dictate the number in the array.

10.  Is there anything else that you would like to share in regards to drifting buoys?

A: It is a great project and it is extremely rewarding to be involved in such activity. The drifters are extremely central for science, climate and day-to day applications (search and rescue, emergency response and weather forecast) and offer an incredible infrastructure to exploit new sensors and technologies to monitor the surface ocean and the way it interacts with the atmosphere. Also I would like to acknowledge the vision of Peter Niiler who started this project and the fundamental support of NOAA and of the US Office of Naval Research.