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A drifter (also float) is an oceanographic device floating on the surface or at a given water depth to investigate ocean currents and other parameters like temperature or salinity. They are also called Lagrangian drifters since they follow the flow in a Lagrangian manner. The depth of drifter is defined by its neutral buoyancy. The device stops sinking when its buoyancy force is in equilibrium with its gravitational force.
Contents
Construction principle
The major component of a drifter are hollow bodies that ensure flotation, underwater-sails to catch water currents, instruments (e.g., data collecting instruments, transmitters to transmit the collected data, and GPS devices), and waterproof containers for instruments. Drifters are a technological evolution of ocean current analysis historically performed through drift bottle experiments, which in turn were built on the principle of a message in a bottle.
Applications
Physical oceanography
Drifters provide real-time information about ocean circulation. The data is a valuable input for weather forecasts as meteorological satellites can at maximum measure the sea surface temperature or the surface roughness of the world oceans, but it is not possible to look into the water column. Data from inside the ocean is helpful for hurricane prediction as well.
Biological oceanography Drifters are frequently used to collect information on biological oceanography, such as environmental changes due to sea surface temperature warming and ocean acidification, the distribution of plankton, and plankton blooms. As shipboard studies become less cost effective, Lagrangian floats (drifters) and seagliders allow for the study of plankton and plankton bloom dynamics over the entire bloom evolution.
A drifter will primarily float in a specific water tract that has unique temperature, salinity, and density. This allows the instruments to measure the evolution of the water parcel with minimal advective effects. This is accomplished by the drifters neutral buoyancy, and a stability disk or drogue which provides drag. The drifter can float at a specific depth in the mixed layer of the water parcel and, if designed, perform vertical profiles to a certain depth and back to its original water depth to continue its float.
For use in plankton, biological, and biogeochemical studies, additional instruments besides temperature, conductivity, and pressure sensors are used. Instruments include sensors for measuring chlorophyll fluorescence, oxygen, nitrate, light, optical backscattering, and beam attenuation. Each deployment may use various instrument combinations depending on the application, scientific study, and budget.
Each drift instrument may measure different properties, like Oxygen, in different ways. For example, Nitrate can be measured by analyzing the ultraviolet absorption of seawater using specific algoriths for water temperature and salinity. Oxygen sensors use pumps or optical sensors to measure oxygen levels in the water parceln. Beam attenuation is determined from an optical sensor observing attenuation at a specific light wavelengthn. Another optical property that can be quantified is particulate organic carbon from light absorption and scattering. Each of these sensors measure a limited number of variables, can measurably drift over time, and have calibration uncertainty, all of which can be overcome thru sensor redundancy, experimental planning, and performing calibration tests near the location of the floatn.
All of these measurements (O2, Nitrate, and POC) can be used to investigate the biological communities in the ocean. Specifically, O2, Nitrate, and POC can be used to estimate net community production and the export of particulate organic carbon. Each of these values can change variably with time and dramatically over the course of a phytoplankton bloom making them difficult to determine with traditional methods but more accessible with drifters.