Traditionally, ocean temperature and salinity profiles have been collected by sampling from research vessels or from ships of opportunity (naval and merchant vessels). This has limitations as often, many miles exist between ship track lines. Profiling float technology, demonstrated successfully during the World Ocean Circulation Experiment (WOCE), can provide a viable alternative means of making ocean measurements at a lower overall cost and has the added advantage of greater potential coverage.
What is Argo?
Argo's global array of profiling floats will provide a major contribution to the Global Ocean Observing System (GOOS) and the Global Climate Observing System (GCOS). Argo is endorsed by the Intergovernmental Oceanographic Commission (IOC) and the World Meteorological Organisation (WMO).
The floats return to the sea surface at regular intervals and data are transmitted via satellite for processing. A significant improvement is expected in data coverage, particularly in the remote ocean regions where floats can be deployed via air drops.
In October 2007 it achieved its goal of 3000 operating floats. These are providing around 100,000 observations each year, throughout all the ice-free deep-ocean areas of the world. Around 800 floats will need to be deployed each year to maintain the 3000 float array.
Unrestricted access to data is fundamental to the Argo project. Individual Data Assembly Centres (DACs) assemble the data from the communications systems and carry out initial processing. Data are forwarded, within 24 hours, to the two Global Data Assembly Centres (GDACs) to allow some users (i.e. forecast scientists) access to the data within two days. Data are also disseminated in real-time via the World Meteorological Office, Global Telecommunications System (WMO GTS). For some research activities a delay is acceptable. During this time the DACs, who are responsible for the integrity of the data, are able to carry out detailed quality control.
The UK part of the project led by the Met Office, Ocean Applications, aims to
- establish by March 2006 an operational system with the capacity to deploy about 50 floats each year, thus maintaining about 150 to 200 UK Argo floats in the water at any one time.
- capture all Argo data in real time in support of operational ocean forecasting, as well as processing UK float data in delayed mode for climatological and hydrographic purposes.
BODC act as the Data Assembly Centre for UK floats in the Argo programme regardless of their location.
The history of a profiling float
The concept of a neutrally buoyant float to measure subsurface ocean currents was developed simultaneously and independently in the mid-1950s by the USA (Stommel, 1955) and the UK (Swallow, 1955). The first floats were built and tested by Swallow and were used extensively to investigate features of the deep ocean. These initial floats have evolved over the years to become a P-ALACE (Argo) float. A summary of float development is provided below
- 1955 — Concept developed and first floats built
- Late 1960s - early 1970s — Transponding float developed; utilising the SOFAR channel for detection.
- 1980s — RAFOS floats developed utilising Argos satellite system.
- Late 1980s — ALACE floats developed returning to the surface at regular intervals.
- 1990s — P-ALACE floats developed carrying CTD sensors; transmitting profiles of temperature and salinity at regular intervals.
To find out more about the history of the profiling float visit the National Oceanography Centre, Southampton (NOCS) Argo web site.
What does a float do?
Argo floats descend to a depth of typically 1500 to 2000 metres and drift at this depth for nine days before rising to the surface. During the ascent a temperature and salinity profile is collected. These data are then transmitted back via satellite (currently Argos) before the float starts another cycle.
Future plans hope to include the ability for the float to be re-programmed at the surface to change, for example, sampling depth or mission duration.
Argo floats are expected to be capable of making as many as 150 cycles and operate for four to five years. Each Argo float (including its antenna) is typically 2 to 2¼ m long and up to 30 kg in weight. Currently there are two commercial manufacturers for UK floats — Webb Research Corporation and Martec.
Stommel, H., 1955. Direct measurement of subsurface currents. Deep-sea Research, 2(4), 284-285.
Swallow, J.C., 1955. A neutral-buoyancy float for measuring deep currents. Deep-sea Research, 3(1), 93-104.