Robots in the ocean
Why do we need a global ocean observing system? by Thomas Mtsonti and Isabelle Ansorge
In the past three decades, discussions of global warming have been restricted mainly to academic debates. Now, however, the same topics provide fuel for public debate and mounting pressure for increasing government action as substantial indicators suggest that climate is undergoing significant variability and change. The flurry of new studies into the climate system have worrying conclusions. However, although the changes seen may be a response of the oceans to global warming, they could just as easily be a natural mode of oceanic variability.
Recent reports have confirmed that sea level is rising at an accelerating rate of 3 mm/year, while August 2012 has seen the Arctic sea ice cover shrink to its lowest level ever. Extreme weather events such as droughts in Africa, intense hurricanes off Florida, unprecedented warm spells in the European winter, dust storms in the Sahara or raging bush fires in Australia are now seen regularly. In the past 10 years we have seen eight of the warmest years recorded since 1860. An understanding of the changes in both the atmosphere and ocean are crucial in order to guide international action and policy. However, a lack of sustained earth observations has hindered the development and validation of the climate models that we need to accurately forecast these changes. In 1998 a plan was put forward to develop a global array of profiling floats throughout the ice-free areas of the deep ocean. This array would, for the first time, provide a systematic robotic monitoring system of the global oceans. That initiative was ARGO.
Where does the name ARGO come from?
The ARGO float programme was designed to operate on the same 10 day cycle as altimetry satellites Poseidon and Jason-1. These satellites are designed to monitor global ocean circulation, but they only measure what happens at the surface of the oceans. ARGO collects data in the upper 2 000 m of the ocean. ARGO is named after the ship in which Jason and the Argonauts set sail in search of the golden fleece, in ancient Greek mythology.
How does ARGO work?
ARGo is a multi-national network of 3 000 profiling floats. ARGO floats are approximately 1.1 m tall and weigh around 40 kg. The casing is made of aluminum tubing and is strong enough to withstand pressures exceeding 2 000 m. The sensors are able to measure temperature, salinity and pressure. An antennae transmits data from the surface via satellite to over 60 ground stations and from there, the data goes through a series of quality checks before becoming freely available to the public.
When deployed at the surface, each float sinks to a parking depth of 1 000 m where it drifts for approximately nine day. Before surfacing the float sinks to 2 000 m. On day 10 the float rises to the surface collecting temperature and salinity measurements on the way up. At the surface, the float transmits its data and position via the ARGOS satellite system before repeating another 10 day cycle.
ARGO floats operate on a technique based on differential buoyancy in which an air bladder inflates or deflates, allowing the instrument to sink and rise again to the surface every 10 days. ARGO floats are designed to have a lifetime of three to four years or between 150-200 cycles. The observations will be used to make ‘weather maps’ of the ocean, to initialise climate forecast models for the ocean-atmosphere system and to improve our understanding of the ocean itself. With the array now complete over 100 000 profiles are received each year.
The advantages of the ARGO system
|Floats are relatively easy to deploy||Due to the nature and length of time spent at the parking depth datasets contain nine day gaps|
|New designs include oxygen, bio-optic sensors||Float trajectories are relatively coarse resolution with a GPS position for the float only recorded every 9-10 days.|
|Improve ocean model and forecasting capabilities||No control of where they go in the oceans|
|Al the data is incorporated into World Ocean Database 2009 and Levitus 2012||Data is only collected on the profiling cast|
|They are cheap compared to the cost of a research ship.||Southern and Arctic Oceans still a problem for floats due to winter sea ice formation|
|They are truly robots in the ocean||Quite cumbersome to carry and transport – the technical team need to decrease size and weight of the floats|
|They can conduct long-lived missions and over 150 10 day cycles||Very difficult and expensive to retrieve floats in the ocean and so the majority sink to the bottom once their lifetime is reached (after approximately four years)|
|They acquire and communicate data to researchers throughout the world without the direct involvement of ships or people||Once deployed, any problems with sensors can not be easily fixed|
|Data is freely available and easy to download||Shelf seas and coastal regions where the depth is <1 000 m are currently not being monitored by Argo due to the parking depth|
Problems with ice – what happens?
One of the major problems with research south of Africa is the lack of access and so observations in the Southern Ocean, especially in winter. The Southern Ocean is remote, the environment is hostile and a vast extent of seasonally varying sea ice means that implementing a year-round observing system is challenging. As a consequence the Southern Ocean remains to under sampled when compared to other ocean regions. The current ARGO float array is not equipped to work in the ice. Sensors and the antennae are fragile and easily crushed by sea ice. A new design of ‘ice float’ is currently being tested. The ice floats are standard ARGO floats that have undergone substantial modifications to their temperature sensors. These floats are programmed to check for the presence of sea ice above them using a sea ice detection algorithm. If ice is detected, the data that the float has collected is stored for up to six months. An ice float can work out if there is ice above by calculating the mean water temperature between 20 and 50 m. If the mean temperature is below -1.78 ˚C the float assumes that there is ice in the surface layers and sinks back down to its parking depth. The inclusion of this ice detecting algorithm has resulted in the increase in the survival probability of all floats deployed in the polar regions by over 200%.
Who pays for ARGO?
ARGO is sponsored by two international programmes: the World Climate Research Programme’s CLIVAR which seeks to understand CLImate VARiability and predictability from years to decades. The second sponsor, GODAE (Global Ocean Data Assimilation Experiment) demonstrates the value of real-time operational ocean prediction. The core aim of GODAE is to provide improved ocean observations and forecasts for the scientific and technical community. Each ARGO float costs about $15 000. The array has on average 3 000 floats, but to maintain this number at least 800 floats need to be deployed each year. The total annual cost of ARGO is about $20 million or roughly $25 000 per float-lifetime, which means that each profile costs around $200 or R1 800. The best news is that the data is absolutely free.
Who is involved in ARGO?
Many new countries have joined the original 10 countries who launched floats in 2000. At the latest count 27 countries have deployed or assisted with float deployments.
Teaching ARGO in South Africa
The launch of the first ever African ARGO programme took place in December 2009 when the first South African floats were procured and launched by SAEON (South African Environmental Observation Network) through SANAP. In the past, the SA Agulhas has provided a platform to seed over 200 ARGO floats on behalf of the UK Met and Argo offices in the Southern Ocean – but the acquisition of two Argo floats by SAEON was a first. Scientists from SAEON and the University of Cape Town deployed the two floats while sailing to SANAE (South African National Antarctic base) at 41˚S (within a large ocean eddy) and at 60˚S. Young researchers on board the SA Agulhas were trained in the techniques of deployment. Dr Sebastiaan Swart is one of the South African scientists who uses ARGO data in his research. Dr Swart’s focus is on the Southern Ocean heat and salt fluxes in a remote area about which little is currently known. Hopefully ARGO will fill some of the gaps.
Teachers and school children from six previously disadvantaged community coastal schools are starting to follow these two ARGO floats as they monitor the track of both floats through the Southern Ocean in a bid to engage and stimulate a new generation of marine scientists. The students are able to download the data from a website and monitor the behaviour of the ocean from their school. Working with the online records as well as interpreting the ocean profile plots and drawing up conclusions from the dataset will undoubtedly boost their skills in science-related activities. Allowing student to access data not only helps them to become comfortable using computers but also provides skill in data analysis and interpretation. The fact that the ARGO Programme is in real time makes this exercise so much more interesting and hands on.
What does the future hold for ARGO?
ARGO is still a young project but its main drive is to reach a permanent global array of 3 000 floats – this will provide over one million ocean profiles over the next 10 years. Although ARGO’s first priority was to complete a basic temperature/salinity array, new sensors such as dissolved oxygen, bio-optic, nutrients, and chlorophyll sensors are necessary to broaden the scientific community base of this programme. ARGO is also limited by the present ARGOS communication system, which restricts the size of data (only 100 depths per profile) per transmission. To ensure reliable data transmission each float must spend several hours at the surface. New communication technologies using iridium coupled with improved GPS navigation promises to remove these restrictions.
Sustainable funding and deplyment opportunities are another challenge.
Where can I find ARGO data?
Argo portal: http://www.argo.net
General information: http://www.argo.ucsd.edu
Coriolis Global Data Centre: http://www.coriolis.eu.org/cdc/argo_rfc.htm
US Global Data Centre: http://www.usgodae.org/argo/argo.html
Dr Isabelle Ansorge is a senior lecturer in the Oceanography Department at the University of Cape Town. She is an observational oceanographer and spends many weeks at sea in the Southern Ocean. Her research interests are in the Subantarctic Belt and in particular the impact changes in ocean circulation have on the ecosystem of the Prince Edward Islands.
Thomas Mtsonti networks with key oceanographic and climatescientists in order to facilitate the integration of marine sciences into school sciences. He has a BSc (Education) from the University of the Western Cape and has worked with people from all sectors of our community. His experience includes teaching high school science and later establishing and managing schools’ outreach programmes.