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Saharan sandstorms at sea

16 February 2006

Snow and rain may be blowing on the February winds in the North Atlantic, but off the coast of Africa it is Saharan dust.

Saharan dust, rich in nitrogen, iron and phosphorus, helps to fertilize the huge plankton blooms that occur in the tropical eastern Atlantic.

MODIS satellite true colour image of dust storm over tropical North Atlantic Ocean, March 2004.

Dr Eric Achterberg, from the National Oceanography Centre, Southampton (NOCS) is leading a study into the dust's effect on nutrients, plankton production and the food chain. The £600,000 project, is part of the Surface Ocean Lower Atmosphere Study (UK SOLAS), funded by the Natural Environment Research Council (NERC) and involves co-ordinating samples taken at sea with atmospheric measurements made from a research aircraft.

The quantity of dust involved, about 500 million tonnes per year, is sufficient to affect the climate. By partly absorbing and partly reflecting sunlight, the dust particles heat the air but cool the ocean surface. They also encourage cloud formation, which reinforces the reflection of light back into space. Such effects can be far-reaching: hurricanes in the Caribbean begin their life off north west Africa, with atmospheric dust being one of many factors influencing their early development. Wind-blown dust from the Sahara desert plays a crucial role in fertilizing large areas of the Atlantic Ocean. The delivery of nutrients, and some metals common on land but scarce in the open ocean, stimulates the production of massive plankton blooms.

"Dust storms are sporadic events," said Eric Achterberg. "And Saharan dust can come from many sources - it can be mixed with soot, from grassland and forest fires; and it can change its chemical and physical properties as it is carried in the atmosphere, at different heights and different moisture conditions. These complications make it difficult to include the dust effects in climate models."

To find out more about the processes involved, researchers from the National Oceanography Centre, Southampton and the Universities of Reading, East Anglia, Southampton, Manchester and Birmingham are carrying out a combined ship and aircraft campaign in the tropical Atlantic around the Cape Verde islands. A BAe 146 research aircraft (jointly funded by NERC and the Met Office) is making sorties from Dakar, Senegal over the Cape Verde islands to investigate the composition, radiative properties and deposition of the dust flowing from west Africa over the tropical Atlantic. Dr Ellie Highwood of the University of Reading and her team in the aircraft are sampling dust at different heights over the ocean - at the same time as Dr Eric Achterberg and other researchers on the German ship RS Poseidon are collecting dust particles falling into the ocean, to study the release of nutrients from the dust and their effects on ocean productivity.

RS Poseidon sailed from Las Palmas in the Canary Islands on 26 January and return there on February 23 chasing dust storms over the ocean on its way to and from the Cape Verde islands. The coordinated ship-aircraft campaign in the Cape Verde region in the period 6-15 February is providing the first synchronous data on the properties and deposition of atmospheric dust particles and their fertilizing influence on phytoplankton in the ocean.

The UK SOLAS studies involve researchers from the National Oceanography Centre, Southampton and the Universities of Reading, East Anglia, Southampton, Manchester and Birmingham. NERC-funded research led by Dr Eric Achterberg (National Oceanography Centre, Southampton) and Dr Ellie Highwood (University of Reading), in collaboration with the UK Meteorological Office, the African Monsoon Multidisciplinary Analysis (AMMA), German SOLAS researchers and other international research groups.

Press release: 8/06

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