The Atlantic Conveyor is one of the most well known ocean circulation currents that transports warm ocean currents from the Caribbean and the Gulf of Mexico across the Atlantic toward Europe, and is believed to bring warm weather, rains and mild winters.
All oceans have these massive ocean currents known as thermohaline circulations, driven by temperature and salinity.These thermohaline circulations greatly influence the formation of sea ice at the world’s poles, and carry ocean food sources and sea life around the planet, as well as affect rainfall patterns, wind patterns, hurricanes and monsoons.
In the North Atlantic, as the warmer waters evaporate, the salinity increases in the colder water and becomes denser. This colder, denser water sinks lower in the ocean and continues the circulation cycle as warmer water replaces it. Knowing the ocean salinity is therefore very important in understanding how these thermohaline circulations work and whether or not they are being affected by global warming, or vice versa.
According to the Metrological Office (UK), and based on Met Office Hadley Centre and Walker Institute for Climate System Research at the University of Reading, ocean salinity changes are being caused by human’s global warming actions.
In particular, the salinity increases in parts of the Atlantic Ocean just north and south of the Tropics of Cancer (Northern Hemisphere) and the Tropic of Capricorn (Southern Hemisphere) are lowering rainfall and increasing evaporation.
Peter Stott, head of climate monitoring and attribution at the Met Office and leader of the study, explains: "Knowing how our oceans are changing over what are essentially vast data-sparse areas is important. It provides us with a window on changes in the hydrological cycle and gives us more certainty in projections of rainfall as the climate changes. In our region (Europe), for instance, this research could help us to refine projections of summer drying extending out from the Mediterranean basin."
Professor Rowan Sutton explains: "The freshening of polar waters in the Atlantic, which has raised fears about a collapse of the warm Gulf Stream current, leading to a significantly colder climate for Europe, looks not to be related to increasing greenhouse gases, but natural variability."
This collapse of the Gulf Stream current has been much debated as to what is causing it to slow down, whether natural or humankind induced. The researchers looked through measurement data going back over 50 years. The analysis seems to confirm that global warming affects worldwide precipitation where higher temperatures increase subtropical evaporation that is transported by the atmosphere circulation to the poles and by the trade winds across Central America to the Pacific where it falls as increased precipitation. The loss of ocean fresh water to evaporation leads to increased North Atlantic salinity.
Over the last 50 years, the subtropical Atlantic salinity has increased by 1%, which sounds small but since the normal variation is much closer to zero, it actually is rather significant.
Soon, ocean and atmospheric researchers will have state-of-the-art satellite measuring equipment to add to these centuries of data. A NASA earth observation satellite, aptly called Aquarius (The Water Bearer), is scheduled to launch in May 2010 and is the first NASA spacecraft able to measure ocean salinity. The spacecraft is actually built by the Argentinian Space Agency or Comision Nacional de Actividades Espaciales (CONAE), and is known as SAC-D.
NASA hopes to improve upon past ocean measurements from ocean going ships and science buoys and intends to measure the remaining 25% of ocean that has never been checked as Aquarius covers the globe once every seven days.
Ocean salinity is actually measured in practical salinity units (psu) and 1 psu is almost equivalent to 1 grams salt in 1 kilogram of seawater. The Pacific Ocean surface salinity is 35psu and the Atlantic is 37psu.
The Aquarius spacecraft will measure the surface salinity to an accuracy of 0.2 psu. Aquarius project manager Amit Sen, NASA JPL, explains, “If you take half a gallon of water and put a pinch of salt in it, that’s about 0.2 psu. We will be able to detect that from space, while flying about 650 kilometers [about 404 miles] above Earth.”
The measurements are made by a passive measurement device called a radiometer that detects microwave radiation given off by the top few centimeters (inch or two) of the sea surface and the power of the reflections are proportional to the surface salinity. An active radar is also carried by Aquarius, called a scatterometer, which will measure ocean ripples to calibrate the reflective surface changes due to ocean surface ‘roughness’.
Resources
NASA Jet Propulsion Laboratory: www.jpl.nasa.gov
Met Office Website: www.metoffice.gov.uk
Trevor Williams is a University of Victoria Mechanical Engineering PhD candidate specialising in renewable energy, power grid modelling and plug-in hybrid electric vehicles. He has a bachelors in Aeronautical Engineering, a Masters in Management Science and over 23 years international experience in the space industry, having worked on Earth observation and telecommunications satellites. He is the author of the Eco-Geek blog.
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