Geoscience Reference
In-Depth Information
- by satellite, visible observations (during the polar day) are often
difficult because of the stratiform cloud cover. Using infrared sensors,
measurement of the surface temperature can be wrong because of the
slight difference between the temperatures emitted by the clouds and
those from the surface. However, using microwaves, the radiometric
observations have proved to be adequate for monitoring the ice
“concentration”, that is to say the quantity of the ice surface by unit of
surface and its extension. The microwave emission is coming from an
ice layer, whose thickness depends on its stratification and associated
distribution of dielectric properties. This complexity has, hitherto,
limited a more “physical”
exploitation of the signal.
It is, therefore, a major challenge to monitor the Arctic and
Antarctic sea ice, keeping in mind the importance of these regions for
the Earth's climate. In 2007, the “International Polar Year”
triggered
an international observation effort, which is still being pursued.
3.3.5.
The example of the impact of the iron supply from Saharan
aerosols
The heart of the subtropical Atlantic Ocean is a region with an
often deep thermocline which lacks any real ocean vortex activity that
might cause important vertical exchanges. As a result, there are few
supplies of nutritive salts coming from the deeper ocean or regions of
upwelling
, such as those off Africa. Therefore, except for a short
period at the end of winter when the thermocline is close to the
surface, weak concentrations of dissolved nutritive elements, in
particular of nitrates and iron, are found at the surface in these regions.
The supplies of aerosols, however, are significant and have different
origins: anthropogenic aerosols rich in inorganic nitrated elements,
above all in winter; natural aerosols coming from Saharan dusts;
aerosols from the Sahel, especially from bush fires. These different
aerosols do not reach the same oceanic regions.
The aerosols coming from Africa are very significant in volume
(between 10 and 50 gm
-2
/year from 5 to 25°N and east of 30°W), and
are deposited with the trade winds as far as the Antilles and even the
