Ocean–Atmosphere Interactions - Ocean in the Earth System - page 124

C OMMENTARY ON F IGURE 3.3.- These maps reflect the disequilibrium
between the partial pressures in water and air. The map on the bottom
illustrates the interannual variability of these fluxes. It is particularly
strong in the equatorial Pacific Ocean, due to the ENSO phenomenon.
The subpolar gyre of the North Atlantic also presents a strong
decennial variability.
3.2.4.2 . Supplies of precipitation
Rain is foremost a source of fresh water (see section 3.2.3), which
generates a decrease in the salinity of surface water, and therefore an
increase in its vertical stratification. In certain tropical regions, it acts
to inhibit the vertical mixing (for example, barrier layers in the west
Pacific). Rain is also a source of gas or dissolved particles, for
example, in the case of solid hydrophilic aerosols that are transported
into the atmosphere. It is also a source of dissolved salts, in particular,
of inorganic nitrogen in the form of nitrates. The latter have increased
over the course of the last decades, to a non-negligible degree in
certain regions, whereby we can expect modifications at the level of
primary production and the biogeochemical cycles in oligotrophic
regions (those poor in nutritive salts). Other salts and important
elements are carried by rain with divergent effects: from iron and
other metals, to micronutrients for the growth of plankton whose
availability is often limited, but also inorganic or organic
micropolluants.
3.2.4.3. Solid deposits
Other additions to the ocean occur in solid form: for example,
aerosols of Saharan origin, or organic particles, such as black carbon. In
certain regions, these supplies could be an important source of the
nutrients or micronutrients useful for the growth of phytoplankton,
notably in the North Atlantic subtropical gyre or in the Northwest
Pacific. Elsewhere, they can, however, act as a ballast and pull
“useful” particles downward from the surface layers. Understanding
the respective part of the supplies to the ocean in solid or dissolved
forms remains an active subject of investigation, as well as the
evaluation of their impacts on the oceanic surface layers and their
biogeochemistry.