Salinity And cLIMATE

The range of temperatures on Earth allows water to be present as a solid (ice) in ice caps and glaciers; liquid (water) in oceans, groundwater, lakes, and rivers; and gas (water vapor) in the atmosphere. The idealized path of a water molecule from one phase to the other is known as the hydrological cycle. The residence times, that is, the average time that the molecules spend in each phase, range from a few days (water vapor in the atmosphere), to several months (seasonal snow cover, rivers), to the thousands of years (oceans and groundwater). Changes in the hydrological cycle affecting precipitation, evaporation, ice cap thawing, and river runoff have the potential to change the salinity of the oceans. The reverse is also true, and salinity changes may have an imprint in the hydrological cycle after thousands of years.

The mechanisms by which salinity affects the hydrological cycle are numerous. Because of its role in density variations, salinity gradients contribute to ocean currents transporting heat, salt, microorganisms, and nutrients across the oceans. In regions of strong precipitation, a layer of low salinity may isolate the uppermost surface of the ocean from the cold ocean below, forming the so-called barrier layer, which blocks the wind-stirring effects that cool the surface by mixing heat downward. This manifests as warmer sea surface temperatures, modifying the surface temperature gradients that drive surface winds. In the equatorial Pacific Ocean, such a phenomenon is of importance in the El Niño-La Niña cycles. Sim ilar salinity effects also occur in the tropical Indian and Atlantic oceans and have potential feedbacks to the hydrological cycles in the region. Tropical surface anomalies may be advected to the deep convection regions, modulating the thermohaline circulation. One of the largest ocean climate events recorded in the Atlantic Ocean is the Great Salinity anomaly, which lasted from 1968 to 1982. A salinity anomaly propagated over thousands of kilometers reached the Labrador Sea and perturbed the thermohaline circulation intensity. The origin and evolution of these anomalies is still not fully understood because of the historical lack of salinity observations, and studies of the mechanisms by which these salinity anomalies evolve are usually based on ocean and climate models.

See ALSo: Climate Change, Effects; El Niño and La Niña; Hydrological Cycle.

BIBLIoGRAPHY. W.J. Emery and R.E. Thomson, Data Analysis Methods in Physical Oceanography (Elsevier, 2001); M. N. Hill, The Sea. Volume 2: The Composition of Sea-Water (Krieger Publishing, 1982); S. Levitus, R. Burgett, and T.P. Boyer, World Ocean Atlas 1994. Volume 3: Salinity (U.S. Department of Commerce, 1994); J.P. Peixoto and A.H. Oort, Physics of Climate (American Institute of Physics Press, 1991).

JOAQUIM BALLABRERA INSTITUT DE ClENCIES DEL MAR

Consejo Superior de Investigaciones Científicas

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