Surface currents, oceanic

In this section we briefly review what controls the density of seawater and the vertical density stratification of the ocean. Surface currents, abyssal circulation, and thermocline circulation are considered individually. 

As a result of these factors (wind, Ekman transport, Coriolis force) the surface ocean circulation in the mid-latitudes is characterized by clockwise gyres in the northern hemisphere and the counterclockwise gyres in the southern hemisphere. The main surface currents around these gyres for the world s oceans are shown in Fig. 10-6. The regions where Ekman transport tends to push water together are called convergences. Divergences result when surface waters are pushed apart. 

Arctic Throughflow Surface currents that move seawater between the Arctic Ocean and the North Pacific and Atlantic Oceans. 

Divergence Horizontal flow of water from a common center or zone that results in upweUing. This occurs in the open ocean at the equator and 60°S as result of surface currents driven by the Trade winds and Westerlies, respectively. 

Indonesian throughflow Surface currents that move seawater from the southwestern Pacific Ocean into the Indian Ocean. 

Fig. 4. Schematic representation of surface currents in the north Atlantic Ocean and stations where surface waters were collected. Areas I to V indicate regions with common temperature, salinity and nutrient characteristics.

Fig. 6.29 World ocean-surface currents viewed from the Antarctic region. Modified from Spilhaus (1942), with permission from the American Geographical Society.

The spills of wastewater, and probably, the special-purpose man-made slicks, timed with satellite and airborne SAR observations, can be used to reveal and study the mesoscale and fine scale features of surface currents in this highly variable Kuroshio area during different seasons as well as in other dynamic oceanic areas. 

Transport processes across the ocean boundaries and within the ocean are central to studies of the global cycles. Such processes as air-sea exchange of gases and aerosols, biological production of particles within the sea, and sedimentation need to be considered. The productivity of the ocean and even climate are influenced by wind-generated surface currents and thermohaline circulation in the deep ocean. The complicated and diverse processes in estuaries influence how much material of riverine origin reaches the sea. 

Surface ocean currents respond primarily to the climatic wind field. The prevailing winds supply much of the energy that drives surface water movements. This becomes clear when charts of the surface winds and ocean surface currents are superimposed. The wind-driven circulation occurs principally in the upper few hundred meters and is therefore primarily a horizontal circulation, although... 

Fig. 3.3 (a) Generalized atmospheric pressure, wind and precipitation patterns on an ocean-covered Earth (n. b. sphere is elongated to aid clarity of wind patterns), (b) Idealized major oceanic surface current systems in the Pacific and Atlantic oceans. The Coriolis effect is responsible for disruption of the meridional circulation cells between 30 and 60° in (a). 

Fig. 5.1 Circumpolar view of the Arctic Ocean showing bathymetry, major surface current (heavy arrows) and bottom current (thin arrows) patterns, and locations of major river inflows.

Recent estimates indicate that of the approximately 200000 tons of mercury emitted to the atmosphere since 1890, about 95% resides in terrestrial soils, about 3% in the ocean surface waters, and 2% in the atmosphere (Expert Panel on Mercury Atmospheric Processes 1994). Some 20-30% of the current oceanic emissions are from mercury originally mobilized by natural sources (Eitzgerald and Mason 1996). Similarly, a potentially large fraction of terrestrial and vegetative emissions consists of recycled mercury from previously deposited anthropogenic and natural emissions (Expert Panel on Mercury Atmospheric Processes... 

In surface current systems away from the continents, Ra becomes a powerful tracer for waters that have been in contact with the continental shelf. The Ra enrichment in surface waters in the equatorial Pacific point to shelf sources off New Guinea, from where the isotope is carried eastward in the North Equatorial Counter Current. In this plume, the vertical distribution of the isotope has been used to derive vertical mixing rates. A very high accumulation of Ra is observed in the transpolar drift in the central Arctic Ocean, a signal derived from the extensive Siberian shelves. 

Wind-driven circulation occurs as a consequence of friction and turbulence imparted by wind blowing over the sea surface. This circulation pattern is primarily horizontal in movement and is responsible for transporting warm water from lower latitudes (warm) to higher latitudes (cold). Surface currents move water and carbon great distances within ocean... 

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