物理海洋学名词-B 70

Glossary of Physical Oceanography and Related Disciplines

Steven K. Baum Department of Oceanography Texas A&M University

May 26, 2004

 

brash ice A type of sea ice defined by the WMO as:
Accumulations of floating ice made up of fragments not more than 2 meters across; the
wreckage of other forms of ice.
See WMO [1970].
brave west winds See roaring forties.
Brazil Basin An ocean basin located off the eastern coast of Brazil in the west-central Atlantic Ocean. It is bounded to the north by the Belem (formerly Para) Rise, at which end there is also a broad depression called the Recife (formerly Pernambuco) Abyssal Plain. This has also been called the Tizard Deep.
See Fairbridge [1966].
Brazil Current A western boundary current that forms the western limb of the subtropical gyre in the South Atlantic Ocean. This current is conspicuously weak as compared with other western boundary currents since only about 4 Sv of the water from the northern limb of the gyre, i.e. the South Equatorial Current (SEC), turns south, with the rest turning north to feed the North Brazil Current (NBC). The BC is not only comparatively weak but also much weaker than might be expected from observed wind fields, more about which later. The portion of the SEC that feeds the BC turns south at about 10–15◦ S. The incipient BC is shallow and flows closely confined to the continental shelf, with direct current measurements at 23◦ S showing that nearly half of its transport of 11 Sv was inshore of the the 200 m isobath. There also seems to be a semi-permanent offshore meander near 22–23◦ S that may be related to local upwelling. South of 24◦S the BC flow intensifies at a rate of about 5% per 100 km, with the intensification apparently linked to a recirculation cell south of about 30◦ S (although there is some evidence for an more extensive recirculation cell extending from 20 to 40◦ S).
Geostrophic transport estimates for the southern BC based on shallow or intermediate zero flow levels (1300-1600 m) have ranged from 18–22 Sv at 33–38◦ S. Evidence for much deeper flow (from the examination of water mass characteristics) has led to estimates ranging from 70–76 Sv at 37–38◦ S with a zero flow level at 3000 m. The latter estimates are at latitudes very close to where the BC separates from the coast and thus may be considered as estimates of the maximum BC flow. The BC separates from the continental shelf between 33 and 38◦ S with the average being near 36◦S. There is some evidence for a seasonal variation in the latitude of this point, with it being generally farther north in the (local) winter than in summer. After it separates from the boundary, it continues to flow in a general southward direction together with the return flow from the Falkland Current, with the southern limit to the warm water it bounds fluctuating between 38–46◦ S on time scales of about two months. After the flow reaches it maximal southern extent it turns back towards the north (as what is sometimes called the Brazil Current Front) and appears to close back on its source flow near 42◦ S. The north–south excursions of its southern limit result in eddies averaging about 150 km in diameter being shed at a rate of about one per week. It was first proposed by Stommel that the reason the BC is weaker than expected from observed wind fields is because of an opposing effect of the thermohaline circulation. The formation of North Atlantic Deep Water requires a net transfer of thermocline water from the South Atlantic to the North as well as net northward fluxes of intermediate and bottom waters. This leads to the situation where the surface circulation of the South Atlantic subtropical gyre is not a closed system because the majority of the SEC flow turns north and crosses the equator due to the demands of the thermohaline circulation. See Peterson and Stramma [1991].

QY@108,20170717 

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