190
Chapter 9: The Atlantic Ocean
not strongly variable across the province although the circulation is sufficiently complex
as to make the derivation of mean mixed depths unsatisfactory: be that as it may, the
regional mean mixed layer appears to be of order 20 m in July, 40 m in January.
Circulation within this region is dominated by the throughflow of water from the
equatorial currents of both North and South Atlantic (NEC, SEC) through the passages
of the Lesser Antilles, and the delivery of this water to the North Atlantic in the Florida
Current. A useful account of flow-through circulation is that of Johns et al. (2002).
The water entering the Caribbean through the Lesser Antilles arc is largely of NEC
origin, of high clarity, and with little vertical structure in the upper 100 m (Borstad,
1982), and this forms about half of the total influx. Nevertheless, the satellite images
show that in many months when active retroflection of the NBC is occurring, the water
mass immediately outside the Lesser Antilles is highly eutrophic and has been retroflected
from the coast. The images suggest that, in this situation, water containing significant
algal biomass enters the northern Caribbean far seaward from the mainland shelf. Water
from the Guiana Current, which originates in the SEC, is turbid and carries much
DOM and passes along the southern coast of the Caribbean (see GUIA) and thence,
after losing much of this material, flows northward along the Nicaraguan coast as the
Yucatan Current and so into the CARB province. These multiple sources for flows of
surface water through this province result in a rather complex pattern of circulation
that is detectable in sea surface topography and in surface chlorophyll. Flow through
the various passages in the island arc induces eddies and vortices, initially of about
100 km diameter but increasing in dimension downstream (Kinder, 1983); indeed, the
ubiquity of mesoscale eddies throughout this province is one of its principal characteristics.
Flow through the Yucatan Channel, and then along the northeast edge of the Campeche
Bank briefly forms a true western boundary current that detaches from topography and
passes northward across the Gulf of Mexico over deep water as the Loop Current. Once
detached, it becomes unstable and forms an anticyclonic loop around the eastern Gulf,
passing from the tip of the Campeche Bank to the western edge of the Florida shelf. Flow
then follows the shelf edge south to the Florida Straits, whence it emerges as the Florida
Current and the Gulf Stream. The Loop Current penetrates furthest into the western Gulf
when it separates from the Campeche Bank at a relatively western position (Molinari and
Morrison, 1988). When the Loop Current penetrates sufficiently far to the north, the
effluent of the Mississippi may become entrained in the flow.
The dynamic shifts of the Loop Current induce the pinching-off of very large meanders
that become major features of the circulation and nutrient budget of the Gulf of Mexico.
These warm-core, anticyclonic rings are generated at about 6- to 18-month intervals
from the Loop Current, mostly in winter (Vukovich, 1988), and move into the western
Gulf, subsequently to pass around the northern coast as they age. They are 100–300 km
in diameter, have a lifetime of about 1 year, and may interact dynamically with the
continental shelf edge (Vukovich and Waddell, 1991). After such a feature is pinched
off, the flow of the Yucatan Current briefly passes directly toward the Straits of Florida.
Upwelling of subpycnocline water in the Gulf of Mexico, due to instabilities in the Loop
Current, occurs at the shelf edge and along the outer shelf in many places throughout
the province, especially off northern Yucatan, but also around Campeche Bank, and
southwest of Florida (Mooers and Maul, 1998).
Cold-dome cyclonic eddies (80–120 km) occur on the eastern/southern side of
the Loop Current, evolving into tongues or quasipermanent meanders of cool water
(Vukovich and Maul, 1985) that are a characteristic feature of the eastern Gulf. Over the
shelves of the northern Gulf of Mexico, cyclonic cells are expected to occur for most of
the year, but will be interrupted by extreme wind events; some may be detached from
the isolated warm-core rings.
Chapter 9: The Atlantic Ocean
not strongly variable across the province although the circulation is sufficiently complex
as to make the derivation of mean mixed depths unsatisfactory: be that as it may, the
regional mean mixed layer appears to be of order 20 m in July, 40 m in January.
Circulation within this region is dominated by the throughflow of water from the
equatorial currents of both North and South Atlantic (NEC, SEC) through the passages
of the Lesser Antilles, and the delivery of this water to the North Atlantic in the Florida
Current. A useful account of flow-through circulation is that of Johns et al. (2002).
The water entering the Caribbean through the Lesser Antilles arc is largely of NEC
origin, of high clarity, and with little vertical structure in the upper 100 m (Borstad,
1982), and this forms about half of the total influx. Nevertheless, the satellite images
show that in many months when active retroflection of the NBC is occurring, the water
mass immediately outside the Lesser Antilles is highly eutrophic and has been retroflected
from the coast. The images suggest that, in this situation, water containing significant
algal biomass enters the northern Caribbean far seaward from the mainland shelf. Water
from the Guiana Current, which originates in the SEC, is turbid and carries much
DOM and passes along the southern coast of the Caribbean (see GUIA) and thence,
after losing much of this material, flows northward along the Nicaraguan coast as the
Yucatan Current and so into the CARB province. These multiple sources for flows of
surface water through this province result in a rather complex pattern of circulation
that is detectable in sea surface topography and in surface chlorophyll. Flow through
the various passages in the island arc induces eddies and vortices, initially of about
100 km diameter but increasing in dimension downstream (Kinder, 1983); indeed, the
ubiquity of mesoscale eddies throughout this province is one of its principal characteristics.
Flow through the Yucatan Channel, and then along the northeast edge of the Campeche
Bank briefly forms a true western boundary current that detaches from topography and
passes northward across the Gulf of Mexico over deep water as the Loop Current. Once
detached, it becomes unstable and forms an anticyclonic loop around the eastern Gulf,
passing from the tip of the Campeche Bank to the western edge of the Florida shelf. Flow
then follows the shelf edge south to the Florida Straits, whence it emerges as the Florida
Current and the Gulf Stream. The Loop Current penetrates furthest into the western Gulf
when it separates from the Campeche Bank at a relatively western position (Molinari and
Morrison, 1988). When the Loop Current penetrates sufficiently far to the north, the
effluent of the Mississippi may become entrained in the flow.
The dynamic shifts of the Loop Current induce the pinching-off of very large meanders
that become major features of the circulation and nutrient budget of the Gulf of Mexico.
These warm-core, anticyclonic rings are generated at about 6- to 18-month intervals
from the Loop Current, mostly in winter (Vukovich, 1988), and move into the western
Gulf, subsequently to pass around the northern coast as they age. They are 100–300 km
in diameter, have a lifetime of about 1 year, and may interact dynamically with the
continental shelf edge (Vukovich and Waddell, 1991). After such a feature is pinched
off, the flow of the Yucatan Current briefly passes directly toward the Straits of Florida.
Upwelling of subpycnocline water in the Gulf of Mexico, due to instabilities in the Loop
Current, occurs at the shelf edge and along the outer shelf in many places throughout
the province, especially off northern Yucatan, but also around Campeche Bank, and
southwest of Florida (Mooers and Maul, 1998).
Cold-dome cyclonic eddies (80–120 km) occur on the eastern/southern side of
the Loop Current, evolving into tongues or quasipermanent meanders of cool water
(Vukovich and Maul, 1985) that are a characteristic feature of the eastern Gulf. Over the
shelves of the northern Gulf of Mexico, cyclonic cells are expected to occur for most of
the year, but will be interrupted by extreme wind events; some may be detached from
the isolated warm-core rings.