Fronts and Frontal Systems
39
Winds of the tropics. This lies athwart the subtropical oceanic gyres and may be associated
with the formation of ephemeral, but biologically significant fronts: such is the case, for
example, in the subtropical North Atlantic to the southwest of Bermuda at 22–32
N
and at comparable latitudes in the North Pacific. Here, the convergent wind regime
generates convergent surface transport. These surface fluxes in turn generate relatively
strong horizontal gradients in surface properties so that, early in the year, frontogenesis
is particularly strong at the 100-km scale and some eastward geostrophic flow is induced
(Voorhuis, 1969; Weller, 1991).
Similarly, in the southern hemisphere parts of each ocean, sea-level anomaly (SLA)
images reveal unambiguous mesoscale eddying in regions where oligotrophic conditions
almost always prevail, below the atmospheric convergence. Lying across each subtropical
gyre is an arc-shaped field of large mesoscale eddies, having strong sea-level signatures
that lie below the atmospheric boundary between the westerlies and the trade winds.
These eddies are of diameter 100–250 km, have SLA signatures of 15–25 cm, and in
the Indian Ocean tend to be strongest off Australia and near the termination of the
field off Southeast Madagascar. The cyclonic eddies frequently lie equatorward of the
anticyclonic eddies, so forming a ridge-trough system in the SLA field. This description
could be adjusted to fit the other two southern subtropical gyres. Especially in the Indian
Ocean, this eddy field is associated with episodic biological enhancement, apparently
forced by appropriate vertical motion associated with eddy dynamics. The analogous eddy
field in the Pacific and Atlantic Oceans is associated only more weakly with biological
enhancement, although “dendritic” blooms may be forced in the SW Atlantic at the same
seasons as those in the SW Indian Ocean. This phenomenon shall be discussed in more
detail in Chapters 9 and 10.
Shelf-Edge and Upwelling Fronts
Water masses of shelf seas are generally more buoyant than the oceanic water offshore,
because freshwater (and hence buoyancy) is supplied by rivers, or in some locations
by the equatorward coastal flow of light polar water. Shelf sea hydrography is therefore
often characterized by buoyancy-driven, longshore currents in which the sea surface
slopes downward offshore. Separation between such coastwise flow and the oceanic water
mass offshore then occurs across a retrograde front in which the density isolines slope
downward inshore. In some cases, as in those parts of the Gulf of Guinea where the
shelf is unusually wide, a series of fronts may lie approximately parallel to the coast
and at varying distances offshore, isolating different water masses. In such cases, we
often observe enhanced nutrients along the outer shelf, perhaps attributable to nutrient
pumping as the nutricline is periodically lifted by the passage of crests of internal waves
interacting with the shelf break. This process has the same frequency as the semidiurnal
tides and may explain the permanence of the increased phytoplankton cell numbers often
observed along the break of slope; it is not to be confused with the consequences of
geostrophic upwelling.
Of course, the discontinuity between oceanic and shelf water that lies above the
upper slope and shelf break off western Europe was one of the first of these systems
to be investigated: one of the earliest suggestions was simply that westerly gales might
overturn (Leslie Cooper actually used the more colorful term capsize) the water column in
autumn. There is not yet complete agreement on the mechanism by which deeper, cooler,
and nutrient-rich water is brought to the surface although the most widely accepted
explanation involves the generation of internal standing waves on the thermocline at
the shelf edge where tidal streams encounter rough topography. These are thought to
originate principally during the period of maximal velocity in the seaward, offshelf tidal
39
Winds of the tropics. This lies athwart the subtropical oceanic gyres and may be associated
with the formation of ephemeral, but biologically significant fronts: such is the case, for
example, in the subtropical North Atlantic to the southwest of Bermuda at 22–32
N
and at comparable latitudes in the North Pacific. Here, the convergent wind regime
generates convergent surface transport. These surface fluxes in turn generate relatively
strong horizontal gradients in surface properties so that, early in the year, frontogenesis
is particularly strong at the 100-km scale and some eastward geostrophic flow is induced
(Voorhuis, 1969; Weller, 1991).
Similarly, in the southern hemisphere parts of each ocean, sea-level anomaly (SLA)
images reveal unambiguous mesoscale eddying in regions where oligotrophic conditions
almost always prevail, below the atmospheric convergence. Lying across each subtropical
gyre is an arc-shaped field of large mesoscale eddies, having strong sea-level signatures
that lie below the atmospheric boundary between the westerlies and the trade winds.
These eddies are of diameter 100–250 km, have SLA signatures of 15–25 cm, and in
the Indian Ocean tend to be strongest off Australia and near the termination of the
field off Southeast Madagascar. The cyclonic eddies frequently lie equatorward of the
anticyclonic eddies, so forming a ridge-trough system in the SLA field. This description
could be adjusted to fit the other two southern subtropical gyres. Especially in the Indian
Ocean, this eddy field is associated with episodic biological enhancement, apparently
forced by appropriate vertical motion associated with eddy dynamics. The analogous eddy
field in the Pacific and Atlantic Oceans is associated only more weakly with biological
enhancement, although “dendritic” blooms may be forced in the SW Atlantic at the same
seasons as those in the SW Indian Ocean. This phenomenon shall be discussed in more
detail in Chapters 9 and 10.
Shelf-Edge and Upwelling Fronts
Water masses of shelf seas are generally more buoyant than the oceanic water offshore,
because freshwater (and hence buoyancy) is supplied by rivers, or in some locations
by the equatorward coastal flow of light polar water. Shelf sea hydrography is therefore
often characterized by buoyancy-driven, longshore currents in which the sea surface
slopes downward offshore. Separation between such coastwise flow and the oceanic water
mass offshore then occurs across a retrograde front in which the density isolines slope
downward inshore. In some cases, as in those parts of the Gulf of Guinea where the
shelf is unusually wide, a series of fronts may lie approximately parallel to the coast
and at varying distances offshore, isolating different water masses. In such cases, we
often observe enhanced nutrients along the outer shelf, perhaps attributable to nutrient
pumping as the nutricline is periodically lifted by the passage of crests of internal waves
interacting with the shelf break. This process has the same frequency as the semidiurnal
tides and may explain the permanence of the increased phytoplankton cell numbers often
observed along the break of slope; it is not to be confused with the consequences of
geostrophic upwelling.
Of course, the discontinuity between oceanic and shelf water that lies above the
upper slope and shelf break off western Europe was one of the first of these systems
to be investigated: one of the earliest suggestions was simply that westerly gales might
overturn (Leslie Cooper actually used the more colorful term capsize) the water column in
autumn. There is not yet complete agreement on the mechanism by which deeper, cooler,
and nutrient-rich water is brought to the surface although the most widely accepted
explanation involves the generation of internal standing waves on the thermocline at
the shelf edge where tidal streams encounter rough topography. These are thought to
originate principally during the period of maximal velocity in the seaward, offshelf tidal