424
velocity at the base of the interior upper layer (eg. Luyten et aL 1983, Luyten and Stommel 1986). This, however, is unadequate because buoyancy influx into the thermocline
can only take place at the base of the much thinner Ekman layer. Haynes and Mclntyre
(1987,1990) and Marshall and Nurser (1992) have introduced a more physical approach
to this problem, using PV fluxes and PV flux lines to determine the creation, through
buoyancy influx, of PV in the mixed layer and its transfer into the permanent thermocline.
2 Mechanisms of Warm Water Mass Formation
WWM formation requires the advection of relatively cold waters. If the surface water is
already warm enough (about 28°C) any additional heat influx from the atmosphere will
be transferred back as latent heat flux. This happens in the equatorial regions where,
despite maximum radiation flux to the ocean, the net annual heat balance is slightly
negative, ie. net heat flux occurs from the ocean to the atmosphere. The main sites
of WWM formation are recognized to be eastern subtropical oceans. These are regions
where relatively cold surface water, which has released heat to the atmosphere at higher
latitudes, gains heat during its recirculation towards the western end of the ocean. In this
Section we will look at three mechanisms of WWM formation: southward advection of
cool surface water by the Azores-Canary Current system, coastal upwelling off northwest
Africa and localized upwelling in the cyclonic eddies generated leeward of islands.
The Azores-Canary Current System
During the last decade there has been an important data collection and analysis effort in
the open ocean areas of the eastern NASG (IGise and Siedler 1982, Stramma 1984, Kiise
et al. 1985,1986, Thiele et al. 1986, Siedler et al. 1987, Bauer and Siedler 1988, Stramma
and Isemer 1988, Stramma and Siedler 1988, Stramma and Muller 1989, Zenk et al. 1991,
Fiekas et al. 1992, Siedler et al. 1992, Siedler and Finke 1993, Arhan et al. 1994, PerezRodriguez and Pelegri 1995). Another number of studies have analyzed data collected
by satellites, leading to the distributions of sea surface elevation, sea surface temperature
(SST) and surface pigment concentrations (Tokmakian and Challenor 1993, Nykjaer and
Van Camp 1994, Hernandez-Guerra and Nykjaer 1995). One main result has been to
provide a view of the circulation in this region (Fig. 2): a NW-SE flow at latitudes higher
than the Madeira islands (Azores Current AC), which progressively turns to the south
and southwest (Canary Current CC), before leaving the region towards the west (North
Equatorial Current NEC); the amount of water that transits the region is about 10 Sv.
This circulation is, however, not the diffuse and slow flow visualized some 50 years ago
(Sverdrup 1947), but a commonly intense (,::::, 0.2 m/ s) and energetic flow characterized
by the existence of large mesoscale variability.
velocity at the base of the interior upper layer (eg. Luyten et aL 1983, Luyten and Stommel 1986). This, however, is unadequate because buoyancy influx into the thermocline
can only take place at the base of the much thinner Ekman layer. Haynes and Mclntyre
(1987,1990) and Marshall and Nurser (1992) have introduced a more physical approach
to this problem, using PV fluxes and PV flux lines to determine the creation, through
buoyancy influx, of PV in the mixed layer and its transfer into the permanent thermocline.
2 Mechanisms of Warm Water Mass Formation
WWM formation requires the advection of relatively cold waters. If the surface water is
already warm enough (about 28°C) any additional heat influx from the atmosphere will
be transferred back as latent heat flux. This happens in the equatorial regions where,
despite maximum radiation flux to the ocean, the net annual heat balance is slightly
negative, ie. net heat flux occurs from the ocean to the atmosphere. The main sites
of WWM formation are recognized to be eastern subtropical oceans. These are regions
where relatively cold surface water, which has released heat to the atmosphere at higher
latitudes, gains heat during its recirculation towards the western end of the ocean. In this
Section we will look at three mechanisms of WWM formation: southward advection of
cool surface water by the Azores-Canary Current system, coastal upwelling off northwest
Africa and localized upwelling in the cyclonic eddies generated leeward of islands.
The Azores-Canary Current System
During the last decade there has been an important data collection and analysis effort in
the open ocean areas of the eastern NASG (IGise and Siedler 1982, Stramma 1984, Kiise
et al. 1985,1986, Thiele et al. 1986, Siedler et al. 1987, Bauer and Siedler 1988, Stramma
and Isemer 1988, Stramma and Siedler 1988, Stramma and Muller 1989, Zenk et al. 1991,
Fiekas et al. 1992, Siedler et al. 1992, Siedler and Finke 1993, Arhan et al. 1994, PerezRodriguez and Pelegri 1995). Another number of studies have analyzed data collected
by satellites, leading to the distributions of sea surface elevation, sea surface temperature
(SST) and surface pigment concentrations (Tokmakian and Challenor 1993, Nykjaer and
Van Camp 1994, Hernandez-Guerra and Nykjaer 1995). One main result has been to
provide a view of the circulation in this region (Fig. 2): a NW-SE flow at latitudes higher
than the Madeira islands (Azores Current AC), which progressively turns to the south
and southwest (Canary Current CC), before leaving the region towards the west (North
Equatorial Current NEC); the amount of water that transits the region is about 10 Sv.
This circulation is, however, not the diffuse and slow flow visualized some 50 years ago
(Sverdrup 1947), but a commonly intense (,::::, 0.2 m/ s) and energetic flow characterized
by the existence of large mesoscale variability.