62
N. Hoepffner et al.
Further north, during the southwest monsoon, the strong and northeasterly flowing
Somali Current is responsible for high marine productivity along the Somali coast,
while low biomass and phytoplankton production is commonly observed in the Gulf
of Aden. The situation reverses during the northeast monsoon in winter (Veldhuis
et al. 1997). Southwest winds during the boreal summer generate upwelling cells
that represent important fishing zones for northeast African countries. MODIS image
from June 2004 (Fig. 3.3) show typically two upwelling events respectively centered
at 5
◦ N and 10
◦ N, starting in May and fading out late September. These features
were already observed and analysed in the early 80’s by Brown et al. (1980) and
Evans and Brown (1980). At both locations, colder and nutrient-rich waters extend
ca. 200 km offshore as filaments with a temperature difference of 4 to 5
◦ C compared
to surrounding waters. A large anti-cyclonic ring known as the Great Whirl (Schott
1983) separates the upwelling cells, whereas other gyres can be observed southeast of
the Socotra Island. Mafimbo and Reason (2010) used satellite data including thermal
imagery to describe the mechanism of these upwelling events showing a strong
covariability between SST and changes in mesoscale winds. This area represents an
important fishing ground for the eastern African countries. A better understanding of
its dynamic is essential to establish an ecosystem approach to fishery management
in this part of the ocean currently under severe overfishing threats.
3.4 Upwelling Dynamics
Upwelling events are driven by alongshore wind stress inducing offshore transport
of surface waters, replaced then by deeper cold water enriched in macronutrients,
iron and inorganic carbon. This oceanographic feature yields optimal conditions to
sustain growth of organisms at all level of the food chain (Cury et al. 2000). A
satellite-based TIR can readily spot changes in the temperature signature resulting
from an upwelling process, which makes SST product an optimum indicator to
assess the variability and strength of such oceanographic phenomena. In particular,
satellite TIR-derived SST has revealed the complexities and spatial extension of the
mesoscale structures associated with upwelling, with off-shore streamers or narrow
filaments that could reach few hundreds km offshore.
Van Camp et al. (1991) have described the upwelling variability off Northwest
Africa using AVHRR data, observing filaments and meanders of cool coastal water
extending well beyond the shelf break, and coupled with the large-scale seasonal
variations of the trade winds (Fig. 3.4a). Hagen et al. (1996) and Barton et al. (1998)
reported recurrent filaments from TIR imagery off Cape Ghir and north of Cape
Bojador, respectively. These features resulting from a strong interactions between
wind and local topography (Troupin et al. 2012) are quasi-permanent and extend few
hundreds km offshore, significantly influencing the hydrodynamics and ecosystem
behavior in the vicinity of the Canary Islands (Pelegri et al. 2005). An analysis of
the temperature gradients on MODIS SST images through edge detection algorithm
(Nieto et al. 2012) confirmed the intense mesoscale dynamics in this region with
N. Hoepffner et al.
Further north, during the southwest monsoon, the strong and northeasterly flowing
Somali Current is responsible for high marine productivity along the Somali coast,
while low biomass and phytoplankton production is commonly observed in the Gulf
of Aden. The situation reverses during the northeast monsoon in winter (Veldhuis
et al. 1997). Southwest winds during the boreal summer generate upwelling cells
that represent important fishing zones for northeast African countries. MODIS image
from June 2004 (Fig. 3.3) show typically two upwelling events respectively centered
at 5
◦ N and 10
◦ N, starting in May and fading out late September. These features
were already observed and analysed in the early 80’s by Brown et al. (1980) and
Evans and Brown (1980). At both locations, colder and nutrient-rich waters extend
ca. 200 km offshore as filaments with a temperature difference of 4 to 5
◦ C compared
to surrounding waters. A large anti-cyclonic ring known as the Great Whirl (Schott
1983) separates the upwelling cells, whereas other gyres can be observed southeast of
the Socotra Island. Mafimbo and Reason (2010) used satellite data including thermal
imagery to describe the mechanism of these upwelling events showing a strong
covariability between SST and changes in mesoscale winds. This area represents an
important fishing ground for the eastern African countries. A better understanding of
its dynamic is essential to establish an ecosystem approach to fishery management
in this part of the ocean currently under severe overfishing threats.
3.4 Upwelling Dynamics
Upwelling events are driven by alongshore wind stress inducing offshore transport
of surface waters, replaced then by deeper cold water enriched in macronutrients,
iron and inorganic carbon. This oceanographic feature yields optimal conditions to
sustain growth of organisms at all level of the food chain (Cury et al. 2000). A
satellite-based TIR can readily spot changes in the temperature signature resulting
from an upwelling process, which makes SST product an optimum indicator to
assess the variability and strength of such oceanographic phenomena. In particular,
satellite TIR-derived SST has revealed the complexities and spatial extension of the
mesoscale structures associated with upwelling, with off-shore streamers or narrow
filaments that could reach few hundreds km offshore.
Van Camp et al. (1991) have described the upwelling variability off Northwest
Africa using AVHRR data, observing filaments and meanders of cool coastal water
extending well beyond the shelf break, and coupled with the large-scale seasonal
variations of the trade winds (Fig. 3.4a). Hagen et al. (1996) and Barton et al. (1998)
reported recurrent filaments from TIR imagery off Cape Ghir and north of Cape
Bojador, respectively. These features resulting from a strong interactions between
wind and local topography (Troupin et al. 2012) are quasi-permanent and extend few
hundreds km offshore, significantly influencing the hydrodynamics and ecosystem
behavior in the vicinity of the Canary Islands (Pelegri et al. 2005). An analysis of
the temperature gradients on MODIS SST images through edge detection algorithm
(Nieto et al. 2012) confirmed the intense mesoscale dynamics in this region with