The Abrolhos Reefs of Brazil
85
6.2 Environmental Setting
Abrolhos is located in the southern part of the trade wind area. The northward migration of the South Atlantic anticyclone cell in summer causes NE
and E winds from October to March. The southward migration of the
anticyclone cell in winter favors winds from the SE dominant between
April and September and the northward advance of polar cold fronts
enhances these winds and adds an SSE component (Nimer 1989). The climate of the Eastern Brazilian coastal region is tropical-humid with average
air temperature between 24 oc in winter and 27 °C in summer. July is the
coldest month and March the warmest. The mean annual rainfall in coastal
regions near Abrolhos is 1, 7 50 mm, with March, April and May concentrating about 35% of the precipitation (Nimer 1989). The average monthly sea
surface temperature ranges from 24.5 oc in August to 27.5 oc in March,
though temperature anomalies of up to 1.5 oc may occur during strong El
Nifio events (Servain et al. 1987).
The waters of the Brazil Current flow over the Abrolhos Bank in a north to
south direction and tides are semidiurnal with a range of 1.6-1.7 m. Owing
to interactions between tides and current flow, which favor N-S circulation
in the Caravelas Channel, the exchange of dissolved and suspended materials between the Coastal and the Outer Arc is more significant than that between coast and reefs (MeyerhOfer and Marone 1996). The principal wave
trains coincide with the wind regime. Spring and summer are dominated by
NE-E waves (significant height of 1m and period of 5 s) which cause southward longshore sediment transport north of Baleia Point. Dominant waves
from theSE-SSE (significant height of 1.5 m and period of 6.5 s) in fall and
winter lead to northward longshore sediment transport south of Baleia
Point (Fig. 6.1A). Despite the humid tropical climate, elevated runoff of fresh
water and terrigenous sediments only occurs during winter storms. During
these storms, large amounts of resuspended bottom sediments reach the
reefs and cause turbid waters. Therefore, and contrary to most of the world's
coral reefs, Abrolhos is an example of reefs surrounded by sediments with
considerable fractions of siliceous sand and mud.
Fig. 6.1. Location of the Abrolhos Reefs (A) and Abrolhos Archipelago with fringing
reefs and isolated chapeiroes (B)
85
6.2 Environmental Setting
Abrolhos is located in the southern part of the trade wind area. The northward migration of the South Atlantic anticyclone cell in summer causes NE
and E winds from October to March. The southward migration of the
anticyclone cell in winter favors winds from the SE dominant between
April and September and the northward advance of polar cold fronts
enhances these winds and adds an SSE component (Nimer 1989). The climate of the Eastern Brazilian coastal region is tropical-humid with average
air temperature between 24 oc in winter and 27 °C in summer. July is the
coldest month and March the warmest. The mean annual rainfall in coastal
regions near Abrolhos is 1, 7 50 mm, with March, April and May concentrating about 35% of the precipitation (Nimer 1989). The average monthly sea
surface temperature ranges from 24.5 oc in August to 27.5 oc in March,
though temperature anomalies of up to 1.5 oc may occur during strong El
Nifio events (Servain et al. 1987).
The waters of the Brazil Current flow over the Abrolhos Bank in a north to
south direction and tides are semidiurnal with a range of 1.6-1.7 m. Owing
to interactions between tides and current flow, which favor N-S circulation
in the Caravelas Channel, the exchange of dissolved and suspended materials between the Coastal and the Outer Arc is more significant than that between coast and reefs (MeyerhOfer and Marone 1996). The principal wave
trains coincide with the wind regime. Spring and summer are dominated by
NE-E waves (significant height of 1m and period of 5 s) which cause southward longshore sediment transport north of Baleia Point. Dominant waves
from theSE-SSE (significant height of 1.5 m and period of 6.5 s) in fall and
winter lead to northward longshore sediment transport south of Baleia
Point (Fig. 6.1A). Despite the humid tropical climate, elevated runoff of fresh
water and terrigenous sediments only occurs during winter storms. During
these storms, large amounts of resuspended bottom sediments reach the
reefs and cause turbid waters. Therefore, and contrary to most of the world's
coral reefs, Abrolhos is an example of reefs surrounded by sediments with
considerable fractions of siliceous sand and mud.
Fig. 6.1. Location of the Abrolhos Reefs (A) and Abrolhos Archipelago with fringing
reefs and isolated chapeiroes (B)