74
C. Medeiros et al.
southern channel branch is more uniform in width and tidal damping
occurs mainly in response to friction.
Tropical systems, like the Santa Cruz Channel, have weak vertical salinity stratification and normal levels of turbulence distribute temperatures
uniformly from surface to bottom (Medeiros and Kjerfve 1993; Araujo et
al. 2000a,b ). Bottom stirring by tidal currents is the most important component of vertical water column mixing, while water surface evaporation
and energy input from surface wind stress contribute much less. Although
precipitation has little effect on stratification, diurnal surface heating may
cause vertical stratification during the dry season and stratification can be
expected to develop during higher freshwater input. Compared to tropical
systems with a single inlet, the differential advection of horizontal densities is seasonally attenuated and horizontal density gradients along the
estuary axis are small (Araujo et al. 2000b ).
The residual circulation in the Santa Cruz Channel is primarily due to
river discharge, intensity of tides, and channel shape and depth. In
general, residual circulation (0.01-0.19m s- 1 ) flows from the Itapissuma
Bridge northward to the Catuama and southward to the Orange entrance.
However, during high freshwater discharge and spring tides the entire
channel has northward residual circulation. At both entrances, residual
currents tend to be highest during high runoff. In contrast, during
low freshwater discharge nearshore water enters the Catuama entrance
in the north, but net inflow through the southern Orange entrance is
absent.
5.4 Hydrology
Overall salinity in the Itamaraca estuary is lower in the rainy (27) than in
the dry season (34). During the dry season, hypersaline conditions (37) at
both entrances are due to evaporation, evapotranspiration by mangroves,
and reduced exchange between channel and reef shelf waters with lower
salinity. The estuary is well mixed and surface to bottom salinity differences are less than 1. The lowest salinity (30) occurs at the innermost part of
the channel during low water neap tide (Medeiros and Kjerfve 1993).
During the rainy season, the northern channel branch tends to be stratified
(~S/S 0 > 0.1) at neap tide. Especially in deeper areas (15m), at the confluence of the Botafogo and Arataca rivers, large freshwater discharge and
reduced tidal mixing energy favor stratification with a surface to bottom
salinity gradient (12). Stratification is less pronounced during high tidal
energy of spring tides (Medeiros and Kjerfve 1993).
C. Medeiros et al.
southern channel branch is more uniform in width and tidal damping
occurs mainly in response to friction.
Tropical systems, like the Santa Cruz Channel, have weak vertical salinity stratification and normal levels of turbulence distribute temperatures
uniformly from surface to bottom (Medeiros and Kjerfve 1993; Araujo et
al. 2000a,b ). Bottom stirring by tidal currents is the most important component of vertical water column mixing, while water surface evaporation
and energy input from surface wind stress contribute much less. Although
precipitation has little effect on stratification, diurnal surface heating may
cause vertical stratification during the dry season and stratification can be
expected to develop during higher freshwater input. Compared to tropical
systems with a single inlet, the differential advection of horizontal densities is seasonally attenuated and horizontal density gradients along the
estuary axis are small (Araujo et al. 2000b ).
The residual circulation in the Santa Cruz Channel is primarily due to
river discharge, intensity of tides, and channel shape and depth. In
general, residual circulation (0.01-0.19m s- 1 ) flows from the Itapissuma
Bridge northward to the Catuama and southward to the Orange entrance.
However, during high freshwater discharge and spring tides the entire
channel has northward residual circulation. At both entrances, residual
currents tend to be highest during high runoff. In contrast, during
low freshwater discharge nearshore water enters the Catuama entrance
in the north, but net inflow through the southern Orange entrance is
absent.
5.4 Hydrology
Overall salinity in the Itamaraca estuary is lower in the rainy (27) than in
the dry season (34). During the dry season, hypersaline conditions (37) at
both entrances are due to evaporation, evapotranspiration by mangroves,
and reduced exchange between channel and reef shelf waters with lower
salinity. The estuary is well mixed and surface to bottom salinity differences are less than 1. The lowest salinity (30) occurs at the innermost part of
the channel during low water neap tide (Medeiros and Kjerfve 1993).
During the rainy season, the northern channel branch tends to be stratified
(~S/S 0 > 0.1) at neap tide. Especially in deeper areas (15m), at the confluence of the Botafogo and Arataca rivers, large freshwater discharge and
reduced tidal mixing energy favor stratification with a surface to bottom
salinity gradient (12). Stratification is less pronounced during high tidal
energy of spring tides (Medeiros and Kjerfve 1993).