Seasonal and fortnightly variability of the hydrodynamic regime at Babitonga Bay, Southern of Brazil

Abstract

This paper provides the first comprehensive observational results of the seasonal variability of hydrography and currents in Babitonga Bay, a microtidal subtropical estuary, and one of the largest estuaries in Brazil with economic and ecological importance. Measurement campaigns were performed during the higher and lower rain conditions in austral summer and winter, respectively. Time series of current velocity and salinity–temperature were obtained near the bottom at the head estuary for each season. Along-estuary hydrographic sections and tidal-cycle cross-estuary transect surveys of current velocity and hydrographic profiles at the mouth were performed in the spring and neap tides. The M2 is the main harmonic constituent. The third-diurnal principal lunar (M3) suffers the highest amplification (∼115%), followed by the shallow water overtides of principal lunar (M4) with ∼82%. The tide mainly controls the variability of salinity, density, and longitudinal currents with no variation between the two seasons. The water temperature presents irregular daily variations associated with daytime heating (summer) and nighttime cooling (summer and winter). The along-estuary salinity distribution characterizes a weakly stratified estuary with small intratidal, fortnightly, and seasonal variations. At the mouth, the flood currents start to inflow laterally in the shallow flank of the estuary, in both spring and neap tides. The maximum velocities measured along-channel are similar for neap and spring tides for both summer and winter seasons. The density induced exchange flow pattern is vertically and horizontally sheared in the spring tides and vertically sheared in the neap with no seasonal variation. The intratidal and the spring–neap cycle are more important than the seasonal scale in governing the hydrodynamic at Babitonga Bay.