Tidal and residual circulation in a semi-arid bay: Coquimbo Bay, Chile

Velocity profiles and time-series data were combined with conductivity‐temperature‐depth (CTD) casts to describe the general circulation at tidal and subtidal scales in a bay of semi-arid climate, Coquimbo Bay (308S), Chile. This was the first study that used a towed acoustic Doppler current profiler (ADCP) in coastal Chilean waters and is one of the very few in semiarid bays. The ADCP was towed for two semi-diurnal tidal cycles in early austral autumn, between March 23 and 24, 1997 along a triangular trajectory that covered most of the bay. Additional data consisted of moored current meters and CTD casts. The observations indicated the presence of a surface layer, above the pycnocline, that showed predominantly diurnal variability forced by the breeze regime and by tides. The tidal circulation in the surface layer featured amplitudes of 10cm/s within an anticyclonic gyre that occupied most of the bay. The subtidal circulation in the surface was characterized by a pair of counter-rotating gyres. The northernmost three-fourths of the bay showed an anticyclonic gyre, and the observations over the southern fourth implied a cyclonic gyre. The subtidal anticyclonic gyre had a counterpart rotating in opposite direction within a lower layer, underneath the pycnocline. The lower layer showed semidiurnal variability in addition to diurnal variability and was insulated by the pycnocline from heat and momentum fluxes through the air‐water interface. Circulations that resemble estuarine and anti-estuarine patterns were found associated with the subtidal gyres. A horizontal divergence related to a 10cm/s near-surface outflow around Point Tortuga, to the south of the bay entrance, allowed the development of upward motion o the Point, as evidenced by the tilt of the isopycnals at the entrance to the bay. # 2000 Elsevier Science Ltd. All rights reserved.