Transverse variability of flow and density in a Chilean fjord

Abstract

Measurements of velocity and density profiles were made to describe the transverse structure of the flow in Aysen Fjord, Southern Chile (45.2°S and 73.3°W). Current profiles were made with a 307.2kHz acoustic doppler current profiler (ADCP) during 20 repetitions of a cross-fjord transect during one semidiurnal tidal cycle. The transect had a ∼320° orientation, 3km length, and its bathymetry consisted of two channels, one on the southern side (230m depth) and the other on the northern side (180m depth), separated by a bank ca. 65m depth, which was located ∼1km from the northern coast. Density measurements to a maximum depth of 50m were made at the extremes of each transect repetition and over the bank. Also, a total of nine CTD stations that covered the surroundings of the bank was sampled 2 days prior to the ADCP sampling. During the sampling period the mean flow showed a three layer structure that was consistent with up-fjord wind-induced exchange: a thin (<8m) weak outflow close to the surface due to river discharge; a layer of inflow (down-wind) underneath attributed to the effect of wind-stress; and a deep compensatory outflow due to the barotropic pressure gradient set up by the wind. The bank caused the strongest wind-induced net inflows and outflows to be shifted to the channels and also disrupted the three-layer structure. Also, the strongest tidal current amplitudes were located over the bank. The near-surface flow and density distributions suggested that the transverse dynamics were ageostrophic in a layer above ∼50m. This was indicated by the wind-induced shifting of the location of salt water intrusion from the northern side to the southern side of the fjord. Density measurements also suggested an alternation to quasigeostrophic conditions in this upper layer during calm winds. Below this layer the dynamics remained quasigeostrophic.