The answer is blowing in the wind: seasonal hydrography and mixing of the inner sea of Tierra del Fuego, Southern Patagonia

Abstract

This study characterizes seasonal hydrography and mixing processes in Almirantazgo Fjord, a sensitive ecosystem in southern Chilean Patagonia. Although estuarine and tidal forcing conventionally explain fjord dynamics, wind stress effects remain less understood in this high-latitude region. The study analyses a comprehensive six-month dataset including a moored time-series of temperature, salinity, and dissolved oxygen, cross-fjord CTD transects, and hydrographic profiles derived from seal-deployed sensors. Observations indicate distinct seasonality, shifting from a stratified water column in summer – defined by low-salinity surface water from glacial melt – to a mixed winter state with significantly reduced vertical stability. The analysis identifies persistent, topographically channelled up-fjord winds as a primary physical driver. By applying the Wedderburn number (Wb) and mechanical energy balance calculations, we determined that strong wind stress perturbs the pycnocline (Wb > 1). During stratified summer periods, strong wind events (above the 90th percentile) generated wind power of the same order of magnitude as that of the estuarine circulation. Under such conditions, wind forcing amplifies vertical mixing, modulates the pressure gradient, and supports oxygenation in the upper and subsurface layers. First-order estimates indicate that the upper brackish layer is flushed in approximately one week, reflecting a dynamic surface exchange governed by the interplay of freshwater buoyancy and wind stress. These results indicate that wind constitutes a primary mechanism regulating the hydrographic structure and biogeochemical function of the Tierra del Fuego inner sea.