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 (W_b) and mechanical energy balance calculations, we determined that wind stress perturb the pycnocline (W_b > 1); furthermore, wind power input rivals that of estuarine circulation during stratified summer periods. Under such conditions, wind forcing amplifies vertical mixing, modulates the pressure gradient, and supports deep-water oxygenation. First-order flushing time estimates suggest slow deep-basin renewal (greater than 5 months), signalling sensitivity to physical forcing. These results indicate that wind constitutes a primary mechanism regulating the hydrographic structure and biogeochemical function of the Tierra del Fuego inner sea.