Seiche excitation in a highly stratified fjord of southern Chile: the Reloncaví fjord

Abstract. We describe a seiche process based on current, temperature, and sea-level data obtained from the Reloncaví fjord (41.6° S, 72.5° W) in southern Chile. We combined 4 months of acoustic Doppler current profiler (ADCP) data with sea-level, temperature, and wind time series to analyze the dynamics of low-frequency (periods > 1 day) internal oscillations in the fjord. Additionally, seasonal conductivity, temperature, and depth (CTD) data from 19 along-fjord stations were used to characterize the seasonality of the density field. The density profiles were used to estimate the internal long-wave phase speed (c) using two approximations: (1) a simple reduced gravity model (RGM) and (2) a continuously stratified model (CSM). No major seasonal changes in c were observed using either approximation (e.g., the CSM yielded 0.73 < c < 0.87 m s⁻¹ for mode 1). The natural internal periods (T_N) were estimated using Merian's formula for a simple fjord-like basin and the above phase speeds. Estimated values of T_N varied between 2.9 and 3.5 days and were highly consistent with spectral peaks observed in the along-fjord currents and temperature time series. We conclude that these oscillations were forced by the wind stress, despite the moderate wind energy. Wind conditions at the end of winter gave us an excellent opportunity to explore the damping process. The observed damping time (T_d) was relatively long (T_d =  9.1 days).