Baroclinic annular variability of internal motions in a Patagonian fjord

Abstract

AbstractTime series of horizontal velocities, echo intensity, wind velocity, and atmospheric pressure were collected for ∼200 days in a Patagonian fjord to explore pycnocline motions produced by the Southern Hemisphere's baroclinic annular mode (BAM). The BAM variability occurs between 20 and 30 days and is associated with fluctuations in atmospheric kinetic energy and in turbulent fluxes of heat. Spectra of horizontal velocities and normalized echo intensity in the fjord's water showed highest energy between 25 and 30 days. This was explained by sustained westerly winds associated with extreme low‐pressure systems (∼900 hPa) that had periodicity related to the BAM. Wind forcing produced >40 cm s−1 along‐channel and cross‐channel currents in the surface layer, which in turn created a wind‐induced setup toward the head of the fjord. The setup was accompanied by a deepening of the pycnocline (from 5 to 15 m depth) with ∼25 to 30 day periodicity, as derived from the normalized echo intensity. The dominant empirical orthogonal function mode of the normalized echo intensity profiles explained 70.8% of the variance and also exhibited a ∼25–30 day periodicity. Further, a wavelet and spectral analysis of 10 years of atmospheric pressure indicated peaks between 25 and 30 days each year, indicating that the BAM consistently influences weather patterns in Chilean Patagonia. This is the first documented case of baroclinic annular variability in a specific region of the Southern Hemisphere, and of its effects on fjord systems.