Lauren Ross

Baroclinic annular variability of internal motions in a Patagonian Fjord

Time 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 Hemispheres 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 fjords 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.

Lateral variability of subtidal flow at the mid-reaches of a macrotidal estuary

Transverse variations of tidal and subtidal flow were investigated in a macrotidal and convergent estuary. This was accomplished by combining data analysis of current velocities and water density with numerical modeling at the mid-reaches of the Gironde Estuary (France). Nonlinear mechanisms responsible for overtide generation and hence subtidal flows were found to vary across the estuary and from neap to spring tides. Subtidal flows were driven by a combination of internal asymmetry, tidal advective accelerations, nonlinear effects of water level variations, quadratic friction, and river discharge. The quarter-diurnal overtide band (D4) in the flow was generated by internal asymmetry and tidal advective accelerations during neap tide. The ratio of quarter-diurnal to squared semidiurnal bands (D4/D22) was largest (>0.3) in sections of the channel showing subtidal outflow. River discharge increased from neap to spring tides causing a subsequent increase of seaward subtidal currents. During spring tide, D4 was generated by tidal advective accelerations and quadratic friction combined with river discharge, rather than by internal asymmetry. The sixth-diurnal overtide (D6) in the flow was comparable to D4 for both neap and spring tides. Largest D6/D23 ratios were found in the shallowest cross-channel locations during both neap and spring tides.

Three‐dimensional tidal flow in a fjord‐like basin with converging width: An analytical model

A three-dimensional analytical model was used to understand tidal dynamics in deep and narrow (fjord-like) basins. This model allows the width of the basin to decay exponentially with along-channel distance from the mouth. Both the length scale of exponential convergence math formula and the friction parameter math formula (vertical eddy viscosity) were the free parameters. Model results show amplification of the tidal amplitude toward the head of the basin. Amplification depends on the narrowing rate of the funnel-like width of the channel and on friction. Cross-channel variations in along-channel tidal flow are also sensitive to the friction parameter. A typical along-channel tidal flow distribution was found across the channel when the vertical eddy viscosity was characteristic of a basin with strong friction, or the Stokes number was larger than 0.1 (St > 0.1). Maximum along-channel tidal velocities (ranging from 0.25 to 0.5 m s−1 depending on width convergence strength) were located in the center of the basin and at the surface. Decreasing values of the Stokes number, St < 0.1, resulted in along-channel velocity maxima located near the lateral boundaries and subsurface in the middle of the channel. These tidal flow distributions were explained by a critical value of St and were verified with observations from Reloncavi Fjord, Chilean Patagonia yielding good agreement.