The setup and relaxation of spring upwelling in a deep, rotationally influenced lake

Abstract

Strong and sustained winds can drive dramatic hydrodynamic responses in density‐stratified lakes, with the associated transport and mixing impacting water quality, ecosystem function, and the stratification itself. Analytical expressions offer insight into the dynamics of stratified lakes during severe wind events. However, it can be difficult to predict the aggregate response of a natural system to the superposition of hydrodynamic phenomena in the presence of complex bathymetry and when forced by variable wind patterns. Using an array of current, temperature, and water quality measurements at the upwind shore, we detail the hydrodynamic response of deep, rotationally influenced Lake Tahoe to three strong wind events during late spring. Sustained southwesterly winds in excess of 10 m s−1 drove upwelling at the upwind shore (characteristic of non‐rotational upwelling setup), with upward excursions of deep water exceeding 70 m for the strongest event. Hypolimnetic water, with elevated concentrations of chlorophyll a and nitrate, was advected toward the nearshore, but this water rapidly returned to depth with the relaxation of upwelling after the winds subsided. The relaxation of upwelling exhibited rotational influence, highlighted by an along‐shore, cyclonic front characteristic of a Kelvin wave‐driven coastal jet, with velocities exceeding 25 cm s−1. The rotational front also produced downwelling to 100 m, transporting dissolved oxygen to depth. More complex internal wave features followed the passage of these powerful internal waves. Results emphasize the complexity of these superimposed hydrodynamic phenomena in natural systems, providing a conceptual reference for the role upwelling events may play in lake ecosystems.

3D Flow Structures During Upwelling Events in Lakes of Moderate Size

“…Cross‐wavelet power in the Integrated Kinetic Energy (IKE) and Integrated Potential Energy (IPE) signals for the field data indicated the existence of periodic responses during the three upwelling events. In particular, a period of 128 hr was concluded to be consistent with a Kelvin wave excited during the upwelling events (Roberts et al., 2021). Estimates of the previously reported internal wave phase speed during E1 and E3 and average speed of the coastal jets, lead to consider that both, the aperiodic response (coastal jet) and cyclonic waves, combine into the flow structure observed at all upwellings (Figures 4–7; Csanady, 1975; Csanady & Scott, 1974).…”

“…The upward fluxes of nutrients and/or chl‐a have been the focus of many limnology studies that address water quality impacts in lakes during upwellings (e.g., MacIntyre et al., 1999; Roberts et al., 2021). However, potential water quality impacts during upwelling relaxation due to rotational flow structures have received less attention.…”

“…We simulated the 3D flow dynamics for three upwelling events captured during the field observation between 30 May 2018 and and 11 June 2018 (Roberts et al., 2021), using 5 days as model spin up time (i.e., simulations started on 24 May 2018 00:00). A 5 − day period was considered enough based on previous simulations in lakes using the numerical model PSi3D‐L, where 2 and 3 days were used as warm‐up period (Rueda, 2009; Rueda & Schladow, 2003).…”

Wind-driven upwelling and surface nutrient delivery in a semi-enclosed coastal sea

“…A striking contrast between the wind-driven pycnocline displacements in the SoG relative to the other systems discussed is the absence of basin-scale internal waves or seiches in our PCA results. Kelvin waves are an established wind response in large lakes including Lake Ontario (Csanady, 1977), Lake Geneva (Bouffard and Lemmin, 2013), Lake Iseo (Valerio et al, 2012) and Lake Tahoe (Roberts et al, 2021), and the presence of along-axis seiches in narrow lakes is also well established (Laval et al, 2008;Stevens and Lawrence, 1997). This absence of detectable wave responses is possibly due to enhanced dissipation or damping as waves travel along the irregular coastline that surrounds the SoG.…”

“…If the lake is wide enough, these seiches can be amphidromic to the point of resembling Kelvin waves, even if the initial pycnocline is an along-axis setup (Roberts et al, 2021;Bouffard and Lemmin, 2013;Valerio et al, 2012).…”