An extraordinary upwelling event in a deep thermally stratified lake

Abstract

[1] A unique combination of temporal and spatial measurements provides a description of an extraordinarily large upwelling event in Lake Tahoe, CA-NV. The 4 d event, which engulfed half of the lake's surface and had an amplitude of 500 m, was recorded with in situ and spaceborne instruments. The vertical mixing that ensued, was characterized by a large transfer of heat across the thermocline, resulting in the replacement of the distinct two-layer thermal structure by a diffuse, temperature gradient. Prior to the event, mixing energy due to the cooling flux at the surface was two orders of magnitude larger than the mixing energy associated with the wind. This dominance by cooling yielded the two-layer structure. During the event, wind energy was of similar magnitude to the cooling energy. The large bottom velocities that were produced at the end of the event were sufficient to re-suspend sediment into the water column.

Upwelling of deep water during thermal stratification onset—A major mechanism of vertical transport in small temperate lakes in spring?

“…From equation , upwelling is more probable for lakes with larger horizontal dimensions L . Observations in large lakes, with horizontal dimensions larger than or comparable to the barotropic Rossby radius [ Gill , ] Ro = f −1 ( gH ) 1/2 , where f is the Coriolis parameter, g is the gravity acceleration, and H is the mean depth of the lake, support an important role of upwelling in the coastal circulation [ Haffner et al ., ; Schladow et al ., ; Plattner et al ., ; Troitskaya et al ., ]. In these lakes, the aspect ratio h / L does not exceed 10 −3 (using the order‐of‐magnitude estimation of the epilimnion depth as 10 1 m), and transient upwelling events have been encountered, driven by strong wind events and affecting primarily the littoral areas without destroying the mean stratification in the central parts of lakes.…”

“…If the slope of isotherms is strong enough, the lower boundary of the epilimnion arrives at the lake surface, bringing the stratified waters into direct contact with the atmosphere and facilitating its mixing with the epilimnion waters in the lateral direction, not restricted by the gravity force. By analogy with the coastal upwelling at the ocean shelf, the effect is called upwelling [ Schladow et al ., ] and may eventually lead to complete destruction of the vertical thermal stratification. Similar to the coastal ocean, upwelling brings nutrient‐rich deep waters to the lake surface [ Webster , ; MacIntyre et al ., ].…”

Optical remote sensing of lakes: an overview on Lake Maggiore

“…Though the 3.7 µm channel is more sensitive to SST, it is primarily used only for night-time measurements because of relatively strong reflection of solar irradiation in this wavelength region, which contaminates the retrieved radiation. Thermal infrared remote sensing applied to freshwater ecosystems has aimed to map surface temperatures (Oesch et al, 2008;Reinart and Reinhold, 2008;Crosman and Horel, 2009), bulk temperatures (Thiemann and Schiller, 2003), circulation surface (Schladow et al, 2004) and to characterize upwelling events (Steissberg et al, 2005).…”

Accessing the Potential of Satellite and Telemetric Data to Evaluate the Influence of the Heat Flux Exchange in the Water Column Mixing and Stratification

“…Thermal infrared remote sensing applied to freshwater ecosystems has aimed to map surface temperatures [3][4][5], bulk temperatures [6], circulation patterns [7] and to characterize upwelling events [8]. However, the application of thermal infrared images to estimate the net heat flux in tropical hydroelectric reservoirs is scarce.…”