Seasonal dynamics of internal waves governed by stratification stability and wind: Analysis of high‐resolution observations from the Dead Sea

Abstract

Internal waves in stratified lakes are affected by the seasonally varying stratification and the wind forcing. We studied the seasonal dynamics of internal waves by means of high‐resolution observations and model simulations for the Dead Sea. A two‐layer hydrostatic model provided high correlations between measured thermocline depth and the lake level oscillations. Seasonally, the amplitude of the thermocline fluctuations were anticorrelated with the density difference between the water layers; the largest fluctuations were observed when stratification was weak in spring/fall and moderate to weak fluctuations in mid‐summer when stratification was fully developed. The surface and the internal waves propagated counterclockwise along the coasts at a speed of ~0.5 m s−1. Power spectra of the observed wind as well as the measured and simulated lake level and thermocline depth show a pronounced diurnal period during summer, suggesting forcing by the diurnally varying wind. During spring and fall, when the water column stability diminishes, a hint of longer wind periods appear in addition to the diurnal mode. Accordingly, the lake level and thermocline depth fluctuations respond at lower frequencies. In the fall, the longer wind periods are close to the lake's first vertical normal mode, suggesting that resonant amplification of the internal waves may explain the observed lower frequency response of the level and thermocline oscillations. Reduction of the stratification stability originating from anthropogenic water diversion over the past four decades, associated with lake level decline and salinity increase, have led to increases of internal waves amplitude and periods.

Lorentzian Filter Correction of Turbulence Measurements on Oscillating Floating Platforms: Impact on Wind Spectra and Eddy‐Covariance Fluxes

“…During the daytime, wind speed is low with <1° tilt. The time delay between wind speed and platform tilt is less than half an hour (i.e., short than the half‐hourly average), which is typical of enclosed lakes, where the waves are generated by wind activity (Arnon et al., 2019; Nehorai et al., 2013). The spectra of vertical wind turbulence and the platform's triaxial rotation (pitch only) are demonstrated for two representative cases (half‐hour spectra): light (Figures 2d and 2f) and strong (Figures 2e and 2g) wind speed.…”

Hydrostatic Densitometer for Monitoring Density in Freshwater to Hypersaline Water Bodies

“…On the first day, the wind speed was low (<4 m•s −1 ) and incoming radiation was high (~850 W•m −2 ), resulting in maximum thermal and density stratification at midday, with a difference between shallow and deep water of 3 • C and 6.5 kg•m −3 . On the second day, the wind was strong (>5 m•s −1 ), higher than the threshold for mixing by wind-waves [8,9,11,[25][26][27], and clouds reduced the incoming radiation (~400 W•m −2 ), resulting in reduced stratification, i.e., depth stratification of 0.3 • C and <3 kg•m −3 . With the intensification of wind speed (7:00 a.m. on 10 April), the density of the shallow water (0.4 m, 0.8 m) increased and, concurrently, the density of the deeper water (5.5 m) decreased, both because of wind-induced mixing.…”

“…Density, temperature, salinity, and equivalent freshwater hydraulic head are physical scalars governing the dynamics of aquatic systems [1][2][3][4][5]. Characterization of spatiotemporal variations in these scalars is crucial for understanding stratification and circulation in aquatic systems [6][7][8][9][10][11][12]. Density is typically determined indirectly by measurements of temperature and salinity, which are then used in an equation of state; the relationship of conductivity to salinity and the equation of state have to be calibrated for each water-ion composition [10,[13][14][15][16].…”

Sedimentology and stratigraphy of a modern halite sequence formed under Dead Sea level fall

“…This unit was deposited when the lower part of the epilimnion, above the depth of the thermocline, was near the lakefloor at the Ein Gedi site. Additionally, internal waves result in offset of the thermocline depth by a few metres over periods of a few hours (Arnon et al, 2014(Arnon et al, , 2019. Thus, there is no sharp transition from deep to shallow halite deposition, and Unit 3 represents halite deposition under the influence of the transition layer.…”