Hydrodynamic measurements and modeling of thermal structure and gas exchange in tropical floodplain lakes
Abstract: Recent reports of exceptional temperatures and dramatic ecological effects in equatorial lakes within Amazon floodplains highlight the urgency of understanding the factors that lead to these high temperatures. We combine high-frequency measurements, heat balance analyses and hydrodynamic modeling to identify conditions and processes responsible for exceptional water temperatures in varied habitats of an Amazon floodplain lake, representative of shallow, tropical lakes. Our results demonstrate that clear skies with strong solar radiation, elevated air temperature, low wind speed, shallow water, and high turbidity are conducive to especially high temperatures. Latent heat exchange under light winds is a key process, though at higher wind speeds latent heat fluxes moderate temperatures. Near-surface water temperatures are often more than air temperatures, hence sensible heat exchanges cause cooling. As climate variability has increased in recent decades, the frequency and intensity of droughts with associated low water levels, clear skies and high insolation, and warm air temperatures have become more common, increasing the likelihood of high water temperatures in tropical lakes.
Exchanges of gases between surficial water and overlying atmosphere depends on the concentration gradient and on physical processes at the interface, usually parameterized as a gas transfer velocity (denoted as k). We report results from studies in Amazonian lakes that combine meteorological data, time-series measurements of temperature profiles, temperature-gradient microstructure profiling, and chamber-based gas fluxes. Surface energy budgets and near-surface dissipation rates of turbulent kinetic energy (ϵ) derived from the microstructure profiler are used to calculate k values with the surface renewal model; k values are also obtained by inverse procedures from chamber measurements. The results indicate that values of ϵ are high (reaching 1x10-5 m2 s-3) as are k values obtained with the surface renewal model and chambers (up to ~ 20 cm hr-1) under light winds and heating. For winds less than 3 m s-1, there was no dependence on wind speed under heating.
Speaker: Prof. Sally MacIntyre & Prof. John Melack, University of California, Santa Barbara
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