Abstract
Factors leading to exceptionally high temperatures in shallow tropical waters are examined with high‐resolution measurements and application of a 3‐dimensional hydrodynamic model in an Amazon floodplain lake. Highest near‐surface (0.05 m) temperatures in open water ranged from 31.0 to 37.5°C. Within mats of floating plants maximum temperatures ranged from 33.7 to 39.8°C, and within flooded forests from 30.6 to 34.1°C. Highest open water temperatures occurred when water was shallow (ca. 1 m), underwater attenuation was high (5.5–7.5 m −1 ), skies were clear (solar radiation ca. 1000 W m −2 ), winds were low (≤ 1 to 2–3 m s −1 ), and air temperatures were in the mid‐30°C range. Heat budgets indicate that latent heat fluxes ranged from ~ −100 W m −2 under light winds to −200 to −250 W m −2 with 4 m s −1 winds. Sensible heat flux was usually negative, as lake temperatures exceeded air temperatures, and much less than latent heat flux. Hydrodynamic simulations quantified how interactions among drivers lead to exceptional warming: reduced wind speeds (≤ 30% of reference) increased near‐surface temperature by 3–4°C, while elevated air temperatures (up to 37.5°C) and high turbidity contributed an additional 1–2°C. The simulations indicate that exceptional water temperatures (up to 42°C) can occur when a shallow, turbid lake under clear skies with high air temperatures and low wind speeds simultaneously maximize heat gain and minimize heat loss. Projections of climatic changes are likely to increase incidences of exceptional water temperatures.