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Stratification and mixing in large floodplain lakes along the lower Amazon River

  • Augusto-Silva, Pétala Bianchi
  • MacIntyre, Sally
  • Rudorff, Conrado de Moraes
  • Cortés, Alicia
  • Melack, John Michael
Publication Date
Jan 01, 2019
Biblioteca Digital da Memória Científica do INPE
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Large, shallow lakes are common in the extensive floodplains throughout the tropics. To determine controls on their mixing dynamics, we instrumented 5 stations in two shallow, connected tropical lakes on the lower Amazon floodplain with meteorological and temperature sensors. A tight relation between changes in thermal structure and LMO/h (the ratio of the Monin-Obukhov length scale to the depth of the actively mixing layer) indicates the sensitivity of thermal structure to wind speed relative to heating and cooling. Four regimes led to variations in mixing: (i) high solar radiation with light winds in the mid-morning to early afternoon resulted in shallow stratification, 0 < LMO/h < 1; (ii) afternoons with higher winds caused the diurnal thermocline to downwell and heat to mix to deeper layers, LMO/h > 1; (iii) by late afternoon, buoyancy flux became negative and LMO/h < −1 and with u∗W and w∗ both >0.06 m s−1, mixing from wind and cooling co-occurred; and (iv) convection dominated mixing on nights with light winds, −1 < LMO/h < 0. Pattern (ii) occurred mid-day if winds were higher. When winds were intermittent and regime (i) predominated mid-day, changes in heat content were primarily determined by one dimensional processes of heating and cooling. When easterly winds were sustained and regime (ii) occurred mid-day, heat was transported west in the day, and colder water upwelled to the west or was advected to the west and north at night. Subtle differences in wind speed determined the extent to which advection moderated the thermal structure. / Pages: 61-72

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