Effects of riverine inflows on the climatology of a tropical Andean reservoir

Field data and numerical modeling were used to investigate the effects of the riverine inflows on the seasonality of the lake mean temperature, thermal structure, and basin‐scale internal waves in a tropical Andean reservoir. Lake temperature and external forcing were measured during contrasting hydrological periods modulated by severe cold (2011–2012) and warm (2015–2016) phases of El Niño–Southern Oscillation (ENSO), a major driver of Andes hydrology. The seasonal heat budget of the reservoir was driven by the competing rates of cooling by the inflow and heating through the surface. As the atmospheric heating rate remained roughly constant, the seasonality of the river inflow heat flux drove the seasonal reservoir temperature. The inflow also intervened on the heat transport within the reservoir and, during dry periods, mixed heat downward from the surface layer and introduced heat at intermediate depths such that a thick metalimnion formed that brought the V2H1 mode into resonance with the diurnal wind forcing. During wet periods, the inflow compressed and cooled the metalimnion, tuning the V1H1 mode to resonate with the diurnal wind. It was also observed that large inflows destroyed temporarily coherent basin‐scale internal wave motions. Because of the dominant role of the inflow on the seasonal reservoir temperature, changes within the reservoir will be more strongly related to changing temperature and volume of the incoming water than atmospheric forcing as in many large natural lakes.