Climatic forcing: effects of el nino on a small, temperate lake.

Temperature profiles measured regularly for 21 years reveal the interannual differences in winter-to-summer heat gain in Castle Lake, California, a small subalpine lake. Year-to-year changes in large-scale climatic surface forcing, especially the amount of snowfall from February through April (which determines the date of thaw) coupled with the early heating and wind mixing after thaw, causes this interannual variation. The seasonal thermal structure for years in which the lake gains significantly more or less heat than normal-all of the El Niño years and several others-shows that the depth of the mixed layer and the mixing of heat into the stratified thermocline region control the storage of heat. The temperature of the mixed layer does not reflect abnormal thermal storage. Variations in mixing during early spring, which controls the heat content at Castle Lake, may also affect the annual average of the primary productivity.