Signals of climate trends and extreme events in the thermal stratification pattern of multibasin Lake Opeongo, Ontario

Previous empirical observations on thermocline response to warmer conditions have been contradictory. Given that a deepening thermocline was observed for a small lake (0.5 km(2)) and a shallowing thermocline was observed for a larger system (73.6 km(2)), these contradictions may be attributable to differences in lake size and the diminishing influence of water clarity on mixing depth. The four basin systems of Lake Opeongo, Ontario, Canada, range in size from 4.4 to 22.1 km(2) and build an empirical suite of stratification responses to climate variation for a wide range of lake sizes. From 1958 to 1996, lake temperature profiles were taken in the four major basins of Lake Opeongo. Canonical correlation analyses linked earlier iceout dates, warm July-August air temperatures, and relatively high amounts of bright sunshine in July and August with warmer midsummer epilimnia and shallower midsummer thermoclines for all four basins. The occurrence of stratification patterns associated with warmer climatic conditions increased (r = 0.44, p < 0.01) in parallel with the recent (post-1965) global warming trend. Such patterns also tended to occur in El Nino years. Stratification conditions with cooler midsummer epilimnia and deeper thermoclines tended to occur in La Nina years and provided a pronounced signal in 1992, the year that experienced a global cooling effect due to the eruption of Mount Pinatubo.