MC 5501
Speaker
Mathew Wells, University of Toronto Scarborough
Title
Thermal mixing patterns in Lake Ontario as revealed by novel year-round observations of thermal stratification
Abstract
Year-round records of thermal stratification in the Great Lakes are rare, and there are few continuous observations of thermal stratification and water column mixing during winter. In this talk I will present analysis from a recent paper of the temperature data from 13 temperature logger chains and from over 150 benthic stations that were deployed across Lake Ontario for the last four years, as well as data from a 600 km long Slocum glider mission from last April. The timing and duration of the fall overturn correlates with the local average water depth, and shallow sites (<50 m depth) overturn up to a month before deep sites (> 100 m depths). Likewise, in spring the shallow sites warm faster. Lake Ontario has partial ice cover, so wind driven mixing stirs the water column throughout winter and inverse thermal stratification is largely absent. Lake Ontario appears to be a warm monomictic lake, rather than having a dimictic mixing pattern – there is no sustained ice cover or inverse stratification that inhibits vertical mixing in winter. I will review the available historic data of winter stratification since 1960, and discuss the evidence that as minimum temperatures warm above 4oC in the centre of Lake Ontario we are entering a profound regime shift in mixing dynamics with the duration of late winer/spring mixing being reduced by several months.