Animation of an upwelling event (see below):
Causes:
Frequency: 7- 8 per year
Winds blowing across the lake surface push water away. Water then rises up from beneath the surface to replace the water that was pushed away. This process is known as “upwelling”.
On the northern shore of Lake Ontario the predominantly westerly winds coupled with the Ekman spiral effect create frequent upwelling events.
In addition, immediately outside the 10-12 m deep Outer Harbour there is a steep under-water escarpment that drops off to 75 m depth within 600 m of the end of Tommy Thompson Park, making the Harbour especially sensitive to upwelling of hypolimnetic waters from Lake Ontario.