We saw a little-known, but fascinating meteorological feature off the coast of Vancouver Island today… a little swirl in the clouds created due to differences in the strength of northwesterly winds along the coast of Vancouver Island.
Speed shear in NW flow parallel to Vancouver Island spun up a nice eddy off the Vancouver Island coast today. Pretty common in the summer but only visible when it forms along a nice marine layer boundary. #wawx @NWSSeattle pic.twitter.com/OLC9GxFH8l
— Charlie Phillips (@GeoduckChuck) June 26, 2020
Our “standard” warm, summertime pattern – and the one in place today – is to have a big ridge of high pressure in the Northeast Pacific and onshore, northwesterly flow off the West Coast. Because Vancouver Island extends from the northwest to the southeast, these winds are roughly parallel to the island. And with winds over the ocean traveling faster than those over land (due to drag forces from terrain and vegetation), we see a change in wind speed along the coast, with winds decreasing as one heads inland.
Credit: University of Washington
In weather-speak, this change in wind speed with distance is known as “wind shear.” Wind shear tends to create localized areas of rotation in the atmosphere. Imagine how different wind speeds across a pinwheel cause it to rotate… the atmosphere works in the same way.
This phenomenon is fairly common during the summer, but it is rarely visible because there are either no clouds to show the eddy or the region is completely obscured in clouds and it is hard to make out any circulation. But today, this eddy was perfectly positioned on a boundary between stratus clouds near the coast and clear skies further offshore, and we could clearly see the counterclockwise rotation.
Featured image: GOES-17 visible satellite at 9:21 am 6/25/2020 from College of Dupage NEXLAB satellite page
