Nor’wester and Upwelling off California

Catching up with the blogs today, I saw that Cliff Mass had written about a big windstorm that hit California on Monday. Unusually for big storms on the California coast, the winds in this one were from the northwest. Through a process called Ekman transport—a weird, nonintuitive one, caused, like so much else in physical oceanography, by the earth’s rotation—winds from the northwest push the surface of the ocean southwest, instead of southeast as you might imagine. As the surface layer slides slowly away from the California coast, cooler water from below rises up to replace it.

I decided to see what such a strong blow from the Northwest would to the ocean, and looked up data from a couple of buoys (the NDBC’s Station 46042, and MBARI’s M1) off Monterey Bay, my old stomping ground.

This graph shows winds (top plot) and water temperature (bottom) at the two buoys over the past few days (x-axis is the date in April). Sustained wind speed at M1 is the solid lines, and 46042, a bit further offshore, is the dotted lines. The triangles show maximum wind gusts at 46042. The blue dots show the wind direction, on the right axis of the first plot—0 degrees is due north, and the angle rotates around clockwise back to north at 360. Dark blue circles are M1, light blue squares are 46042.

Wind and upwelling

Beginning Sunday afternoon, the wind starts building steadily, topping out at more than 18 m/s (35 kts) Monday morning, with gusts up to 23 m/s (45 kts). The direction is steady at around 330°, right out of the northwest. And almost immediately, that building wind starts pulling water offshore and up from the depths—which you can see in the water temperature, which falls steadily for two days as the cooler, deeper water rises to the surface.

Those pictures are pretty nice, but you know what’s even cooler? Seeing the same thing from space. The clouds parted Tuesday, giving NASA’s Aqua satellite a clear shot of the sea-surface temperature off California:

California Current SST

That narrow band of purple and blue next to the coast is the upwelled water, stirred and distorted by all kinds of mesoscale eddies, jets, and squirts. I really love this picture—you can see the large-scale temperature gradient from north to south, the California Current bringing cooler water down from Oregon, the narrow band of upwelling, and the small swirls and eddies. Within a couple days, the nutrients in that upwelled water will be fueling a big phytoplankton bloom all along the coast.

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