Finding the Upwelling Filament

You may be wondering where the ship is going on this expedition. The answer is that it is always changing! Rather than stationary sampling (Eulerian) we are sailing with one continuous water mass and observing the changes within it through time (Lagrangian). Where we are going depends on finding an upwellingfilament (a special chunk of water that is rich in nutrients) and following that chunk of water wherever it moves. 

So what is upwelling?

Upwelling begins with wind. When strong winds blow North to South along the CA coastline, a process called “Ekman transport,” causes deep water to be brought to the surface and pushed offshore. This water is cold, dense, and rich in nutrients (elements like nitrogen and phosphorous needed to make proteins and DNA). These nutrients can fuel growth of primary producers like microscopic algae. The algae is food for small animals, which become food for bigger fish, dolphins, etc—fueling a productive ecosystem! If this chunk of upwelled water moves westward offshore, we call it a “filament” and follow it with our ship.

Black arrow: Wind direction
Blue arrows: Ekman transport

In Search of a Filament!

We can predict when/where upwelling will occur by looking at wind trajectories, and satellite data.

Wind trajectories

Unfortunately, due to continuous cloud cover, we have been unable to utilize satellite data, including sea surface temperature and sea surface chlorophyll. Instead, we have relied solely on data from wind, deployed instruments, and sampling we do on the ship. When we started, we did not observe upwelling, but by Wednesday, July 21 signs of emerging upwelling were observed: a patch of relatively cold water with a high concentration of nitrogen! We can measure nutrient concentrations in the water by conducting CTD casts (see previous post) that give data that look like this:

The y-axis is depth from 0 to 100 meters, and the x-axis shows temperature, nitrogen, and fluorescence (a proxy for chlorophyll from algae). 

We continue to track the filament once we have found it, and observe the changes to the chemistry and biology as the water mass evolves.

-Monica Thukral (Allen lab)

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