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Adam Wright
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(Wednesday) 9.14.11

Ocean Science 101: Upwelling, Downwelling, and other random acts of shrinkage

I’ve been getting a lot of questions this summer about upwelling and downwelling and while I have written a lot about the effects of these phenomenon I haven’t really took the time to sit down and explain exactly what they are…so here it goes, I will try not to get two nerdy in the explanation but hey no promises.

Upwelling

Most of you know or have heard the term upwelling and that it usually means that it is time to break out a thicker wetsuit, put away the boardshorts, and in some areas get ready for the combination ice-cream headache duckdive and the “claw” that your hands twist into after an extended cold water surf session. Just in case you haven’t heard of upwelling I will give you the quick and dirty definition first, which is: Upwelling is a process where cold water at the bottom of ocean is pulled from the lower depths and is brought to the ocean surface.

If you really want to get into the oceanography-nerdery there are actually 5 types of upwelling being researched by oceanographers:

1. Coastal upwelling
2. Large-scale wind-driven upwelling in the ocean interior
3. Upwelling associated with eddies
4. Topographically-associated upwelling
5. Broad-diffusive upwelling in the ocean interior.

The main types of upwelling that affect us as surfers are coastal upwelling and upwelling that are associated with eddies (which for Southern California are essentially the same thing.)

Coastal Upwelling

I know what you are saying…”Wow Adam…Coastal Upwelling affects surfers. The coastal waters, where the waves break affects us, dude you must be a genius!” Yeah I know it is a big duh…but this is the main type of upwelling that we have to worry about. We can actually mix in the “upwelling associated with eddies” into the coastal upwelling category since most surface/weather eddies occur close to land like the infamous Catalina Eddy/Coastal Eddy/June Gloom that occurs in Southern California.

Just to be clear, this isn’t exclusive to Southern California, it occurs all over the world. In fact here is a map, with the areas highlighted in red where we see consistent upwelling…but just the basic nature of the process means that any spot that has a coastline and winds can experience the ball numbing fun.

Ok the main basic concepts to upwelling are that wind blowing across the surface of the ocean moves water AND that nature dislikes a vacuum…so if you move water from one place more water from “somewhere else” will try to fill the place it left and equalize water levels.

Where this gets a little funky is when you start dealing with the wind and a process called coriolis. (I know coriolis sounds like a fungal infection) but it actually is a force of nature that affects all things that A) Have mass…like air molecules and B) are in motion…like air molecules being blow by the wind. I has to do with our planet being a spinning ball of fun…just trust me (or Wikipedia it). I won’t go too deep into theory but coriolis makes anything with mass that is in motion in the Northern Hemisphere veer to the right…and anything that is in the Southern Hemisphere veer to the left. Our Human perception is that the wind is blowing in a straight line but in actuality it is curving. And yes this is why low-pressure storms in the Northern Hemi spin Counterclockwise and systems in the Southern Hemi spin Clockwise.

I could go into the whole picture yourself trying to throw a ball to your friend on a spinning merry-go-round example but it will just give us all headaches…so trust me…Air Molecules in Motion in the Northern Hemisphere veer to the right and that is important because the direction of the wind along the coast is what causes upwelling.

Ok now that I have glossed a lot more nerdiness that even I am willing to put up with let’s get to the meat and potatoes and start talking about things that make some sense. So for California (Southern and Northern) the main cause of upwelling is N-NW winds blowing along our Coastline. Why you ask? Well remember that stupid coriolis effect…well while these NW winds blow down our coast they are actually veering off to the right away from the coast. I know “WTF?” right.

Let me get more complicated just one more time…because there is strange energy transfer taking place when these NW winds crank up. Basically wind blowing on water moves some of the water molecules (little ripples, texture, chop), which means some of the water molecules are affected by coriolis, but it also translates some of the wind energy motion into pure “swell” energy, which has very little to almost no mass, which means it just travels off along the path the wind was blowing and doesn’t really change direction. I know it doesn’t make a lot of sense but fortunately a lot smarter people that me have done the math and can prove it. I just copied my homework off of theirs in 3rd period. The part to take away from this is that while the wind is getting hammered by coriolis the water underneath isn’t affected as much…just enough to help the upwelling process.

So remember the whole “veer to the right thing”? Well if you take that and apply it to the California Coast the NW winds are moving down the coast in a perceived NW direction but from a physics perspective they are actually hooking out to the WSW. This is important because it means the water along the coast, which isn’t getting hooked as badly, is also being pulled AWAY from the coast along that WSW direction.

It is this “pulling away” motion along the surface that causes upwelling

So the surface water is being moved away (and WSW) along the Coast of California…and since the surface water is taking the trip, the ocean tries to replace that newly caused vacuum, with water from below…F-ing cold water…that is both nutrient rich and hasn’t seen any sunlight to warm it up. (the Nutrient rich part is why all of the coastal upwelling zones are great fishing areas…the cold part is why upwelling sucks). Just to get the Jeopardy Daily-Double…this movement process is sometimes referred to (by us science nerds) as the “Ekman Transport”.

So just so we are clear, let’s summarize…the NW winds, due to a bunch of factors , forces the warmer surface water away from and down along the coastline, which in turn creates a vacuum that pulls cold water from the below the surface and we all freeze our butts off. (Remember that the NW winds really only affect areas in the Northern Hemi…and it more impactful on the eastern side of an ocean basin…if you apply this to the Southern Hemi, replace NW with SE-S winds and you will get the same effect).

Downwelling – the opposite of upwelling (as if you couldn’t tell)

Since the main part of Coastal Upwelling is surface water being pulled “AWAY” from the coast…then, yep you guessed it, Downwelling is when you push warm surface water TOWARD the coast, piling it up and forcing the colder water back down into the depths.

Since the whole cold/warm water transfer effects of downwelling are the opposite of upwelling…all of the other effects that cause downwelling are opposite. In Southern California it is the S winds that, thanks to coriolis and the Ekman transport, veer right, toward land bringing warm water from the much clearer (therefore able to absorb more sunlight energy) surface waters that sit farther offshore.

You can see the warmer waters waiting just offshore of Socal…all they need is the S winds to drive them in. (Also note the darker/cooler colors that run next to Central/Northern California Coasts…you can tell that when this image was taken the NW winds and upwelling were in full swing along the those areas)

The Southern California Eddy – Why it sort of sucks and sort of doesn’t

Now that you know the processes of upwelling and downwelling you can see why the Southern California Eddy isn’t always a “Bad Thing”. From a surf perspective it isn’t all rainbows and cupcakes to see an eddy form, since many of our beaches have some sort of South Exposure, it means onshore winds and textured crumbly surf along with the depressing grey marine layer.

On the positive side those S winds drive in warm water from further offshore…and if we have a S-swell, particularly a shorter-period one, it seems to enhance the process. The eddy itself is sort of a protectant from the constant NW winds that blow along most of California…actually giving Socal a chance to break out the trunks once in a while.

And finally to answer the question that a lot of people ask me…”Why does the water temps change so rapidly, I was trunking it on Wednesday and by Friday I want my 4/3…what the heck?”

The warm water pulses, are actually pretty fragile things…the water gets deep off our coast pretty quick thanks to a narrow continental shelf, and our dominate wind pattern is to have some sort of NW onshore flow blowing most of the year. So the Eddy has to stall out the NW winds, and keep them down through the whole day, usually for several days, and that will let the warm water in, but keep in mind that it is a pretty shallow band of warm water right at the surface, dive down about 5-10 feet and you will feel the water temps plummet. So with that skim of fragile warm water it usually only takes one afternoon of gusty NW winds and all of the Eddy’s work will be wrecked and we will be piling on the rubber once again.

Big thanks to NOAA and the National Weather Service and their various departments that deal with ocean currents and upwelling for the images…and to Dr Garrison at OCC…for hammering me with the coriolis concepts, while using small words, until it makes sense…If you ever get a chance attend his class or read his excellent Oceanography textbook…the ocean will make a lot more sense

  • shirtless in san ysidro

    Dang! Go for a Doctorate or something, bro! U smart.

  • Douglasazo

    makin’ me proud adam.

  • E

    “Dr Garrison at OCC…for hammering me with the coriolis concepts, while using small words, until it makes sense…If you ever get a chance attend his class or read his excellent Oceanography textbook…the ocean will make a lot more sense”

    TRUTH! One of my favorite courses I have taken in college. Fantastic teacher, great material.

  • AJ

    Nice article!!

  • http://smartvark.com Stefan

    Awesome stuff. Still waiting on the full eddy writeup.

  • robin york

    Great job educating us on this tricky concept, Adam, thanks for breaking it down so lucidly. I’m still a bit clueless on how Eddies form, can you make that a topic of your next article?
    Thanks a lot, bro, and also BigUp for providing the most insightful, comprehensive and most entertaining forecasts in SoCal!!!


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