Thursday, June 2, 2011

Ocean Circulation Primer

Microwave Feedhorns
I just read a NASA article about the new Aquarius satellite going up June 9th, the first satellite designed to taste the earth. Well, that's how I like to think of it. They actually use some of that Radio Frequency stuff I was talking about yesterday (microwaves), but I do like the idea of an invisible tongue licking the earth. Anyway, the NASA article isn't that good. It doesn't say enough about ocean circulation to make me care. Maybe it's because I know that they're leaving out the best part. I think there is a very simple bit of information they could have included that would make this compelling but they just didn't even mention it.

What does salt do to water? 
It lowers the freezing point.

People up North probably get this intuitively since they deal a lot with ice and salt. But my nieces in South Florida are clueless about cold. My oldest niece posted a status update on Facebook revealing that her little sister asked her if she knew how to play Lick a Tree. I assume she heard about The Christmas Story where the kid in the earflap hat is dared to put his tongue on a frozen flagpole. But little girls in South Florida know nothing about cold. I can't speak to her weird adaptation of the dare. But it makes me think they're missing an important lesson. I told my older niece to put a big metal spoon in the freezer for a few hours and try licking it. If it's quick to warm up and release her tongue then she can see if her sister wants to learn to play Lick a Spoon. (I found a lot of videos on YouTube of people licking flagpoles in the snow and it is really horrifying. Not putting them on here.) Anyway, it makes me think the writers really need to hit the basics when they're talking about an ocean salinity satellite.

I spent months in grad school studying ocean circulation. Even though I was a bona fide physicist there were a few simple facts that made me go "Huh!" It's not hard to grasp, I just never thought about it. But it's cool. I'm glad I know now. Articles about Aquarius could stand to include a salty taste of these things. Here is my ocean circulation primer to go along with the upcoming ocean salinity satellite launch.

What's Interesting about Circulating Oceans
  1. Hot water expands. It becomes less dense. Most modern sea level rise is explained simply by the expansion of the water that was already in the ocean, not because of the extra water in the ocean from the ice melting off of continents.
  2. Ocean circulation is based on density. Hot water floats on cold water. When there are extra minerals in the water is gets more dense from that too. Of course pressure makes things denser, but in the case of seawater it's less important than temperature and salinity. Cold, salty water is about as dense as it gets.  
  3. Sea ice is fresh. When the ice crystals form on the surface of the North Sea in the winter, for example, the salt molecules are excluded. Fresh ice floats on top of the salty seawater. If it's really windy and cold the surface becomes a strong salty brine and this water gets a good deal colder than 32°F or 0°C. There's plenty of water in the ocean -2°C. It gets super cold in the dark, windy, North Sea in the winter, sinks to the bottom and runs downhill underneath the warmer water on top, just like it was rain running down a mountain.
  4. The different densities don't mix up nearly as much as you'd think.
  5. They name the water. Yes, these very cold, very salty layers of water are so distinct and "live" for so long it is like a pet and they name it. The names came about before geek culture so instead of being named after Star Wars characters they have literal names like North Atlantic Deep Water, which is the most common water in the ocean. North Atlantic Bottom Water is even denser than the deep water. From the southern hemisphere you get Antarctic Bottom Water and Antarctic Deep Water. There's also an Antarctic Intermediate Water that floats about 1 km deep all the way up to the equator. There's also one named water mass that is dense from just being salty without being cold, the Mediterranean Intermediate Water. Evaporation in the Mediterranean drives the salinity up enough that the water becomes dense enough to sink to 2 or 3 km and flow west into the Atlantic like a long salty tongue. That's just for the Atlantic. There are names in the Pacific too but that water isn't as layered and moves a lot more slowly than the Atlantic stuff so they aren't in such a big hurry to figure it out.
  6. There is a global conveyor belt of moving water. It takes about 1000 years for a molecule of water to make the round trip. This circulation moves nutrients around the ocean and is seriously linked to the climate. If the deep water stops being formed in the North Atlantic in the winter because it doesn't get cold enough some serious shit could go down. But we don't know what exactly or how long it would take. That's the main motivator for collecting all this accurate ocean surface data. We need it to develop computer models that accurately match the physics of the system as it stands today. Then we can try different things to see what it takes to break the global conveyor belt. (NOAA link)
  7. Cold water can hold a lot more dissolved air than hot water. The deep water formed on the surface of the North Sea and Antarctica contains a lot of dissolved air -- nitrogen, oxygen, carbon dioxide, argon whatever is in the air at the time the water sinks. This more-gas-in-cold-water phenomenon should be intuitive to people who drink a lot of fizzy drinks. If you open a hot bottle of Coke it fizzes a lot more than a cold one will. And if you don't put it in the refrigerator it will go flat very fast. But if you keep it cold and under pressure the carbon dioxide stays dissolved in it for days. Ocean water is the same. That's why the hot surface water around the equator is utterly boring for fishing but all around Antarctica you have whales and penguins and seals and all the plankton and fish they're eating - a food web fiesta.
I found the Science Writers Guide for this project. It has excellent data on the system and all the extra goodies they've got on there, like cameras. And it has none of the stuff I just described. No wonder the articles I'm reading are kind of dry.

I feel close to this instrument because back in the early '90s I helped my friend Dave Kunkee make a wave tank on the roof of the EE building to test a radiometer he built for his PhD. It was to measure wave height from space. Knowing wave height is part of the process of getting the salinity so Aquarius has one of those on there as well as other passive and active microwave instruments. Now Dave is an Associate Director at The Aerospace Corporation and designs these things that improve our understanding of the earth.


  1. I'm still mulling over that first factoid. It strikes me, emotionally, as much more alarming that the ocean might be warming up enough to expand than that it might be warming up enough to melt some surface ice.

    I don't like imagining our oceans as open bottles of Coke going flat---and relatively fast---in my pantry. Especially when that cold ocean Coke is chock full of yummies for the sea life.

  2. I just want to applaud Barbara. Maybe genuflect a bit too.

    Damn good teacher of complex issues.

  3. Thanks for the compliments! I really didn't think anybody would read this one. Credit should go to Dr. Philip Froelich for being the one who taught it to me. He'd probably be appalled that I'm attempting to spread the word on this considering I made a 40% on my first test in his class. The guy that made 100% doesn't speak English very well though so I guess that's how I win the Explainer award.

  4. Pretty sure that Blue Planet devoted one episode to this topic, or part of one episode. I'll have to go back through it to see.

    As an Oceanographer wannabe (Chemistry scared me away), I am attracted to this subject and appreciate when someone cares enough to write about it.