Science Friday: Global Circulation Currents

I'd like to start this post by promising that the boringness of this post's title in no way reflects the awesomeness of this topic.

The Earth includes several circulation systems—currents on a global scale that carry air, water, or even rock thousands of miles before returning to their starting points. These have interesting and sometimes surprising effects on people. Example 1: a giant floating trash heap in the middle of the Pacific Ocean.

Boats crossing the northern Pacific Ocean regularly encounter massive junk accumulations resembling “trash soup”. This is a relatively recent development—its existence was predicted in 1988, but it was not observed until 1997, and didn't get much media attention until last year. A big pile of trash in the middle of an ocean is, of course, a bad thing—besides the choking hazards to fish and birds, water-insoluble toxins can bond to plastic and poison them too. Further, invasive species can hitch a ride on the floating trash and get carried across the ocean.

So...why did it form? Although it's no secret that humans have a rude habit of dumping trash pretty much everywhere, the ocean is a huge place, and high concentrations of trash are found in this area. We don't know how big it is (it can't be determined remotely, and surveying the entire ocean is a ridiculous proposition), but it's really big, and it's in a pretty particular part of the remote North Pacific.

The answer is a type of ocean current called a gyre—a giant circular surface current. Besides the North Pacific gyre, the South Pacific, North Atlantic, South Atlantic, and Indian Oceans all have gyres of their own. Trash gets carried by other ocean currents into the gyre, where it gets trapped, and accumulates.

Have a look at that map. Besides the fact that the gyres are in the middle of oceans, you may notice that none of the gyres significantly crosses the equator, and that the northern gyres spin clockwise, and the southern gyres counterclockwise. This is due to the Coriolis effect, a product of the Earth's rotation which causes moving objects to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere. You'll notice the same thing if you roll a ball around on a spinning merry-go-round (here's an excellent demonstration.)

Another example of global circulation currents is a big part of what's driving the nasty weather we're having now from the remnants of Hurricane Ida. Ida started in the southwest Caribbean Sea, moved north (hitting Nicaragua and Honduras on its way up) and eventually ran into Mississippi and Alabama. Now, what's left of it is doing its best to turn North Carolina and Virginia into a swamp, and is making bike rides in Chapel Hill most unpleasant.

A hurricane is a circulation current by itself: hot air rises, cool air sinks, and the whole thing spins due to the Coriolis effect. However, they're part of a much bigger system, one also driven by heat flow: the set of atmospheric cells and prevailing winds.

The Earth's main energy source is, of course, the Sun. Sunlight doesn't shine on the Earth evenly: the equator gets a lot more heat from it than the poles do. However, heat tends to flow from hot things to cold things, and a lot of atmospheric processes are driven by this heat flow. Most of those processes are referred to as “weather.”

One process that we usually don't think of as weather, but helps determine it, is convection. This basically the process of hot air at the equator rising, getting pushed toward the poles by more rising hot air, and sinking when it gets cold at high latitudes. Meanwhile, air near the Earth's surface is drawn toward the equator to fill the space left by the hot air rising; as it gets close to the equator, it gets heated up and rises, and the process continues.

In a non-rotating Earth, this would form rotating air currents (called cells) where hot equatorial air rises, moves to a pole, gets cold, sinks, and moves to the equator. However, the Earth's rotation and the Coriolis effect complicate things. So, instead of having a single giant cell in each hemisphere, it gets sheared into three: a tropical (Hadley), a temperate (Ferrel), and a Polar cell.

Convection drives the air north and south. The Coriolis effect deflects it right or left, depending on hemisphere. So, this process creates large scale winds, called prevailing winds.

We're at a pretty temperate latitude here in NC, so we're in a Ferrel cell. This means the prevailing winds, which are westerly (from the west) blow weather around here roughly to the east. However, at tropical latitudes, the prevailing winds (the Trade Winds, which are easterly) blow things like hurricanes toward the west. That's why hurricanes that form over the hot ocean water off the African coast hit land in North America.

Hurricanes don't just form off of Africa though—any hot ocean water will do, including the Caribbean, as our good friend Ida will tell you. I said before that heat moves from the hot equator toward the cold poles. Convection cells are one way to move the heat away from the equator. Hurricanes are another.

It takes a lot of heat to evaporate water. I learned this the hard way a couple weeks ago when I accidentally made a casserole too soupy, and I couldn't fix it by evaporating excess water despite leaving it in the oven for hours. However, when you do evaporate a lot of water, like what it takes to form a hurricane, you're left with a bunch of water vapor with a huge heat energy content. Consequently, when the water vapor condenses and it rains, it releases that heat energy. A rainstorm over a typical US city can release more energy than a nuclear bomb into the upper atmosphere.

Hmmm. So hurricanes soak up energy when they form, move north, dump tons of rain on some poor unsuspecting North Carolinians. Some, like Bill earlier this year, make it all the way to Canada, bringing heavy rain, wind, and general mayhem all the way up, releasing tons of heat energy in the process. Turns out that hurricanes are just another of nature's cruelly ingenious ways to move heat from hot to cold, with a little help from the Coriolis effect.

I mentioned another process similar to this in my first post. The Earth's mantle has convection currents also. It's a matter of some debate whether the entire mantle is convecting, or just the upper part. But since the crustal plates are sitting on top of the mantle, they get dragged around too, and when they grind against each other, earthquakes occur.

So, the next time somebody asks you how Pacific trash heaps, hurricanes, prevailing winds, and earthquakes are related, you'll be able to stop them cold: they're all driven by global circulation currents.

In the news this week:

--Daily Kos has a nice post about life during the Cretaceous period. Dinosaurs lived on land, and a giant shallow sea covered much of the American West, where truly awesome creatures like 55 foot-long Mosasaurs and Ammonites lived.

--Some Michigan State scientists developed robot fish to take over the world, as soon as they finish collecting temperature and chemical data in the oceans.

--The US Department of Energy is sponsoring a national geothermal resource database including data from the USGS and state geological surveys. This may seem boring, but developing infrastructure like this is critical to making clean energy take off.

--Do cell phones cause brain tumors? Probably, but not definitively enough to stave off decades of industry denial and refusal to include safety features, likely to be followed by years of bitter lawsuits and a crash in public opinion of the industry. There's a solid chance this is smoking all over again. Use speakerphone or a headset, or at least hold the phone away from your ear!

--In case anyone's wondering, I don't link to much global warming news here, despite it being the issue I care about the most. That's because nearly all the news is very bad and getting worse, and I get depressed reading more than the headlines and linking to it. That goes for both the politics and the science. Bottom line, as far as I can tell: the Republicans and many Democrats are complete assholes for not acting on it, weak legislation capping carbon emissions likely won't be enough on its own, but will be an important part of the equation, innovation in the alternative energy and carbon sequestration fields will have to drive the shift to a low carbon economy, and we're going to feel some effects of it anyway. But, we might stop the ice sheets from melting irreversibly; if we succeed in that, the rest of the damage will mostly heal, assuming we don't find some new way to destroy the environment.

Some very good news from earlier this year is that the stimulus bill included huge investments in high speed rail and a modern electric grid. The former will eliminate some car and plane traffic; the latter will make electricity transmission more efficient and, more importantly, allow decentralized clean energy generation on a large scale.