Q: In looking at a world map showing ocean surface currents, it appears that with very few exceptions, currents do not cross the equator. Why is this?
— Submitted by Chris W., 15, Beaverton, Ore.
The first thing to keep in mind is that ocean currents are very thin compared to the 12,742-kilometer (7,917-mile) diameter of Earth. They’re only about four kilometers (13,000 feet) deep at most, and surface currents can be just a few hundred meters thick. As with gas and vapor currents on other planets, they tend to move in an east-west direction.
These currents have trouble crossing the equator, but they can. The answer for why they usually don’t involves a concept that’s relatively new in the science world. It’s a principle called potential vorticity conservation. It’s related to a very well-known concept of angular momentum conservation.
It starts with the rotation of the earth. Think of the planet as a spinning top and think of the water as columns that are no taller than the ocean depth. Water at the North Pole spins along with the planet like an ice skater. But as one moves closer to the equator, the water is less and less aligned with the spin of the planet. As one moves from the equator to the South Pole, the alignment returns, but now the spin is upside down. Thus surface currents in the Southern Hemisphere are predominantly counterclockwise and in the Northern Hemisphere they are clockwise, for the most part.
As an ocean current moves toward the equator, the angle of the water column relative to the earth’s rotation changes. The water’s tendency is to compensate for that. That’s where conservation of potential vorticity comes in. The water develops vorticity or “spin.” That spin stifles north-south motion. The water that does make it to the equator often turns to flow along the equator. The part that does cross the equator trails off into swirls known as eddies.
When currents come up against boundaries, such as continents, they can overcome the forces that keep them otherwise bound to one hemisphere, but not for long. The North Brazil Current is a prominent example, where a western land boundary supports the current flowing northward from the South Atlantic to the equator. But soon after crossing the equator, it “eddies out” into gigantic swirls that are several hundred kilometers across. These rings move on northward towards the Caribbean.
– Lynne Talley, physical oceanographer, Climate, Atmospheric Sciences, and Physical Oceanography (CASPO) division
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great info, i will try to understand about this information
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So why don’t the equatorial oceans boil away and the polar oceans freeze solid?
Now that was something I was not aware of . I had read this article about The Ocean’s Accelerator , does this also affect ocean currents?
What about the moons gravitational pull pulls the water and then drops it because of its weight.
wow , thanks for your sharing
Thats amazing. Thanks for sharing your insight.
Thanks! I usually needed to write on my website something like that. Can I implement an integral part of your posting to my site?
Thank you for educating us!!
Now I never thought about this aspect of currents. Informative and exciting too. It’s nice to learn more new stuffs.
Thanks for the info. This is what my kid was asking about. Now i have answer for him.
Wow. Never knew currents didn’t cross the equators. Thanks for the detailed explanation.
I actually had no idea that the currents weren’t able to cross the equator. I would never have thought it possible since it’s a body of water.
I learnt something new today – thankyou!
It’s strange that I’ve never heard of this before. It make sense I suppose, but I’ve never seen the system explained before.
Cheers!
Fascinating stuff. I live in Joao Pessoa in north east Brazil, just about at the point where the north and south equatorials collide. There are times when the division is clearly visible in terms of marine life and local currents.
Thansk for this post, I really enjoyed it and it’s explained a lot.
Coriolis force is stronger in the poles than the ecuator, and in the north is oposite to the south emisfer. I think that is an other reason why.
Just wondering how big of an influence the gravitational effect of the moon is. Each year the gap between us and the moon widens, hence does the gravitational force. But does increasing distance influence us exponential or is there a cut-off line?
Excellent news. Thank you.
Thank you. I will have to study the answer to fully understand all the points you brought up, but I already feel like I know a lot more than I did a few days ago. Thank you.