Changes in the ocean can be felt on land
Terrence Joyce discusses link between ocean currents, climate.
Terrence M. Joyce is Director of the WOCE Hydrographic Programme Office at Woods Hole Oceanographic Institution. Joyce studies the relationship between ocean currents and climate. He spoke with Earth & Sky’s Eleanor Imster in July, 2005.
Imster: When we think about climate change, a lot of us think about changes in the air. We don’t consider the role of the ocean.
Joyce: That’s because we don’t live in the ocean. If the public were fish, they wouldn’t care about the air.
Imster: But ocean currents affect the air and climate…
Joyce: ...and vice versa, that’s right. It’s an ongoing area of research. What we know is that the atmosphere affects the ocean pretty much the same everywhere, maybe a little bit more at high latitudes. The reason it affects the ocean more at high latitudes is that when the atmosphere cools the ocean, the ocean can sink to great depths. And so, you get a lot of interaction at great depths, not just due to wind driving, but also due to the heating and cooling. And if the ocean is not so fresh that it can’t sink, then the ocean gets denser and it forms very deep mixed layers. For example, in the Labrador Sea, the mixed layers can be deeper than two kilometers
The ocean affects the atmosphere also. We know that is most sensitive in the tropics. When you get warm water in the tropics, and it gets warm enough, you can actually have more atmospheric convection, and that can go to great heights.
That’s sort of the El Nino situation. Where upwelling currents stop off of Peru, all that cold water essentially disappears. And all the warm water, which is piled all the way on the other side of the Pacific Ocean, comes racing across. When it gets to the eastern side, the whole convection system in the atmosphere follows it. So, what was rainfall over in the western Pacific now is rainfall in the eastern Pacific. And when it’s raining in the eastern Pacific, we have a nice, wet spring in the southwest US and the desert blooms. When we don’t have that rain the eastern Pacific, it’s kind of a drought. And so, we know in the tropics, then, ocean currents move the temperature around and the atmosphere really responds.
Where we don’t know a whole lot yet is outside the tropics. We don’t have clear indication of the ocean affecting the atmosphere in the mid to high latitudes. When ocean currents move warm water to the north, for example, off the Grand Banks, the atmosphere responds, but doesn’t seem to respond really strongly. In the tropics, there’s definitely a two–way communication. In high latitudes, it’s mainly the atmosphere affecting the ocean. Not entirely, but mainly.
Imster: So, there’s a kind of partnership between the ocean current and the air? They respond to changes back and forth?
Joyce: Yes. We know, for example, that some years the Gulf Stream is further south than it normally is. Well, you may think that since it’s further south, the water is colder and Britain, for example, has colder winters. And that’s true. But if you look in detail, it looks like the Gulf Stream shifting to the south occurs after the atmosphere has undergone its change. So, the Gulf Stream lags the atmosphere by about a year. So, yes when the Gulf Stream is north, the atmosphere is warmer. Yes, when the Gulf Stream is south the atmosphere is colder. But then you ask me who was causing what, I’d have to say, at least as far as Britain is concerned, the atmosphere that’s leading the Gulf Stream, not the other way around.
Imster: These days, scientists can get detailed information about ocean currents – temperature, speed, etc. — using satellites. Does that help you predict how currents and climate will change and interact in the future?
Joyce: Well, one of the problems that we have is our record is so short. And currents themselves have a lot of variation over the course of a year, or over the course of five to ten years, there are huge swings. And our record really is so short it’s hard to say if there are trends You can certainly see some times, some years, four or five years, when the system is certainly in one stage, and it’s definitely in another stage for another five years. And we think of that as climate. We can see changes in the currents clearly associated with climate. And as I mentioned, in the tropics, the currents are actually at the root of the problem. They are actually responsible for moving the warm or cold sea surface temperature around, and they affect the atmosphere.
But if you say, “can we tell from the existing record on currents if there’s a change due to global warming in the strength of a major current like the Gulf Stream? It’s not very clear. People have made statements about that, but one should be very skeptical. Because we know there’s a lot of variability from one five–year period to the next five–year period, or one three–year period to the next three–year period. You have to have a long time series to have any good reliability to a statement about changes. You can’t just rely on the last five years. We know, for example, there’s been a lot more wind in the southern hemisphere at the Antarctic circumpolar current, which zips around the southern hemisphere, has been speeding up. But is this due to global warming? Best I can say to you is, ask me in ten years and I’ll be able to answer it.
Imster: Given that there’s partnership between ocean currents and climate, as the Earth is warming and the currents will respond somehow, and something will change. Is there a consensus among scientists about what this “something” might be?
Joyce: First of all, when you make statements about “the Earth is warming”... Yes, on average, it’s warming. But there are certain places where it’s not warming, or it may even be cooling. For example, we’ve seen that large areas in the north Atlantic and the north Pacific in recent years have gotten cooler. And this may actually be a consequence of global warming. It may not be uniform warming everywhere. This is the way our system works. In some places, it gets really warm, and in other places, it may get cool. And if you take an average over everything, then you can see, on average, it’s warming up.
One scenario that’s been given is that if we have a continued amount of global warming, we may have a lot more El Ninos, so we have a lot more of this shift, this shut off of the upwelling off of South America, and we end up with warm water appearing in the eastern Pacific, not in the west. That may be more common if we have global warming.
On the other hand, a lot of the models say that the deep circulation that’s caused by sinking water in high latitudes, say in the north Atlantic, is going to decrease. What that will do is cause cooling in the northern north Atlantic, not a warming. So, this will run against the trend elsewhere. Both of these things are actually consequences of a globally warming Earth.
So, I wouldn’t say that there’s a consensus that we know in detail about these things. We can believe something maybe about the whole globe, or very large–scale averages, but in detail, people are still skeptical. I think that’s probably where, in the next decade, a lot more interest is going to be, in trying to get these models of the atmosphere and the ocean believable and working at much more reasonable scales. Where we can say, “What’s going to happen to the air temperature of New England?” As opposed to the air temperature of the whole eastern United States?
Imster: So what do we do, when we hear that the climate is changing, and maybe pretty fast, but even the smartest scientists don’t know exactly how that will alter our world. Do we just wait?
Joyce: Well, there are some sensible things we can do. There are ways we can be proactive, we don’t just have to be at the complete mercy of Nature. If you’re thinking of buying a house near the ocean, pick one that’s a little higher above sea level than one or two feet. If you’re worried about temperature extremes and heating and cooling, think about efficient energy systems in homes. We can also think about getting a hybrid car instead of replacement for the big SUV. They’re more efficient, they produce less CO2, they’re going to have less of an impact on earth’s climate. So, I think that there is something that every person can do.
And if enough people do that, there will be big changes.. One person can’t go out and control climate, but a lot of people can have an effect. Obviously. We’re having an effect right now for better or for worse. We can change that.
Imster: It ‘s tough though, because there’s not an easy or sure answer.
Joyce: But we’ll never know enough to be completely sure. If we wait, then we’ll be waiting hundreds of years, and it’ll be too late. We have to use our best guess and the information we have right now.
