Climate Change The Intertropical Convergence Zone, or ITCZ

Published on April 3rd, 2013 | by Joshua S Hill


Northern Hemisphere Is Becoming Warmer Than Southern Hemisphere

You may not have consciously thought about it, but I imagine that to some of you out there who have an environmentally conscientious brain, the fact that the Northern Hemisphere has more landmass and the Southern Hemisphere has more ocean would have triggered some interesting questions.

For example, if there is such regional variation in a country as boring as the United States of America, surely there would be some difference across the whole planet.

New research from climatologists at the University of California, Berkeley, and the University of Washington, Seattle, have determined that, yes, in fact, the Northern Hemisphere is becoming warmer than the Southern Hemisphere, a factor which could significantly alter tropical precipitation patterns.

The scientists believe that such a shift in hemispheric similarity could increase or decrease seasonal rainfall in areas such as the Amazon, sub-Saharan Africa, or East Asia. Such a shift could see these areas wetter or drier than they are today.

“A key finding is a tendency to shift tropical rainfall northward, which could mean increases in monsoon weather systems in Asia or shifts of the wet season from south to north in Africa and South America,” said UC Berkeley graduate student Andrew R. Friedman, who led the analysis.

“Tropical rainfall likes the warmer hemisphere,” added John Chiang, UC Berkeley associate professor of geography and a member of the Berkeley Atmospheric Sciences Center. “As a result, tropical rainfall cares a lot about the temperature difference between the two hemispheres.”

The Intertropical Convergence Zone, or ITCZ

The thunderstorms of the Intertropical Convergence Zone form a line across the eastern Pacific Ocean.
Image Credit: NASA Earth Observatory

The research shows that a warmer Northern Hemisphere is going to cause atmospheric overturning in the north to weaken and strength in the south, pushing rainbands northwards. University of Washington’s Dargan M. W. Frierson, who was part of the team to publish their results in the Journal of Climate, believes that the regions most affected by this shift are likely to be on the bands’ north and south edges.

“It really is these borderline regions that will be most affected, which, not coincidentally, are some of the most vulnerable places: areas like the Sahel where rainfall is variable from year to year and the people tend to be dependent on subsistence agriculture,” said Frierson, associate professor of atmospheric sciences. “We are making major climate changes to the planet and to expect that rainfall patterns would stay the same is very naïve.”

The study was reliant upon over 100 years of data and model simulations, which allowed the authors to compare the yearly average temperature difference between the Northern and Southern hemispheres with rainfall throughout the 20th century.

Their results showed abrupt changes in the difference coincided with rainfall disruptions in the equatorial tropics. The largest drop in temperature difference was a one-quarter degree drop in the late 1960s which coincided with a 30-year drought in the African Sahel which caused famine throughout the region, increased deforestation in North Africa, and decreased monsoons in East Asia and India.

As global temperatures rose over the course of the 20th century (top), the temperature between the two hemispheres changed little until the 1980s, though it has been rising since.  Image Credit: Andrew Friedman.

As global temperatures rose over the course of the 20th century (top), the temperature between the two hemispheres changed little until the 1980s, though it has been rising since.
Image Credit: Andrew Friedman.

“If what we see in the last century is true, even small changes in the temperature difference between the Northern and Southern hemispheres could cause measureable changes in tropical rainfall,” Chiang said.

Is there a need to worry though? Are these correlations minimal and unlikely to ever increase in volume?

According to most computer models which are simulating past and future climate, the team found a steady rise in hemispheric temperature difference through to the end of the century. Even if we humans manage to get our act together enough to lower our greenhouse gas emissions, models predict a 1 degree Celsius increase in difference by 2099.

Chiang makes an interesting point in discussing the future of climate sciences. With a greater percentage of land mass in the Northern Hemisphere more susceptible to warming than the ocean-laden Southern Hemisphere, he and his colleagues believe that climate scientists need to not only look at the rise of global mean temperature but specific hemispheric variability as well.

“Global mean temperature is great for detecting climate change, but it is not terribly useful if you want to know what is happening to rainfall over California, for example,” Chiang said. “We think this simple index, interhemispheric temperature, is very relevant on a hemispheric and perhaps regional level. It provides a different perspective on climate change and also highlights the effect of aerosols on weather patterns.”

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About the Author

I'm a Christian, a nerd, a geek, a liberal left-winger, and believe that we're pretty quickly directing planet-Earth into hell in a handbasket! I work as Associate Editor for the Important Media Network and write for CleanTechnica and Planetsave. I also write for Fantasy Book Review (, Amazing Stories, the Stabley Times and Medium.   I love words with a passion, both creating them and reading them.

  • Herrie

    Not being a Christian Geek (from Dutch ‘gek’ — simpleton or zany!) or a left-winger (but a liberal of the more tranquil kind) and not being addicted to writing Amazing Fantasies (if I didn’t hate emoticons, I would have inserted a placatory smiley here) … I am extremely skeptical about theories that blame us humans for the warming that has been observed in the last two decades or so (coming hard on the heels of a long cooling trend, I think? ) and expectations of the coming Armageddon that (significantly?) started gathering momentum round about the end of a millennium. The two great millennial panics (1000 AD and 2000 CE) have been really hard on us! If one looks at the huge and obviously unpredictable swings in temperature during the most recent glaciations and interglacials, one must surely wonder how much natural factors beyond our command or ability to predict have to do with variations in temperature over relatively short periods. Should the fact that we still have no clue about the real engine driving the ice ages not give us pause? By playing safe and misapplying the precautionary principle we could land ourselves in serious trouble if, with little warning, fickle Nature decides to swing back the other way and we need all the warmth we can get. Perhaps it would be more sensible to do what we humans do best — concentrate on coping as best we can with climatic swings. Perhaps we should pay more attention to civil engineering, architecture and innovative farming and food-production methods to safeguard our future against unpredictable extremes of both heat and cold? H. sapiens has been highly successful at coping with climatic extremes, and there can be little doubt that we are going to need this skill for the rest of our existence on earth. Why not start now? If we now throw money at fruitlessly trying to stop the tide coming in once, and reduce efforts to eradicate poverty and disease, we may well be the makers of our own hell on earth. As I see it, the precautionary principle should tell us that, while we must obviously learn to be more responsible in our dealings with Nature, we cannot make any assumptions whatsoever about what our Fickle Old Mother has up her sleeve. Beware the hubris of science!.

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