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Introduction：
Migration of Monsoon
季风(又称季候风)是周期性的风,随着季节变化,并且盛行风向(40%以上风频)季节切变达 120 度以上。主要发生在季风亚洲(东亚、南亚地区)、西非几内亚和澳大利亚的北部沿海地带等地。
Background information：
Monsoon is traditionally defined as aseasonal reversing wind accompanied by corresponding changes in precipitation,but is now used to describe seasonal changes in atmospheric circulation andprecipitation associated with the asymmetric heating of land and sea. Usually,the term monsoon is used to refer to the rainy phase of a seasonally-changingpattern, although technically there is also a dry phase.
New research indicates that the Sahara, thebarren and desert covered region of northern Africa, was once lush and wasfilled with vegetation and lakes.
You may be surprised to hear that cavepaintings in the Sahara depict hippos drinking in waterholes and large herds ofelephants and giraffes. A strange finding perhaps, in such a lifeless place,but new research indicates that the wide belt of barren desert in northern Africahas not always been so barren.
Researchers from Columbia University andMIT have discovered that the climate in Sahara was quite lush between 11,000and 5,000 years ago, a period known as the African Humid Period. On either sideof that period, the region has been a desert, as we know it today. The mostamazing fact about the change in northern Africa is how abruptly it occurred.The researchers believe the region reverted into a desert in just a generationor two.
Knowledge points：
1. 季风现在用来描述大气流通和不对称加热陆地和海洋的降水季节变化。
2. 最新研究指出撒哈拉沙漠之前是充满富饶的植被和湖水的土地
3. 撒哈拉地区的洞穴壁画描绘了河马,大量的大象和长颈鹿。这些动物都是需要生活在丰富的水源旁边的。
4. 11000 到 5000 年前,撒哈拉沙漠的气候很湿润,被叫做非洲湿润时期。从湿润到干旱仅仅只经历了一个或者两个世纪的时间。
Vocabulary prediction：
Greenery 绿色植物温室
Parameter 参数
Fossilized 石化的僵硬的
Variation 变化
Pollen 花粉
Dynamic 动态;动力
Aquifer 蓄水层
Evaporate 蒸发
Test Point – TPO6L4
Professor
What do I mean? Okay, a monsoon is aseasonal wind that can bring in a large amount of rainfall. Now if the monsoonmigrates, well, that means that the rains move to another area, right?
So what caused the monsoon to migrate?Well, the answer is: the dynamics of earth’s motions, the same thing thatcaused the Ice Age by the way. The earth’s not always the same distance fromthe sun, and it’s not always tilting toward the sun at the same angle. Thereare slight variations in these two parameters. They’re gradual variations buttheir effects can be pretty abrupt. And can cause the climate to change in justa few hundred year.
Student
That’s abrupt?
Professor
Well, yeah, considering that other climateshifts take thousands of years, this one is pretty abrupt. So these changes inthe planet’s motions, they caused the climate to change.
But it was also compounded. What the Saharaexperienced was uh...a sort of “runaway drying effect”. As I said the monsoonmigrated itself, so there was less rain in the Sahara. The land started to getdrier, which in turn caused a huge decrease in the amount of vegetation,because vegetation doesn’t grow as well in dry soil, right? And then, lessvegetation means the soil can’t hold water as well, the soil loses its abilityto retain water when it does rain. So then you have less moisture to helpclouds form, nothing to evaporate for cloud formation. And then the cyclecontinues, less rain, drier soil, less vegetation, fewer clouds, less rain etc.etc..
Knowledge points
1.地球和太阳的距离不是恒定的,也不是和太阳保持同一个倾斜角度。在这两个参数之间有细微的变化。这些变化是逐渐的但是可以快速发生。
2.撒哈拉沙漠经历了一种“干燥作用”。当土地开始变得干旱, 这反过来引起植被数量的大量减少。然后,植被减少意味着土壤不能充分的保留水分。下雨的时候,土壤失去了保留水分的能力。帮助云层形成的水分减少,然后这个循环一直继续,雨水减少,土壤变干,植被减少,雨水减少等等。
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