Shulou(Shulou.com)11/24 Report--

As we all know, singer Xiao Jingteng was nicknamed "God of Rain". Someone counted his concert itinerary from January 2012 to July 2013. It rained 26 days out of 43 days, accounting for more than 60% of rainy days. The name of "God of Rain" is absolutely worthy of the name. The National Weather Administration unexpectedly publicly followed Xiao Jingteng on Weibo, the official campaign. The most "deadly".

However, "God of Rain" is still a word of ridicule. After careful analysis, some experts said: rather than "Xiao Tianwang" attracting rain, it is better to say that he is particularly "greedy" for those rainy cities and rainy days-he holds concerts almost every time in time when the local rainy season.

At this point, you may wonder why there is more precipitation in some places, less in some places, more in summer in some places and more in winter in some places.

Fig. 1 Spatial distribution of annual precipitation in China, then let me introduce you to ITCZ, the real "god of rain" in nature.

01. What is ITCZ? ITCZ (Intertropical Convergence Zone) is tropical convergence zone / equatorial convergence zone. The convergence zone mainly refers to the convergence of the wind field, such as the north wind meets the south wind and the east wind meets the west wind, so that the air flow of different wind directions flows to the same area, forming the convergence of the air flow / wind field. Therefore, the equatorial convergence zone, as its name implies, means that the wind converges near the equator. Specifically, between the two subtropical highs in the northern and southern hemispheres, the winds blowing from the subtropical high belt to the equatorial low pressure belt (that is, the north-south trade wind flow) usually converge at the highest sea surface temperature (that is, the lowest pressure) to form a tropical convergence zone.

Fig. 2 Tropical convergence zone in the northeast Pacific in September 2014

Fig. 3 A schematic diagram of the three-circle circulation of the earth's atmosphere under the action of the Coriolis force. The equatorial convergence zone is formed by the convergence of the Hadley circulation in the northern and southern hemispheres in the lower southeast and northeast trade winds. Among them, Hadely cell: Hadley circulation, Ferrel cell: Ferrer circulation; Polar cell: polar circulation. Source: https://www.britannica.com/ science / Ferrel-cell. When the trade wind boundary layer wind field near the equator converges, the moist air convection rises to form cumulonimbus clouds. Under certain conditions, these cumulonimbus clouds form convective clouds and form a large-scale ascending movement, which will bring obvious rain weather.

Figure 4 (a) Geographic distribution of the global surface monsoon system. (B) the region where the dry-wet index of the superimposed rainfall analysis data (CMAP) is positive (according to the Climate Prediction Center) [1] as the largest and most important rain belt on earth, the movement of ITCZ controls the beginning and duration of the tropical rainy season, which directly affects monsoon rainfall and weather changes in countries such as China, India and Australia. When there are persistent subtle changes in the position of ITCZ in the northern and southern hemispheres in different seasons, it is likely to cause devastating natural and Moo Myoung-Dae disasters. In history, the Mayan civilization in the 9th-10th century probably collapsed because ITCZ moved southward, resulting in a long drought in Central America, and the rise and fall of many dynasties in China may also be related to the north-south migration of ITCZ.

So how does the change in the location of ITCZ affect precipitation?

02, ITCZ and precipitation ITCZ almost circle the earth, like a belt tied to the earth, but the position of this "belt" is not immutable. As the earth rotates, the direct latitude of the sun changes seasonally, and there will be differences in temperature between the northern and southern hemispheres. The annual average position of ITCZ changes according to the temperature comparison between the northern and southern hemispheres, usually moving toward the warmer hemispheres, resulting in the seasonal migration of ITCZ. In the eastern Pacific, for example, ITCZ moves to the Northern Hemisphere in summer and stays in the Southern Hemisphere in winter, just as the belt is raised to the chest in summer and slips to the knee in winter.

The seasonal displacement of ITCZ forms the well-known monsoon [4]. The monsoon circulation includes the summer monsoon and the winter monsoon, and its influence on precipitation is mainly determined by the water vapor caused by the wind. The summer monsoon (the southeast monsoon of East Asia and the southwest monsoon of South Asia) blows from the ocean to the mainland, and the water vapor is sufficient and easy to condense, resulting in precipitation. The monsoon climate distributed on the east coast of the mainland is characterized by high temperature and rain in summer. The winter monsoon (the northwest monsoon in East Asia and the northeast monsoon in South Asia) blows from the mainland to the sea, which has little water vapor, is not easy to condense precipitation, and is dry and less rainy.

China is located in one of the most typical monsoon areas, which is controlled by the southeast monsoon from the ocean in summer and the northwest monsoon from Siberia-Mongolia in winter. The temporal and spatial distributions of monsoon precipitation and trade wind intensity in the middle and low latitudes vary with the migration of ITCZ positions in response to the seasonal and long-term variation of solar radiation [5]. The seasonal variation characteristics of precipitation in China are as follows: precipitation is concentrated in summer and less in winter. The precipitation in eastern China is mainly controlled by the summer monsoon, which brings abundant water vapor from the ocean and forms abundant precipitation after landing. In winter, China is controlled by the winter monsoon, which comes from the continent of higher latitudes, and the water vapor is relatively less, so it is not easy to form precipitation, so there is more precipitation in summer and less in winter.

03. ITCZ and paleoclimate ITCZ are the key components of global atmospheric circulation, which control the distribution of rainfall between high and low latitudes and affect the lives of nearly half of the world's population. Therefore, the analysis of the movement law and influencing factors of ITCZ in the geological history period is of great value for predicting the future climate.

Using the geochemical records of marine sediment cores obtained at a depth of 1660 meters near the eastern edge of Papua New Guinea in the Western Pacific warm Pool, scientists have reconstructed the migration history of the low-latitude rain belt in the Western Pacific over the past 280,000 years. It is found that its change is controlled by the earth's orbital period around the sun and is also affected by the Siberian high pressure in Eurasia. It is pointed out that the South Australian summer monsoon, which also belongs to the East Asian-Australian monsoon system, is more affected by the inclination of the earth's rotation axis, showing a significant 41000-year (inclination) cycle. (students who don't understand this concept can check the previous tweets: "do you play with the top?" It's the size of the earth.

When the tilt angle of the earth's rotation axis is large, the total amount of sunshine absorbed by the high latitudes of the northern hemisphere in winter is very small, and the Siberian high in Eurasia will enhance the intensity of the East Asian winter monsoon, which goes all the way south across the equator and carries plenty of moisture, pushing the low-latitude rain belt of the western Pacific further south, bringing showers to North Australia. On the contrary, the area of Australia is much smaller than that of Eurasia and is located in the middle latitudes, so it is difficult to influence the variation of East Asian summer monsoon through the Australian winter monsoon.

Some studies have analyzed the δ 18O and Mg / Ca ratios of planktonic foraminifera in boreholes at the southwest edge of the Yintai warm pool, and reconstructed the rainfall records on the southern margin of the tropical convergence zone in the past 410,000 years. Compared with the rainfall records on the northern margin of the tropical convergence zone based on loess reconstruction in China, the rainfall on the southern and northern margin of ITCZ shows an inverse phase change in the orbital inclination period, indicating that the zonal migration of ITCZ in the past 410,000 years is dominated by the earth inclination. The high dip angle strengthens the northward transport of the Agulus current and warm water masses, which promotes the strengthening of AMOC, which leads to the northward migration of ITCZ.

Fig. 5 the comparison of rainfall records between the southern and northern margin of the tropical convergence zone is limited, so far is the introduction of ITCZ. In fact, ITCZ plays a very important role in the global climate mechanism, and scientists are stepping up their research on it. I believe that one day, the "god of rain" of nature will unveil its mystery in front of us.

Reference

An Zhisheng, Wu Guoxiong, Li Jianping, Sun Youbin, Liu Yimin, Zhou Weijian, Cai Yanjun, Duan Anmin, Li Li, Mao Jiangyu, Cheng Hai, Shi Zhengguo, Tan Liangcheng, Yan Hong, ao Hong, Chang Hong, Feng Juan. Global monsoon dynamics and climate change [J]. Journal of the Earth Environment, 2015. 6 (06): 341-381.

[2] Broccoli A J, Dahl K A, Stouffer R J. Response of the ITCZ to Northern Hemisphere cooling [J]. Geophysical Research Letters, 2006, 33 (1).

[3] Schneider T, Bischoff T, Haug G H. Migrations and dynamics of the intertropical convergence zone [J]. Nature, 2014, 513 (7516): 45-53.

[4] Wang Pinxian. Geological evolution of the global monsoon [J]. Science Bulletin, 2009. 54 (05): 535-556.

[5] Chiang J C H. The tropics in paleoclimate [J]. Annual Review of Earth and Planetary Sciences, 2009, 37: 263,297.

[6] Mamalakis A, Randerson J T, Yu J Y, et al. Zonally contrasting shifts of the tropical rain belt in response to climate change [J]. Nature Climate Change, 2021, 11 (2): 143151.

[7] Liu Y, Lo L, Shi Z, et al. Obliquity pacing of the western Pacific Intertropical Convergence Zone over the past 282000 years [J]. Nature communications, 2015, 6 (1): 10018.

[8] Zhang P, Xu J, Holbourn A, et al. Obliquity induced latitudinal migration of the Intertropical Convergence Zone during the past ∼ 410 kyr [J]. Geophysical Research Letters, 2022, 49 (21): e2022GL100039.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), author: Zhixing

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