Related articles 0. Figures 0. Information related to the author. Supplementary material 0. Result List. Previous article Next article. Related articles. Share this page. Meanwhile, sub-Saharan Africa becomes particularly hot and dry in winter as air descends from high in the atmosphere to the ground as part of the Hadley Circulation. The locations of the heaviest tropical rainfall from December to February top and June to August bottom. Images: UCAR. Rajeev told the Indian press on Friday June 12, , after a full night of rain.
He referred to winds even though people cared more about the rains that had just started. Air warmed in the tropics rises, flows towards the poles, then downward in the subtropics, and back to the equator. Image: UCAR. Sunlight, and the energy it brings to Earth, is the driving force behind the Hadley Circulation.
Sunlight heats land and ocean surfaces near the equator. The warmed surface releases energy into the atmosphere, in the form of heat and evaporated water. As it flows toward the poles, this air cools and drops down toward the surface of the Earth in the subtropics, near 30 degrees latitude north or south of the equator. As air rises near the equator and then flows poleward, it leaves an area of fewer air molecules at the equator. This is a region of low pressure because there is a smaller mass of air left over the equator.
Air from the subtopics, north and south of the equator, flows in to fill the space, completing the loop of Hadley Circulation. Water vapor condenses as air rises and cools in the ITCZ, forming clouds and falling as rain. The ITCZ can be seen from space as a band of clouds around the planet.
This is where monsoon rainfall occurs. If the Earth were not rotating, winds would blow directly towards the Intertropical Convergence Zone from the north and south. But the Earth is rotating — making a full turn on its axis each day — which turns the wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It also affects the movement of ocean currents and the direction of rotation in hurricanes.
Motions that span hundreds to thousands of miles feel the Coriolis force. It does not impact smaller scale phenomena like tornadoes. And contrary to legend, the direction that water spins in a flushing toilet is due to toilet design, as toilets are much too small to feel the Coriolis force.
The way the trade winds turned to the west on their way to the equator was of great interest to George Hadley, an 18th Century British lawyer who dabbled in meteorology. He proposed that it was the spin of the Earth that caused the winds to turn as they blew towards the equator. He produced what was essentially the first global theory of atmospheric circulation. Over the years other scientists have refined and further developed these ideas, but Hadley did get some of the basics correct.
Today, the Hadley Circulation in the tropics is named after George Hadley. The Hadley Circulation doesn't stay in the same place year-round, but varies with the seasons. This is the key to understanding why many tropical regions around the world have patterns of wet monsoon summers and dry winters.
During December and January, the Southern Hemisphere is heated more strongly by the sun than the Northern Hemisphere, so the hottest air — the air that rises in the ITCZ — is found a little south of the equator. As the ITCZ changes location through the year, the winds and rains and the location of monsoon wet weather changes, too. As the Intertropical Convergence Zone ITCZ changes location through the year, the winds, rains, and the location of wet monsoon weather changes, too.
Remember that the Coriolis force changes direction on the equator: It turns winds toward the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
So when air crosses the equator as it flows from the cold winter hemisphere toward the ITCZ in the summer hemisphere, it experiences a change in the Coriolis force. This causes the trade winds to reverse direction and blow toward the west in the winter hemisphere and to the east in the summer hemisphere. This seasonal reversal of the winds was historically very important for trade between Africa and Asia; ships would sail from Asia to Africa in winter and then undertake their return voyage when the summer monsoon changed the wind from westward to eastward.
The animation above shows how the ITCZ, winds, and rain patterns change through the months of the year. Video: UCAR. The summer monsoon is what people often think of as monsoon conditions: large amounts of rain. But the winter monsoon, where dry conditions prevail, is part of the pattern too. During winter, air descends over tropical continents as the part of the Hadley Circulation that is outside of the ITCZ. Descending air causes high pressure, and makes clouds and rain uncommon.
The dry conditions during winter can even lead to drought if they are too intense or persist for too long. Geography affects the amount of rainfall that an area receives as the ITCZ moves through the seasons. Low-level winds blow south towards the ITCZ, picking up moisture as they move over the warm, tropical ocean. Meanwhile in India, dry air descending over land means there is little precipitation.
Lau , K-M. Li , : The monsoon of East Asia and its global associations—A survey. Yang , : Climatology and interannual variability of the Southeast Asian summer monsoon.
Weng , : Recurrent teleconnection patterns linking summertime precipitation variability over East Asia and North America. Japan , 80 , — Yang , , and S. Shen , : Seasonal and Intraseasonal climatology of summer monsoon rainfall over East Asia.
Yang , : Dynamical and boundary forcing characteristics of regional components of the Asian summer monsoon. Climate , 13 , — Lorenc , A. Madden , R. Julian , : Detection of a 40—50 day oscillation in the zonal wind. Matsumoto , J. Geography Bull. McBride , J. Global Perspectives on Tropical Cyclones, R. Elsberry, Ed. Montaigne , E. Murakami , M. Japan , 54 , 15 — Neumann , C. Holland, Ed. Ninomiya , K. Murakami , : The early summer rainy season Baiu over Japan.
Monsoon Meteorology, C. Chang and T. Krishnamurti, Eds. Nitta , T. Japan , 65 , — Ramage , C. Rao , X. Reed , R. Saha , K. Van den Dool , , and S. Saha , : On the annual cycle in surface pressure on the Tibetan Plateau compared to its surroundings. Climate , 7 , — Sikka , D. Tao , S. Chen , : A review of recent research on the East Asian summer monsoon in China. World Climate Research Programme No. White , G.
Xie , P. Arkin , : Global precipitation: A year monthly analysis based upon gauge observations, satellite estimates, and numerical model outputs. Five-day mean values of station rainfall in southern China, Taiwan, and Japan stations are indicated in the map shown in the upper-left corner. Averaged rainfall from top nine stations in southern China and Taiwan and bottom five stations in Japan is expressed by histograms.
Rainfall P for Hong Kong, Lung Chow south China , and Tainan Taiwan , and yr tropical cyclone TC occurrence frequencies of Taiwan and Hong Kong are added to the bottom of the top panel, while the averaged surface pressures of the five stations in Japan are superimposed in the lower panel.
This figure is a modification of Ramage's Fig. Areas of 2. Different phases of the summer monsoons in the three regions are indicated by active, break, and revival. The day-averaged rainfall derived from three regions in Fig. Occurrence frequencies of fronts lightly stippled histogram and tropical cyclones heavily stippled histogram over these three regions are added. Geographic distributions of the day accumulated fronts stippled areas and the tropical cyclone dots and open circles superimposed on the corresponding mb streamline charts.
The aforementioned stations are marked in Fig. Five-day-averaged rainfall histograms of 25 surface stations in Taiwan. Rainfall and surface pressure of six low plains and the tallest East Asian mountain Yu Shan; m stations denoted by stippled circles are used in Fig.
The location and elevation of the Yu Shan station in Taiwan are denoted by a thick, solid vertical line in d — f. Same as in Fig. Rainfall variation of the East Asian summer monsoon has long been believed to be caused by the transition of weather regimes in company with the evolution of monsoon circulation.
However, this claim was neither comprehensively analyzed nor convincingly demonstrated. The monsoon life cycle in the southern part of East Asia is basically developed by the sequential passages of the mei-yu rainband in early summer, the western Pacific subtropical high in midsummer, and the tropical cyclone activity in late summer.
In view of the role played by the subtropical high in developing the monsoon break, the phase lag of the monsoon life cycle between the south and north is a result of this subtropical high's northward progression. Two rainfall maxima in the revival phase of the Japanese monsoon are generated by different mechanisms: the first is a result of the collaborative contribution of the frontal and tropical cyclone activity, but the second is primarily due to the midlatitude frontal activity.
Despite the vertical phase reversal of monsoon circulation in the middle troposphere, two intraseasonal 10— and 30—day monsoon modes propagating across East Asia do not undergo a vertical phase reversal until — mb. In addition to these new findings, further research efforts are needed to explore the modulations of the frontal and tropical cyclone activity by intraseasonal modes and the monsoon revival in northeast Asia by the interannual climate mode of the summer monsoon circulation through the tropical cyclone and midlatitude frontal activity.
Email: tmchen iastate. The demand for water in the Asian monsoon region by more than half of the world's population makes monsoon rain extremely vital to the human activity in this region.
The onset of summer monsoon rain, therefore, often becomes the first concern of most monsoon research. Consequently, monsoon onset dates in different Asian regions were compiled by numerous efforts, for example, over the Indian subcontinent by Rao , over Indochina and East Asia by Tao and Chen , and over Southeast Asia by Lau and Yang Actually, different regions within the Asian monsoon system exhibit diversified characteristics of monsoon rainfall, namely the different timing of the onset, active, break, revival, and withdraw phases.
Undoubtedly, a better understanding of the cause of monsoon rainfall variations is useful to authorities of the monsoon region to develop the proper management policy of water storage and supply. Despite being a part of the annual variation of the atmospheric circulation, time variations of monsoon rainfall in different Asian regions exhibit pronounced intraseasonal fluctuations.
Based on satellite images, Sikka and Gadgil found that the Indian monsoon rainfall is regulated by the northward propagation of transient monsoon troughs and ridges from the equator. These transient monsoon troughs and ridges are propagated by the 30—day monsoon mode Krishnamurti and Subrahmanyam and coupled with the eastward propagation of the global Madden—Julian oscillation Lorenc ; Krishnamurti et al. In addition to this intraseasonal monsoon mode, Murakami and Chen and Chen also demonstrated the possible modulation of the Indian monsoon rainfall by the westward-propagating 10—day monsoon mode Krishnamurti and Ardanuy The Southeast Asian summer monsoon undergoes three basic life cycles.
Like the Indian monsoon, the time variation of this monsoon is regulated by the northward-propagating 30—day monsoon mode from the equator and the westward-propagating 10—day monsoon mode Chen and Chen Examining east China rainfall, Lau et al. Later, it was shown by Chen et al.
Regardless of the effects of the intraseasonal monsoon modes on the East Asian monsoon rainfall variation presented by previous studies, a comprehensive depiction of the East Asian monsoon rainfall variation and its evolution with the East Asian monsoon circulation are still absent.
Based on rainfall measurements of East Asian stations in south China, Taiwan, and Japan, and records of the tropical cyclone occurrence over the western Pacific—South China Sea region, Ramage presented his observations of the monsoon rainfall variation in this region Fig.
During the summer, the mei-yu rainband and intertropical convergence zone ITCZ lie along the northwest and southwest peripheries of the western Pacific subtropical high, respectively. Following the seasonal march, these two rainbands progress northward in company with this subtropical high. The sequential passages of the mei-yu rainband, the subtropical high, and the ITCZ form the monsoon life cycle in the southern part of East Asia.
This simple mechanism of the East Asian monsoon life cycle has long been adopted by the Asian meteorological community e. In contrast, the monsoon rainfall variation in the northern part of East Asia exhibits different characteristics.
The Korean monsoon changma covers only July and August and the monsoon revival in Japan exhibits two rainfall maxima. Evidently, the aforementioned mechanism of the East Asian monsoon rainfall life cycle may not be applicable to the monsoon rainfall variation in the northern part of East Asia.
Differences in the monsoon rainfall variations between the northern and southern parts of East Asia lead us to raise several questions:. It has been five decades since Ramage presented his observations of the East Asian monsoon rainfall variation. Few attempts e. Numerous studies e. These efforts need a comprehensive climatology of the East Asian summer monsoon rainfall variation.
This paper is organized in the following fashion. The transition of the monsoon rainfall contribution from the mei-yu regime to the tropical cyclone season is shown in section 3. The contrast of monsoon rainfall variations between the low plains and the tallest East Asian mountain, and the impact of intraseasonal monsoon modes on this contrast, are illustrated in section 4.
A summary of the findings in this study and some concluding remarks are offered in section 5. The monsoon rainfall variation during the warm season in East Asia is generally characterized by two active rainfall periods separated by a break spell 1 :.
It was inferred by previous studies e. Since the break varies from a month in south China and Taiwan to about half of a month in Korea, does the shortening of the monsoon break spell suggest that the northward progression of the western Pacific subtropical high ends in Korea? Ramage pointed out that the monsoon break is a transition period from the prior break in the mei-yu regime to the postbreak tropical cyclone season. Following the break spell, the East Asian monsoon circulation certainly undergoes a regime transition.
Does the monsoon rain also go through a characteristic transition during the monsoon break? Further analysis of this question will be performed later, but a quick look of this possible transition may be derived from Fig. Let us compare the rainfall amount between the onset and middle of the break, P A , and between the middle of the break and withdrawal, P R. It will be demonstrated later in section 3 that this special zone is relatively coincident with the East Asian frontal zone.
In other words, the frontal activity contributes more rainfall along this special zone prior to the break. It is generally believed that monsoon rain variations in different East Asian regions are caused by the northward progressions of both the mei-yu rainband and the western Pacific subtropical high. On the other hand, Ramage suggested that the second active phase of the monsoon rain in south China and Taiwan is revived by the ITCZ. Actually, the ITCZ has rarely been considered by previous studies to exert any impact on the East Asian monsoon rainfall.
In order to clarify Ramage's observation, latitude—time y — t cross sections of the CMAP rainfall averaged over three longitudinal zones are displayed in Fig. Histograms of 5-day rainfall averaged over areas around Taiwan, Korea, and Japan are also shown for reference in the bottom row of Fig.
Commencement of the monsoon active, break, and revival phases in these three areas are indicated. These y — t diagrams of CMAP rainfall are characterized by the salient features depicted below. The passage of the mei-yu rainband is followed by a break spell monsoon break that also propagates northward. Perhaps, simultaneous northward progressions of the mei-yu rainband and the dry zone have convinced the meteorological community that time variations of the East Asian summer monsoon rain are determined by these two monsoon elements.
In contrast, the cause of the monsoon rainfall revival after the monsoon break has not clearly been addressed by previous studies e. This northward rainfall intrusion forms the first rainfall maximum in the revival phase of Japan observed by Ramage The cause of this abrupt decrease in monsoon rainfall will be investigated further in section 2c. The East Asian summer monsoon break is stressed as a transition from the mei-yu regime to the tropical cyclone season, but the monsoon revival, on the other hand, is suggested to be in concert with the northward progression of the ITCZ.
What is the link between the tropical cyclone activity and the ITCZ? This question will be addressed in section 3.
The monsoon life cycles of the Indian subcontinent e. In contrast, the life cycle of the East Asian monsoon is established by northward progressions of the mei-yu rainband and ITCZ.
Since all of these monsoons belong to the Asian monsoon system, it is interesting to see that the mechanisms responsible for the formation of monsoon life cycles in these regions differ greatly from each other. Instead, right after the occurrence of the monsoon revival, a rainband migrates southward from Japan and attains its maximum in mid-to-late September.
This rainfall maximum forms the second of Ramage's observations along the south coast of southern Japan. Evidently, during the revival phase, the first maximum rainfall in southern Japan is coupled with the northward migration of the western Pacific ITCZ, while the second maximum rainfall may be caused by midlatitude processes. The distinction between mechanisms responsible for these two rainfall maxima needs some clarification.
Lau et al. In contrast, Chen et al. Evidently, the impact of these two intraseasonal modes on the East Asian monsoon is likely to exist over the entire monsoon season. However, further clarification of the role played by the day mode in the East Asian monsoon evolution is needed.
The revival of the East Asian monsoon rainfall as indicated by Fig. According to Y. Ding Chinese Academy of Meteorological Science, , personal communication the monsoon revival north of the Yangtze River basin undergoes a pronounced interannual variation in time and northward extension of the monsoon rain.
How this interannual variation of the monsoon revival in northeast China is linked to the East Asian summer climate variability e. The formation and transition of the East Asian summer monsoon rainfall regimes discussed in the previous two sections are coupled with the evolution of the large-scale monsoon circulation. Despite the conclusions reached by numerous studies that the East Asian monsoon rainfall variations are a result of northward progressions of the mei-yu rainband and the subtropical high, the y — t diagrams of the CMAP rainfall reveals that this mechanism covers only the first active monsoon phase and break.
The monsoon revival in East Asia is caused by a different mechanism associated with the development of other monsoon circulation components. These components consist of the South China Sea—western Pacific monsoon trough embedded by the ITCZ , the ridge intruding into northeast China adjacent to the Yellow Sea , the closed surface anticyclone developed from the northeast China ridge , and the development of the Manchuria China summer monsoon low into an open trough in late summer.
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