Monsoon

About: Monsoon is a research topic. Over the lifetime, 16087 publications have been published within this topic receiving 599888 citations.

On the Maximum Cloud Zone and the ITCZ over Indian, Longitudes during the Southwest Monsoon

Abstract: An investigation is presented of the daily variation of the maximum cloud zone (MCZ) and the 7W mb trough in the Northern Hemisphere over the Indian longitudes 70–90°E during April–October for 1973–77. It is found that during June–September there are two favorable locations for a MCZ over these longitudes–on a majority of days the MCZ is present in the monsoon zone north of 15°N, and often a secondary MCZ occurs in the equatorial region (0–10°N). The monsoon MCZ gets established by northward movement of the MCZ occurring over the equatorial Indian ocean in April and May. The secondary MCZ appears intermittently, and is characterized by long spells of persistence only when the monsoon MCZ is absent. In each of the seasons studied, the MCZ temporarily disappeared from the mean summer monsoon location (15–28°N) about four weeks after it was established near the beginning of July. It is reestablished by the northward movement of the secondary MCZ, which becomes active during the absence of the monsoo...

Influence of the intertropical convergence zone on the East Asian monsoon

Abstract: A palaeoclimate record spanning the past 16,000 years with nearly annual time resolution has been obtained from Lake Huguang Maar in China. The magnetic properties and titanium content of the lake sediments are thought to reflect changes in the East Asian winter monsoon strength, thus providing a complement to palaeoclimate archives that record the strength of the rain-bearing summer monsoon. The records point to an anti-correlation between winter and summer monsoon strength, best explained by migration of the intertropical convergence zone (ITCZ), a belt of low-pressure air at the Equator. Interestingly, the decline of China's Tang dynasty and the Mayan civilization broadly coincide with drought periods recorded in Lake Huguang Maar and Cariaco basin sediments, respectively. Did major shifts in the position of the ITCZ catalyse simultaneous events in civilizations on opposite sides of the Pacific Ocean? The Asian–Australian monsoon is an important component of the Earth’s climate system that influences the societal and economic activity of roughly half the world’s population. The past strength of the rain-bearing East Asian summer monsoon can be reconstructed with archives such as cave deposits1,2,3, but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct. Here we present high-resolution records of the magnetic properties and the titanium content of the sediments of Lake Huguang Maar in coastal southeast China over the past 16,000 years, which we use as proxies for the strength of the winter monsoon winds. We find evidence for stronger winter monsoon winds before the Bolling–Allerod warming, during the Younger Dryas episode and during the middle and late Holocene, when cave stalagmites suggest weaker summer monsoons1,2,3. We conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone. Similar migrations of the intertropical convergence zone have been observed in Central America for the period ad 700 to 900 (refs 4–6), suggesting global climatic changes at that time. From the coincidence in timing, we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the Tang dynasty in China and the Classic Maya in Central America.

Climate change and the South Asian summer monsoon

Abstract: More than one billion people live in regions affected by the South Asian summer monsoon. This Review provides an overview of our understanding of summer monsoon rainfall variability and its causes, and considers how the monsoon will change as a consequence of global warming.

Monsoons and the dynamics of deserts

Abstract: The existence of subtropical deserts, such as the Sahara, has often been attributed to the annual-mean, zonal-mean Hadley circulation which shows strong descent in the subtropics. However, the zonal-mean Hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter, but practically no zonal-mean subtropical descent during summer when rainfall over the eastern Sahara and the Mediterranean is least. Charney (1975) proposed a biosphere-albedo feedback mechanism whereby local anthropogenic effects related to over-grazing could affect the radiative balance, enhancing summertime diabatic descent and leading to desertification of the subtropics in general. The present study, which uses an idealized model, suggests a monsoon-desert mechanism for desertification whereby remote diabatic heating in the Asian monsoon region can induce a Rossby-wave pattern to the west. Integral with the Rossby-wave solution is a warm thermal structure that interacts with air on the southern flank of the mid-latitude westerlies causing it to descend. This adiabatic descent is localized over the eastern Sahara and Mediterranean, and over the Kyzylkum desert to the south-east of the Aral Sea, by the mountains of north Africa and south-west Asia. Trajectories indicate that the monsoon-desert mechanism does not represent a simple ‘Walker-type’ overturning cell. Instead, the descending air is seen to be mainly of mid-latitude origin. It is speculated that the monsoon-forced adiabatic descent may result in clear air and, therefore, a local diabatic enhancement which effectively doubles the strength of descent. With this mechanism, desertification can be forced by remote changes in monsoon strength rather than by local effects. This conclusion is supported by the observed dramatic strengthening of descent over the Mediterranean and east Sahara during the onset of the Asian monsoon and, on the longer timescale, by relating prehistoric lake-levels to Milankovitch-monsoon forcing. The latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern Sahara and the strength of a ‘tropic-wide’ monsoon. The monsoon-desert mechanism may not be confined to the Asian monsoon alone and the existence of other monsoon-climate regions over the globe may, in a similar way, explain the observed summertime strengthening of the oceanic sub-tropical anticyclones and the existence of western continental deserts and ‘Mediterranean-type’ climate regions.