Monsoon

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

Anthropogenic aerosols and the weakening of the South Asian summer monsoon

Abstract: Changes in monsoon rainfall are caused by human-produced aerosols slowing the tropical atmospheric circulation. Observations show that South Asia underwent a widespread summertime drying during the second half of the 20th century, but it is unclear whether this trend was due to natural variations or human activities. We used a series of climate model experiments to investigate the South Asian monsoon response to natural and anthropogenic forcings. We find that the observed precipitation decrease can be attributed mainly to human-influenced aerosol emissions. The drying is a robust outcome of a slowdown of the tropical meridional overturning circulation, which compensates for the aerosol-induced energy imbalance between the Northern and Southern Hemispheres. These results provide compelling evidence of the prominent role of aerosols in shaping regional climate change over South Asia.

Choice of South Asian Summer Monsoon Indices

Abstract: In the south Asian region, two of the major precipitation maxima associated with areas of intensive convective activity are located near the Bay of Bengal and in the vicinity of the Philippines. The variations of monthly mean outgoing longwave radiation in the two regions are poorly correlated, particularly in the decade of 1980s. The enhanced convection over the Bay of Bengal and Indian subcontinents is coupled with reinforced monsoon circulation west of 80°E over India, the western Indian Ocean, and the tropical northern Africa. In contrast, the enhanced convection in the vicinity of the Philippines corresponds to intensified monsoon circulation primarily east of 80°E over southeast Asia including the Indochina peninsula, South China Sea, Philippine Sea, and the Maritime Continent. To better reflect regional monsoon characteristics, two convection indices (or associated circulation indices that are dynamically coherent with the convection indices) are suggested to measure the variability of the...

Correlation between Arabian Sea and Greenland climate oscillations of the past 110,000 years

Abstract: Palaeoclimate studies have revealed the general high-frequency instability of Late Pleistocene climate—for example, the so-called Dansgaard–Oeschger and Heinrich events—on timescales of a few millennia, centuries or even decades1,11. Here we present evidence for a general relationship between low-latitude monsoonal climate variability and the rapid temperature fluctuations of high northern latitudes that are recorded in the Greenland ice records. Sediment cores from the northeastern Arabian Sea show laminated, organic-carbon-rich bands, reflecting strong monsoon-induced biological productivity, that correlate with the mild interstadial climate events in the northern North Atlantic region. In contrast, periods of lowered southwest monsoonal intensity, indicated by bioturbated, organic-carbon-poor bands, are associated with intervals of high-latitude atmospheric cooling and the injection of melt water into the North Atlantic basin. Our records suggest that Dansgaard–Oeschger and Heinrich events are strongly expressed in low-latitude (monsoonal) climate variability, suggesting the importance of common forcing agents such as atmospheric moisture and other greenhouse gases.

Interannual and Interdecadal Variations of the East Asian Summer Monsoon and Tropical Pacific SSTs. Part I: Roles of the Subtropical Ridge

Abstract: The interannual relationship between the East Asian summer monsoon and the tropical Pacific SSTs is studied using rainfall data in the Yangtze River Valley and the NCEP reanalysis for 1951‐96. The datasets are also partitioned into two periods, 1951‐77 and 1978‐96, to study the interdecadal variations of this relationship. A wet summer monsoon is preceded by a warm equatorial eastern Pacific in the previous winter and followed by a cold equatorial eastern Pacific in the following fall. This relationship involves primarily the rainfall during the pre-Mei-yu/Mei-yu season (May‐June) but not the post-Mei-yu season (July‐August). In a wet monsoon year, the western North Pacific subtropical ridge is stronger as a result of positive feedback that involves the anomalous Hadley and Walker circulations, an atmospheric Rossby wave response to the western Pacific complementary cooling, and the evaporation‐wind feedback. This ridge extends farther to the west from the previous winter to the following fall, resulting in an 850-hPa anomalous anticyclone near the southeast coast of China. This anticyclone 1) blocks the pre-Mei-yu and Mei-yu fronts from moving southward thereby extending the time that the fronts produce stationary rainfall; 2) enhances the pressure gradient to its northwest resulting in a more intense front; and 3) induces anomalous warming of the South China Sea surface through increased downwelling, which leads to a higher moisture supply to the rain area. A positive feedback from the strong monsoon rainfall also appears to occur, leading to an intensified anomalous anticyclone near the monsoon region. This SST‐subtropical ridge‐monsoon rainfall relationship is observed in both the interannual timescale within each interdecadal period and in the interdecadal scale. The SST anomalies (SSTAs) change sign in northern spring and resemble a tropospheric biennial oscillation (TBO) pattern during the first interdecadal period (1951‐77). In the second interdecadal period (1978‐96) the sign change occurs in northern fall and the TBO pattern in the equatorial eastern Pacific SST is replaced by longer timescales. This interdecadal variation of the monsoon‐SST relationship results from the interdecadal change of the background state of the coupled ocean‐atmosphere system. This difference gives rise to the different degrees of importance of the feedback from the anomalous circulations near the monsoon region to the equatorial eastern Pacific. In a wet monsoon year, the anomalous easterly winds south of the monsoon-enhanced anomalous anticyclone start to propagate slowly eastward toward the eastern Pacific in May and June, apparently as a result of an atmosphere‐ocean coupled wave motion. These anomalous easterlies carry with them a cooling effect on the ocean surface. In 1951‐77 this effect is insignificant as the equatorial eastern Pacific SSTAs, already change from warm to cold in northern spring, probably as a result of negative feedback processes discussed in ENSO mechanisms. In 1978‐96 the equatorial eastern Pacific has a warmer mean SST. A stronger positive feedback between SSTA and the Walker circulation during a warm phase tends to keep the SSTA warm until northern fall, when the eastward-propagating anomalous easterly winds reach the eastern Pacific and reverse the SSTA.

The onset and interannual variability of the Asian summer monsoon in relation to land-sea thermal contrast

Abstract: The onset and interannual variability of the Asian summer monsoon in relation to land–sea thermal contrast and its contributing factors are studied using a 14-yr (1979–1992) dataset. The onset of the Asian summer monsoon is concurrent with the reversal of meridional temperature gradient in the upper troposphere south of the Tibetan Plateau. The reversal is the result of large temperature increases in May to June over Eurasia centered on the Plateau with no appreciable temperature change over the Indian Ocean. In spring the Tibetan Plateau is a heat source that is distinctly separate from the heat source associated with the rain belt in the equatorial Indian ocean. The Tibetan heat source is mainly contributed by sensible heat flux from the ground surface, while the oceanic heat source is due to the release of latent heat of condensation. It is the sensible heating over the Plateau region in spring that leads to the reversal of meridional temperature gradient. Despite its intensity the condensatio...