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Monsoon Trough (monsoon + trough)

Distribution by Scientific Domains
Earth and Environmental Science100%

Selected Abstracts

An analysis of late twentieth century trends in Australian rainfall

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2009
Andréa S. Taschetto
Abstract Trends in Australian precipitation from 1970 to 2006 are examined using a daily rainfall dataset. Results suggest a linkage between changes in the monsoon trough and rainfall trends over northwestern Australia. The late twentieth century drought observed along the Queensland coast is a response to changes in the atmospheric circulation that generates anomalous subsidence at high and middle levels of the atmosphere, thus inhibiting convection over the region. In addition, an anomalous anticyclonic circulation at low levels over Queensland tends to weaken the easterlies in the tropical western Pacific, thus diminishing the transport of moist air onto the coast. Trends in the frequency and magnitude of different rainfall events are also examined. This reveals that changes in total rainfall are dominated by trends in very heavy rainfall events across Australia. For example, some parts of western Australia reveal an increase in heavy rainfall events that are not accompanied by a rise in modest rainfall events, resulting in changes in the shape of the distribution towards a more skewed precipitation distribution. On the other hand, the frequency of extreme rainfall events along the Queensland coast has declined during summer and autumn consistently with the total rainfall decrease, indicating changes in the position of the precipitation distribution rather than its shape. Copyright © 2008 Royal Meteorological Society [source]

Seasonal march and its spatial difference of rainfall in the Philippines

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2007
I. Akasaka
Abstract On the basis of the pentad rainfall data averaged from 1961 to 2000, the seasonal march of rainfall in the Philippines is analyzed in this study. The relation to the atmospheric circulation at the 850 hPa level is also discussed. To investigate the temporal and spatial features of rainfall, the Empirical orthogonal function (EOF) analysis was applied to rainfall data. The result showed two dominant modes in the seasonal march of rainfall. The first mode reveals the increase of rainfall amount in the entire Philippines during summer monsoon while the second mode represents the contrast between the west and east coasts in the seasonal march of rainfall. The rainy season starts simultaneously over the entire west coast in the middle of May and withdraws gradually from northern stations around November. And on the east coast, the rainfall amount increases in autumn and winter rather than in summer. These regional differences between west and east coasts are considered to correspond to the seasonal change of Asian summer monsoon and orographic effect. The seasonal march of rainfall in the Philippines is characterized by the sudden change of atmospheric circulation around the Philippines. Particularly, the onset and peak of rainy season on the west coast are influenced by the eastward shift of the subtropical high and the evolution of the monsoon trough with southwesterly, respectively. The increase of rainfall on the east coast is related with the weakened monsoon trough around early September. Copyright © 2006 Royal Meteorological Society [source]

Intraseasonal oscillations and the South China Sea summer monsoon onset

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2005
Wen Zhou
Abstract This paper investigates the role of intraseasonal oscillations (ISOs) in the onset of the South China Sea summer monsoon (SCSSM). Two major components of ISO (10,20-day and 30,60-day modes) are identified. The coupling of these two intraseasonal modes during the pre-monsoon period of the SCSSM are investigated by examining the filtered outgoing longwave radiation (OLR), low-level circulation, apparent heat source and apparent moisture sink from October of a previous calendar year to September of a calendar year. The zonal and meridional propagations of the 10,20-day and 30,60-day modes are found to be different, which reflects their different roles in the establishment and development of the SCSSM. The northwestward propagation of the 10,20-day mode is associated with the weakening of the subtropical high over the western Pacific, while the northeastward propagation of the 30,60-day mode originates from convection over the equatorial Indian Ocean. A hypothesis is then proposed to explain the observed variabilities in the SCSSM onset. When the equatorial Indian Ocean exhibits a 30,60-day mode oscillation, an initially weak convection develops into a large convection band (or monsoon trough). Meanwhile, a convective disturbance of the 10,20-day mode is induced when this monsoon trough extends to the western Pacific. These two processes then collaborate to cause a weakening of the subtropical anticyclone over the South China Sea. Because the monsoon trough associated with the 30,60-day mode subsequently propagates northward into the Bay of Bengal (BOB), the induced vortex together with the 10,20-day westward-migrating convection from the equatorial western Pacific will substantially increase the effect of horizontal advection of moisture and heat, thus destabilizing the atmosphere and weakening the subtropical ridge there. Westerlies can then penetrate and prevail over the SCS region, and the SCSSM onset occurs. Copyright © 2005 Royal Meteorological Society. [source]

Interannual variability of lower-tropospheric moisture transport during the Australian monsoon

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2002
Christopher R. Godfred-Spenning
Abstract The interannual variability of the horizontal lower-tropospheric moisture transport associated with the Australian summer monsoon has been analysed for the 1958,99 period. The 41-season climatology of moisture flux integrated between the surface and 450 hPa showed moderate levels of westerly transport in the month before Australian monsoon onset, associated with cross-equatorial flow in the Sulawesi Sea and west of Borneo. In the month after onset the westerly moisture transport strengthened dramatically in a zonal belt stretching from the Timor Sea to the Western Equatorial Pacific, constrained between the latitudes 5 and 15 °S, and associated with a poleward shift in the Intertropical Convergence Zone and deepening of the monsoon trough. Vertical cross-sections showed this transport extending from the surface to the 500 hPa level. In the second and third months after onset the horizontal flow pattern remained similar, although flux magnitudes progressively decreased, and the influence of trade winds became more pronounced over northern Australia. Nine El Niño and six La Niña seasons were identified from the data set, and composite plots of the affected years revealed distinct, and in some cases surprising, alterations to the large-scale moisture transport in the tropical Australian,Indonesian region. During an El Niño it was shown that the month prior to onset, in which the moisture flux was weaker than average, yielded to a dramatically stronger than average flux during the following month, with a zone of westerly flux anomalies stretching across the north Australian coast and Arafura Sea. The period of enhanced moisture flux during an El Niño is relatively short-lived, with drier easterly anomalies asserting themselves during the following 2 months, suggesting a shorter than usual monsoon period in north Australia. In the La Niña composite, the initial month after onset shows a tendency to weaker horizontal moisture transport over the Northern Territory and Western Australia. The subsequent 2 months show positive anomalies in flux magnitude over these areas; the overall effect is to prolong the monsoon. Comparison of these results with past research has led us to suggest that the tendency for stronger (weaker) circulations to arise in the initial month of El Niño (La Niña) events is a result of mesoscale changes in soil moisture anomalies on land and offshore sea surface temperature (SST) anomalies, brought about by the large-scale alterations to SST and circulation patterns during the El Niño,Southern Oscillation. The soil moisture and SST anomalies initially act to enhance (suppress) the conditions necessary for deep convection in the El Niño (La Niña) cases via changes in land,sea thermal contrast and cloud cover. Copyright © 2002 Royal Meteorological Society. [source]

High-resolution regional climate simulations of the long-term decrease in September rainfall over Indochina

ATMOSPHERIC SCIENCE LETTERS, Issue 1 2009
Hiroshi G. Takahashi
Abstract We address the long-term decrease in September rainfall over the Indochina Peninsula. Distinct long-term decreases in rainfall along the monsoon trough across the Indochina Peninsula have been observed. We performed long-term simulations and discuss the effects of long-term changes in both the local surface conditions and large-scale circulation. Using a 30-year simulation for September for the period from 1966 to 1995 with land-use conditions fixed at present-day values and neglecting the recorded deforestation, we successfully simulated the observed long-term decrease in rainfall. We therefore conclude that the weakening tropical-cyclone activity over the Indochina Peninsula region is probably responsible for the decrease in rainfall. Copyright © 2008 Royal Meteorological Society [source]