The 10–20-Day Mode of the 1979 Indian Monsoon: Its Relation with the Time Variation of Monsoon Rainfall

Chen, Tsing-Chang; Chen, Jau‐Ming

doi:10.1175/1520-0493(1993)121<2465:tdmoti>2.0.co;2

Cited by 177 publications

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“…It appears that the northwestward propagation mode of the 10-20-day band and the northeastward propagation mode of 30-60-day band may be a mechanism to link with the monsoon evolution in moistening the atmosphere over the SCS as far as the onset is concerned. Chen and Chen (1993) suggested that the northward-migrating monsoon trough of the 30-60-day mode is a possible mechanism responsible for the development of the low in the Northern Hemisphere of the double-cell structure of the 10-20-day mode and its northward propagation. As can be seen from Figures 4 and 7, the convection over the equatorial western Pacific of the 10-20-day mode migrates northward concomitantly with the 30-60-day monsoon trough or off-equatorial ITCZ, which might also indicate that the northeastwardpropagating monsoon trough of the 30-60-day mode initiates the development of the disturbance over the equatorial western Pacific of the 10-20-day mode.…”

Section: The Role Of the Isomentioning

confidence: 99%

Intraseasonal oscillations and the South China Sea summer monsoon onset

2005

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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.

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Section: The Role Of the Isomentioning

confidence: 99%

Intraseasonal oscillations and the South China Sea summer monsoon onset

2005

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“…Takahashi and Yasunari (2006) reported that the first rainy peak, in late May to early June, is associated with strong monsoon southwesterlies, and the second rainy peak, in mid-August to mid-September, is probably influenced by tropical cyclone (TC) activity. Previous studies (e.g., Krishnamurti et al 1977;Saha et al 1981;Chen and Chen 1993) have shown that the monsoon depressions over the Bay of Bengal and the Indian subcontinent are closely related to westward-propagating disturbances that cross the Indochina Peninsula and originate from the South China Sea and western North Pacific. Fudeyasu et al (2006) reported that August and September are the most active months for westward-propagating TCs around the Indochina Peninsula.…”

Section: Introductionmentioning

confidence: 95%

Decreasing Trend in Rainfall over Indochina during the Late Summer Monsoon: Impact of Tropical Cyclones

Takahashi

Yasunari

2008

Journal of the Meteorological Society of Japan

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We examined the decreasing trend in rainfall during the late summer monsoon season (September) in Thailand from 1951 to 2000 and associated changes in tropical cyclone (TC) activity. Thailand receives significant rainfall from May to October and experiences two rainy peaks in late May to early June and in September. A previous study reported a decreasing trend in September rainfall in Thailand and, based on a regional climate model, suggested that the trend was associated with local deforestation. However, the long-term trend may also be affected by changes in large-scale circulation. Thus, the purpose of this study was to investigate changes in large-scale circulation associated with the decreasing rainfall trend.Westward-propagating TCs from the South China Sea and the western North Pacific brought most of the rainfall over Thailand in September. TCs include tropical depressions, tropical storms, severe tropical storms, typhoons, and residual lows. 70% of the rainfall amount in September was estimated to be associated with TCs.The 50-year time-series of September rainfall over Thailand showed a significant decreasing trend. TC activity defined by 700-hPa relative vorticity, showed a weakening trend over the Indochina Peninsula. TC tracks also suggested the weakening of TC activity over this area. The long-term trend in rainfall during the late summer monsoon season was closely associated with changes in TC activity over the Indochina Peninsula; these changes were likely caused by changes in the major course of TCs. Concurrent with the changes in TC tracks, there was a change in the TC steering current around the Philippines archipelago and Taiwan. This led to the TC activity over the Indochina Peninsula being inactive, probably resulting in the long-term decrease in rainfall over Thailand.

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“…Sengupta et al [2004] indicates that biweekly meridional current variability is well simulated in their moderate resolution ocean general circulation model and that the signal propagates to the west with the phase speed and zonal wavelength of 2.5$5.0 m/s and 4000$5000 km, respectively. They also suggest that the biweekly variability has characteristics of the Mixed Rossby-gravity wave and is generated by meridional wind stresses associated with the atmospheric ISVs [Chen and Chen, 1993;Chatterji and Goswami, 2004]. Another mechanism for the generation of the biweekly meridional current variability in the upperlayer of the eastern equatorial Indian Ocean is proposed by Miyama et al [2006], in which the Mixed Rossby-gravity wave is excited in the western Indian Ocean and a raypath of its energy penetrate into the deep ocean and, then, is reflected at the bottom to reach the upper eastern Indian Ocean.…”

Section: Introductionmentioning

confidence: 99%

Intraseasonal meridional current variability in the eastern equatorial Indian Ocean

et al. 2008

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[1] Intraseasonal variability in meridional current in the eastern equatorial Indian Ocean is investigated by use of results from a high-resolution ocean general circulation model. Both the simulated and observed meridional current variability demonstrate energy peaks in two distinct time-scales; one in the biweekly (6-8 d) period above the thermocline and the other in lower frequency band (20-70 d) within the subsurface layer. In addition, the intraseasonal current variability can be seen even in the deep ocean at 2000 m and 4000 m depths. Composite analysis of the biweekly current field demonstrates that the horizontal structure is consistent with the one for the Mixed Rossby-gravity wave, with the phase speed and wavelength close to theoretical values of the Mixed Rossby-gravity wave at 15-d period. This meridional current variability shows large coherence with the local meridional wind stress, suggesting the upper-ocean responses to the local wind-forcing. A part of the energy of the biweekly variability penetrates into the deeper layer along the raypath of the 15-d Mixed Rossby-gravity wave, causing the intraseasonal variability observed in the current meter data at 4000 m depth. On the other hand, the lower frequency intraseasonal variability in the subsurface layer is also captured in the model simulation forced by climatological winds. Large barotropic energy conversion rate appears during boreal autumn and early winter in a region southeast of Sri Lanka, suggesting importance of instability due to the seasonally changing current systems to generate mesoscale eddy-like disturbances that result in the subsurface meridional current variability.

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The 10–20-Day Mode of the 1979 Indian Monsoon: Its Relation with the Time Variation of Monsoon Rainfall

Cited by 177 publications

References 0 publications

Intraseasonal oscillations and the South China Sea summer monsoon onset

Intraseasonal oscillations and the South China Sea summer monsoon onset

Decreasing Trend in Rainfall over Indochina during the Late Summer Monsoon: Impact of Tropical Cyclones

Intraseasonal meridional current variability in the eastern equatorial Indian Ocean

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