A model study of the 30-50 day oscillation In the tropical atmosphere

Anomalous interactions between the Indian summer monsoon (ISM) circulation and subtropical westerlies are known to trigger breaks in the ISM on subseasonal time-scales, characterised by a pattern of suppressed rainfall over central-north India, and enhanced rainfall over the foothills of the central–eastern Himalayas (CEH). An intriguing feature during ISM breaks is the formation of a mid-tropospheric cyclonic circulation anomaly extending over the subtropical and mid-latitude areas of the Asian continent. This study investigates the mechanism of the aforesaid Asian continental mid-tropospheric cyclonic circulation (ACMCC) anomaly using observations and simplified model experiments. The results of our study indicate that the ACMCC during ISM breaks is part of a larger meridional wave train comprising of alternating anticyclonic and cyclonic anomalies that extend poleward from the monsoon region to the Arctic. A lead–lag analysis of mid-tropospheric circulation anomalies suggests that the meridional wave-train generation is linked to latent heating (LH) anomalies over the CEH foothills, Indo-China, and the Indian landmass during ISM breaks. By conducting sensitivity experiments using a simplified global atmospheric general circulation model forced with satellite-derived three-dimensional LH, it is demonstrated that the combined effects of the enhanced LH over the CEH foothills and Indo-China and decreased LH over the Indian landmass during ISM breaks are pivotal for generating the poleward extending meridional wave train and the ACMCC anomaly. At the same time, the spatial extent of the mid-latitude cyclonic anomaly over Far-East Asia is also influenced by the anomalous LH over central–eastern China. While the present findings provide interesting insights into the role of LH anomalies during ISM breaks on the poleward extending meridional wave train, the ACMCC anomaly is found to have important ramifications on the daily rainfall extremes over the Indo-China region. It is revealed from the present analysis that the frequency of extreme rainfall occurrences over Indo-China shows a twofold increase during ISM break periods as compared to active ISM conditions.

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Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon

Uppara

Vellore

Krishnan

et al. 2019

Earth Syst Environ

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The Abnormal Indian Summer Monsoon of 2000

Krishnan¹,

Mujumdar²,

Vaidya³

et al. 2003

J. Climate

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Diagnostic analysis of observations and a series of ensemble simulations using an atmospheric general circulation model (GCM) have been carried out with a view to understanding the processes responsible for the widespread suppression of the seasonal summer monsoon rainfall over the Indian subcontinent in 2000. During this period, the equatorial and southern tropical Indian Ocean (EQSIO) was characterized by persistent warmer than normal sea surface temperature (SST), increased atmospheric moisture convergence, and enhanced precipitation. These abnormal conditions not only offered an ideal prototype of the regional convective anomalies over the subcontinent and Indian Ocean, but also provided a basis for investigating the causes for the intensification and maintenance of the seasonal anomaly patterns. The findings of this study reveal that the strengthening of the convective activity over the region of the southern equatorial trough played a key role in inducing anomalous subsidence over the subcontinent and thereby weakened the monsoon Hadley cell. The leading empirical orthogonal function (EOF) of the intraseasonal variability of observed rainfall was characterized by a north-south asymmetric pattern of negative anomaly over India and positive anomaly over the region of the EQSIO and accounted for about 21% of the total rainfall variance during 2000. The GCM-simulated response to forcing by SST anomalies during 2000 is found to be consistent with observations in reasonably capturing the seasonal monsoon anomalies and the intraseasonal variability. Further, it is shown from the GCM experiments that the warm Indian Ocean (IO) SST anomalies influenced the regional intraseasonal variability in a significant manner by favoring higher probability of occurrence of enhanced rainfall activity over the EQSIO region and, in turn, led to higher probability of occurrence of dry spells and prolonged break-monsoon conditions over the subcontinent. In particular, the simulated breakmonsoon anomaly pattern of decreased rainfall over the subcontinent and increased rainfall over the EQSIO is shown to intensify and persist in response to the IO SST anomalies during 2000. These results clearly bring out the significance of the IO SST anomalies in altering the regional intraseasonal variability and thereby affecting the seasonal mean monsoon. Further studies will be required in order to investigate the detailed physical mechanisms that couple the variability of convection over the IO region with the local SST boundary forcing and the large-scale monsoon dynamics.

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Dynamical Response of the South Asian Monsoon Trough to Latent Heating from Stratiform and Convective Precipitation

Ayantika

Krishnan

2011

Journal of the Atmospheric Sciences

114

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Simulation experiments using a simplified atmospheric GCM and supplementary diagnostic analyses of observations are performed to understand how the South Asian monsoon trough (MT) responds dynamically to latent heating from mesoscale convective systems (MCSs). Observations reveal that the MT during active monsoons is characterized by a deep cyclonic vorticity extending from the surface to 350 hPa and organized MCSs covering over 3500-4000 km along the Indo-Gangetic plains. The MCSs during active monsoons are composed of a relatively higher abundance of stratiform-type precipitation (mostly nimbostratus) as compared to the convective type. The results suggest that a stratiform-type heating profile is very effective in promoting upward development of continental-scale cyclonic circulation well above the midtroposphere over the MT region. The vertical development involves a dynamical uplift of the layer of cyclonic circulation and is induced by midlevel (600-500 hPa) convergence and vorticity stretching above 500 hPa. By varying the population of stratiform and convective rain types in the simulation, the horizontal scale of midlevel vorticity response is shown to increase significantly with stratiform population; in contrast, the midlevel response is more localized when the MCS is dominated by deep convective clouds. For large stratiform populations, the midlevel response is found to extend far westward up to the northern flanks of the African ITCZ, indicative of Rossby wave dispersion of PV anomalies that are generated near the level of maximum heating gradient. From the present findings, one can conclude that the vertical deepening of MT during active monsoons is not merely a localized phenomenon; instead it represents a large-scale dynamical response to organized MCSs that exert pivotal influence on the upward development of cyclonic circulation well above the midtroposphere.

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A model study of the 30-50 day oscillation In the tropical atmosphere

Cited by 3 publications

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Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon

Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon

The Abnormal Indian Summer Monsoon of 2000

Dynamical Response of the South Asian Monsoon Trough to Latent Heating from Stratiform and Convective Precipitation

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