Abstract
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.
Highlights
By performing detailed diagnostic analyses using observed data sets and reanalysis products, the present study provides a mechanistic understanding of the Asian continental mid-tropospheric cyclonic circulation (ACMCC) pattern
Our results indicate that the ACMCC is part of a larger pattern of meridionally dispersing Rossby waves that extends from the tropics into higher latitudes during Indian summer monsoon (ISM) breaks
It is further noted that the aforesaid large-scale meridional circulation pattern during ISM breaks is largely forced by latent heating (LH) anomalies over centralnorth India and the central–eastern Himalayas (CEH) foothills
Summary
The active and break monsoon cycles of the Indian summer monsoon (ISM) rainfall variability are a well-known aspect of the monsoon subseasonal variability and have. An important aspect of Rossby wave-train excitation in response to diabatic heating over the tropics and monsoon areas is the formation of meridionally extending patterns of large-scale circulation anomalies towards the extra-tropics and polar latitudes (e.g., Sardeshmukh and Hoskins 1988; Liu et al 2001; Enomoto et al 2003; Krishnan et al 2009, Krishnamurti et al 2015). While a decrease in upper tropospheric divergence during ISM breaks due to suppressed monsoon convection over the Indian subcontinent can induce a downstream Rossby wave response towards East Asia (Krishnan et al 2009), it is not yet clear if the monsoonal heating anomalies can have any further consequences on large-scale circulation patterns. Details about the data sets and numerical experiments are described
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