On breaks of the Indian monsoon |
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| Authors: | Sulochana Gadgil P V Joseph |
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| Institution: | (1) Centre for Atmospheric and Oceanic Sciences and Centre for Ecological Sciences, Indian Institute of Science, 560 012 Bangalore, India;(2) Department of Atmospheric Sciences, Cochin University of Science and Technology, 682 016 Cochin, India |
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| Abstract: | For over a century, the term break has been used for spells in which the rainfall over the Indian monsoon zone is interrupted.
The phenomenon of ’break monsoon’ is of great interest because long intense breaks are often associated with poor monsoon
seasons. Such breaks have distinct circulation characteristics (heat trough type circulation) and have a large impact on rainfed
agriculture. Although interruption of the monsoon rainfall is considered to be the most important feature of the break monsoon,
traditionally breaks have been identified on the basis of the surface pressure and wind patterns over the Indian region. We
have defined breaks (and active spells) on the basis of rainfall over the monsoon zone. The rainfall criteria are chosen so
as to ensure a large overlap with the traditional breaks documented by Ramamurthy (1969) and Deet al (1998). We have identified these rainbreaks for 1901-89. We have also identified active spells on the basis of rainfall over
the Indian monsoon zone. We have shown that the all-India summer monsoon rainfall is significantly negatively correlated with
the number of rainbreak days (correlation coefficient -0.56) and significantly positively correlated with the number of active
days (correlation coefficient 0.47). Thus the interannual variation of the all-India summer monsoon rainfall is shown to be
related to the number of days of rainbreaks and active spells identified here.
There have been several studies of breaks (and also active spells in several cases) identified on the basis of different criteria
over regions differing in spatial scales (e.g., Websteret al 1998; Krishnanet al it 2000; Goswami and Mohan 2000; and Annamalai and Slingo 2001). We find that there is considerable overlap between the rainbreaks
we have identified and breaks based on the traditional definition. There is some overlap with the breaks identified by Krishnanet al (2000) but little overlap with breaks identified by Websteret al (1998). Further, there are three or four active-break cycles in a season according to Websteret al (1998) which implies a time scale of about 40 days for which Goswami and Mohan (2000), and Annamalai and Slingo (2001) have
studied breaks and active minus break fluctuations. On the other hand, neither the traditional breaks (Ramamurthy 1969; and
Deet al 1998) nor the rainbreaks occur every year. This suggests that the `breaks’ in these studies are weak spells of the intraseasonal
variation of the monsoon, which occur every year.
We have derived the OLR and circulation patterns associated with rainbreaks and active spells and compared them with the patterns
associated with breaks/active minus break spells from these studies. Inspite of differences in the patterns over the Indian
region, there is one feature which is seen in the OLR anomaly patterns of breaks identified on the basis of different criteria
as well as the rainbreaks identified in this paper viz., a quadrapole over the Asia-west Pacific region arising from anomalies
opposite (same) in sign to those over the Indian region occurring over the equatorial Indian Ocean and northern tropical (equatorial)
parts of the west Pacific. Thus it appears that this quadrapole is a basic feature of weak spells of the intraseasonal variation
over the Asia-west Pacific region. Since the rainbreaks are intense weak spells, this basic feature is also seen in the composite
patterns of these breaks. We find that rainbreaks (active spells) are also associated with negative |
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| Keywords: | Indian monsoon intraseasonal variation active and weak spells monsoon breaks |
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