Meteorol Atmos Phys 95, 53–61 (2007) DOI 10.1007/s00703-006-0197-4 Printed in The Netherlands Disaster Preventive Research Institute, Kyoto University, Gokasho, Uji, Kyoto, Japan Active, weak and break spells in the Indian summer monsoon V. S. Prasad  and T. Hayashi With 7 Figures Received June 22, 2005; revised October 10, 2005; accepted March 2, 2006 Published online: July 31, 2006 # Springer-Verlag 2006 Summary Active weak and break phases of the Indian summer mon- soon for the period 1958–2002 are studied using ERA -40 data. The criteria for identifying the break are proposed and tested using the 850 hPa level horizontal wind shear. Inde- pendent datasets such as All-India Rainfall, NOAA Out- going Long-wave Radiation and CMAP rainfall datasets are used for the verification of the proposed criteria. 1. Introduction The Indian summer monsoon (ISM ) rainfall oscil- lates in the form of ‘‘active’’ and ‘‘weak’’spells. These active and weak spells of the monsoon are associated with the tropical convergence zone (TCZ ; Sikka and Gadgil, 1980). The inter-annual variability in the ISM is mainly due to the occur- rence of long, dry spells in the latter (Krishnamurti and Bhalme, 1976). Apart from the above intra- seasonal variations, which occur every year, a special situation traditionally known as ‘‘break monsoon’’ occurs (Gadgil and Joseph, 2003). These breaks are periods when the monsoon trough is located close to the foot of the Himalayas, which leads to a striking decrease in rainfall over most of India but increase along the Himalayas and parts of northeast India and the southern peninsula (Rao, 1976). An exhaustive survey of the observed characteristics of the breaks in the ISM was conducted by Ramamurthy (1969) for the 1888–1967 period and by De et al (1998) for the 1968–1996 period. They identified break as a day in which the trough of low pressure was not seen on the surface chart over India and in which easterlies were practically absent (hereafter, R þ D break). Although interruption of rainfall is recognized as the most important feature of these ‘‘breaks’’, the criterion used by the Indian Meteorological Department (IMD ) for identify- ing break is the synoptic situation associated with such a rainfall anomaly and is defined clearly by Rao (1976). Recently, there are many studies of breaks iden- tified on the basis of different criteria over regions differing in spatial scale. Webster et al (1998) use the term ‘‘break (active) spells’’ to denote weak (strong) spells of convection and 850 hPa zonal winds over a large-scale region (65–95  E, 10–20  N). On the other hand, Goswami and Mohan (2000) define breaks on the basis of the strength of the 850 hPa wind at a single grid point 15  N, 90  E. Anamalai and Slingo (2001) use the term ‘‘break’’ to denote weak spells of daily average rainfall. Krishnan et al (2000) defines the break days as days with positive Outgoing Long-wave Radiation (OLR ) anomalies exceed- ing 10 Wm 2 over the region 73–82  E, 18–28  N.  On leave from the National Center for Medium Range Weather Forecasting, A-50 Institutional Area, Phase-II, Sector 62, Noida, U.P., India.