Abstract
Active and break phases of the Indian summer monsoon are characterized by enhancement and decrease of precipitation over the monsoon trough region. Using genesis data of monsoon low pressure systems (LPS) and circulation data for the period 1954 to 1993, it is shown that the frequency of occurrence of LPS is nearly 3.5 times higher in the active phase of monsoon as compared to the break phase. In addition, the tracks of these synoptic systems are also strongly spatially clustered along the monsoon trough during the active phase of the monsoon. The enhanced (decreased) frequency of occurrence of LPS during active (break) phases is due to modulation of meridional shear of zonal winds and cyclonic vorticity along the monsoon trough by the intraseasonal oscillations (ISO).
3
Introduction
Active and break episodes, characteristic of subseasonal variations of the Indian summer monsoon, are associated with enhanced (decreased) rainfall over central and western India and decreased (enhanced) rainfall over the southeastern peninsula and eastern India [Singh et al. 1992 ; Krishnamurthy and Shukla 2000] . The intraseasonal variations of rainfall (active-break cycles) are strongly coupled to the intraseasonal variations of circulation [Hartmann and Michelson 1989; Webster et al. 1998; Sperber et al. 2000; Goswami and Ajayamohan 2001] . How large scale intraseasonal variations of circulation result in enhancement or decrease in rainfall over the Indian subcontinent?
The main rain producing systems over the Indian monsoon region are the synoptic scale lows and depressions, collectively referred to here as low pressure systems (LPS) with typical time and length scales of 3-5 days and 2000 km. The maximum wind speed associated with lows is less than 8.5 m/s while the maximum wind speed in depressions is between 8.5 m/s and 16.5 m/s. Lows and depressions are shear instabilities energized by moist convection [Shukla 1987; Goswami et al. 1980; Mak 1987] . Large meridional shear of the eastward component of winds and high cyclonic vorticity at low levels over the monsoon trough region favor growth of these instabilities. Recent studies [Webster et al. 1998; Goswami and Ajayamohan 2001; Sperber et al. 2000] have shown that the spatial structure of monsoon ISO is such that they strengthen the seasonal mean circulation in one phase while weakening it in the opposite phase. Therefore, the monsoon ISO has the potential to modulate the frequency of occurrence of LPS by alternately enhancing 4 and weakening the zonal wind shear and low level cyclonic vorticity in the monsoon trough (MT). Indication of association between ISO regimes and synoptic activity during the Indian summer monsoon was presented in some previous studies [Murakami et al. 1984; Murakami et al. 1986; Yasunari 1981] . Murakami et al.(1986) show an approximate inverse relationship between the low frequency ISO anomalies and energy of synoptic disturbances over the Indian monsoon region during 1979. However, the number of years of data used in these studies were rather small and the robustness of the signal not established. Moreover, the spatial clustering was not addressed. 
Data Used
Daily NCEP/NCAR reanalysed winds [Kalnay et al. 1996] 
Results
The frequency of occurrence of the monsoon synoptic systems as a function of phase of the ISO is examined in this section, using an index of ISO activity. The relative vorticity at 850 hPa represents monsoon activity quite well on intraseasonal time scales [Webster et al. 1998; Goswami and Ajayamohan 2001; Sperber et al. 2000] .
Therefore, we define a monsoon intraseasonal index (MISI) as the 10-90 day filtered relative vorticity at 850 hPa averaged over 80 How do different frequencies of monsoon ISO contribute to the clustering? To 8 answer this question, MISI was reconstructed with 30-60 day, 10-20 day and less than 10 day filtered vorticity and frequency distribution of genesis of LPS (similar to Fig.3) were calculated and the results are summarized in Table-I . Ratio between active and break conditions as defined by different frequency bands is noteworthy. Its is seen that both 10-20 day and 30-60 day oscillations result in some clustering. However clustering is enhanced significantly when they are combined (i.e 10-90 day). Probability of genesis is significantly enhanced when phases of both 10-20 day and 30-60 day modes cooperate.
Also, high frequencies (period less than 10 days) by themselves do not give rise to any clustering.
Conclusions
What leads to the increase (decrease) in rainfall over the MT region during active (break) conditions? To gain insight to this question, we hypothesize that the monsoon ISO achieve this by modulating the genesis of LPS through modulation of the large scale monsoon flow. An index of monsoon intraseasonal circulation is constructed for 40 summer monsoon seasons using 850 hPa vorticity from NCEP/NCAR reanalysis.
Using LPS genesis data for this period and the index of intraseasonal variability of circulation, the frequency distribution corresponding to different phases of ISO is found.
It is shown that the genesis of an LPS is more than three times as likely in an active phase than in a break phase. Increase (decrease) in the meridional shear of zonal wind and low level cyclonic vorticity along the MT during active (break) phase lead to enhanced (decreased) cyclogenesis and higher (lower) frequency of occurrence of 

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