Intraseasonal Oscillations and Interannual Variability of the Indian Summer Monsoon
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Abstract

The prediction of seasonal mean summer monsoon precipitation has been probably the most challenging and most difficult problem in tropical meteorology. The experience in attempted prediction of Indian summer monsoon has indicated that the predicatbility of monsoon climate is limited by 'internal' variability. The monsoon intraseasonal oscillations (ISOs) that are very vigorous during summer monsoon season arise from internal feedback between convection and dynamics. If the monsoon ISOs could influence the seasonal mean and its interannual variability, the part of seasonal mean monsoon variability goverened by ISOs would be unpredictable. Major outstanding questions are (a) Is there a physical basis for the monsoon ISOs to influence the seasonal mean? (b) is there any empirical evidence of association between some statistics of the ISOs and interannual variability of the Indian sumer monsoon? (c) If the ISOs do influence the seasonal mean, what is the fraction of total interannual variability contributed by ISOs? This study adresses the above issues in a systematic manner.

How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and it's inter-annual variability of the Indian summer monsoon is investigated using 42-years of daily circulation data from NCEP/NCAR reanalysis and 24-years of satellite derived outgoing long wave radiation data. Based on zonal winds at 850 hPa over the Bay of Bengal, a criterion is devised to define 'active' and 'break' monsoon conditions. The underlying spatial structure of a typical ISO cycle in circulation and convection that is invariant over the years is constructed using a composite technique. Typical ISO's have large scale horizontal structure similar to the seasonal mean and intensifies (weakens) the mean flow during it's 'active' ('break') phase. A typical 'active' ('break') phase is also associated with enhanced (decreased) cyclonic low-level vorticity and convection in the northern position of the tropical convergence zone (TCZ ) and decreased (increased) convection in the southern position of the TCZ. The cycle evolves with a northward propagation of the TCZ and convection from the southern to the northern position of the TCZ. It is shown that the intraseasonal and interannual variations are governed by a common mode of spatial variability. The spatial pattern of standard deviation of intraseasonal and interannual variability of low-level vorticity is shown to be similar. The spatial pattern of the dominant mode of ISO variability of the low-level winds is also shown to be similar to that of the inter-annual variability of the seasonal mean winds. The similarity between the spatial patterns of the two variability indicates that higher frequency of occurrence of 'active' ('break') conditions would result in 'stronger' ('weaker') than normal seasonal mean. This possibility is tested by calculating two dimensional probability density function (PDF) of the ISO activity in the low-level vorticity. The PDF estimates for 'strong' monsoon years and 'weak' monsoon years are shown to be asymmetric in both the cases. It is seen that the 'strong' ('weak') monsoon years are associated with higher probability of occurrence of 'active' ('break') conditions. This result, indicates that the frequency of intraseasonal pattern determine the seasonal mean. As the ISOs are essentially chaotic, it raises an important question on predictability of the Indian summer monsoon.

Quantitatve estimates of potential predictability reveal that the monsoon cliimate is marginally predictable on monthly and seasonal time scales as the contribution from the boundary forcing in this region is comparable to that from the internal dynamics. It is further shown that most of the internal low frequency variability in the Indian summer monsoon region arise from the ISOs.

While the monsoon ISOs seem to lead to decrease in the predictability of monthly or seasonal mean monsoon climate, it is possible that the same ISOs lead to extended range prediction of spells of synoptic activity. The main rain bearing system during the monsoon season are the Low Pressure Systems (LPS) consisting of lows and depressions. Since the genesis of the LPS depends on the horizontal shear and low-level vorticity, it is possible that more LPS form in the active phase relative to the break phase. In other words, large scale circulation associated with the ISOs could modulate the frequency of genesis of LPS. Using more than 40 years of LPS genesis statistics and daily circulation data, here we show that the dry and wet spells are the result of clustering of lows and depressions caused by modulation of the large scale monsoon flow by the intraseasonal oscillations. The slow evolution of the ISOs may permit extended range prediction of the ISO phases and through them dry and wet spells of the Indian summer monsoon.

More details on http://caos.iisc.ernet.in/hpg/students/ajay.html
E-mail: ajayan@caos.iisc.ernet.in or ajayan_rs@yahoo.com