The tenth IMSC, Beijing, China, 2007 - International Meetings on ...

<strong>The</strong> intra-seasonal variability of the Indian monsoon rainfall plays a dominant role in
deciding the seasonal strength of the monsoon. <strong>The</strong> Indian monsoon is known to exhibit two
dominant periodicities of intra-seasonal oscillations 30-60 days and 10-20 days on the spatial
scale of entire landmass. In this paper the nature and intensity of these dominant periodicities
have been studied on smaller spatial domains during all-India droughts and floods. <strong>The</strong> high
resolution data 1o x 1o latitude/longitude over Indian domain for the recent period of
1951-2003 provided by India Meteorological Department have been used in this analysis. <strong>The</strong>
powerful tools of mathematical statistics (thoroughly discussed by Storch and Zwiers, 1999)
are applied to analyse the daily rainfall time series of high resolution data over Indian domain.
<strong>The</strong> multi-taper spectrum analysis method (Ghil et al, 2002) has been applied to identify the
periodicities over four homogeneous regions. This method shows that 30-60 days periodicity is
dominant over west coast and central India during droughts while the floods are characterized
by high frequency oscillations (Kulkarni et al, <strong>2007</strong>) To examine the time variability of dominant
periodicities over these four regions, wavelet analysis has been applied to daily time series
over every grid for each of all-India drought and flood years. <strong>The</strong> average spectrum shows that
over west coast the 30-60 days oscillations are dominant in later half of the season,
August-September during all-India droughts while during floods they dominate at the end of
the season, September (Kulkarni et al, 2006). Over Central India 30-60 days oscillations are
dominant in the vital months of the season, July and September during droughts <strong>The</strong>
space-time variability of two most dominant modes of ISV of Indian monsoon is studied by
applying Empirical Orthogonal Function (EOF) technique to the 53 spatial patterns of
explained variance by these two modes. <strong>The</strong> most dominant pattern explains 23.6% (30-60
days mode) and 11.5% (10-20 days mode) variance. <strong>The</strong> corresponding principal component
time series reveal that both the oscillations have weakened after 1975. Kripalani et al (<strong>2007</strong>)
have shown that only 6 coupled climate models under IPCC AR4 (cccma_cgcm3.1, cnrm_cm3,
mpi_echam5, miroc3_medres, miroc3_hires and ukmo_hadcm3) are able to simulate the
seasonal mean monsoon precipitation over India reasonably well. We are trying to examine
the capability of these models to simulate the characteristics of intra-seasonal variability of
Indian monsoon precipitation which are discussed above. Also the intra-seasonal variability in
future will be studied in SRES projection scenarios. <strong>The</strong> work is under progress and the results
will be presented at the conference.
References:
Ghil M and 10 others 2002. Advanced spectral methods for climatic time series. Review of
Geophysics .40 : 1-1 – 1-41
Kripalani R H, Oh J H, Kulkarni Ashwini, Sabade S S and Chaudhari H S <strong>2007</strong> South
Asian summer monsoon precipitation variability: Coupled climate model simulations and
projections under IPCC AR4 <strong>The</strong>oretical and Applied Climatology DOI
10.1007/s00704-006-0282-0
Kulkarni Ashwini, Sabade SS and Kripalani RH 2006 Intra-seasonal vagaries of Indian
summer monsoon rainfall IITM Research Report ISSN 0252-1075, No RR-114, 51 pp
124