Signature of ENSO signals in the coral growth rate record of Arabian Sea and Indian monsoons

We examine here three sets of recently published data: (1) Updated Indian Rainfall (IRF) time series of the entire country covering the time span of 1826-1994, (2) coral growth rate time series for a period of 42 years spanning 1948-1990 from the Arabian Sea, and (iii) NINO3 temperature records to investigate the signature of ENSO response of the Indian monsoon. Multiple spectral techniques (e.g., multi-taper method (MTM), maximum entropy method (MEM), wavelet and cross spectra) are used to identify the coherent cyclic and nonstationary modes in these records. MTM analysis of IRF time series resolves statistically significant variability (>90% C.I) (i) at multi-decadal (66-70 year's) scales related to the well-known global temperature variability of internal atmospheric-ocean origin, (ii) relatively weak signals at 13 and 22 years (solar cycles) and (iii) the 2.5 to 7.5-year cycles associated with the ENSO frequency band. The MTM spectra of the coral growth rate record also reveal statistically significant periodicities (>90% C.I.) within 1.8-4.2 ENSO frequency band, and a relatively weak signal at 12.8 years. MEM analysis confirms the stability of above spectral peaks. Wavelet spectral analyses of the above time series reveal nonstationary "localized modes" of ENSO evolution corresponding to 2-7 years and higher order terms. Although matching periodicities are present in these records, cross-spectral analysis of IRF and NINO3 temperature records exhibits significant "coherency" (>80% CI) only at periods 5.4 years and 2.7 years, suggesting the significant role of ENSO dynamics in organizing the subtle Indian monsoon at these frequencies. These results may provide significant implication for the modeling of Indian monsoon.