Analyzing the non-linear trend and multiscale teleconnections of regional monsoon indices using empirical mode decomposition

Abstract

This paper first performs the analysis of linear and non-linear trends of four representative monsoon Indices namely Indian Monsoon Index (IMI) and Webster-Yang index (WYI), Western North Pacific Monsoon Index (WNPMI) and Australian Monsoon Index (AMI) for the period 1948–2015. The study evaluates the trend in the time series of different monsoon months and seasonal (JJA/JJAS/DJF) series, of all the indices using Mann–Kendall test, Sen’s slope method, linear fitting and the Empirical Mode Decomposition (EMD) method. The trend analysis showed significant reduction in all the series of WYI index and the August month series of IMI. The results of sequential version of MK test revealed that the WYI time series of different months and monsoon season showed statistically significant reduction in the past with differences in commencement year of statistical significance of MK value. The comparison of long term trend of monsoon indices and ElNino Southern Oscillation (ENSO) displayed an opposing trend for different indices except for WNPMI. Further for a detailed examination of association of different indices with ENSO in multiple time scales, the running correlations between the oscillatory modes of monsoon indices and ENSO are performed by employing the time dependent intrinsic correlation analysis. The association between ENSO and monsoon indices at different time scales is found to be strongly negative and consistent for WNPMI and AMI while for rest of the indices, the nature of association is found to be positive in inter annual and inter decadal time scales.