E-Paper
Monsoon troughs, depression-related variability are primary causes of rainfall variability: Study
The study highlighted that the relationship between ENSO and ISMR has not remained consistent throughout the period from 1901 to 2018
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Latest study by scientists from the Indian Institute of Tropical Meteorology (IITM) Pune highlighted that the El Niño Southern Oscillation (ENSO) has different influences on the Indian Summer Monsoon Rainfall (ISMR) at the regional level in India. At the same time, monsoon troughs and depression-related variability have emerged as the primary cause of rainfall variability over central India, revealed the study.
While scientists have gained success in understanding the overall insights into the ENSO–ISMR relationship, the regional variability, and its long-term changes were yet to be decoded. Therefore, a study was conducted by scientists including K S Athira, MK Roxy, Panini Dasgupta, JS Saranya, Vineet Kumar Singh and Raju Attada to understand the Indian regional ENSO–ISMR relationship to identify and improve the monsoon forecast skills. The study was published on August 4, in the Nature Science journal.
The scientists carried out a correlation analysis to examine the relationship between ENSO and ISMR, Empirical Orthogonal Function (EOF) analysis to identify the major drivers of ISMR variability, a power spectrum analysis to distinguish the frequency present in the time series for the rainfall of different regions, and wavelet analysis to identify the corresponding timescale in which these frequencies are present in the time series.
For this, rainfall data was obtained from the India Meteorological Department (IMD). Monthly and daily gridded rainfall data with a resolution of 0.25˚ × 0.25˚58 was used for the analysis from 1901–2018 during the June to September months, covering 118 years.
The study highlighted that the relationship between ENSO and ISMR has not remained consistent throughout the period from 1901 to 2018. The ENSO–ISMR inverse relationship started getting stronger from 1901 to 1940, became stable from 1941 to 1980, and then the relationship has weakened in the recent epoch (1981 onwards).
However, this change in ENSO–ISMR relationship is spatially non-uniform.
Whereas over north India, the ENSO–ISMR relationship is becoming strong in recent decades and has shown significant multi-decadal variation. On the contrary, the association between the central India rainfall and ENSO has diminished in recent decades. We find that the role of monsoon trough and depression-related variability has emerged as the primary cause of rainfall variability over central India surpassing the role of ENSO-related variability, mentioned in the study.
Former IITM researcher Vineet Kumar Singh, who was also a part of the study team, said, “This year, the influence of El Niño was limited until July. This is primarily attributed to the absence of significant atmospheric response to the evolving El Niño conditions till last month. This observation aligns with this study, indicating the restricted effect of El Niño on rainfall within the core monsoon zone. However, during the latter part of the monsoon season, El Niño could bring some adverse impact on rainfall.”
Variation in spatial patterns and influences on the rainfall
The study revealed that there are two major spatial patterns of ISMR which explain about 24% of the total inter-annual variability. The first mode (PC1) implies reduced rainfall across the Indian subcontinent, especially the central and western parts of India, accounting for 15.11% of the total variability.
The second mode (PC2) indicates a dipole pattern with reduced rainfall over the Indo-Gangetic plain and increased rainfall over south India accounting for 8.48% of the total variability. The PC1 pattern of ISMR is significantly influenced by ENSO, Indian Ocean Dipole (IOD), Pacific Decadal Oscillation (PDO), Inter-decadal Pacific Oscillation (IPO), and variation in the monsoon trough. While the PC2 pattern is significantly associated with North Atlantic Oscillation (NAO), IOB, PDO, IPO, Monsoon Depression Frequency (MDF), and the strength of the monsoon trough.
