Characterizing the onset and demise of the Indian summer monsoon

Abstract

An objective index of the onset and demise of the Indian summer monsoon (ISM) is introduced. This index has the advantage of simplicity by using only one variable, which is the spatially averaged all‐India rainfall, a reliably observed quantity for more than a century. The proposed onset index is shown to be insensitive to all historic false onsets. By definition, now the seasonal mean rainfall anomalies become a function of variations in onset and demise dates, rendering their monitoring to be very meaningful. This new index provides a comprehensive representation of the seasonal evolution of the ISM by capturing the corresponding changes in large‐scale dynamic and thermodynamic variables. We also show that the interannual variability of the onset date of the ISM is associated with El Niño–Southern Oscillation (ENSO) with early (late) onsets preceded by cold (warm) ENSO.

High‐resolution regional‐coupled ocean–atmosphere simulation of the Indian Summer Monsoon

“…The corresponding RMS error and spatial correlation of the climatology from the RCOAM simulation with the corresponding climatology from the IMD rainfall dataset is also indicated. As noted in Figure and akin to the observational analysis in Noska and Misra (), the onset evolution is characterized by sustained heavy rainfall (∼10–20 mm day −1 ) over parts of northeastern India prior to the onset date of the ISM. But the southwestern (Malabar) coast witness a moderate rise in rain rate about 5 days prior to the onset date that continues to rise almost 20 days after the onset date.…”

The atmospheric branch of the hydrological cycle over the Indus, Ganges, and Brahmaputra river basins

“…4 and 5) over the basins and is permitted to identify the date associated with rainfall increase because of the monsoon onset (the day after the minimum accumulated rainfall anomalies) and demise (the day of the maximum accumulated rainfall). According to Noska and Misra (2016), this index is capable of representing the annual rainfall variability across the region and thus must be adequate for our target regions.…”

“…To determine the day on which the increase in rainfall indicates the beginning of the monsoon involvement for each basin, we utilized the method proposed by Noska and Misra (2016). This method is based on daily cumulative anomalies (C m) of the average precipitation for each basin along the year and according to Eqs.…”

“…Taniguchi and Koike (2006) argue that the rapid enhancement of the wind speed related well with the abrupt beginning of the rainy season and it represents a clear transition in atmospheric conditions or the beginning of ISM. To determine the onset and demise dates, we applied an objective index to the basins from Noska and Misra (2016), which was previously adapted for the Asian monsoon region in Misra and DiNapoli (2014) and builds upon the index proposed by Liebmann et al (2007). The analysis is based on the cumulative anomalies of daily rainfall averaged (see Eqs.…”

“…To determine the onset and demise dates, we applied an objective index from Noska and Misra (2016) for the basins, which is based on the cumulative anomalies of averaged daily rainfall (see Eqs. 4 and 5).…”