Rainfall Characteristics Over The Indian Subcontinent And The Influence Of The Madden Julian Oscillation

Abstract

Understanding the influence of the Madden Julian Oscillation (MJO) on various rainfall characteristics is intriguing and important over the Indian subcontinent. Previous studies have documented the various aspects of rainfall, but a comprehensive understanding of diurnal cycle of rainfall (DCR) and extreme rainfall events (ERE) during MJO phases is still warranted across the seasons. The rainfall occurs in conjunction with the primary climatic modes (e.g. ENSO, MJO, and IOD) might invigorate or enervate the rainfall, which has many societal repercussions. Therefore, an in-depth understanding of the influence of primary climate modes on rainfall is essential to mitigate the vast impacts. With this rationale, the dissertation aims to quantify the scale interaction of the MJO during the diurnal and ERE over the Indian subcontinent. The Tropical Rainfall Measuring Mission precipitation data were used to manifest DCR and ERE. The European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction and the National Center for Atmospheric Research reanalysis data sets were used to investigate large-scale circulations associated with the rainfall. In order to delineate the MJO phases, the Real-time Multivariate MJO index was employed. The phases of the MJO are classified into active and suppressed, depends on the location of active convective centers over the oceanic regions of Indo-Pacific. The rainfall frequency percentage and the rainfall contribution index were used to understand the diurnal cycle of rainfall. The peak over the threshold, changes in probability, logistic regression models, standardized precipitation index and composite analysis were used to understand the statistical and atmospheric circulation during extreme rainfall events.The perusal of results reveals that the positive rainfall anomalies and enhanced rainfall peak during the active phases of the MJO. However, negative rainfall anomalies and declined rainfall peaks were evident during the MJO suppressed phases. The pattern is consistent across the seasons and prominently evident over the Bay of Bengal, equatorial Indian Ocean and east coastal regions of India. The diurnal cycle of rainfall shows the mid-late afternoon peak over the land regions and the late evening to early morning peak over the oceanic regions. In addition, during the Indian Summer Monsoon (ISM), the nocturnal peak was evident over the Himalayan mountains and Western Ghats regions. Similarly, the secondary rainfall peak during the daytime was evident over the central Bay of Bengal regions. Importantly, this study identified the presence of seashore and offshore regimes during the DCR over the Arabian Sea and Bay of Bengal coastal regions Similarly, the analysis of sub-daily rainfall extremes reveals that ERE intensity was higher than the daily ERE during pre-monsoon and ISM. The frequency percentage index of hourly rainfall events during different octets shows the mid to late afternoon (MLA) and late evening-early morning (LE-EM) peaks during ISM. This signifies that the sub-daily extremes were phase-locked with the diurnal cycle, notably during the ISM. The probability of detection analysis affirms the above results. Such as the daytime extremes were prominent over the land, and nocturnal extremes were occurring over coastal and oceanic regions. The investigation of synoptic circulations shows that the nocturnal extremes are stronger over the CIR and the WCWG regions, whereas the daytime extremes are stronger over the SIP. Indeed, the synoptic circulations and mechanisms, the nocturnal extremes are governed by air pump of the second kind, and daytime follows the inertial oscillation theory during the ISM. Akin to this, the analysis of ERE using the peak-over threshold method reveals the highest intensity of events prevails during the ISM over the Western Ghats, Northeast India and monsoon core regions. During the post-monsoon seasons in the eastern coastal regions of India, the intensity of rainfall events was higher. The influence of MJO was invariably evident in the distribution of extreme rainfall events. For instance, the frequency and rainfall contribution of the ERE were found higher during the active phases of the MJO. The south of 20o N was identified as rainfall hotspot regions, where active MJO influences were predominant. The logistic regression and cumulative probability changes affirm the above results. This study also aims to understand the precursors, mechanisms and synoptic-scale circulations manifested in ERE. The Indian subcontinent is divided into three homogenous regions such as Southern Indian Peninsula (SIP), Western Coast and the Western Ghats regions (WCWG) and the central Indian regions (CIR) for further analysis. The standardized precipitation index (SPI) was computed using the gamma distribution function to separately identify the top hundred SPI events over each region. The analysis reveals that the presence of meridional dipole plays a vital role in occurrences of ERE in the ISM over the SIP and WCWG regions. The anomalous high over the head Bay of Bengal and anomalous low over the south of 14o N ensure a conducive environment for the ERE. Notably, ERE were prominent over the break–active transition phases. Over the CIR, the intricate interaction between the oscillating monsoon trough, westward-moving monsoon depressions in the presence of secondary cyclonic vortices causes widespread extremes. During the post-monsoon seasons, the formation of tropical low-pressure systems over the North Indian Ocean (NIO) plays a vital role in the occurrences of ERE across the regions. The formation of mesoscale convective systems over zones of the dry-moist environment leads to rainfall extremes in the pre-monsoon season. In addition, the southward intriguing of subtropical westerly jet and northward propagating pre-monsoon organized convection from Indian Ocean regions causes the ERE. The large-scale circulations during the active and suppressed phases indicate that the contextual synoptic background provided by the MJO might facilitate or impede the occurrence of rainfall extremes over the homogenous regions during ISM. The passage of active MJO plays a prominent role in the genesis and intensification of the tropical low-pressure systems over the NIO. Overall, the present study enhances the existing knowledge on diurnal and extreme rainfall characteristics and their association with the MJO over the Indian subcontinent. In addition, in the current warming scenario, the evidence suggests that the frequency of the MJO will escalate; therefore, this study has practical implications in the seasonal to sub-seasonal prediction, flood forecasting, water resources management and mitigation and to validate advanced high-resolution ocean-atmospheric coupled models.