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In this study, we introduce a localized definition of the onset and retreat of the northern Australian rainy season that is solely based on gridded rainfall analysis. Our analysis shows that the local onset/retreat of the rainy season has considerable spatial heterogeneity. Onset is earlier and the length of the rainy season is longer to the west of the Gulf of Carpentaria than to its east. Furthermore, we also find the local onset/retreat is influenced by the wet and dry spells of the 30–60-day intraseasonal oscillation. Much of the retreat of the rainy season occurs in the dry phases of the intraseasonal oscillation. However, intriguingly, a majority of the local onset of the rainy season occurs during dry phases of the intraseasonal oscillation. The ENSO teleconnection with the variable-length northern Australian rainy season also exhibits spatial heterogeneity and significant differences from rainfall anomalies using the fixed-length boreal winter season. The onset, the retreat, the length, and the seasonal rainfall anomalies of the rainy season display a stronger correlation with the ENSO SST anomalies for the region east of 140°E relative to its west. The strong covariability of the local onset date with the corresponding seasonal length and seasonal rainfall anomalies over northern Australia offers the advantage of monitoring the onset of the northern Australian rainy season to provide an outlook for the forthcoming season. The proposed local definition of onset/retreat of the northern Australian rainy season is simple, objective, and unambiguous and is ideally suited for real-time monitoring of the evolution of the season.

Indian summer monsoon exhibits strong northward propagation in intraseasonal timescale [intraseasonal oscillations (ISO)] from the equatorial Indian Ocean (EIO) to the foothills of the Himalayas. Initiation of this northward march is often marked by a strong convective activity over the EIO, which could be associated with Madden–Julian Oscillation. Satellite derived rainfall and reanalysis products are used to unravel different characteristics of ISO in terms of strength, extent and speed at different times. Eastern EIO convective events are grouped into two categories based on the intensities of the simultaneous rainfall anomalies over central India (CI). Northward propagation from EIO is observed to be much stronger and slower when CI experiences strong dry anomaly at the time of initiation of northward march of convection near the equator. Essentially, a first northward propagating dry lobe seems to impact the behavior of the next arriving wet lobe. Distinctive features in Rossby waves emanated from the eastern EIO and different surface and atmospheric conditions over the region in the two different cases are observed. Strong dry conditions over CI favors strong easterly vertical wind-shear, which eventually helps destabilizing the atmosphere. Upper tropospheric meridional temperature gradient over the region modulates the variation in vertical wind-shear via thermal wind balance. These conditions are favorable for generation of strong boundary layer convergence to the north of the convective band, producing strong northward propagation. These results indicate the dual role of convection near the equator and dryness over CI in modulating the northward march of rainfall over India.

In this study, rainfall estimates by the Tropical Rainfall Measuring Mission are used to understand the spatiotemporal structures of convection in the intraseasonal time scale and their intensity during the boreal summer over South Asia. A quantitative analysis on how these intraseasonal modes modulate the central Indian rainfall is also provided. Two dominant modes of variability with periodicities of 10–20 and 20–60 days are found, with the latter strongly modulated by sea surface temperature. The 20–60-day mode shows northward propagation from the equatorial Indian Ocean linked with eastward-propagating modes of convective systems over the tropics. The 10–20-day mode shows a complex space–time structure with a northwestward-propagating anomalous pattern emanating from the Indonesian coast. This pattern is found to be interacting with a structure emerging from higher latitudes propagating southeastward, the development of which is attributed to the vertical shear of the zonal wind. The two modes exhibit profound variability in their intensity on the interannual time scale and they contribute a comparable amount to the daily rainfall variability in a season. The intensity of the 20–60 and 10–20-day modes shows a significantly strong inverse and direct relationship with the all-India June–September rainfall, respectively. This study establishes that the probability of the occurrence of substantial rainfall over central India increases significantly if the two intraseasonal modes simultaneously exhibit positive anomalies over the region. The results presented in this paper will provide a pathway to understand, using observations and numerical model simulations, intraseasonal variability and its relative contribution to the Indian summer monsoon. It can also be used for model evaluation.

The decreasing trend in rainfall in the last few decades over the Indo-Gangetic Plains of northern India as observed in ground-based observations puts increasing stress on groundwater because irrigation uses up to 70 of freshwater resources. In this work, we have analyzed the effects of extensive irrigation over the Gangetic Plains on the seasonal mean and intra-seasonal variability of the Indian summer monsoon, using a general circulation model and a very high-resolution soil moisture dataset created using extensive field observations in a state-of-the-art hydrological model. We find that the winter-time (November�March) irrigation has a positive feedback on the Indian summer monsoon through large scale circulation changes. These changes are analogous to a positive North Atlantic Oscillation (NAO) phase during winter months. The effects of the positive NAO phase persist from winter to spring through widespread changes in surface conditions over western and central Asia, which makes the pre-monsoon conditions suitable for a subsequent good monsoon over India. Winter-time irrigation also resulted in a reduction of low frequency intra-seasonal variability over the Indian region during the monsoon season. However, when irrigation is practiced throughout the year, a decrease in June�September precipitation over the Gangetic Plains, significant at 95 level, is noted as compared to the no-irrigation scenario. This decrease is attributed to the increase in local soil moisture due to irrigation, which results in a southward shift of the moisture convergence zone during the active phase of monsoon, decreasing its mean and intraseasonal variability. Interestingly, these changes show a remarkable similarity to the long-term trend in observed rainfall spatial pattern and low-frequency variability. Our results suggest that with a decline in the mean summer precipitation and stressed groundwater resources in the Gangetic Plains, the water crisis could exacerbate, with irrigation having a weakening effect on the regional monsoon.

This study uses precipitation estimates from the Tropical Rainfall Measuring Mission to estimate the intensity and examine the spatiotemporal patterns in the modes found in intraseasonal timescale over the Indian monsoon region during boreal summer. Here, using multichannel singular spectrum analysis, two dominant modes of oscillations are found in the intraseasonal timescale with periodicity of 10-20-days and 20-60-days, respectively. 20-60-days mode shows northward propagation from the equatorial Indian Ocean linked with the eastward propagating modes of convective systems over the tropics. 10-20-days mode shows very complex structure with a northwestward propagating anomaly pattern emanating from the Indonesian coast moving towards central India. This pattern is found to have a possible interaction with a structure emerging from higher latitudes propagating southeastwards. The two intraseasonal modes contribute comparable amount to the total rainfall variability. The intensity of the 20-60-days (10-20-days) mode show significantly strong inverse (direct) relationship with all India June-September rainfall and both the modes exhibit profound variability in their intensity in interannual scale. This study also establishes that the probability of getting good amount of rainfall (no rainfall) over central India increases significantly if the two intraseasonal modes exhibit positive (negative) anomalies over the region. Relation between the ISO intensities and sea surface temperature is also discussed. This study points towards the fact that the knowledge of ISO phases can increase the skill in the probabilistic forecasting of rainfall over India.