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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.

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.