Indian Summer Monsoon Onset Delayed by the Weakening of Hadley Circulation

The timing of the Indian Summer Monsoon (ISM) onset significantly impacts agriculture, food production, the economy, and livelihoods in India. Parker et al. (2016) highlighted the role of both mid-level dry northwesterly winds and low-level moist southwesterly winds in influencing the climatological ISM onset. However, the question of what drives the delay in ISM onset remains unclear and uncertain. Is it primarily due to mid-level dry northwesterly winds, low-level moist southwesterly winds, or a combination of both? In this study, we find that the weakening of low-level moist southwesterly winds is the primary factor, while the mid-level dry northwesterly winds remain unaltered during delayed onset years. This weakening of southwesterly winds is associated with the low-level circulation anomaly caused by the anomalous high pressure over the Arabian Sea, which is further linked to the weakening of the Hadley circulation. The relatively low pressure over the Mascarene High reduces the cross-equatorial pressure gradient, weakening the Hadley circulation, which in turn weakens the southwesterly winds, thereby delaying the ISM onset. Understanding the underlying mechanisms of delayed monsoon onset provides critical insights for improving Indian monsoon modelling and prediction.