An observational study of the impacts of Madden-Julian Oscillation (MJO) on South Asian monsoon onset

The onset of Indian Summer Monsoon (ISM) marks the beginning of the rainy season for the Indian subcontinent, bringing rainwaters that are critical for local agriculture, human consumption, energy generation and industrial development. The inter-annual variability of the onset dates is substantial, with the earliest ever onset date having been recorded in mid May and the latest one in late June. Despite its importance in climate and human livelihood, the mechanisms involved in the abrupt onset of ISM are still not well understood. It has been hypothesized that the Madden-Julian Oscillation (MJO), and possibly the associated northward propagation of convection in boreal summer, might be a contributing factor to the onset of ISM.

In this study, we use reanalysis and satellite-base data to explore statistical relationships between these two important tropical circulations. Preliminary results suggest that years in which the monsoon has a later-than-normal onset have a stronger eastward MJO propagation. In other words, there seems to be a so far unrecognized association between late ISM onset and strong MJO activity. Correlations between the MJO and other important aspects of the monsoonal circulation, such as Indian Ocean Sea Surface Temperatures, lower- and upper-level winds, meridional and zonal wind shears are explored. By shedding further insights into the connection between the MJO and the ISM, this study has the potential to improve seasonal predictions skills of monsoon onset, which to date remain limited.