Reconciling Different Views on the Onset of Boreal Summer Monsoon: The Example of Thailand

The onset of the rainy season in monsoonal climates is a critical stage of the crop calendar, and its real-time monitoring, forecast and understanding are fundamental tasks for either end-users, stakeholders and experts in climatology. Onset may be ideally defined as a sharp switch between opposite winds in lower and upper troposphere and/or a sudden increase of rainfall, nearly synchronous over a sufficiently large scale. Thailand is an excellent field to illustrate the different facets of the onset. We used the daily rainfall over a network of 75 stations (1981–2021) and upper- and lower-level winds to define local- to country-scale onsets. Some significant pre-monsoon rainfall occurred 1–4 weeks in the mean before the clear settlement of the monsoon circulation occurring usually in mid-May. These pre-monsoon rainfall are not systematically purely local and may sometimes start sustained rainfall or be interrupted by a long dry spell, especially over Central Thailand. The use of a criterion of post-onset dry spell allows us to filter out some of the ‘false’ onsets, i.e., when the initial 3-day wet spell receiving at least 30 mm is followed by a 14-day dry spell in the following 30 days. This definition optimises the amount of interannual covariance amongst the rain gauges but is clearly not optimal for the end-users, because 30 days are needed to declare the onset at local scale and even more at the province to country scale. An operational solution discards the dry spell criteria over a reduced set of only five stations located on the windward southern Peninsula. Such definition is similar to the ‘Monsoon over Kerala’ for the Indian monsoon, and this onset occurs around 9 April in mean, and its interannual variations are highly correlated (r = 0.72) with the spatially averaged local onsets over mainland Thailand > 11° N.