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

ABSTRACT 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.