The Second Rainy Stage Onset in the Central Highlands of Vietnam.

Two distinct rainfall stages over the Central Highlands (CH) of Vietnam during the rainy season have been objectively defined using the high‐resolution Vietnam Gridded Precipitation data set for 1983–2010 (28 years): a second rainy stage (SRS) embedded in the conventional rainy season. Surprisingly, the pronounced interannual variation in the SRS onset date has led to three apparent regimes: an early (late) SRS with a 1 month longer (shorter) rainfall period occurring in early July (until mid‐August) and a normal SRS starting in late July. Almost all the early SRS years occur during El Niño developing phases, particularly during the Niño3.4 sea surface temperature (SST) increase from January through December. Water vapor budget analyses reveal that the interannual variation in the divergent water vapor flux is in response to the warmer July tropical Pacific SST anomalies, resulting in rainfall enhancement over the CH and eventually inducing early SRS onset. Plain Language Summary: The Central Highlands (CH) of Vietnam contribute up to 90% of the country's total coffee production and 25% of its total hydropower potential. A second rainy stage (SRS) is observed in late July in this region, which is distinct from the conventional rainy season that occurs in late April–early May. Because the onset of the SRS has a strong impact on coffee yield and hydropower potential in the CH, this study examines the climatology of and interannual variation in the SRS onset date during 1983–2010. An early (late) SRS with a 1 month longer (shorter) rainfall period occurs in early July (until mid‐August). Almost all the early SRS years occur during El Niño developing phases. An association between the early SRS onset years and strengthening of the water vapor flux convergence induced by the warmer July tropical Pacific SST anomalies is discovered. Key Points: A second rainy stage (SRS) is embedded within the conventional rainy seasonEarly (late) SRS with a 1 month longer (shorter) rainfall period occurs in early July (until mid‐August); a normal SRS starts in late JulyAlmost all the early SRS years occur during El Niño developing phases [ABSTRACT FROM AUTHOR]