The Effect of an Active Phase of the Madden-Julian Oscillation on Surface Winds over the Western Coast of Sumatra Island

This study examined the effect of an active phase of the Madden-Julian Oscillation (MJO) on surface winds over the western coast of Sumatra Island using surface meteorological observations, meteorological radar observations, and balloon sounding data obtained from the Years of the Maritime Continent (YMC) Sumatra 2017 field campaign. Wind gusts (sudden and large increases in surface wind speed with durations of several to tens of minutes; squall) were observed frequently at Bengkulu from late November to early December 2017 during a local active phase of the MJO. The wind gusts occurred simultaneously with heavy precipitation when the eastward propagating convection from the Indian Ocean moved over the observation site. Upper air observations showed strong northwesterly winds in the lower troposphere in the 1,000–4,000 m layer. The surface wind gusts extended from the surface up to an elevation of 560 m, with the highest wind speed occurring approximately 300 m above the ground. Small changes in surface air pressure were observed during the wind gusts. The results suggested that momentum transfer from the upper atmosphere by downdrafts was the main cause of the surface wind gusts on the western coast of Sumatra Island during an active phase of the MJO.