Abstract
Madden-Julian Oscillation (MJO) plays an important role in modulating precipitation at Maritime Continent (MC) not only on a larger scale, but also in the diurnal cycle. Diurnal rainfall offshore propagation is one of the most evident features near coasts. This study investigates the impacts of MJO on diurnal rainfall and its offshore propagation at the western coast of Sumatra during boreal winters using ERA5 reanalysis. The real-time multivariate MJO (RMM) index was applied to locate the active MJO convection through eight different phases, in the western hemisphere and Africa in P8–P1, at the Indian Ocean in P2–P3, at MC in P4–P5, and the western Pacific Ocean in P6–P7. The rainfall characteristics, including the daily rate, the absolute and normalized diurnal variation amplitudes, and the strengths of diurnal offshore propagation, not only depend on active/inactive MJO stages but also vary under different MJO phases, through the combined modulations of large-scale backgrounds and local-scale land–sea circulations. The offshore rainfall propagation is associated with meso-large-scale gravity waves generated from land–sea thermal contrast and thus is affected by the radiation effect of cloud under different MJO phases. The stronger wave signals in P8–P1 and P6–P7 enhance the diurnal rainfall variation amplitudes away from the coast, while the strong coupling of moist convection with gravity waves contributes greatly to the diurnal rainfall cycle in P2–P3.
Highlights
Madden–Julian Oscillation (MJO) [1,2] is a dominant intra-seasonal tropical oscillation, characterized by an eastward propagation of about 5 m s−1 and a zonal shift of 12,000–20,000 km within 30–100 days [3]
This paper focused on the influences of winter Madden-Julian Oscillation (MJO) on the diurnal rainfall variation and its offshore propagation near western coast of Sumatra during 2011–2020 using ERA5 reanalysis
Days under strong MJO background were separated into four stages according to the locations of convective disturbance, respectively, at western hemisphere in P8–P1, at Indian Ocean in P2–P3, at Maritime Continent (MC) in P4–P5, and at western Pacific Ocean in P6–P7
Summary
Madden–Julian Oscillation (MJO) [1,2] is a dominant intra-seasonal tropical oscillation, characterized by an eastward propagation of about 5 m s−1 and a zonal shift of 12,000–20,000 km within 30–100 days [3]. The most significant oscillations and influences usually occurred during boreal winters [9,10,11,12,13]. Those findings improved our understanding of the formation and development of precipitation controlled by the large-scale forces during an MJO event. The diurnal variation of rainfall is a common phenomenon in coastal regions, including MC; it is often characterized by offshore propagation near the coasts, whose mechanisms are closely related to gravity waves [21,22,23,24], background-wind modulations [21,23,25], land–sea breezes [26,27,28,29], and cold pool outflows [30,31,32]
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