Tropical coastlines robustizing stratospheric quasi-biennial cycle

Robust existence of the equatorial-stratospheric quasi-biennial oscillation (QBO) ^{1, 2} affecting the global climate ^3,4,5,6 has been explained by heating-independent wave-mean flow interaction ^7,8,9 , but requested bidirectional (east/westward propagating) waves generated in the troposphere have never been ascertained. The strongest mode of heating on the equatorial lands with diurnal cycles drives the sea-land breeze circulations (SLBCs) centered at the Indonesian maritime continent (IMC) with the longest coastline ^10,11,12 , which organize convective clouds maintaining global atmospheric energy and water cycles ¹³ . Here we show from improved objective re-analysis data (JRA-55) ¹⁴ that SLBCs are the bidirectional (land/seaward) waves maintaining QBO. Among these “SLBC waves”, slow zonal and all meridional waves are absorbed by interactions with the tropospheric Walker and Hadley circulations, and only suitably fast zonal waves are propagated into the stratosphere. The momentum flux divergence per unit mass ±2 m/s/month corresponds to a zonal wind acceleration ±30 m/s/1.25 year of QBO. These findings reveal that QBO is dependent on insolation, Earth’s rotation (daytime length), and the distribution of coastline as the triple boundary among land, sea and air, which are changed with tectonics and climate. Equatorial SLBC-resolving models make climate change prediction more accurate.