In the zonal direction, the downward branch of the Walker circulation above the Indian Ocean is only 20 degrees wide, whereas the Pacific counterpart is 90 degrees wide. This zonal sharpness is notable because atmospheric disturbances smaller than the planetary scale, such as the Asian Summer Monsoon, can interact with the planetary-scale Walker circulation through this branch. As a moist circulation, this zonal sharpness is imprinted on a unique zonal discontinuity of the tropical rain belt above Northeast Africa. Therefore, in this study, we refer to this narrow downward branch as the “Wall”, investigate its climatology and interannual variability, and aim at determining its reason for existence. The strongest season of the lower tropospheric Wall in boreal summer is sustained by horizontal cold advection associated with the Asian Summer Monsoon. Two weak phases of the Wall correspond to two rainy seasons at the Eastern Horn of Africa, which are not reproduced well by state-of-the-art global climate models. As for interannual variability, one standard deviation change of a strength of the downward motion at the Wall is associated with about one degree of sea surface temperature variation in the tropical Pacific, and the regression and correlation coefficients are highest in boreal autumn. Nevertheless, total variance is explained more by local sea surface temperature. Experiments using a convection-permitting atmospheric model show that vertical mixing forced by mountain waves in East Africa are necessary for sustaining the Wall. After flattening the East African topography, zonal discontinuity of the tropical rain belt disappears.

Sea surface temperatures (SSTs) of the Gulf Stream and the Kuroshio are shown to be synchronized for the decadal time scale. This synchronization, which we refer to as the Boundary Current Synchronization (BCS), is associated with meridional migrations of the atmospheric jet stream. The singular value decomposition (SVD) between SST and zonal wind shows that, within the context of known climate modes, BCS can be understood as the covariability shared by the Pacific Decadal Oscillation and the Northern Annular Mode. Nevertheless, because the SVD time series exhibit high correlations with the zonal-mean meridional SST difference between the subtropics and the midlatitudes, BCS can be understood more simply as an oceanic annular mode. Air temperature regressed on the BCS index exhibits a similar spatial pattern to temperature observed in July 2018.

A possibly important dynamical process for the Madden–Julian oscillation (MJO) convective initiation is proposed. An MJO event during the “CINDY2011” field campaign is triggered by eastward-moving lower-tropospheric mixed Rossby-gravity (MRG) wave packets, and its leading precursor is predominance of upper-tropospheric MRGs in the Indian Ocean (IO). Simple three-dimensional model experiments reveal that the upper-tropospheric MRGs in the IO are amplified particularly in the western IO (WIO) by their westward advection and wave accumulation due to the upper-level convergence in mean easterlies of the Walker circulation. The model also predicts downward dispersion of the amplified upper-tropospheric MRGs and resultant lower-tropospheric MRG wave packet formation. This MRG evolution consistently explains the MJO initiation process during CINDY2011, which is further verified by ray tracing for MRGs. Upper-tropospheric circumnavigating Kelvin waves assist the proposed mechanism by promoting MRG-wave accumulation (advection) in their westerly (easterly) phases via enhanced zonal convergence and weakened easterlies (enhanced easterlies).

Climatological features regarding the sharp downward branch (SDB) of the Walker circulation above the Indian Ocean are comprehensively investigated. Compared to the Pacific downward branch, SDB has two distinctive features: two-peak seasonality and deep subsidence extension. The two weak phases of SDB in boreal spring and fall correspond well to the two rainy seasons at the Eastern Horn of Africa, which is not reproduced well by state-of-the-art global climate models. Unlike the Pacific counterpart, the annual-mean subsidence of SDB extends to the surface, and is supported by horizontal cold advection associated with the Asian Summer Monsoon. Two experiments using a convection-permitting atmospheric general circulation model show that mountains in East Africa, particularly the Ethiopian Highlands, is necessary for the existence of SDB. The dry and clear climate in the Northeast Africa, which is imprinted as a discontinuity of the Intertropical Convergence Zone, is sustained by the East African topography.