A Wall‐Like Sharp Downward Branch of the Walker Circulation Above the Western Indian Ocean

Abstract

AbstractIn the zonal direction, the downward branch of the Walker circulation above the Indian Ocean is only 20° wide, whereas the Pacific counterpart is 90° wide. This zonal sharpness is notable because atmospheric disturbances smaller than the planetary scale can interact with the planetary‐scale Walker circulation through this branch. This zonal sharpness is also imprinted on a unique zonal discontinuity of the tropical rain belt above Northeast Africa. Therefore, 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, whereas another peak in boreal spring is explained by the seasonal migration of zonal winds. Two weak phases of the Wall correspond to two rainy seasons at the Eastern Horn of Africa, which are not reproduced well by the state‐of‐the‐art global climate models. Experiments using a convection‐permitting atmospheric model show that vertical mixing forced by mountain waves in East Africa is necessary for sustaining the Wall. After flattening the East African topography, zonal discontinuity of the tropical rain belt disappears. As for interannual variability, one standard deviation variability of vertical motions at the Wall is associated with one degree of sea surface temperature in the tropical Pacific, and the relationship is strongest in boreal autumn. Nevertheless, total variance is explained more by sea surface temperature in the tropical Indian Ocean.