A Wall-like Sharp Downward Branch of the Walker Circulation above the
Western Indian Ocean
Abstract
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.