2:10 PM - 2:25 PM
[AAS01-03] A Wall-like Sharp Downward Branch of the Walker Circulation above the Western Indian Ocean
★Invited Papers
Keywords:Walker Circulation, Intertropical Convergence Zone, Eastern Horn of Africa
In the zonal direction, the downward branch of the Walker circulation above the Indian Ocean is only 20 degrees thick, whereas the Pacific counterpart is 90 degrees thick. This zonal sharpness is remarkable 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 Intertropical Convergence Zone, which has direct implications for the dry climate in the Northeast Africa. Therefore, in this study, we refer to this zonally-thin 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 to interannual variability, a mass-weighted vertical mean of vertical motion at the Wall exhibits a tight linkage to the tropical Pacific, though total variance is explained more by local sea surface temperature.
Model experiments using a convection-permitting atmospheric general circulation model show that mountains in East Africa are necessary for the existence of the Wall. Vertical mixings forced by mountain waves play a fundamental role in strengthening the Wall. After flattening the East African topography, zonal discontinuity of the Intertropical Convergence Zone disappears.
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 to interannual variability, a mass-weighted vertical mean of vertical motion at the Wall exhibits a tight linkage to the tropical Pacific, though total variance is explained more by local sea surface temperature.
Model experiments using a convection-permitting atmospheric general circulation model show that mountains in East Africa are necessary for the existence of the Wall. Vertical mixings forced by mountain waves play a fundamental role in strengthening the Wall. After flattening the East African topography, zonal discontinuity of the Intertropical Convergence Zone disappears.