The climatological impact of the upper-tropospheric Rossby wave propagation on the Red Sea trough in winter

There is a prominent feature in the climate of the northeast Africa that, once activated, may cause heavy rainfall occurrence in large parts of the eastern Mediterranean (EM) and the Arabian Peninsula. The feature is Red Sea Trough (RST) which is an inverted trough of low pressure at lower troposphere wave over the northeast Africa and the Red Sea. This study investigates the way Rossby wave propagation in the upper troposphere may change the circulation of the lower troposphere and thus the surface weather in the northeast Africa, the Red Sea and the EM regions on a monthly time scale. The interaction between the wave activity flux at the 300 hPa level and the lower tropospheric circulation is investigated with consideration of the meteorological quantities at different levels of the troposphere in a wide region from the western North Atlantic to the EM. For this, two critical phases of the wave activity flux are defined. The critical positive (negative) phase corresponds to the months during which the amount of wave activity entering the region of study is higher (lower) than the climatological mean plus (minus) one standard deviation. The results show that during the critical positive phase, in the northeast Africa, the mean meridional circulation is strengthened throughout the troposphere. Further consequences of the positive phase are the strengthening and interconnection of the divergence cores of the Atlantic and Mediterranean storm tracks, strengthening of the RST and thus locally-unstable conditions leading to higher monthly precipitation in large parts of the study region. •Rossby wave activity flux and its divergence in the upper troposphere are investigated.•Two phases are defined for the wave activity entering the Red Sea and northeast African region.•The differences in tropospheric circulation between the two phases are diagnosed.•A definite relation is established between the critical phases and the RST activity.•The two critical phases are linked to the North Atlantic and Mediterranean storm tracks.