A Preliminary Study Of The Multiscale Variability Of The East African Monsoon Precipitation

The prevailing winds in the East African region change twice a year due to thermal differences caused by the migration of the intertropical convergence zone and the distribution of land and sea,resulting in strong East African monsoon.The East African monsoon is a largescale weather system that controls the western Indian Ocean,including East Africa,and regulates precipitation in East Africa.The East African monsoon precipitation occurs in two seasons,the long rain(March-May)and the short rain(October-December),each of which is characterized by multi-timescale variability,leading to unusual drought and flood events in East Africa,which have a significant impact on the production and livelihood of the local population.The East African monsoon is characterized by significant interannual and interdecadal variability and is influenced by the oceanic climate system at different time scales.Studies have shown that the short rain precipitation of the East African monsoon is closely related to ENSO and the Indian Ocean dipole,while the long rain precipitation has significant inter-decadal and intra-seasonal variability.Due to the lack of observations,the variability and influence mechanisms of the western Indian Ocean monsoon system,especially monsoon precipitation,have not been adequately studied internationally.This study took the variability of precipitation in East Africa and its influence mechanism as the starting point.Based on the observations,reanalysis data and numerical model results,analyzing the interannual variability of monsoon precipitation in East Africa,and revealing the influence process of different types of El Ni(?)o events on monsoon precipitation in East Africa,and providing new information for further development of monsoon precipitation prediction and forecast in East Africa.The main results of this study are as follows:(1)Based on buoy observations,the diurnal variation of the East African monsoon is analyzed and the international common atmospheric reanalysis data are evaluated.The results show that the diurnal variation of the East African monsoon system is clear,and the reanalysis data have some bias in reproducing the in-situ observations.The reanalysis data show that sealevel pressure and temperature deviate less from the actual,while the sea-surface wind field is more biased.Madden-Julian Oscillation in the tropical Indian Ocean may influence the accuracy of the surface wind description.(2)There are significant interannual variation in East African monsoon precipitation and noticeable differences in the response to different types of El Ni(?)o events.In the same year of different types of El Ni(?)o events occurred,there are significant differences in the spatial distribution of short rain precipitation,specifically: during the occurrence of eastern,central type I and central type Ⅱ El Ni(?)o events,the short rain precipitation in East Africa and the western Indian Ocean increases clearly,with more increase in the eastern type and less increase in the central type,among which type Ⅰ increases the least.Moisture budget analysis revealed that anomalous meridional advection and anomalous upward motion are the main influencing factors affecting water vapor transport during the short rain,together leading to an increase in rainfall,and the difference in the contribution of the dynamical terms are the main reason for the different effects of the three types of El Ni(?)o events on rainfall.Compared to the central Type Ⅰ and Ⅱ El Ni(?)o events,the eastern Type El Ni(?)o occurs during which the anomalous updrafts and anomalous northerly winds control a greater extent and rainfall is enhanced.(3)Based on the FIO numerical model,this study analyzed the simulations of precipitation in East Africa by the AMIP and CMIP 6 experiments.The results show that the simulations of the AMIP and CMIP 6 experiments for the long rain precipitation in the western Indian Ocean differ significantly from the actual,underestimating the importance of the long rain precipitation to the annual precipitation.The AMIP model has systematic biases in simulating the vertical transport of climatic water vapor by anomalous vertical motions and the transport of climatic water vapor by anomalous meridional winds,which in turn underestimate or overestimate the short rain precipitation under the influence of different types of El Ni(?)o events.(4)In 2000,Kenya experienced a historically drought.This study shows that the drought,which occurred during the long rain,was mainly due to a shortage of meridional water vapor supply.