Multi-time Scale Variation Characteristics Of Somali Jet And Its Impact On The Asian Monsoon

Using ERA-40, ERA Interim and NCEP/NCAR wind and Geopotential height field data,the composite indices are built to describe the activity characteristics of Somali jet. Thespace-time evolution characteristics and the multi-time scale activities of Somali jet isanalyzed synthetically and its influences on Asian climate especially on summer rainfall inChina are studied. Also, the possible factors that result in the change of jet are explored.Firstly, the indices of intensity and establish time as well as position of Somali jet aredefined and the spatial and temporal variations are also researched. The intensity of Somalijet exists period of9pentad and quasi6years and10~15years, but the interdecadalvariation characteristics are the most obvious. It sets up on the24th pentad and retreats on the62th pentad and has an earlier tendency. Eastern boundary position of the jet exists stronginterannual variability and the trend of west retreated become clear gradually. Each of thethree indices can describe the development features of Somali jet from different angles, andthe intensity has significantly positive correlation with the South Asian monsoon while thespace position has positive relation with the South China Sea monsoon.Secondly，the correlation coefficients between jet indices and time coefficient of summerrainfall spatial distribution pattern in China are calculated and the distribution of summerprecipitation as well as circulation in China on Somali jet abnormal years are discussed usingsynthetic analysis. The time coefficient of first three China summer precipitation EOFanalysis mode exist significant correlation with different indices sequence of jet.Theinfluence of position oscillation of jet on precipitation in China are asymmetric. When it setsup early, the summer precipitation become less in Northern China and more in SouthernChina and vice versa, also when the intensity gets weak, the rainfall become less in NorthernChina and more in Northern China and vice versa.The circulation of abnormal years of jet have better corresponding relation with rainfallin China and have close relationship with the Western Pacific subtropical high, the activitiesof Southern China warm current and cyclone or anticyclone circulation abnormality overrainfall region.Take the influence of its establishment for example, when the jet sets up early,the cyclonic circulation anomaly in the eastern Somali exists in the middle and lowatmosphere and east wind anomaly in the equatorial become stronger. The north wind anomaly in northern area are unusually strong while wind field convergence exist in coastalarea of Southern China. The Western Pacific subtropical high gets weak and the northwardvapor transportation of Southern China sea and bay of Bengal region is weak. When the jetsets up late, the convergence in northern region is stronger and the southern region is underthe control of anomaly north wind，which is not beneficial to the vapor transportation.Thirdly，the influences of Somali jet variations on rainfall in China in interannual andinterdecadal scale as well as in sub-monthly scale are studied. The preliminary analysisshows that the strength variations in May and June during early days of establish period havegood correlation with summer precipitation. The Somali jet intensity in interdecadal scale hasclose relation with same scale variation of China’s precipitation.Also, the relationshipbetween the Somali jet strength and rainfall in China’s typical correlation area exist obviousinterdecadal variation characteristics. In addition, China has more rainfall when in positiveanomaly situation but with less rainfall when in negative events in the jet main activitiesperiod in synoptic scale.Finally, the effect of the ground surface temperature and sea level pressure oncharacteristic of Somali jets are studied. The summer sea and land thermal and pressuredifference in Somali area in summer is closely related to the jet intensity and the two factorson continent of Africa and the Indian Ocean are respectively closed to the establishment ofjet.