A Climatological Study On The Tropical Easterly Jet And Asian Summer Monsoon

Based on NCEP/NCAR reanalysis data, the structure and variation of Tropical Easterly Jet (TEJ) and its relationship with the distribution of rainfall in Asia and Africa are analyzed. In addition, the Asian summer monsoon indices are reappraised and the secular variation of monsoon is studied. Finally, the difference between NCEP/NCAR and ERA-40 reanalysis data is made for reference. The main results are as follows:(1) TEJ covers large tropical areas extending from the dateline to the west coastal regions in Africa. Its core is located at 150~100hPa above 10-15° N in southern Indian peninsula with the maximal speed larger than 30m/s. The first three EOF modes of TEJ indicate three different circulation patterns: EOF1 shows its basic condition, i.e. easterly, which is an upper branch of the Asian monsoon system, controls the tropics;EOF2 shows the circulation associated with ENSO;EOF3 indicates the circulation associated with AAO.(2) TEJ has intraseasonal oscillation (ISO) with periods of both 40-60 days and 20-30 days and interannual variation with periods of 2-4 years and 7-10 years. Generally it is negatively correlated with ENSO: TEJ weakens in warm events and strengthens in cold events. TEJ has obvious interdecadal variation, with 1978 its climate jump year. It is strong TEJ period before 1978, and TEJ weakens significantly after the climate jump.(3) In Asia and central Africa, the primary rainfall, convection, upper divergence and ascending motion appear in the right side of the entrance region and the left side of the exit region of TEJ. However it is not the case in West Africa. There are two opposite vertical circulations in the entrance region and exit region, which is closely associated with the distribution of rainfall.(4) To modify the monsoon index WYI, 150~100hPa instead of 200hPa is chosen to redefine a monsoon index DHI=U*850-U*(150+100). DHI is in agreement with WYI in describing the interannual variation of monsoon, while it is superior to WYI in describing the interdecadal variation and climate jump of Asian summer monsoon. DHI indicates that the monsoonal circulation weakens after its climate jump in 1980, which is caused by the weakening of the easterly at 150~100hPa, while easterly at 200hPa does not weaken significantly. After the year of 1980 in general, easterly in the upper troposphere weakens in Asia, showing the weakening of summer monsoon;the land-sea pressure difference and thermal difference decrease, resulting in the weakening of monsoon;the upper divergence and watervapor transport decrease in Indian peninsula, central Indo-China peninsula, North China and Northeast China, indicating the weakening of summer monsoon as well.