The Seasonal And Interannual Features Of Ascending Motion In The Intertropical Convergence Zone In The North Pacific

In this paper the seasonal and interannual features of ascending motion (ω)intensity and position in the intertropical convergence zone (ITCZ) in the North Pacific, the influence of ENSO on ITCZωand the interaction between ITCZ co and cross-equatorial flow, Australian High have been investigated, in the last part of which, the anomalous features of subtropical high, cross-equatorial flow, Australian High and ITCZωduring the Huaihe River Basin flood in 2003 and 2007.The intensity of ITCZωis calculated with the sum of value ofωwhich is less than-0.02 multiplied by the grid area divided by the area at the range of 140-170°E (140-90°W),0-30°N in the western (eastern) Pacific(The area is only taken to 15°N in the eastern Pacific when it reaches to the land).And the index of ITCZωposition is presented by the latitude where there is minimum value of theωaverage of 140-170°E (140-90°W) in the western (eastern) Pacific. In long-term average the strongest intensity of ITCZωin the western (eastern) Pacific is on the top(bottom) of troposphere and the weakest one is on the bottom(top). The most northern position in the western Pacific is almost the same as each other on three levels. In the eastern Pacific the most northern (southern) position is on the top (bottom). The strongest intensity in the western (eastern) Pacific is in August (August and September) and the most northern position is in September (September). The weakest intensity and the most southern position are in February (February and March). In the eastern Pacific the intensity is also weaker in November. The time that ITCZ w intensity reaches to strongest (weakest) corresponds to the rapid northward(southward) shift of position. When the position is more northern (southern), the intensity is stronger (weaker).And there are changes in position ahead of that in intensity.By Empirical Orthogonal Function analyses (EOF) for climatic data and anomaly data, it is found that the first mode of climatic data is the basic meteorological state ofωand the second mode is the seasonal fluctuation ofωposition. The first mode of anomaly data is east-west dipole mode and in an 4.8-year cycle by spectrum analysis of the time series. In Elnino years, the intensity in the western Pacific is weaker than normal years and in Lanina years it is stronger. The position of ITCZωin the eastern Pacific is more southern than normal years in Elnino years and is more northern in Lanina years. The position of ITCZωin the western Pacific and the intensity of ITCZωin the eastern Pacific have interdecadal change after 1999. In general, ENSO has great influence on the position of ITCZωin the eastern Pacific and the intensity of ITCZωin the western Pacific.The intensity of ITCZωis more affected than that of position by the cross-equatorial flow and Australian High. The correlation coefficient between the cross-equatorial flow and the intensity of ITCZωof 925hPa is larger than that of 850hPa.The location of subtropical high ridge was southward of the mean and maintained steadily, northern boundary of 588 located north of the Huaihe River Basin and its western boundary was westward of the mean. The cross-equatorial flows east of 90°E and cold air form the Southern Hemisphere were weaker. ITCZ and ITCZωwere weaker and more southern than the mean. These factors led to Huaihe River Basin flood in 2003 and 2007.There were differences among these factors between 2003 and 2007. The subtropical high ridge was more southern in 2003 than 2007. The date that 588 northern boundary of subtropical high moved over 30°N in 2007 was earlier than that in 2003 and the number of times which moved over 30°N in 2007 were more than that in 2003. The subtropical high was weaker in 2007 and its stability was worse. The cross-equatorial flow near 125°E was also weaker than normal.ITCZ w in 2007 was weaker than that in 2007 and the cold air form the Southern Hemisphere in 2003 was weaker than that in 2007 although it lasted longer.