Mechanisms For The Inter-decadal Variation Of The Relationship Between The Frequency Of Northwest Pacific Intense TC And ENSO

The Mechanism for the inter-decadal variation of the relationship between the intense tropical cyclone (TC) frequency over the western North Pacific (WNP) and the El Nino-Southern Oscillation (ENSO) are studied based on NCEP/NCAR reanalysis monthly data, Hedley Centre of SST monthly data and TC Best Track from JTWC during1949-2008. Our study found that the correlation coefficient between the WNP intense TC frequency and the Nino3.4SST index is0.69during1949-1968(the first period) and is0.70during1989-2008(the third period), respectively, while it is only0.07during1969-1988(the second period). Moreover, the intense TC frequency is much lower in the second period (with an average of4.85per year) than in the first and third periods (with averages of7.60and7.75per year, respectively). Not only the evolution of TC frequency is generated in the second phase shifts, but also the differences of strong TC generated location and the path show between the second period and the other two periods.The second period, strong TC mainly generated in the east of160°E, the south of20°N.The path is given priority to with moving westward.In the other two stage, the scope of strong TC mainly generated slants wide, extended to the north, the path is to give priority to turning to the north.Through the analysis of the inter-decadal variability, the results show that the low intense TC frequency and the impact of ENSO on intense TC frequency was largely suppressed in the second period are mainly associated with high sea surface temperature (SST)in the Indian Ocean during August-October,These lead to two anomalous anticyclones over the WNP and the tropical central Pacific, respectively. At the same time, The higher sea level pressure and tropospheric temperature, the lower middle humidity ratio,these factors to the typhoon possible Intensity (Maximum Potential Intensity, MPI) index have negative contribution in the north Pacific and south the China sea, The lower relative humidity, stronger vertical wind shear, and upper lever convergence/lower level divergence, these factors lead to the second period in the southeastern part of the Pacific northwest Potential Index (Genesis Potential Index, GPI) Index is negative anomalies. To generate the position and the transition of the path, from the steering current can be seen:the east wind shear abnormal in the second stage near the equator, lead to strong TC position by west to north,the path westward march is given priority to. Compared with the second period, the equatorial central Pacific is noticeably warmer during August-October in the third period, which leads to more TC geneses in the southeast quadrant of the WNP and also excites an anomalous large-scale cyclonic circulation in the lower troposphere over the WNP as the atmospheric Rossby response and this leads to cyclones over the WNP, thus facilitates the formation and strengthening of TC in the WNP and the effect of ENSO is improving. At the same time, The MPI over the whole tropical Pacific is larger in the third period than in the second period. This is mainly a result of the warmer SST and middle humidity ratio in the WNP. Middle-level relative humidity is more, the weaker vertical wind shear and850hPa absolute vorticity is abnormal, these factors lead to the third stage of GPI index positive in the southeast quadrant of the tropical cyclone generated in WNP. From steering current can be seen, there is a big cyclonic circulation and the equatorial westerly winds, is conducive to the path of the TC turn, and generate position by southeast. At the same time also for different types of ENSO phenomenon are analyzed,The results show that:the strong TC frequence in the central Pacific heating (CPW)than the eastern Pacific Ocean heating EPW is more conducive, Compared strong TC frequency in the eastern Pacific cold (EPC) and the eastern Pacific Ocean heating (EPW) is not conducive to the increase of the TC. At last,this article uses ECHAM mode is ECHAM4.6version.Using this model to simulate the possible mechanism of the three stages, the control experiment (CTL) was run with the observed climatological monthly mean SST. The first experiment, called the warm tropical Indian Ocean run (TIO), is forced by the observed climatological monthly mean SST superimposed by the tropical Indian Ocean SST difference between the second and the first periods. The second experiment, called the warm central Pacific run (CP), is forced by the observed climatological monthly mean SST superimposed by the central Pacific SST difference between the third and the second period. The third experiment, called the warm tropical Indian Ocean-central Pacific run (TIO-CP), is forced by the observed climatological monthly mean SST superimposed by the SST difference of the central Pacific and the tropical Indian Ocean between the third and the second period. Simulation results show that:The first stage to the second phase of change mainly because of the second phase in the Indian Ocean SST between August and October obviously is warmer than the first stage, And comparing the change of the second and the third stage, is due to the third stage between August and October in the equatorial Pacific SST warm significantly.