Relationship Between Tropical Cyclone Activity Over The South China Sea And MJO Propagation

In order to study the tropical cyclone (TC) activity, the characteristics of intra-seasonal oscillation (MJO) over the South China Sea (SCS) was examined to discuss the relationship between the MJO and cyclones over the SCS by using the tropical cyclones data from year 1949 to 2009 and NCEP/NCAR reanalysis data et al. A new index named U index was created to describe the MJO activity over the SCS. The filtered time series of the zonal wind at 850hPa was divided into 8 phases. The tropical cyclone activity and atmospheric circulation features were studied according to the phase variation. In addition, the oscillation simulation ability of the regional climate model was tested and three groups of sensitive simulation tests were carried out to find out the most influential signal based on the lateral boundaries.The main conclusions are as follows: (1) The new U index was able to describe the MJO activity over the SCS effectively; (2) The MJO process was divided into 8 phases using the U index. The phase variation was found to be in accordance with the tropical cyclone activity over the SCS with less TC formation and landfall during inactive period (phase 4-6) while the situation was quite contrary during active period (phase 7-3); (3) The composite analysis indicated that there were negative OLR anomaly (strong convection), weak subtropical high and cyclonic shear during active MJO period, which created a favorable circumstance for TC generation. The majority of TC genesis locations came from the Northwestern Pacific and the TC from the SCS also increased evidently. The phase 2 and 6, regarded as the representation of the most active and inactive period of U index respectively, had the most significant characteristics of each period. Therefore TC activity was quite different accompany with the eastward propagation of convective centers of MJO; (4) The U index was found to have a phase deviation with the CPC-MJO index. As the CPC-MJO120 index had the highest correlation coefficient with other three indexes, it was treated as the deputy of the index to represent the SCS. The CPC-MJO index was found to be incapable of grouping the TC activity over the SCS into two main classes, although phase 3 and 7, as the delegation of the most active and inactive period, revealed such a tendency to some extent; (5) Convection was of great importance both for MJO and TC grow up. The composite analysis of the horizontal and vertical convective heating distribution according to different MJO phases was studied. The results showed that in different phases, the heating source was quite different. In combination with the convergence and divergence of the moisture transport fluxes, the modulation of TC activity by MJO can be revealed effectively from the energy aspect. Thus, the U index was more suitable for describing the MJO propagation eastward over the SCS and the wind at 850hPa corresponded well with the MJO phases as well; (6) Numerical simulation results showed that the intraseasonal oscillation signal comes from the north and west boundaries have the most influential impact on the MJO intensity over the South China sea, especially that from the north. It is supposed to be the propagation of the intraseasonal oscillation from the middle-high latitude to the low-latitude and the eastward propagation from the India ocean to the South China sea.