Research On Impacts Of SST On LFO In Tropical Atmosphere And Relation Between MJO In Southern Hemisphere And That In Northern Hemisphere

At first, periods and stability of Low-frequency Oscillation in Tropical Atmosphere are analyzed by a baroclinic ageostrophic model with the effect of SST and SSTA in this paper. Then, changes of periods and intensity of LFO at atmosphere are respectively studied in different period of SST change of the equatorial east and west Pacific during El Nino in 1997/1998. At last, relation between MJO in South China Sea and that in Southern Hemisphere are explored in early and late onset of Summer Monsoon over South China Sea . The main results are as follows:(1) In dynamical study about SST on LFO in the tropical ,we introduced a baroclinic ageostrophic model with the important effects of SST (SST heating and SST anomaly) for the characteristics of multi-scale LFO in the tropics. It is shown that: The Low-frequency Kelvin is eastward and Low-frequency Rossby is eastward under the effect of SST heating. And when SST is lower, wave is stable, and frequency and velocity are decreasing with the increase of SST ; when SST is higher, wave is instable, and period is decreasing with the increasing of SST. The impacts of SSTA on ultra-wave system are more evident than them on synoptic scale. So the effects of SSTA on LFO are studied by longwave approximation. Result indicates that SST anomaly can lead to longer period, moreover SST anomaly can easily produce instable LFO.(2) During El Nino in 1997/1998, change of period of LFO in atmosphere under the equatorial east Pacific are related not only with SST of the east equatorial Pacific but also with the rate of change of SST: During onset of El Nino, SST raised rapidly and periods of MJO increased. During development of El Nino, SST continued to raise slowly and periods of MJO continued to increase. During maturation of El Nino, SST is the highest, but periods of MJO decreased. Then, and periods of MJO increase when SST fall rapidly and El Nino die out. At last, when SST of the equatorial east Pacific resumed low temperature, periods of MJO also resumed low period. On the other hand, changes of periods of LFO in atmosphere under the equatorial west Pacific are mainly related with SST: When SST is lower, period of LFO are increasingwith the increase of SST; when SST is higher, period of LFO are decreasing with the increase of SST. These results are in accord with results of dynamical study basically.(3) The studies of the connection of MJO between SH and NH show: During Summer Monsoon over SCS, MJO show main oscillation of zonal wind, and the time when low-frequency easterly become westerly can imply approximately onset date of Summer Monsoon over SCS. The relation of MJO between SH and NH commonly exists and has interannual change: In earlier onset years, the increase of baroclinity round Mascarene High and Australian High separately lead to increase of southerly round Mascarene High and Australian High. Then, cross-equatorial flows become stronger about Somali and 85°E, respectively. Shortly, 105°E cross-equatorial flows becomes stronger. At last, the effects of 85°E and 105°E cross-equatorial flows together cause the establishment and strengthening of Summer Monsoon over SCS. In later onset years, the intensification of low-frequency westerly in SCS is dosed related to the reinforcement of barodinity of southern-hemisphere Mascarene high, and cross-equatorial flow northward around Somali. And the influences of Barodinity of Australian high, and 85°E, 105°E cross-equatorial flow are little.