Study On Climatic Anomaly Of South Asian High

From the NCEP/NCAR and ECMWF/ERA40 reanalysis data, the characteristics and their corresponding causes of seasonal, interannual and interdecadal changes of south Asian high (SAH) are studied by the aid of statistical analysis and numerical modelling. The results are as following:This paper categorizes the various indices of SAH into two types, the intensity and domain indices including main center intensity index, the area index, the unit area intensity index, the westward extention index and the eastward extention index, all of which describe the intensity and domain of the SAH; the location indices, including main center longitude and latitude location index and the ridge location index, all of which describe the location of the SAH. Significant correlations are witnessed in the summer intensity and area indices, the stronger (weaker) the intensity of SAH, the larger (smaller) the total area of SAH. There is remarkable linkage between the intensity and the longitude location of SAH in winter, the enhancement of the intensity of SAH corresponding to the shift of the longitude location of SAH toward east, and negligible linkage between the intensity and the location of SAH in summer. The winter-spring-summer persistent anomaly of the intensity of SAH is revealed and has a close association with ENSO. The response of strength of SAH to ENSO starts from December, with the maximum anomalies value during January-May period, and the secondary maximum during June-September period. CAM3.0 simulations argue that the consecutive seasons anomaly of SAH intensity is caused by ENSO events, in winter, the mature period of the ENSO, the equatorial eastern Pacific SST play a important role to the anomaly of SAH; in spring, the decaying phase of ENSO events, both the equatorial eastern Pacific and the Indian Ocean play important role in the anomaly of SAH; in summer, the terminate of the ENSO events occur in the tropical eastern Pacific, and the Indian Ocean play a important role in the anomaly of SAH.The interannual variation of the intensity of SAH has a close relation to ENSO events. During the summer decaying stage of ENSO warm (cool) phase, the SAH becomes stronger (weaker) with its area larger (reducer). In the low and middle latitude, the zonal vertical circulation anomalies show upward (downward) motion in Indian Ocean and subsidence (ascent) in the eastern Pacific and western African, accompanied by two tropically symmetrical anticyclones(cyclones) over the middle-west Pacific to East Indian Ocean, cyclones(anticyclones) over the middle-west Indian Ocean and anticyclones(cyclones) over the Africa at 850hPa and corresponding cyclones-anticyclones-cyclones ( anticyclones-cyclones-anticyclones) abnormity at upper-level (at 200hPa) from the western Pacific to Africa. CAM3.0 simulations confirm that During the summer decaying stage of ENSO, the SST of Indian Ocean play a important role in the interannual variations of the intensity of SAH.The intensity of SAH undergo a substantial interdecadal intensification since the late 1970s, the intensity is stronger, the area is lager during 1978-2006 relative to the 1950-1978. The interdecadal variation of SAH is in close correlation with the Asian, African and Australian monsoon systems. Since the late 1970s, the low and middle latitude atmosphere present four-celled pattern of meridional vertical circulations in the potential function and divergence anomalous fields, with main ascending area over the East Pacific Ocean, the Indian Ocean(especially in winter)and the South China Sea to West Pacific Ocean (especially in summer) and main subsidence area over the Africa monsoon region and the Middle Pacific, and the pronounced patter of anomalous stream function fields, manifest as two tropically symmetrical cyclones over the East Pacific, two tropically symmetrical anticyclones over the Africa, two tropically symmetrical cyclones over Indian Ocean (especially in winter), remarkable anticyclone over the South China Sea to the West Pacific (especially in summer) and two tropically symmetrical anticyclones over the Middle Pacific. Numerical simulations verify the influence of tropical and subtropical SST on the late 1970s climate transition of SAH, the East Pacific Ocean SST is the dominate factor, the Indian Ocean SST also play a certainly important role.