The Spatial And Temporal Variation Characteristics Of The South Asia High And Western Pacific Subtropical High On Multiple Time Scales And Their Relations With East Asian Summer Monsoon

The Western Pacific subtropical high (WPSH) and South Asian high (SAH) are closely related to the East Asian summer (June-July-August, JJA) monsoon (EASM). Previous researches found a close relationship between the WPSH and SAH on the inter-annual time scale in present-day climate. However, their behaviors and relationship are rarely discussed on millennial time scale during the Holocene. In this study, we explore the spatial and temporal variation characteristics of the WPSH and SAH and their relations with EASM on millennial time scale for the period of Holocene from 9.5 ka B.P. to 0 ka B.P. using the model results from a series of numerical experiments in an atmosphere-ocean-sea ice coupled climate model, the Kiel Climate Model (KCM). In addition, the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis dataset is used in this study to compare with the model results. Finally, considering the orbital forced change in insolation, we analyse the mechanism of spatial and temporal evolution of WPSH and SAH and their relations with EAWM.Using the NCEP/NCAR monthly mean reanalysis data from 1948 to 2013 and climate simulation results during the Holocene, the characters of the South Asia high (SAH) and western Pacific Subtropical high (WPSH) were analyzed. The behavior of the SAH showed a complete opposite trend with the WPSH, displaying a westward shift before 5 ka B.P. and an eastward shift after then. According to the data from 1948-2013, it is found that there are significant interannual and interdecadal variations in the zonal shift of the SAH, which suffered abrupt changes in the late 1970s and early 1990s. The contrast in the behavior between the WPSH and SAH on millennial time scale during the Holocene resembles to that on inter-annual time scale in present-day climate. The westward (eastward) extension of the SAH was accompanied by an eastward (westward) extension of the WPSH both on decadal and millennial time scales.Numerous studies have shown abnormal movement of SAH and WPSH can directly affect the strength of the Asian summer monsoon, thereby affecting the summer precipitation in Eastern China. In order to explore the specific way affecting EASM and precipitation, simulation is carried out during Holocene using the EASM index defined by Wang B. The evolution of the EASM on millennial time scale during the Holocene is closely associated with the zonal migration of the WPSH and SAH. The model results show a weakening trend of EASM since 9.5ka B.P.. The stronger EASM during the early Holocene (9.5-7 ka B.P.) was accompanied by a gradual westward (eastward) shift of the SAH (WPSH). In the mid-Holocene (7-4 ka B.P.), the EASM shows a multi-centennial time scale oscillations with the SAH (WPSH) relatively located between 112°-115°E(145°-155°E). During the late Holocene (4.5～0 ka B.P.), the EASM intensity decreases continuously with the SAH (WPSH) shifting eastwards (westwards) gradually. In addition, through analysing of the wind fields during Holocene, we reveal the effects of SAH and WPSH on EASM. On interannual scale, the EASM index has obvious interannual oscillation, but the correlation with SAH is not strong, because of the factors affecting the summer monsoon is quite complex. In addition, the analysis of the summer precipitation during 1948-2013 showed that accompanied by an westward shift of the WPSH and a eastward shift of the SAH, the precipitation increases in the middle and lower reaches of the Yangtze river and decreases in north China and south China. On the contrary, the precipitation increases in south China and decreases in the middle and lower reaches of the Yangtze river and north China.By analyzing Holocene evolution of spring insolation at the equator and the tropical sea surface temperature (SST) during summer season (JJA), it is suggested that the spatial and temporal evolution of the SAH and WPSH is closely related to the orbitally induced changes in spring insolation at the equator. Model result shows that since the early Holocene the spring insolation at the equator and the western pacific SST gradually decreased until mid-Holocene, and then they gradually increased until present day. This trend is consistent with the zonal migration of SAH and the summer mean surface temperature of tropical Indian-western Pacific ocean(averaged over 70° E-140° E and 5° N-20° N). The SAH has a positive correlation with the summer mean surface temperature of tropical Indian-western Pacific ocean, which then forces a Matsuno-Gill-type response that strengthen the SAH. The result means that the increasement of the equatorial spring insolation may result in the summer mean surface temperature of tropical oceans leading to the eastward (westward) shift of SAH (WPSH).In summary, the evolutions of the WPSH and SAH on millennial time scale are probably controlled by spring insolation at the equator. The formation of the SAH is closely related to the land surface heating of the Tibetan Plateau that is affected by the tropical Indian Ocean, whereas the WPSH is associated with the Hadley Circulation that is influenced by the tropical ocean. The tropical oceans receive most of the solar insolation in spring that exerts a sustained impact on the tropical ocean in summer. The extension of the WPSH and SAH is closely related to their strength. Therefore, the tropical oceans can influence zonal migration of the WPSH and SAH via affecting their intensity.