Dynamic Effects of the South Asian High on the Ozone Valley over the Tibetan Plateau

In this study,the TOMS/SBUV（Total Ozone Mapping Spectrometer/Solar Backscatter Ultraviolet Radiometer） data and SAGE（Stratospheric Aerosol and Gas Experiment） II data were employed to calculate the monthly total zonal ozone deviations over the Tibetan Plateau and the 150-50-hPa zonal ozone variations.The results show that there is a significant correlation between the two,with a correlation coefficient of 0.977.From 150 to 50 hPa,the ozone valley over the Tibetan Plateau（OVTP） becomes the strongest based on the SAGE II data,and the South Asian high（SAH） is the most active according to the 40-yr reanalysis data of the European Centre for Medium-Range Weather Forecasts（ERA40）,so a correlation between the SAH and the OVTP may exist.The WACCM3（Whole Atmosphere Community Climate Model version 3） simulation results show that both SAH and OVTP could still present within 150-50 hPa with reduced strength even when the height of the Tibetan Plateau was cut down to 1500 m.It is also shown that the seasonal variation of SAH would result in a matched seasonal variation of the OVTP,which suggests a meaningful effect of SAH on the OVTP.Meanwhile,it is found that the atmospheric circulation would impose different effects on the OVTP,depending on the SAH＇s evolution stages and movement directions.At 150-50 hPa,as the SAH approaches the plateau,the SAH zonal（meridional） transport would make the OVTP deeper（shallower）,while the vertical transport of ozone produces a deeper（shallower） OVTP at the lower（higher） level;the combined dynamic effects lead to a weakened OVTP.When the SAH stabilizes over the plateau,the zonal（meridional） transport results in a shallower（deeper） OVTP while the vertical transport would create a deeper（shallower） OVTP at the middle（bottom and top） levels;the combined dynamic effects produce a deeper OVTP.As the SAH retreats from the plateau,the OVTP becomes deeper（shallower） under the zonal（meridional） effect or shallower under the vertical effect;the combined dynamic effects contribute to a deeper（shallower） OVTP at the middle（bottom and top） levels.The SAH would have a weak effect on the OVTP over the plateau when positioned over the tropical Pacific.

Current Progresses in Study of Impacts of the Tibetan Plateau on Asian Summer Climate

The current progresses in the study of impacts of the Tibetan Plateau on Asian summer climate in the last decade are reviewed. By analyzing evolution of the transitional zone between westerly to the north and easterly to the south （WEB）, it is shown that due to the strong heating over the Tibetan Plateau in spring, the overturning in the prevailing wind direction from easterly in winter to westerly in summer occurs firstly over the eastern Bay of Bengal （BOB）, accompanied with vigorous convective precipitation to its east. The area between eastern BOB and western Indo-China Peninsula thus becomes the area with the earliest onset of Asian monsoon, which may be referred as BOB monsoon in short. It is shown that the summertime circulations triggered by the thermal forcing of the Iranian Plateau and the Tibetan Plateau are embedded in phase with the continental-scale circulation forced by the diabatic heating over the Eurasian Continent. As a result, the East Asian summer monsoon is intensified and the drought climate over the western and central Asian areas is enhanced. Together with perturbations triggered by the Tibetan Plateau, the above scenarios and the associated heating have important influences on the climate patterns over Asia. Furthermore, the characteristics of the Tibetan mode of the summertime South Asian high are compared with those of Iranian mode. Results demonstrate that corresponding to each of the bimodality of the South Asian high, the rainfall anomaly distributions over Asia exhibit different patterns.

The Earliest Onset Areas and Mechanism of the Tropical Asian Summer Monsoon

The multi-yearly averaged pentad meteorological fields at 850 hPa of theNCEP/NCAR reanalysis dada and the TBB fields of the Japan Meteorological Agency during 1980-1994 areanalyzed. It is found that if the pentad is taken as the time unit of the monsoon onset, then thetropical Asian summer monsoon (TASM) onsets earliest, simultaneously and abruptly over the wholearea in the Bay of Bengal (BOB), the Indo-China Peninsula (ICP), and the South China Sea (SCS), eastof 90°E, in the 27th to 28th pentads of a year (Pentads 3 to 4 in May), while it onsets later inthe India Peninsula (IP) and the Arabian Sea (AS), west of 90°E. The TASM bursts first at the southend of the IP in the 30th to 31st pentads near 10°N, and advances gradually northward to the wholearea, by the end of June. Analysis of the possible mechanism depicts that the rapid changes of thesurface sensible heat flux, air temperature, and pressure in spring and early summer in the middleto high latitudes of the East Asian continent between 100°E and 120°E are crucially responsiblefor the earliest onset of the TASM in the BOB to the SCS areas. It is their rapid changes thatinduce a continental depression to form and break through the high system of pressure originallylocated in the above continental areas. The low depression in turn introduces the southwesterly tocome into the BOB to the SCS areas, east of 90° E, and thus makes the SCS summer monsoon (SCSSM)burst out earliest in Asia. In the IP to the AS areas, west of 90° E, the surface sensible heatflux almost does not experience obvious change during April and May, which makes the tropical Indiansummer monsoon (TISM) onset later than the SCSSM by about a month. Therefore, it is concluded thatthe meridian of 90° E is the demarcation line between the South Asian summer monsoon (SASM, i.e.,the TISM) and the East Asian summer monsoon (EASM, including the SCSSM). Besides, the temporalrelations between the TASM onset and the seasonal variation of the South Asian high (SAH) arediscussed, too, and it is found that there are good relations between the monsoon onset time and theSAH center positions. When the SAH center advances to north of 20°N, the SCSSM onsets, and tonorth of 25° N, the TISM onsets at its south end. Comparison between the onset time such determinedand that with other methodologies shows fair consistency in the SCS area and some differences inthe IP area.