南海夏季风爆发与热带太平洋两类海温型关系的年代际差异
Interdecadal Change in the Relationship between the South China Sea Summer Monsoon Onset and Two Types of Pacific Sea Surface Temperature Anomaly
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摘要: 南海夏季风爆发日期在1993/1994年出现年代际偏早的转变,利用海温和再分析资料的研究证实西北太平洋增暖和两类海温型的年代际差异可能是导致此种变化的重要成因。进一步的研究揭示出在南海夏季风爆发出现年代际变化的背景下,南海夏季风爆发日期与太平洋海温的关系也出现明显的变化:1993/1994年之前的第一年代东太平洋(EP)型海温异常起主导作用,而1993/1994年之后的第二年代两类海温型均影响了季风爆发,但以中太平洋(CP)型海温异常为主。第一年代,东太平洋型增温(EPW)通过抑制西北太平洋-孟加拉湾的对流活动,在菲律宾海、孟加拉湾西部激发出两个距平反气旋,使越赤道气流建立偏晚、孟加拉湾低槽填塞、西北太平洋副热带高压增强,进而导致南海夏季风爆发偏晚,且其影响可从4月维持到5月;而中太平洋型增温(CPW)对季风爆发前期的流场无显著影响。第二年代,CPW通过抑制菲律宾-孟加拉湾东部的对流活动,在菲律宾-孟加拉湾激发出一个距平反气旋,使孟加拉湾低槽填塞、南海地区副高增强,进而阻碍季风爆发,且显著影响仅出现在4月;EPW对4月大气环流场的影响与第一年代较为接近,在菲律宾-孟加拉湾一带产生的风场、对流场异常稍弱于CPW,但其影响无法持续到5月。Abstract: The onset dates of the South China Sea summer monsoon(SCSSM) show a significant advancement around 1993/1994.Using sea surface temperature(SST) and reanalysis data, this interdecadal change was investigated and found to be probably attributable to the warming in the Northwest Pacific Ocean and the interdecadal differences between two types of SST anomaly in the Pacific Ocean.The results further revealed that the relationship between the timing of SCSSM onset and SST variability over the Pacific Ocean experienced a pronounced interdecadal change, accompanied by the interdecadal variation of the SCSSM onset.Eastern Pacific warming/cooling(EPW/EPC) had a primary impact on SCSSM onset in the first epoch(before 1993/1994).However, while both types of Pacific SST event affected SCSSM onset in the second epoch(after 1993/1994), the effect of central Pacific warming/cooling(CPW/CPC) was more significant.During the first epoch, EPW suppressed convective activity over the Northwest Pacific Ocean and Bay of Bengal.This suppressed convection then excited two anticyclonic circulations, located over the Philippines and the west of the Bay of Bengal, respectively.These patterns led to a delayed establishment of cross-equatorial flow, together with a significant weakening of the Bay of Bengal trough and an intensification of the western Pacific subtropical high, indicating that SCSSM onset was later than usual.The impact of EPW on the circulation variability before SCSSM onset could last from April to May.However, CPW seemingly had an insignificant influence on the preceding circulations.During the second epoch, CPW suppressed convective activity over the region from the Philippines to the eastern Bay of Bengal, which induced an anomalous anticyclonic circulation in that region.The anomalous anticyclone blocked the low trough over the Bay of Bengal and strengthened the subtropical high over the South China Sea, indicating the onset of SCSSM was earlier.The significant impact of CPW on the preceding circulations appeared in April rather than May.The impact of EPW on the circulation in April during the second epoch was similar to that during the first epoch.The anomalous wind field and convection induced by EPW, which could not sustain until May, were weaker than that forced by CPW.