西太平洋暖池冰期旋回中的类ENSO式演化及其驱动机制

ENSO-like patterns and its driving mechanism in Western Pacific Warm Pool during the glacial cycles

作为全球接受太阳辐射最多、表层海水温度最高的区域,西太平洋暖池区通过厄尔尼诺-南方涛动(El Nino-Southern Oscillation,ENSO)和季风等过程影响着全球气候的变化。越来越多的沉积记录证明,在地质历史时期西太平洋暖池也存在类似于现代ENSO过程的“类ENSO式”变化。而目前类ENSO式变化与冰期—间冰期旋回之间的响应关系和驱动机制及其与东亚季风的关联仍存在争议。本文利用位于暖池核心区的B10岩心浮游有孔虫氧同位素、Mg/Ca(质量分数比)和黏土矿物参数重建了暖池区氧同位素8期以来的古气候记录,并结合已有的热带海表温度记录、中国石笋氧同位素和南大洋地区海表温度记录,研究了西太平洋暖池冰期旋回中类ENSO状态的演化规律及其与东亚季风的关系,并探讨了暖池区类ENSO演化的驱动机制。结果发现:冰期时,西太平洋暖池区温跃层变浅,赤道东、西太平洋温差减小,同时,东亚夏季风减弱,暖池区降水量相对减少,与现代El Nino时期气候态类似;间冰期时,西太平洋暖池区温跃层加深,赤道东、西太平洋温差增大,东亚冬夏季风增强,暖池区降水量相对增加,与现代La Nina时期气候态类似。频谱分析结果表明,西太平洋暖池区海表温度的变化具有偏心率周期(96 ka)。冰消期时,低纬度太阳辐射量的增加,增大了纬向上的SST梯度,并使得次表层海水储存了更多的热量,积累的热量会通过调节次表层环流向暖池区的热传输,最终调控赤道太平洋地区Walker环流强度和ENSO活动的长期变化。而冰期时,南大洋地区降温所引起的东南信风和大洋环流异常可能对类ENSO式起到调控的作用。

The Western Pacific Warm Pool(WPWP)region affects the global climate change through El Nino-Southern Oscillation(ENSO)and monsoons,as it receives the most solar radiation and has the warmest surface temperature in the world.Abundant sedimentary records indicate WPWP also experienced ENSO-like events throughout geological history.However,the relationships between ENSO and glacial-interglacial cycles or East Asian Monsoon(EAM),as well as the driving mechanism behind ENSO-like patterns,still remain unclear.In this study,we reconstructed the paleoclimate records since Marine Isotope Stage(MIS)8 by using oxygen isotope,Mg/Ca ratio from planktonic foraminifera,and clay mineral proxy data obtained from sediment core B10 at WPWP.Combined with previously published paleoenvironment records of tropical region,δ^(18)O data of Chinese cave stalagmite,and sea surface temperature records of the Southern Ocean,we studied the evolution of ENSO-like patterns during glacial-interglacial cycles and their relationship with EAM,and discussed the driving mechanism behind ENSO-like patterns.We found that during the glacial period the thermocline in WPWP became shallower,with the temperature gradient decreasing between the Eastern Equatorial Pacific and WPWP.Meanwhile,weakened East Asian Summer Monsoon(EASM)was accompanied by reduced rainfall,similar to modern El Nino.During the interglacial period,the thermocline in WPWP deepened,the temperature gradient along the equatorial Pacific increased,and EASM strengthened with increased precipitation,which was very similar to modern La Nina.Spectral analysis revealed the sea surface temperature in WPWP followed a significant eccentricity cycle(96 ka).During the interglacial period,increasing solar radiation in tropical region increased the latitudinal sea surface temperature gradient,and led to heat accumulation in subsurface water.The accumulated heat in turn regulated heat transfer to WPWP via subsurface circulation and ultimately controlled the long-term changes in the Walker circulation intensity and ENSO-like patterns in equatorial Pacific.During the glacial period,the southeast trade wind and ocean circulation anomalies caused by the cooling of the Southern Ocean might also regulate the ENSO-like patterns.