【作者】 孟庆佳；

【导师】 王凡； Mojib Latif；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 物理海洋学， 2010， 博士

【摘要】 本文利用德国莱布尼茨海洋研究所(IFM-GEOMAR)的海洋-大气耦合模式Kiel Climate Model (KCM)和由观测海表面温度驱动的ECHAM5大气环流模式,分别研究了北赤道流分叉季节、年际变化特征和Walker环流百年变化趋势,及其相关的海洋、大气变化和耦合响应过程;分析指出控制北赤道流分叉年际变化的两个海洋环流模态,即NMK模态和MK模态,并通过分析与之相关的海洋大气变化过程,分析了它们的形成机制以及与El Ni?o-Southern Oscillation (ENSO)循环系统的关联;结合海表面温度、海平面压强、降水、风场等各种观测数据,分析了Walker环流在20世纪对全球变暖的响应,指出Walker环流受SST纬向梯度变化的影响而增强。主要结果如下:KCM海气耦合模拟结果表明,北赤道流分叉在东亚季风的驱动下表现出明显的季节变化,春、夏季节偏南,而秋、冬季节偏北。同时,北赤道流分叉还存在明显的2-4年和10年左右的年际变化,在El Ni?o期间位于较高纬度,而在La Nina期间位于较低纬度。北赤道流分叉年际变化与菲律宾以东海洋经向流速异常的前2个EOF模态密切相关。其中,第一EOF模态,即NMK模态方差贡献率为51%,具有近4年的周期,反应了副热带和热带流涡的整体变化。该模态的主要特征是NMK流系增强,北赤道流分叉位于较低纬度。该模态与海洋、大气的回归与相关分析结果表明,北太平洋副热带流涡和热带流涡增强,大气环流异常呈相应的偶极子分布,暖池及赤道太平洋、黑潮海域增温多雨,棉兰老流及菲律宾海海域降温少雨。NMK模态滞后于ENSO循环2个季节左右,其形成、演变过程受ENSO影响显著。在ENSO形成和爆发过程中,通过遥相关作用,产生西太平洋大气环流异常,副热带海洋流涡发生相应变化,进而通过ENSO正反馈,热带海洋流涡发生变化,产生局地的海洋-大气动力热力偶极子现象,NEC分叉和局地气候发生相应变化。黑潮和棉兰老流的变化对其流经海域的温度、热含量产生影响,进而影响降雨。当ENSO事件衰退和转换时,通过赤道海域反向的正反馈,使得赤道西北太平洋的大气环流异常逐渐消亡,随之海洋环流偶极子崩溃,引起副热带和热带流涡的先后变化,进入NMK模态反位相。尽管NMK模态滞后ENSO循环2个季节,且具有密切的海气耦合响应关系,但仍具有一定的独立性。第二EOF模态,即MK模态方差解释率是26%,具有准两年的周期,反映了NMK流系在大洋西边界附近的区域变化,同观测的棉兰老流流量准两年振荡周期一致,该模态的主要特征为棉兰老流增强,源地黑潮减弱,北赤道流分叉位于较高纬度。该模态与海洋、大气的回归与相关分析结果表明,从黑潮到菲律宾沿岸南向带状异常冷中心、降水减少,赤道呈El Nino-like SST,热带西北太平洋呈气旋式大气和海洋环流异常、降水增多。MK模态与东亚季风的准两年振荡紧密相连:东亚季风异常和菲律宾海的大部分海域上空出现大气环流异常,影响NEC分叉的南北向移动,使MC发生变化,MK模态形成,通过海洋过程,对赤道海域的SST、热含量、风场和海流产生影响,有利于ENSO的产生,通过遥相关作用在菲律宾海域形成反向大气环流异常,进入MK模态循环的反位相。尽管MK模态有利于ENSO的产生,但并不是其产生的充分条件。近来研究表明Walker环流在20世纪中减弱,本文利用各种观测数据结合由观测SST驱动的大气环流模式ECHAM5,发现Walker环流在20世纪很可能是增强而不是减弱的。观测的赤道印度-太平洋海盆间的海表面温度自1870年起呈现出纬向不对称现象:赤道太平洋增温较少,而在赤道印度洋增温显著。这导致沿赤道的海盆间海表面温度梯度的显著增强,而这种温度梯度的变化,正如由观测SST强迫的大气模式模拟那样,会驱动异常的大气环流,使得Walker环流增强,。模拟结果表明,伴随着西太平洋表层风场的变化,印度洋降水异常增多,而西太平洋降水异常减弱。这个结果对于全球变暖的大环境非常重要,海盆间的相互作用对于赤道海域区域气候的改变很可能具有关键性的作用。更多还原

【Abstract】 The seasonal and interannual variability of the bifurcation latitude of the North Equatorial Current (NEC), Walker Circulation variation during 20th century and relative ocean-atmosphere coupled responses are investigated with a control simulation in the Kiel Climate Model (KCM) and an atmospheric general circulation model (AGCM) forced by observed SSTs. There are two oceanic circulation modes about the interannual variability of the NEC bifurcation latitude (NBL), which is NMK Mode related to the variations of the NEC-Mindanao Current (MC)-Kuroshio Current (KC) (NMK) system and MK Mode related to the variations of MC and KC. We analyze the mechanism and the connection with El Ni?o-Southern Oscillation (ENSO) through ocean-atmosphere coupled responses within the two modes. We re-investigate the behavior of the Walker Circulation using different observational datasets and ensemble integrations. The Walker Circulation may have strengthened forced by the spatial structure of the long-term trend in tropical SST.As it shows in KCM, the seasonal cycle of the NBL is dominated by East Asian Monsoon through the local Ekman Pumping with in southern part during spring and summer, while in northern part during autumn and winter. The NEC bifurcated in higher (lower) latitude during El Ni?o (La Ni?a) events, with a period time as 2-4 years and a longer time in the interannual and interdecadal variability of NBL.The interannual variability of NBL is highly related to the first two Empirical Orthogonal Function (EOF) modes for the meridional velocity averaged within a 4°-longitude band off the Philippine coast. The first mode named NMK Mode with a variance of 51% is mainly related to variations of the Subtropical Gyre (STG) and Tropical Gyre (TG) as a 4-year period time. It shows an enhancement of the NMK system with low latitude of NEC bifurcation. The linear regressions of ocean and atmosphere onto NMK Mode show strengthens of the STG and TG with a dipole for the atmospheric and oceanic circulations with warming and more precipitation in Warm Pool, equatorial Pacific and KC region while cooling and less precipitation in MC region. It is highly influenced by ENSO cycle with 2-season lag for the formation, evolvement and decay of the NMK Mode. During the developing and mature of ENSO warm (cold) event, there are both atmospheric and oceanic anti-cyclone (cyclone) circulations in northern part of the equatorial Western North Pacific due to the teleconnection, then cyclone (anti-cyclone) circulations in southern part due to the ENSO positive feedback and Hadley Circulation change to compose the dipoles for atmospheric and oceanic circulations. Thus, both subtropical gyre and tropical gyre are intensified (weakened) in turn, corresponding with the NMK Mode and southern (northern) part of NBL. The climate changes are high related to the oceanic-atmospheric coupled responses with a warming (cooling) and more (less) precipitation in East China Sea, but cooling (warming) and less (more) precipitation in Philippine coast. The anomalous atmospheric circulation decreases during the decay of ENSO warm (cold) events. Then the NMK Mode starts to decay due to weaken of the STG and TG with a northward shift of NBL. The NMK Mode is highly influenced by the ENSO cycle with high connection to the ocean-atmosphere coupled responses. However, it shows the independence upon ENSO to some extent.The second mode named MK Mode with a variance of 26% is mainly related to variations of the western currents of NMK system as a quasi-biennial oscillation (QBO). It shows an increase of MC but decrease of KC with high latitude of NBL. The linear regressions of ocean and atmosphere onto MK Mode show a cold band from KC to the Philippine coast, an El Ni?o-like SST pattern in the equator and both atmospheric and oceanic cyclone circulation in the Tropical Western Pacific with more precipitation. The MK Mode is highly correlated to the QBO of East Asian Monsoon with 1-2 seasons leading ENSO cycle. The atmospheric cyclone (anti-cyclone) circulation in the Philippine Sea corresponding with the strong (weak) East Asian winter monsoon anomaly resulted in the NBL at high (low) latitude with decrease (increase) of KC while increase (decrease) of MC, cooling (warming) and less (more) precipitation, meanwhile, there is a warming (cooling) and more (less) precipitation in east of Philippine coast. Then, it forms local air-sea dynamics and thermal positive feedbacks which make the MK Mode persistence and warming (cooling) in Equatorial Central and Eastern Pacific via oceanic process. This may excite the El Ni?o (La Ni?a) event and an anti-cyclone (cyclone) in the Philippine Sea through teleconnection, which will lead East Asian winter monsoon to be weak (strong) and then into the other phase of MK Mode. MK Mode is favor of ENSO through oceanic process via changes of MC due to the NBL shifts by wind variations. However, it is not a substantial condition. Recent studies indicate a weakening of the Walker Circulation during the 20th century. Here, we present evidence from sea surface temperature (SST) observations and an atmospheric general circulation model that the Walker Circulation may have intensified rather than weakened. Observed Equatorial Indo-Pacific Sector sea surface temperature since 1870 exhibited a zonally asymmetric evolution: While the Equatorial Pacific shows only a weak warming, the Equatorial Indian Ocean exhibited a rather strong warming. This has resulted in a strong increase of the SST gradient between the two ocean basins along the Equator. The change in the SST gradient drove an anomalous atmospheric circulation, with an enhancement of both Walker and Hadley Circulation, as inferred from ensemble experiments with the atmosphere model forced by the observed SSTs. Anomalously strong precipitation is simulated over the Indian Ocean and anomalously weak precipitation over the western Pacific, with corresponding changes in the surface wind pattern. Sensitivity to the forcing SST, however, is noticed. The results may be important in the context of Global Warming, as regional climate changes in the Equatorial Sector may critically depend on inter-basin interactions.更多还原