【作者】 马浩；

【导师】 吴立新；

【作者基本信息】 中国海洋大学 ， 气象学， 2011， 博士

【摘要】 风应力和淡水通量是南大洋的两种重要外强迫。风应力直接驱动了全球最强的一支海洋环流——南极绕极流,淡水通量是末次冰消期冰融水事件的重要成因之一,同时也是全球变暖情景下南极融冰的重要产物。然而长期以来,对南大洋风应力和淡水通量的气候效应缺乏深入系统的研究。本文围绕南大洋风应力和淡水通量在全球气候系统中的作用这一关键科学问题,利用海-气完全耦合模式FOAM1.5 (Fast Ocean-Atmosphere Model, version1.5),借助以“部分耦合”和“部分阻挡”为核心的模式手术方案,通过一系列敏感性实验揭示南大洋风应力和淡水强迫对全球海-气耦合系统的影响。为了探讨南大洋风应力的气候效应,我们在耦合模式中去除40°S以南海-气界面的风应力,发现风应力的消失将引起局地垂向对流混合和南极绕极流以南上升流的减弱,导致南大洋出现大范围海温冷异常、海冰覆盖面积向低纬扩展。由于海洋层结加强,在上层海洋变冷的同时,次表层海洋发生了显著的增暖。南大洋的海温冷异常激发了对流层大气的相当正压槽响应,伴随着南半球西风的显著增强。西风加强又进一步加剧了SST冷异常,从而构建了局地的正反馈过程。我们的研究同时发现：南半球中高纬度风应力对北大西洋经向翻转环流(Atlantic Meridional Overturning Circulation, AMOC)有着极强的调控作用。去除海-气界面风应力之后,风应力对深层海水的抽吸作用也随之消失,导致南极经向翻转环流(Antarctic Meridional Overturning Circulation, AnMOC)几乎完全消失,AMOC流量也减小了约50%。AMOC的减弱使跨赤道热量输送减弱,有利于南大西洋增暖,然而由于在南大西洋的副热带地区存在风-蒸发-SST(Wind-Evaporation-SST, WES)反馈过程,将南半球中纬度地区的冷异常传递到热带地区,从而抑制了南大西洋暖异常的产生,因此AMOC减弱导致的南北半球海温跷跷板并未出现。为了研究南大洋淡水强迫对局地和远程气候系统的影响,我们利用耦合模式,在60°S以南的海洋中均匀施加1.0Sv淡水通量异常。模式结果表明：南大洋表层淡水通量强迫使局地海洋层结加强,抑制了南极附近的深对流过程、南大洋的垂向对流混合过程和南极绕极流以南的上升流,从而在局地海洋中出现了表层变冷、次表层增暖的斜压响应,与此同时,大气中的西风得到加强。在上层海洋平流过程和大气过程的共同作用下,南大洋高纬地区的冷异常被输送到ACC区域。之后,南半球中纬度的海温冷异常能够通过上层海洋与低层大气之间的接力遥相关机制传递到热带地区。WES反馈激发了副热带地区的冷异常,同时使南半球副热带-热带流环(Subtropical-Tropical Cell, STC)加强,STC的加速与平均潜沉过程共同导致了热带海洋海温冷异常的出现。南大洋淡水强迫也引起了北半球气候的变化。大气遥相关过程使北半球的热带外海区在最初几十年中表现为冷异常,随着南大洋次表层暖水向北半球输运并通过垂向对流混合过程上升到表层,北半球的初始冷异常逐渐转为暖异常。南半球和北半球分别出现了海温冷异常和暖异常,从而构建了显著的南北半球海温跷跷板。南大洋表层淡水扰动使海洋层结加强,从而南极底层水减弱,南极底层水与北大西洋深层水之间的相互调制使AMOC在最初几十年中得到加强,随着淡水通量在上层海洋平流的作用下不断流入北大西洋,AMOC的流量逐渐减小。气候平均态的改变也使气候变率发生了变化。在南大洋淡水强迫情景下,ENSO的振幅加强,同时频率向低频方向移动。振幅加强的原因是温跃层的纬向倾斜加强、同时整个热带温跃层整体抬升,从而增强ENSO变率；频率减慢的原因是赤道与赤道外之间的经向热力梯度减小,降低了充放电的效率,使ENSO的频率减慢。此外,两种类型的厄尔尼诺——东太平洋厄尔尼诺和中太平洋厄尔尼诺对南大洋淡水强迫表现出不同的响应。整体上来说,淡水强迫对前者影响较大,而对后者影响较小。不仅如此,局地淡水强迫也使南半球环状模发生了显著的变化。环状模年际变率的振幅显著减弱,同时年代际变率开始增强。环状模的变化具有垂向一致性。更多还原

【Abstract】 Wind stress and freshwater work as two important forcings over the Southern Ocean (SO). Wind stress directly drives the most powerful circulation among global oceans, i. e., the Antarctic Circumpolar Current (ACC). Freshwater flux over the SO, which once led to the melt-water-pulse events during the last deglaciation, is the important product of Antarctic ice-melting induced by global warming. However, so far, there have been few comprehensive and systematic researches discussing the climatic impact of wind stress and freshwater flux over the SO.In this paper, we focus on the role of SO wind stress and freshwater flux in global climate system as a key scientific issue. A series of sensitivity experiments, combined with the "modeling surgery" strategy based on the "Partial-Coupling" and "Partial-Blocking" schemes are carried out in the fully coupled ocean-atmosphere model, FOAM1.5 (Fast Ocean-Atmosphere Model, version 1.5), to investigate the influence of wind stress and freshwater flux over the SO on global ocean-atmosphere coupled system.In order to explore the climatic impact of SO wind stress, we remove the wind stress south of 40°S in the coupled model. Experimental results demonstrate that the disappearance of wind stress weakens both local vertical diapycnal mixing and the upwelling south of ACC, leading to the SO covered by large cold SST anomalies and equatorward expansion of sea ice. Due to the strengthened oceanic stratification, cooling in the surface layer triggers significant warming in the subsurface layer. Meanwhile, cold SST anomalies induce the quasi-barotropic atmospheric "trough" response in the troposphere, coupled by the acceleration of westerly wind. The enhancement of westerly wind further intensifies local SST cooling, and forms local positive feedback. Our modeling results suggest a strong control of southern high-latitude wind stress on the Atlantic Meridional Overturning Circulation (AMOC). When surface wind stress is turned off, the pumping effect of wind stress on deep water is also eliminated, which leads to the Antarctic Meridional Overturning Circulation (AnMOC) almost disappears. Meanwhile, the transport of AMOC is also reduced by 50%. The weakened AMOC drives less cross-equator heat transport, which helps to generate anomalous warming in the South Atlantic. However, the Wind-Evaporation-SST (WES) feedback in the subtropical South Atlantic works to transmit the southern mid-latitude cold anomalies to tropics, which inhibits warm anomalies in the South Atlantic, and in this way, the bi-polar SST seesaw induced by weakened AMOC can not be formatted.To investigate local and remote impacts of freshwater forcing over the SO, 1.0Sv freshwater flux is uniformly imposed over the Antarctic Ocean (south of 60°S). The modeling results demonstrate that surface freshwater perturbation enhances local stratification, inhibiting deep convection near the Antarctic, vertical diapycnal mixing in the SO and upwelling south of ACC, and leads to baroclinic response in local ocean with anomalous cooling and warming prevails in the upper layer and subsurface layer, respectively. Meanwhile, an intensification of westerly wind is also detected. The high-latitude cooling can be conveyed to the ACC region by the joint effect of upper-ocean advective process and atmospheric process. Subsequently, southern mid-latitude cold anomalies can be further transmitted to the tropics by the upper ocean-lower atmosphere coupled relay teleconnection mechanism. That is, WES feedback triggers southern subtropical cooling, and accelerates Subtropical-Tropical Cell (STC) in the Southern Hemisphere. The combined impact of intensified STC and mean subduction causes the tropical ocean displays anomalous cooling.Freshwater forcing over the SO can also remotely influence the climate of the Northern Hemisphere. In the initial several decades, northern extratropical region shows anomalous cooling due to the atmospheric teleconnection. As the Antarctic subsurface warming propagates northward and comes up to the surface due to vertical diapycnal mixing process, the initial cooling in the Northern Hemisphere is gradually replaced by anomalous warming. The cold and warm anomalies in the Southern and Northern Hemisphere generates a significant bi-polar SST seesaw.Surface freshwater perturbation over the SO strengthens oceanic stratification, which weakens the formation of Antarctic Bottom Water, and the competition between Antarctic Bottom Water and North Atlantic Deep Water leads to the enhancement of AMOC in the initial several decades. As the fresh anomalies from the SO spread to the North Atlantic with upper-ocean advection, the transport of AMOC gradually decreases.The shift of modeling climatology induces important change of climate variability. Under the SO freshwater forcing scenario, the amplitude of ENSO is significantly intensified, and meanwhile, the frequency of ENSO shifts towards the low-frequency range. The enhancement of ENSO can be attributed to the sharper zonal-tilt of tropical thermocline, and the general uplift of entire tropical thermocline also contributes to intensify ENSO. The period of ENSO is enlarged primarily because the upper-ocean meridional temperature gradient between equatorial and off-equatorial region is decreased, which is favorable to extend the period of ENSO. Besides, two types of El Nino, i. e., the eastern Pacific El Nino and center Pacific El Nino display different response to freshening over the Antarctic Ocean. Generally speaking, freshwater perturbation can significantly influence eastern Pacific El Nino but has little effect on center Pacific El Nino.In addition, local freshwater forcing triggers distinct change of the Southern Annular Mode (SAM). The interannual variability of SAM is largely weakened, but the interdecadal variability is amplified. The change of SAM is similar at different vertical layers.更多还原