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南半球西风急流和南极绕极流的涡旋动力学
杨小怡
学位类型博士
导师王东晓
2007
学位授予单位中国科学院研究生院
学位授予地点广州
学位专业物理海洋学
关键词南极臭氧损耗+南半球绕极西风+南半球环状模(Sam)+ekman抽吸速率+deacon cell+等密面坡度+南极绕极流+涡饱和+ep通量+正压不稳定+斜压不稳定
其他摘要本论文利用大气再分析资料ERA40以及一部分实测资料来确定南大洋风应力在1980-2000出现显著增强的年代际趋势,并深入探讨了这一年代际变化的原因。分析结果显示,人类活动导致南极臭氧在1980年后明显减少,且主要出现在南半球春季,这一现象通过正反馈过程影响平流层低层大气环流,从而使南半球绕极西风呈现增强趋势。其中正反馈过程可描述为:春季臭氧损耗引发南极极涡增强,温度下降,中高纬度的位涡和温度水平梯度也增强,这造成极涡的孤立和绕极西风的加速;另外,增强的水平和垂直位涡梯度使对流层向上传播的扰动更多地向赤道和向下反射,南极平流层动力增温效应减弱,温度更趋降低,而温度的下降使得平流层臭氧损耗加剧。这种正反馈机制引起绕极西风持续增强的异常信号通过平流层与对流层的耦合向下传播到对流层上层,然后由对流层波流相互作用和异常经圈环流的地转效应在南半球环状模(Southern Annular Mode, 以下简称SAM)的联系下将纬向风异常信号传递到边界层,从而触发南大洋风应力的年代际变化。 风应力异常对南大洋环流的影响通过对SODA(Simple Ocean Data Assimilation)资料的分析来揭示,SAM正位相(风应力增强)时,表层Ekman速度和Ekman抽吸速率均增强南移,对应浅层经向翻转流(Deacon Cell)也增强南移,并且表现出与风应力相一致的增强趋势。Deacon Cell的增强导致南大洋等密面坡度加大,水平密度梯度增强,因而南极绕极流(Antarctic Circumpolar Current, 以下简称ACC)斜压输运在年际尺度上也表现出与SAM的显著正相关关系。 但另一方面,ACC输运并没有明显的年代际增强趋势,这可以从南大洋的“涡饱和”现象来解释。从ECCO(Estimating the Circulation and Climate of the Ocean)资料利用Eliassen-Palm(EP)通量剖面图来诊断南大洋平均流与涡旋的相互作用,发现了同时的正压不稳定和滞后的斜压不稳定耗散机制。其中,正压的正反馈机制可表述为:海表风应力驱动向东的纬向流异常,整个南大洋区域的涡旋扰动均增强,而增强的扰动使得纬向急流变窄增强,水平切变增大,这样就使得正压转换率更趋增强,从而使平均动能向涡动能转化,部分平衡海表风能输入。而与正压不稳定相比,斜压不稳定在ACC动量平衡中占据更重要的地位。斜压转换率对SAM的风应力响应在滞后一年时间开始出现明显的正异常,这表示平均势能向涡动能转化,其量值与海表风能量值相当。此斜压不稳定耗散机制为ACC输运和南大洋涡动能对风应力滞后2-3年的响应,以及ACC输运在年代际上保持稳定不变的形势提供了一定的理论解释。; Using ERA40 reanalysis dataset and some in-situ observations, the obvious decadal strengthening of Southern Ocean surface wind stress during 1980-2000 is identified, and its cause has been explored. The analysis results indicate that Antarctic ozone has been depleted continuously after 1980s due to human activity, and the most severe depletion occurred in Southern spring. Through a positive feedback process, this ozone variability interplayed with lower-stratospheric atmosphere circulation, leading to the acceleration of Southern circumpolar westerly jet. The ozone-related positive feedback loop can be depicted as following: spring Antarctic ozone depletion induces the strengthening and cooling of polar vortex, the enhancement of mid-latitude potential vorticity gradient and temperature meridional gradient and hence the acceleration of circumpolar westerly. Moreover, the increasing horizontal and vertical gradients of potential vorticity tend to refract wave equatorward and downward, rendering less dynamical heating in Antarctic stratosphere. The resulting cooling further intensifies the Antarctic ozone depletion. The anomalous westerly signals at lower-stratosphere level propagate downward through the stratosphere-troposphere coupling process. Thereafter tropospheric wave-mean flow interaction and the meridional circulation (Ferrel cell) anomalies collaborates in the medium of Southern Annular Mode (SAM) to relay the westerly signals downward to the surface layer, triggering the decadal variability of Southern Ocean surface wind stress. The general oceanic response to this wind stress anomalies is investigated by applying the SODA (Simple Ocean Data Assimilation) data. SAM positive phase (strong wind) causes the strengthening and poleward shift of surface Ekman velocity and Ekman pumping rate, thus the strengthening trend of Deacon Cell in consistent with the wind stress decadal variability. This strengthening of Deacon Cell steepens the Southern Ocean isopycnal slope and enhances the meridional density gradient, resulting in the interannual positive correlation between Antarctic Circumpolar Current (ACC) baroclinic transport and SAM index. On the other hand, ACC transport did not exhibit the decadal strengthening trend, which may be explained by the “eddy saturation” effect. Eliassen-Palm flux (EP flux) sections calculated from ECCO (Estimating the Circulation and Climate of the Ocean) data are applied to diagnose the eddy-mean flow interaction in the Southern Ocean. The simultaneous barotropic instability and the delayed baroclinic instability mechanisms are revealed. The barotropic positive feedback process can be described in terms of barotropic conversion rate. The surface wind stress drives the eastward mean flow anomaly, which facilitate the enhancement of eddy perturbation in the Southern Ocean. The eddy perturbation, in turn, forces the stronger and narrower zonal jet and increasing horizontal shear. The resulting higher barotropic conversion rate indicates the enhanced conversion from mean kinetic energy to eddy kinetic energy, which partly balances the surface wind energy input. Baroclinic instability, compared with barotropic instability, is more important in the ACC momentum balance. Baroclinic conversion rate respond lag one year to SAM-related wind stress with significant positive anomalies, which means the mean potential energy being converted to eddy kinetic energy with comparable value with wind energy input. This baroclinic instability mechanism can provide a explanation for the lag 2-3 years response of ACC transport and eddy kinetic energy to surface wind stress variability as well as the stability of ACC transport in the decadal timescale. 
文献类型学位论文
条目标识符http://ir.scsio.ac.cn/handle/344004/15831
专题学位论文(博士)
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GB/T 7714
杨小怡. 南半球西风急流和南极绕极流的涡旋动力学[D]. 广州. 中国科学院研究生院,2007.
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