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一个中尺度风场反馈作用的参数化方法及其对南美西海岸海表温度模拟暖偏差的影响
崔超然
Subtype博士
Thesis Advisor张荣华
2020-08
Degree Grantor中国科学院大学
Place of Conferral中国科学院海洋研究所
Keyword中尺度海气耦合 吉洪诺夫正则化方法 南美西海岸 海洋模式模拟 大尺度海气耦合
Abstract

中尺度海表温度(SST)扰动和海表风应力扰动之间的相互作用对于调节大气和海洋状态具有重要作用,然而当前的大部分气候模式还不能有效地表征这一中尺度海气耦合过程。特别在南美西海岸海域的中尺度海气耦合非常强烈,气候模式模拟一直存在一个明显的SST暖偏差。

本文采用局地加权回归(LOESS)的滤波方法,从高分辨率的Quik-SCAT风场和AMSR-E SST 卫星观测数据中提取了南美西海岸大气和海洋的中尺度扰动信号,揭示了该海域存在强烈的中尺度海气耦合过程。通过最小二乘法回归分析计算了该海域的中尺度海气耦合系数,发现该海域的中尺度耦合过程具有明显的季节变化。其中,中尺度SST扰动和海表风应力扰动之间的耦合系数在冬季约为0.0095 N•m-2/°C、夏季约为 0.0082 N•m-2/°C。基于利用卫星观测数据得到的中尺度海气耦合关系,本文构建了一个中尺度风场反馈的参数化方法,即根据切风向SST梯度扰动和顺风向SST梯度扰动求得中尺度风应力旋度和散度扰动;再采用吉洪诺夫正则化方法,从中尺度风应力旋度和散度扰动中求得中尺度风应力扰动的两个分量;并将观测与重构的中尺度风应力扰动强度之间标准差的比例系数作为矫正系数。结果表明,通过该参数化方法重构的风应力扰动在空间分布上与SST扰动具有很好的相关关系,并且可以合理地表征中尺度风应力扰动的振幅。

将这一参数化方法运用到ROMS中,建立了包含中尺度风场反馈效应的海洋动力模式,并以南美西海岸的海洋模拟为例进行研究。结果表明,在数值模式中加入中尺度海气耦合后,模拟的南美西海岸海域的SST在整体上出现了降低的现象,其中在秘鲁海域SST降低约0.3°C,而在智利海域SST降低约0.7°C。通过混合层热收支分析发现,南美西海岸海域SST的变化由海表热通量、垂向热通量扩散和水平平流作用所主导,其中智利海域的SST变化由水平平流作用所主导,而秘鲁海域的SST变化由垂向热扩散作用所主导。海表热通量的变化(包括潜热通量、感热通量和向上的长波辐射)对SST的变化主要起抑制作用。垂向流场的变化主要是由中尺度风应力旋度扰动引起的Ekman抽吸作用所调控。增强的上升流可以将更多的次表层冷水带到海洋表层,通过水平平流效应放大,间接对SST的降低起到重要作用。中尺度风应力扰动可以从动量和热通量相关的两个过程对SST产生影响。敏感性试验表明动力反馈同时发生在秘鲁和智利海域,而热力反馈主要发生在智利海域,中尺度海气耦合试验中SST变化是动力反馈和热力反馈试验中SST变化之和。考虑到南美西海岸海域在大尺度气候模拟研究中的重要作用,本文进一步研究了在大尺度海气耦合模式中考虑中尺度海气耦合对海洋的反馈效应。结果表明,在大尺度海气耦合模式中加入中尺度海气耦合效应后,模拟的SST的冷却效应会得到加强,其中秘鲁海域的SST降低约0.4°C,而智利海域的SST降低约0.8°C。分析发现SST的降低主要受垂向热通量扩散和水平平流作用的影响。

以上结果表明,本文建立的中尺度风场反馈的参数化方法可以很好地应用于海洋模式和海气耦合模式中,为改善中尺度海气耦合及大尺度海气相互作用的研究提供了一种有效的方法。

Other Abstract

The interaction between the mesoscale perturbations of sea surface temperature (SSTmeso) and wind stress (WSmeso) has great influences on the ocean and atmosphere. However, most current climate models cannot adequately capture the characteristics of mesoscale wind stress-SST coupling. In the in the western coast of South America with strong mesoscale coupling, there is a consistent warm SST bias in most of climate model simulations.

Using daily Quik-SCAT wind data and AMSR-E SST data, SSTmeso and WSmeso fields in the western coast of South America are extracted by using a locally weighted regression method (LOESS). The spatial patterns of SSTmeso and WSmeso indicate strong mesoscale SST-wind stress coupling in the region. The least-squares regression analysis is used to calculate the mesoscale air-sea coupling coefficient in the sea area, and it is found that the mesoscale coupling in the sea area has obvious seasonal changes. The coupling coefficient between SSTmeso and WSmeso is about 0.0095 N•m-2/°C in winter and 0.0082 N•m-2/°C in summer. Based on mesoscale coupling relationships, we build a parameterized method for the mesoscale wind stress induced feedback effect. The mesoscale perturbations of wind stress divergence and curl can be obtained from the SST gradient perturbations, which can be further used to derive wind stress vector perturbations using the Tikhonov regularization method. The computational examples are presented in the western coast of South America. By matching the spatially averaged maximum standard deviations of reconstructed WSmeso magnitude and observations, a reasonable magnitude of WSmeso can be obtained when the rescaling factor is used.

This parameterized method is used in Regional Oceanic Modelling Systems (ROMS) and can be used to represent mesoscale wind stress–SST coupling. It is found that SST can be cooled down in the western coast of South America if the feedback effect of mesoscale coupling is included in the numerical simulaitons. The reduced maximum value of SST can reach 0.3℃ in the Peru sea and 0.7 ℃ in the Chile sea. The mixed layer (ML) heat budget analysis indicates that horizontal advection, vertical heat diffusion and surface heat flux are the main terms that explain the change in SST. The SST changes in Chile sea and Peru are dominated by horizontal advection and vertical diffusion, respectivly. Furthermore, the mesoscale coupling can lead to a strengthened vertical velocity which is mainly controlled by the Ekman pumping induced by WSmeso. The strengthened Ekman pumping acts to bring subsurface cold water to the sea surface, leading to a cooling of SST. Additionally, SST change is damped via surface heat flux adjustment. Analyses of the sensitivity experiments demonstrate that SST change in the momentum feedback experiment occurs both in the sea of Peru and Chile, while SST change in the thermal feedback experiment mainly occurs in the Chile sea. The SST difference between the mesoscale coupling experiment and control experiment is generally a sum of the SST differences between the two sensitivity experiments and control experiment. The feedback induced by mesoscale coupling is also studied in a large-scale coupled model. The results indicate the cooling effect of SST can be strengthened in the large-scale coupled ocean-atmosphere model. The reduced maximum value of SST can reach 0.4℃ in the Peru sea and 0.8 ℃ in the Chile sea. The horizontal advection, vertical heat diffusion and surface heat flux are still the main terms that explain the change in SST. The empirical wind stress perturbation model developed in this study can be used to improve the simulation of mesoscale coupling in the ocean model and climate model.

MOST Discipline Catalogue理学 ; 理学::海洋科学
Language中文
Document Type学位论文
Identifierhttp://ir.qdio.ac.cn/handle/337002/164776
Collection海洋环流与波动重点实验室
Recommended Citation
GB/T 7714
崔超然. 一个中尺度风场反馈作用的参数化方法及其对南美西海岸海表温度模拟暖偏差的影响[D]. 中国科学院海洋研究所. 中国科学院大学,2020.
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