IOCAS-IR  > 海洋环流与波动重点实验室
Ningaloo Niño海表温盐变异机理及中尺度涡旋的作用
郭亚茹
学位类型博士
导师王凡
2021-05
学位授予单位中国科学院大学
学位授予地点中国科学院海洋研究所
关键词Ningaloo Niño 东南印度洋 海-气相互作用 海洋中尺度涡旋 印尼贯穿流
摘要

Ningaloo NiñoNN)是东南印度洋(SEIO)年际时间尺度上最主要的气候变率模态,对该区域的天气和气候变化产生强烈作用,并对珊瑚生态系统和渔业资源造成显著影响。深入研究NN的温盐变异机制具有重要的科学价值和现实意义。

本文首先采用高分辨率(312 km)区域海洋环流模拟系统ROMS研究了由NN引起的海表面温度(SST)变化机制。在高质量边界条件和大气资料驱动下,模式能较好地模拟1993-2016年期间的八次NN事件及其主要时空特征。为了定量评估各种物理过程对SST增暖的贡献,本文开展了一系列模式敏感性实验。实验结果表明,在大多数NN事件中,表面潜热通量是控制SST演变的最主要过程,其贡献>60%;而印尼贯穿流(ITF)和局地风场强迫的贡献均在10%-20%之间,它们的作用是次要的。潜热通量的减少是驱动SST增暖的主要原因,它主要是由该区域气温的升高所导致(贡献占潜热通量绝大部分),而气温的升高则来源于异常北风携带的低纬度暖湿空气。NN成熟阶段时,高SST会导致潜热通量的增加,导致暖异常进入衰减阶段。这些研究明确解释了调控NN海温演变的关键物理过程。

本研究发现,NN能够引起大尺度海洋盐度异常。在NN成熟阶段,SEIO区域表现出大范围的海表盐度(SSS)降低,振幅达0.15–0.20 psuROMS敏感性实验结果表明,SEIO区域SSS的降低主要是由NN期间局地降水量增加(>50%)和ITF淡水输送增强(>40%)共同造成的;其它过程如局地风场和蒸发的影响是次要的,总贡献<10%。其中,ITF的加强能增加印度洋东边界向南的淡水平流,这对引起澳大利亚西海岸附近强烈的盐度降低(> 0.20 psu)至关重要。由于ITF的重要调制作用,SEIOSSS与厄尔尼诺-南方涛动(ENSO)指数如Niño-3Niño-4Niño-3.4指数的相关性分别为0.570.770.70,普遍高于SSTENSO的相关性(-0.27, -0.42-0.35),这也反映了NN海温和盐度变异机理的差异。通过开展人为抑制盐度变化的理想性实验,探讨了盐度变异对NN增暖的反馈作用。结果表明,盐度变异对SEIO区域的上层海洋层结影响有限,因此对海温变化的影响较小。

ROMS模拟中还显示出NN暖异常中显著的中尺度结构,并被卫星微波观测资料所证实。因此,本文开展了高、低分辨率模式试验的对比分析,研究了海洋中尺度过程对NN事件的作用。结果表明,在卫星观测和高分辨率(~3 km)海洋模拟中,来源于ITF的暖信号可以通过大量中尺度涡旋的形式从东边界向西传播。由于涡旋传播慢、强耗明显,很少能将变化能量传播到离岸较远的区域,故暖信号主要局限在沿岸附近。但在粗分辨率(~100 km)海洋模式,由于无法分辨中尺度涡,变化能量主要以长罗斯贝波形式迅速西传,耗散较弱,因此更多能量输运到大洋内部,低估了沿岸暖信号。涡旋的重要性还在于引起局部增暖热斑、促进海表潜热释放和中尺度海-气相互作用,这些过程是粗分辨率模式中无法体现的。进而,通过多成员集合模拟,证实了由海洋内部不稳定产生的涡旋对SSTSSS异常空间分布的显著影响,并评估了其对NN可预报性的影响。根据计算的信噪比,澳大利亚西海岸附近的变化具有较高的可预报性,SEIO内区的变化可预报性较低。这些结果指出了中尺度涡旋对NN的模式模拟的重要性。

综上所述,本文系统阐明了SEIO区域NN的温度和盐度变异机理,并揭示了中尺度涡旋对NN的重要作用。研究结果不仅有助于我们对NN机理的深入理解和认识,也有助于加强对NN的预测能力。

其他摘要

Ningaloo Niño (NN), as a dominant climate mode of interannual variability in the southeast Indian Ocean (SEIO), has a strong influence on the regional whether and climate change, and also significantly affects the coral ecological system and fishery resources. It is of great scientific value and practical importance to study the mechanisms of temperature and salinity variation of NN.

Firstly, this study utilized a high-resolution (3~12 km) regional oceanic modeling system (ROMS) to explore the sea surface temperature (SST) variability of NN. With the high-quality boundary conditions and atmospheric forcing, the model simulates well eight NN events and their major spatialtemporal characteristics with a high fidelity during 1993–2016. A series of hierarchical model experiments are carried out to quantitatively evaluate the effects of key processes on SST warming. The results reveal that the predominant process controlling SST evolution is the surface latent heat flux, with a contribution of >60%; while the roles of both Indonesian Throughflow (ITF) and local wind forcing range from 10% to 20%, thus playing a secondary effect. Reduce of surface latent heat is essential in driving the growth of SST warming, which is mainly caused by the warming of air temperature from the low-latitude warm and moist air carried by the northerly wind anomaly (accounting for the majority of latent heat flux). The established SST warming in the mature stage of NN prompts the increase of latent heat loss, which further initiates the begin of decay stage. These analyses clearly explain the key physical processes that regulate the SST evolution of NN.

This study shows that NN can also induce profound variability in ocean salinity. At the mature stage of NN, the SEIO region displays large-scale sea surface salinity (SSS) freshening of 0.15–0.20 psu. ROMS sensitivity experiments show that this SSS freshening is mutually caused by the increased local precipitation (>50%) and enhanced fresh-water transport of the ITF (>40%) during NN events; the effects of other processes, such as local winds and evaporation, are secondary (<10%). In addition, the ITF feeds the southward fresh-water advection near the eastern boundary, which is crucial in causing the strong freshening (> 0.20 psu) near the Western Australian coast. Owing to the strong modulation effect of the ITF, SSS of the SEIO bears a higher correlation with the El Niño-Southern Oscillation (ENSO), such as 0.57, 0.77, and 0.70 with Niño-3, Niño-4, and Niño-3.4 indices, respectively, than that of SST (-0.27, -0.42, and -0.35). This reflects the difference between mechanisms of temperature and salinity variation of NN. In order to explore whether the salinity variability has a feedback effect on the warming of NN, an idealized model experiment with artificial damping for salinity anomaly was conducted in this study. The results indicate that ocean salinity has limited impact on ocean near-surface stratification in the SEIO and thus minimal feedback effect on the warming of NN.

ROMS simulation also displays prominent mesoscale noises in the warming signature of NN, which is confirmed by satellite microwave SST data. Therefore, the diagnostic analysis of high and low resolution model experiments is carried out in this study to explore the importance of oceanic mesoscale eddy on the NN. The results show that in the satellite observation and high-resolution (~3 km) simulations, the warming signatures originated largely from the ITF are transmitted westward from the eastern boundary by mesoscale eddies. Few eddies propagate long distances offshore due to strong dissipation and slow propagation, and as a result NN signatures are predominantly confined near the coast. On the contrary, in coarse-resolution (~100 km) simulations that cannot resolve eddies, anomalous energy propagates westward swiftly as long Rossby waves with much weaker dissipation, and more energy spreads to the ocean interior with the suppression of coastally trapped warming signatures. Eddies also induce local SST warming “hotspots”, promoting surface latent heat release and mesoscale air-sea interactions. These processes are not resolved by coarse-resolution models. However, a multi-member ensemble simulation confirms the strong influence of mesoscale eddies arising from ocean internal instability in the spatial redistribution of SST and SSS anomalies and evaluates their impacts on the predictability of NN. Through computing “signal-to-noise” ratio, the results indicate high predictability over the west coast of Australia and degraded predictability in the ocean interior. These results highlight the importance of resolving mesoscale oceanic processes in the simulation of NN.

In conclusion, this study clarifies systematically the physical mechanisms of temperature and salinity variability of NN in the SEIO and reveals the importance of mesoscale eddies on the NN. The research findings can not only deepen our full understanding of NN variability but also strengthen the model ability to predict the NN.

学科门类理学
语种中文
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/170744
专题海洋环流与波动重点实验室
推荐引用方式
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郭亚茹. Ningaloo Niño海表温盐变异机理及中尺度涡旋的作用[D]. 中国科学院海洋研究所. 中国科学院大学,2021.
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