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基于潜标观测的El Niño事件期间西太平洋赤道环流变异研究
吕宜龙
Subtype博士
Thesis Advisor王凡
2019-06
Degree Grantor中国科学院大学
Place of Conferral中国科学院海洋研究所
Degree Name理学博士
Keyword潜标观测 El Niño 季节内变化 Mjo 西风爆发 海洋环流
Abstract

热带西太平洋是驱动大气环流和亚洲季风的关键海域,也是ENSOEl Niño与南方涛动)发生发展的重要区域。为了研究该海区海-气特征,特别是El Niño现象的演变,从20世纪80-90年代开始国际学界实施了多个海-气观测计划,这极大地促进了对ENSO的动力学机制认识和预报能力。如美国和日本布放的TAO/TRITON浮标阵列就提供了热带太平洋大量的、高分辨率连续海洋观测资料。然而TAO/TRITON阵列在热带西太平洋的观测比较稀疏,缺少热带西太平洋可用于研究目的的长期、高质量次表层海洋观测数据。中国科学院海洋研究所(IOCAS)建立的热带西太平洋科学观测网,在西太平洋130E140E142E段面以及雅浦-马里亚纳海沟连接区布放了20余套深海潜标,该观测网提供了近年来ENSO事件中相关的海洋温、盐和海流的演变过程,有效的补足了对ENSO事件现场观测的短板。本工作分析了该观测网在2014-2016年间上层海流观测数据,重点关注2015/2016年超强El Niño事件期间西太平洋赤道环流变异的特征与相关动力学过程,针对潜标观测之中发现的新的变异现象开展了深入的研究:(12015El Niño发展期上层海流的季节内变化(ISV);(22016El Niño事件衰退期西太平洋赤道上层强烈的东向流异常。本文主要取得如下结果:

1)通过分析潜标观测资料和对比历史观测资料,发现了1997-1998年和2015-2016El Niño事件中上层海洋ISV存在明显差异。与1997-1998年事件相比,2015-2016年事件的海洋ISV强度在赤道太平洋海盆上显著减弱约30-50%,并且ISV变化最大值从1997年的140°E移动到2015年的160°E附近。特别是在西太平洋142°E站位两者强度差异最明显,统计结果表明其减弱了43%。西太平洋的洋流ISV与季节内西风事件的强迫有较强的关系。海洋ISV的差异也通过平流作用影响了海表温度(SST)的差异,1997年海流的平流作用明显强于2015年,且对几次季节内增暖有明显的贡献。这些差异可以在很大程度上由不同来源的季节内西风爆发造成的海洋ISV强度和位置的差异解释。2015年海表纬向风ISV总体上弱于1997年,1997年的季节内西风爆发起源于印度洋并且发生在西边界附近(140°E),而2015年引起西风爆发的扰动主要来自副热带东北太平洋并且引起上层洋流ISV最大值出现在赤道160°E附近。这些ISV差异可能与2015-2016El Niño事件期间副热带东北太平洋的年代际变暖和印度洋大气季节内振荡的减弱有关。海洋ISV在两次El Niño事件之中扮演的不同角色,可能是影响ENSO多样性的因素之一。

2)通过分析IOCAS在赤道、142°E站位的潜标观测数据发现,2016年夏季即El Niño衰退期海洋上层出现了异常的东向强流,其最大流速达到了67 cm/s,垂向范围向上扩展达到表层,表层西向流几乎消失。这个现象,JAMSTEC1998年夏季的潜标观测中也出现了。进一步分析热带西太科学观测网2014-2016年期间3套潜标观测资料发现,赤道以北的2°N140°E站位和赤道以南的1°S142°E站位的观测中在2016年夏季也有明显东向流增强,其最大流速分别为90 cm/s63 cm/s,而更北的3°N143.55°E站位东向流速却从88 cm/s减弱到了大约30 cm/s,这表明西太平洋赤道环流变异存在复杂的空间结构。进一步分析OSCAR表层流数据、GODAS海洋同化数据和1.5层非线性约化重力模式结果都表明El Niño衰退期西太平洋西边界上层海洋出现了一支跨赤道的逆时针环流异常。因此,潜标在赤道及其南、北两个站位观测到了东向流速增强,而更北却观测到了东向流减弱。利用El Niño不同时期和区域的风场驱动1.5层模型结果表明,El Niño成熟期赤道东太平洋的东风异常是驱动该环流变异风场的具体时间和关键区域,其激发的赤道外暖Rossby波西传到达西边界后反射成赤道暖Kelvin波,造成了赤道西太平洋南北两个海表高度异常(SLA)高值区。SLA通过改变压力场的分布而造成赤道上的西向流异常和赤道以南的沿岸东向流异常,组成了跨赤道逆时针环流异常。此外,通过理想化模式实验表明,西边界海陆分布对该逆时针环流起着重要作用。因此,El Niño衰退期西太平洋西边界的跨赤道逆时针环流异常是El Niño成熟期赤道东太平洋风场和西边界海陆分布共同作用的结果。

综上所述,本论文通过分析西太平洋潜表阵列发现了2015-2016El Niño事件中西太平洋赤道环流的两个变异现象,即(12015El Niño发展期上层海流的季节内变化弱于1997年;(2El Niño事件衰退期西太平洋赤道上层的出现跨赤道的逆时针异常环流。其中现象(1)是由两次事件中不同来源的季节内西风爆发导致,现象(2)是由El Niño成熟期赤道东太平洋风场和西边界海陆分布共同作用的结果。这些结果为ENSO期间海洋环流变异及相关动力学机制的研究提供了新的认识,为后续研究西太平洋海洋环流结构与多尺度变异打下基础,突显了高分辨率的上层海洋连续潜标观测在环流结构变异和在ENSO事件演变研究中的重要性。

Other Abstract

The tropical western Pacific Ocean is a key sea area that drives the atmospheric circulation and the Asian monsoon. It is also an important area for the development of ENSO (El Niño and Southern Oscillation). In order to study the sea-air characteristics of the sea area, especially the evolution of the El Niño phenomenon, a number of sea-air observation programs have been implemented by the international academic community since the 1980s and 1990s, which has greatly promoted the understanding and forecasting ability of the dynamic mechanism of ENSO. TAO/TRITON buoy arrays, such as those deployed by the United States and Japan, provide large, high-resolution continuous oceanographic observations of the tropical Pacific. However, TAO/TRITON buoy arrays are sparsely observed in the tropical western Pacific, lacking long-term, high-quality subsurface ocean observation data for the study purposes in the tropical western Pacific. The Tropical Western Pacific Scientific Observation Network, established by the Institute of Oceanography, Chinese Academy of Sciences (IOCAS), has deployed more than 20 deep-sea moorings in 130°E, 140°E and 142°E sections of the western Pacific Ocean, as well as in the Yap-Mariana Trench connection area. The observation network provides the evolution of ocean temperature, salt and ocean currents associated with ENSO events in recent years, effectively complementing the short-board observations of ENSO events. This work analyzes the upper ocean current observation data of the observation network from 2014 to 2016, focusing on the characteristics and related dynamic processes of the equatorial circulation variation in the western Pacific during the 2015/2016 super El Niño event. In-depth research was carried out on the new variability found in the subsurface observations: (1) intraseasonal variation (ISV) of the upper ocean current during the El Niño developing phase in 2015; (2) The strong eastward current anomaly of the upper equatorial western Pacific in the decaying phase of the 2016 El Niño event. This paper mainly achieved the following results:

(1) By analyzing the subsurface observation data and comparing the historical observation data, it is found that there are significant differences in the upper ocean ISV between the 1997-1998 and 2015-2016 El Niño events. Compared with the 1997-1998 event, the marine ISV intensity of the 2015-2016 event was significantly reduced by approximately 30-50 in the equatorial Pacific basin, and the maximum ISV moved from 140°E in 1997 to near 160°E in 2015. In particular, the difference in intensity of the 142°E stations in the western Pacific was the most obvious, and the statistical results showed that it was weakened by 43. The ocean current ISV in the western Pacific has a strong relationship with the forcing of the westerly wind event. The difference in marine ISV also affects the difference in sea surface temperature (SST) through zonal advection. In 1997, the advection of ocean currents was significantly stronger than in 2015, and it contributed significantly to several intraseasonal warmings. These differences can be explained to a large extent by differences in the intensity and location of marine ISVs caused by the onset of intraseasonal westerly winds from different sources. In 2015, the sea surface zonal wind ISV was generally weaker than in 1997. In 1997, the westerly winds originated in the Indian Ocean and occurred near the western boundary (about 140°E). The westerly wind event caused by the turbulence in 2015 mainly came from the subtropical northeast Pacific Ocean and caused the maximum value of the upper ocean current ISV to appear near the equator 160°E. These ISV differences may be related to the decadal warming of the subtropical northeastern Pacific during the El Niño event during 2015-2016 and the weakening of intraseasonal oscillations in the Indian Ocean atmosphere. The different roles played by marine ISVs in the two El Niño events may be one of the factors affecting the diversity of ENSO.

2By analyzing the subsurface observation data of IOCAS at the equator, 142°E station, it is found that an abnormal eastward strong current occurred in the upper layer of the El Niño decaying phase in the summer of 2016, and the maximum current velocity reached 67 cm/s. The vertical range extends up to the surface layer, and the surface westward current almost disappears. This phenomenon was also observed by JAMSTEC in the summer of 1998. Further analysis of three sets of subsurface observation data from the Tropical West Pacific Scientific Observation Network during 2014-2016 revealed that the 2°N, 140°E stations north of the equator and the 1°S and 142°E stations south of the equator also had significant eastward current enhancements in the summer of 2016, with maximum current speed of 90 cm/s and 63 cm/s, respectively. While the northerly 3°N, 143.55°E station experienced a decrease of eastward velocity from 88 cm/s to about 30 cm/s, this indicates that the western Pacific equatorial circulation has a complex spatial structure. Further analysis of OSCAR surface current data, GODAS ocean reanalysis data and 1.5-layer nonlinear reduced gravity model results show that there is a cross-equatorial counterclockwise circulation anomaly in the upper ocean of the western boundary of the El Niño decaying phase. Therefore, the subsurface moorings showed an increase of the eastward current velocity on the equator and at the two stations in the north and the south, while the eastern current was observed to be weakened in the north. The results of the 1.5-layer model driven by the wind stress of different El Niño phases and regions show that the easterly anomaly wind of the equatorial eastern Pacific during the mature phase of El Niño event is the specific time and key area for driving the variation of circulation. Its excited off-equatorial warming Rossby wave passes to the west boundary and then reflects into the equatorial warm Kelvin wave, resulting in two high sea surface height anomalies (SLA) in the equatorial western Pacific. By changing the distribution of the pressure field, the SLA causes the anomalous westward flow on the equator and the eastward flow anomaly south of the equator, forming a cross-equatorial counterclockwise circulation anomaly. In addition, experiments by idealized models show that the sea-land distribution of the western boundary plays an important role in this counterclockwise circulation. Therefore, the cross-equatorial counterclockwise circulation anomaly over the western boundary of the western Pacific in the El Niño decaying phase is the result of the interaction between the equatorial eastern Pacific wind stress of the El Niño mature period and the sea-land distribution of the western boundary.

In summary, this paper finds two variations of the equatorial circulation in the western Pacific during the 2015-2016 El Niño event by analyzing the western Pacific subsurface mooring array, namely (1) During the El Niño developing phase in 2015, the intraseasonal variation of the upper ocean current was weaker than that of 1997; (2) The cross-equatorial counterclockwise anomalous circulation of the upper equatorial formation in the western Pacific Ocean during the decaying phase of the El Niño event. The phenomenon (1) is caused by the westerly wind events from different sources in two events. The phenomenon (2) is the result of the interaction between the equatorial eastern Pacific wind stress of the El Niño mature phase and the sea-land distribution of the western boundary. These results provide a new understanding of the study of ocean circulation variation and related dynamics during ENSO, laying the foundation for the subsequent study of the western Pacific ocean circulation structure and multi-scale variation, highlighting the high-resolution upper ocean continuous subsurface observations in the circulation and structural variation and importance in the evolution of ENSO events.

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/337002/156905
Collection海洋环流与波动重点实验室
Recommended Citation
GB/T 7714
吕宜龙. 基于潜标观测的El Niño事件期间西太平洋赤道环流变异研究[D]. 中国科学院海洋研究所. 中国科学院大学,2019.
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