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Presentation Mode : All
Conference Day : 05/08/2021
Time Slot : AM1 08:30 - 10:30
Sections : OS - Ocean Sciences










Ocean Sciences | Thu-05 Aug




OS17-A006 | Invited
The Niño 4 West Sea Surface Temperature Variability

Ming FENG1#+, Ying ZHANG2, Harry HENDON3, Michael MCPHADEN4
1Commonwealth Scientific and Industrial Research Organisation, Australia, 2South China Sea Institute of Oceanology, Chinese Academy of Sciences, China, 3Bureau of Meteorology, Australia, 4National Oceanic and Atmospheric Administration, United States


Sea surface temperature (SST) variability in the western half of the Niño 4 region (Niño-4W) is important in driving wind anomalies in the far west equatorial Pacific, leading to enhanced Leeuwin Current/Ningaloo Niño, and winter-spring rainfall variability on the Australian continent. Niño-4W SST variability had negative skewness, due to the strong penetration of the cold-tongue into the western Pacific warm pool during strong La Niña, whereas strong El Niño being less likely to generate significant SST anomalies in the region. Zonal advection played a dominant role in driving Niño-4W SST cooling during the strong La Niña events; vertical processes also contributed, whereas the air-sea heat fluxes provided weak negative feedback. Among the advection terms, the advection of mean temperature by zonal velocity anomalies, or zonal advective feedback, played a key role during the development phase of the strong La Niña, causing the negative skewness in the Niño-4W.

OS17-A001 | Invited
Variations of the North Equatorial Current Bifurcation and the Ssh in the Western Pacific Associated with El Niño Flavors

Xin WANG1#+, Bo TONG2, Dongxiao WANG3, Lei YANG1
1Chinese Academy of Sciences, China, 2South China Sea Institute of Oceanology, Chinese Academy of Sciences, China, 3Sun Yat-sen University, China


This study presents different variations of the North Equatorial Current Bifurcation (NECB) in the development of the canonical El Niño, the Central Pacific El Niño I (CP‐I El Niño), and the Central Pacific El Niño II (CP‐II El Niño) in autumn and investigates the dynamic mechanism by analyzing Simple Ocean Data Assimilation data sets. It is suggested that the meridional shifts of the NECB are negatively related to the sea surface height (SSH) near the coast of the Philippines. The NECB shifts northward in autumn of the development phase of both canonical and CP‐II El Niño events, whereas it moves insignificantly for CP‐I El Niño events. During the development phase of canonical and CP‐II El Niños, the remote positive wind stress curl anomalies in the central and western tropical Pacific between 12°N and 15°N are responsible for the variations of the NECB, which can induce the westward Rossby waves of negative SSH anomalies and thus result in the northward shift of the NECB several months later. However, the local wind stress curl anomalies near the Philippine coast are significantly negative and result in locally positive SSH anomalies for CP‐I El Niños. Such anomalous positive SSHs are offset by the negative counterparts that result from the remote wind forcing in the western tropical Pacific. Therefore, the meridional shifts of the NECB are not significant in autumn for CP‐I El Niños. A one and a half layer‐reduced gravity model is used to conduct experiments to further confirm the above results.

OS17-A005
The Extremely Positive Indian Ocean Dipole in 2019: Its Impact on Atmospheric Conditions and Waves

Donghyuk KIM+, Hajoon SONG#, Min-Jee KANG, Hyun-Kyu LEE, Hye-Yeong CHUN
Yonsei University, Korea, South


The Indian Ocean Dipole (IOD) in 2019 was extremely positive, which is the strongest at least since 1979. This strongly positive IOD leads to not only the local changes such as precipitation in the eastern and western equatorial Indian Ocean, but the potential impacts on the climate of remote areas through teleconnections. However, the range of the impacts and the associated processes caused by 2019 positive IOD have not yet been explored in detail. In this study, we utilize the ERA5 reanalysis data to investigate the direct and indirect changes in atmospheric variables due to the positive IOD and find that the anomalies of surface wind, latent heat flux, and outgoing longwave radiations were extremely large. The dispersion relation of the waves associated with the deep tropical convection in this period shows that the positive IOD can be the source of the anomalous stationary Rossby waves with a zonal wavenumber 3. Considering that the Rossby waves can guide the anomalies to travel to the remote regions, it is plausible that the 2019 positive IOD affected the extreme events such as drought in South America and the disruption of the quasi-biennial oscillation.

OS17-A007
Global and Regional Salinity Changes Related to ENSO

Huayi ZHENG1+, Shiyi ZHANG1, Jiang FAN1, Lijing CHENG2#
1Lanzhou University, China, 2Institute of Atmospheric Physics, Chinese Academy of Sciences, China


As the most prominent interannual climate variation on Earth, El Niño–Southern Oscillation (ENSO) alters the pattern of salinity variability worldwide by both atmospheric and oceanic dynamics and processes. Investigating salinity provides a new perspective for understanding the ENSO evolution and prediction. Previous studies have mainly focused on the sea surface salinity anomaly and mixed layer salinity anomaly during ENSO events, the vertical structure of salinity changes and the basin-wise salinity budget are less explored. This study analyzes the global and regional salinity changes related to ENSO from surface to 2000m, and explores their connection with the surface freshwater flux. The lateral and vertical patterns of ENSO-related salinity changes are also analyzed. Results confirm a strong negative sea surface salinity anomaly (SSSA) in the tropical central and western Pacific Ocean during EI Niño, caused by anomalously eastward currents and negative freshwater flux anomaly. The salinity changes at the near-surface layer (0-100m) is opposite to subsurface (100–300m) for the global average, probably associated with the tropical thermocline changes. SSS in the tropical oceans decreases during El Niño, and the tropical Atlantic Oceans shows an increase of salinity during El Niño, partly offsetting the tropical Pacific and Indian freshening for the tropical oceans as a whole. This study, to our knowledge, provides the first comprehensive analysis on the ocean salt budget during ENSO at global and regional scales.

OS17-A004 | Invited
Intraseasonal Variation of Surface Chlorophyll-a Associated with Coastal Upwelling Along the Southern Coast of Java

Takanori HORII1#+, Eko SISWANTO1, Iskhaq ISKANDAR2, Iwao UEKI1
1Japan Agency for Marine-Earth Science and Technology, Japan, 2Sriwijaya University, Indonesia


Coastal upwelling along the southern coast of Java brings cold and nutrient-rich subsurface water upward and plays an important role in controlling ocean surface heat balance, biogeochemical balance, coastal ecosystem, and regional fisheries in the southeastern Indian Ocean. To understand the coastal upwelling, we investigated satellite-based Chlorophyll-a data south of Java, with a focus on the seasonal to intraseasonal-scale variability. Based on the weekly timeseries, spectrum analysis showed that annual, semi-annual, and intraseasonal variations were significant in the Chlorophyll-a variations. The peaks of the Chlorophyll-a variations were accompanied with decreases in the local sea level and sea surface temperature, indicating a coastal upwelling of cold and nutrient-rich subsurface water at the intraseasonal timescale. Year-to-year differences of the intraseasonal variations associated with the Indian Ocean Dipole will also be discussed.

OS17-A003
Impacts of Detoured Madden-julian Oscillations on the South Pacific Ocean

Lei ZHOU#+
Shanghai Jiao Tong University, China


Madden-Julian Oscillations (MJOs) are a major component of tropical intraseasonal variabilities. There are two paths for MJOs across the Maritime Continent; one is a detoured route into the Southern Hemisphere and the other one is around the equator across the Maritime Continent. Here, it is shown that the detoured and non-detoured MJOs have significantly different impacts on the South Pacific convergence zone (SPCZ). The detoured MJOs trigger strong cross-equatorial meridional winds from the Northern Hemisphere into the Southern Hemisphere. The associated meridional moisture and energy transports by the intraseasonal meridional winds are favorable for reinforcing the SPCZ. In contrast, the influences of non-detoured MJOs on either hemisphere or the meridional transports across the equator are much weaker. The detoured MJOs can extend their impacts to the surrounding regions by shedding Rossby waves. Due to different background environmental vorticity during detoured MJOs in boreal winter, more ray paths of Rossby waves traverse the Maritime Continent connecting the southern Pacific Ocean and the eastern Indian Ocean, but fewer ray paths of Rossby waves cover Australia. Further studies on such processes are expected to contribute to a better understanding of extreme climate and natural disasters on the rim of the southern Pacific Ocean and Indian Ocean.

OS17-A008
Pdo Modulation on the Relationship Between Enso and Typhoon Tracks

Chaoming HUANG1,2#+, Hailong LIU3, Xidong WANG1
1Hohai University, China, 2Shanghai Jiao Tong University, China, 3Yunnan University, China


Tropical cyclones (TCs) trajectories in western North Pacific (WNP) in 1950-2017 were clustered into 7 clusters (clusters A-G), including three recurved trajectories and four straight-moving tracks. These clusters were distinguishing well on numbers of TC, intensity, lifetime, genesis position/month, landing and track. Most of negative vertical wind sheer (VWS) accompanied by the eastward steer flow in the composite differences between positive and negative phase of ENSO. Sea surface temperature (SST) anomaly composite analysis and accumulated cyclone energy (ACE) and so on of each cluster demonstrated that there are four clusters (A, C, E and G) with a closer relationship with La Niña (El Niño), within which, only two clusters (E and G) are modulated by PDO. Differences of quantity, genesis location, tracks and its shape between PDO(+)ENSO(+) and PDO(+)ENSO(-) are more obvious than that between PDO(-)ENSO(+) and PDO(-)ENSO(-), such as clusters E and G. The leading two modes of empirical orthogonal functions (EOF) analysis of TCs track density is connected with ENSO significantly and this relation is more obvious in PDO warm phase. It illustrated that in PDO warm phase, differences of TC with ENSO phase change have a bigger influence to TCs track than in PDO cold phase. This more notable differences can be results from steer flow and VWS that they had more sensitive change in PDO warm phase, local SST and 850-hPa wind will also be affected by these environmental factors.

OS17-A009
Variability of Oceanic Mixed Layer in the Southern Ocean and Its Impact on Primary Productivity

Yuxin SHI1#+, Hailong LIU2
1Shanghai Jiao Tong University, China, 2Yunnan University, China


The mixed layer depth (MLD) in the Southern Ocean (SO) at different time scales is investigated using the Simple Ocean Data Assimilation (SODA) and World Ocean Database 2018 (WOD18) from 1980 to 2018. The distribution of MLD shows significant seasonal variations with a deep ML band (40°S-60°S) beneath the westerlies in winter and spring (>150 m). And MLD near Antarctica is shallow (< 50 m) all year round because the sea ice cover weakens the stirring effect of wind. The MLD variability of the interannual band is at least three times larger than that of the decadal band. During the 39 years of this study, the MLD trend is more obvious in winter and spring. The Pacific Ocean presents a dipole distribution, shallowing in the west and deepening in the east, while the southern Australian sector tends to deepen. This is caused by a combination of net heat flux and wind stress.

OS02-A002
The Cause of a Long-period Tsunami by the July 2020 Mw 7.8 Shumagin Earthquake

Iyan MULIA1#+, Mohammad HEIDARZADEH2, Kenji SATAKE3
1RIKEN Cluster for Pioneering Research, Japan, 2Brunel University of London, United Kingdom, 3The University of Tokyo, Japan


A large Mw 7.8 submarine earthquake ruptured the Shumagin seismic gap region located along the Aleutian subduction zone on 22 July 2020. The earthquake generated a relatively small tsunami, but with unprecedented longer period waves than that of typically expected from its causal earthquake magnitude. When compared with a larger event such as the 2011 Mw 9.0 Tohoku-oki earthquake that produced tsunami wave periods of less than 30 min, the July 2020 Shumagin earthquake resulted in ~60 min periods. The main objective of the study is to reveal the underlying physics of such a unique event. Here, we examined the cause of the anomalous ocean wave through a tsunami source modeling by an inversion analysis from geodetic and both offshore and coastal tsunami data. Based on our source modeling, we found that the main slip region was located beneath the Shumagin Islands to the east of the estimated epicenter by the United States Geological Survey. The main slip patch was confined at depths between 30–40 km, with a maximum slip amount of approximately 2 m. The corresponding coseismic surface displacement predominantly took place at a mean water depth of ~200 m within the broad continental shelf extending ~150 km offshore. Consequently, to satisfy the linear shallow water theory, the initial displacement on the shallow water depths was responsible for the long-period waves because the water depth is inversely proportional to the period. Furthermore, our result implies that it was unlikely for the earthquake to rupture updip (<20 km depth) as it would displace the seafloor beyond the continental shelf, and thus generate shorter tsunami wave periods.

OS02-A031
The Role of Frontal Thrusts in Tsunami Earthquake Generation

Raquel FELIX1#+, Judith HUBBARD2, James Daniel Paul MOORE3,4, Adam SWITZER3
1NTU, Singapore, 2Earth Observatory of Singapore, Singapore, 3Nanyang Technological University, Singapore, 4Nanyang Technological University, Singapore


The frontal sections of subduction zones are the source of a poorly understood hazard: “tsunami earthquakes,” which generate larger-than-expected tsunamis given their seismic shaking.  Slip on frontal thrusts has been proposed as a way to increase wave heights in these events, but thus far, this mechanism has not been quantified. We explore the impact of frontal thrust slip on tsunami wave generation by quantifying both wave heights in 2D and expected tsunami wave energies in 3D. First, we show that the narrow band of seafloor uplift generated by slip on a frontal thrust is heavily damped by the water column, leading to relatively low initial tsunami heights and resulting tsunami energies. Although the specific geometry of the frontal thrust ramp can modify seafloor deformation, water damping reduces these differences, and tsunami energy is insensitive to details in thrust ramp geometry, like fault dip in most cases, geological evolution, sedimentation, and erosion. Our results show that the tsunami energy depends primarily on three large-scale features: décollement depth below the seafloor, water depth, and coseismic slip. Since frontal ruptures of subduction zones are expected to include slip on both the frontal thrust and the down-dip décollement, we compare the tsunami energies generated by these two fault segments. We find that in most cases, thrust ramps generate significantly lower tsunami energies than the paired slip on the décollement. Using the 2010 Mw 7.8 Mentawai tsunami earthquake as a case study, we show that although slip on the décollement and the frontal thrust together can generate the required tsunami energy, <10% was contributed by the frontal thrust. Overall, we demonstrate that the wider, lower amplitude uplift patch produced by slip on the décollement must play a dominant role in the tsunami generation process for tsunami earthquakes in subduction zones around the world.

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