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Presentation Mode : All
Conference Day : 04/08/2021
Time Slot : AM1 08:30 - 10:30
Sections : OS - Ocean Sciences










Ocean Sciences | Wed-04 Aug




OS03-A010 | Invited
Ocean Microstructure in the Interleaving Zone of the Ross Ice Shelf Cavity

Craig STEVENS1,2#+, Alena MALYARENKO1,3, Yingpu XIAHOU2,1, Christina HULBE4
1National Institute of Water and Atmospheric Research, New Zealand, 2University of Auckland, New Zealand, 3Victoria University of Wellington, New Zealand, 4University of Otago, New Zealand


Ocean mixing within ice shelf cavities influences their circulation and has implications and feedbacks with melting of the ice. Hydrographic observations in 2017 from the mid-water column of the Ross Ice Shelf cavity, the largest on the planet by area, show a region of interleaving layers.  Though clearly variable, similar structure was observed in the only other direct measurements in the central cavity forty years prior.  While the sampling conditions were challenging for microstructure (~370 m of borehole), a small number (10) of microstructure profiles were recorded in conjunction with the hydrographic and other sampling. There is clear evidence of turbulent mixing in the temperature microstructure data. Here we look at the potential driving roles of double diffusion and shear to consider if they are responsible for the observed small-scale variability.  The implications for overall vertical diffusion are then examined.

OS03-A005
Turbulent Mixing Variability in an Energetic Standing Meander of the Southern Ocean

Ajitha CYRIAC1#+, Helen PHILLIPS1, Nathaniel BINDOFF1, Kurt POLZIN2
1University of Tasmania, Australia, 2Woods Hole Oceanographic Institution, United States


Turbulent mixing is a crucial mechanism that controls the distribution of heat, salt, sediments and organisms throughout the world oceans. It also plays an important role in driving large-scale oceanic processes such as watermass transformation, overturning circulation and stratification. Here we present mixing estimates from field observations of a standing meander near the Macquarie ridge, a major topographic obstacle for the Antarctic Circumpolar Current (ACC). It is a region of high eddy kinetic energy and interior upwelling. We collected more than 1400 profiles of temperature, salinity and velocity of the upper ocean using state of the art Electromagnetic Autonomous Profiling Explorer (EM-APEX) profiling floats in the upper 1600 m. By applying a finescale parameterization, we estimated the spatial and temporal variability of diapycnal mixing along the float tracks and investigated the sources.  Elevated turbulent mixing is mostly associated with regions of subantarctic front and mesoscale eddies. In the upper layers, wind-generated downward propagating near-inertial waves dominate the enhanced mixing. The mixing is also high in cyclonic eddies associated with upward propagating internal waves. The dissipation rate estimates over rough and smooth topography has similar magnitudes suggesting that topography plays less role in mixing the upper 1600 m.

OS03-A006
Internal Lee Waves Generated by Shear Flow Over Small-scale Topography

Hui SUN#+, Qingxuan YANG, Wei ZHAO, Xiaodong HUANG, Jiwei TIAN
Ocean University of China, China


When the bottom current flows over small-scale topographic features (with characteristic horizontal scales of 0.1–10 km), it breeds internal lee waves, which furnish the energy cascade from mesoscale to dissipative scale, resulting in enhanced turbulent mixing. Considering the deviation between the observation and the predicated value by a linear lee wave theory using a uniform background flow, we improve this theory by taking a shear background flow into account. The results reveal that the shear flow has a substantial influence not only on the frequency and wavenumber but also on the magnitude of lee waves; hence, on the energy flux and dissipation of lee waves. From the bottom up, when the background flow decreases, the frequency, vertical wavelength, vertical group velocity, and energy flux of lee wave all decrease; a bottom-up increasing background flow, on the other hand, leads to diametrically opposite results. Additionally, simulation using three different background flow indicates the bottom-up decreasing flow is more prone to the occurrence of turbulent mixing. Further, a parameterization for lee wave-driven mixing, which reflects the effects of shear flow, is put forward and applied to a global estimation. The estimate suggests the bottom shear flow can make the lee wave energy flux decline with the increasing distance above the bottom, with a global mean rate of about -5% per 100 m. The shear flow-generated lee waves are potentially a candidate for the enhanced mixing (as high as 10-3 m2 s-1) in the Drake Passage, Gulf Stream, Kuroshio Extension, and so on.

OS03-A007
Spatiotemporal Characteristics of Internal Gravity Waves in the Northwestern Pacific Observed by Argo Floats

Yang WANG1#+, Zhenhua XU2, Qun LI3, Robin ROBERTSON4
1Institute of Oceanology, Chinese Academy of Sciences, China, 2IOCAS, China, 3Polar Reasearch Institute of China, China, 4Xiamen University Malaysia, Malaysia


Argo floats measure the hourly pressure and temperature at the parking depth near 1000m dbar. The data are sufficient for interpreting the vertical displacements and spectrum features of internal gravity waves. More than 10000 parking-phase observations of 208 Argo floats are used to estimate the spatial-temporal characteristics of internal gravity waves in the northwestern Pacific. Enhanced vertical displacements caused by internal gravity waves were found near the energetic internal tide sources, such as Luzon Strait, Izu ridge, Philippine coast and Bonin ridge. The internal gravity waves possess evident seasonality and spring-neap trends. The diurnal and semidiurnal peaks in the spectrum exhibit differently with latitudinal dependence due to different mechanisms. The energy of internal gravity waves is estimated by fitting the vertical displacements to the full-depth water. The observed e-folding scales of the internal gravity wave energy are about 600-1200 km in the deep basin. The analysis provides a new view for understanding the basin-scale spatial-temporal variability of internal gravity waves with observational evidence.

OS03-A003
Three Dimensional Simulation on the Generation and Propagation of Internal Tides and Solitary Waves Northeast of Taiwan Island

Wenjia MIN1#+, Qun LI2, Robin ROBERTSON3
1Institute of Oceanology, Chinese Academy of Sciences China, China, 2Polar Reasearch Institute of China, China, 3Xiamen University Malaysia, Malaysia


The slope area northeast of Taiwan was known as a hotspot for internal tides and internal solitary waves (ISWs), while their specific sources and generation mechanism of ISWs remain unclear. We investigate the generation and evolution processes of internal tides and ISWs with realistic configuration based on the high resolution non-hydrostatic numerical simulations. The ISWs northeastern Taiwan show a complex pattern according to the satellite image and our numerical results. ISWs propagate to various direction, and both shoreward and seaward propagating ISWs are generated on the continental slope. The ISWs observed on the continental slope-shelf region northeastern Taiwan can be generated by two ways. One is the local tide-topography interaction, and the other is the disintegration of remote internal tides generated over the I-Lan Ridge. The generated internal tides propagate northward to the Okinawa Trough, and can reach the continental slope-shelf region. During the propagation of the internal tides, the internal tides start to steepen and internal solitary waves are formed about 80 km north of I-Lan Ridge. The amplitude of the generated internal solitary waves is about 30 m. Furthermore, the Kuroshio is important to modulate the propagation and evolution of internal tides and ISWs, especially to the complexity of the ISW spatial pattern. We revealed most of the generated internal wave energy is dissipated locally over the double-canyon region, and strong mixing occur over the canyons.

OS03-A008
Internal Tide Energetics and Intensified Mixing in the Philippine Sea

Zhenhua XU1#+, Jia YOU2, Robin ROBERTSON3, Qun LI4, Yang WANG5
1IOCAS, China, 2Chinese Academy of Sciences, China, 3Xiamen University Malaysia, Malaysia, 4Polar Reasearch Institute of China, China, 5Institute of Oceanology, Chinese Academy of Sciences, China


The spatially inhomogeneous and seasonally variable diapycnal diffusivities in the upper Philippine Sea were estimated from ARGO float data using a strain-based finescale parameterization. Based on a coordinated analysis of multi-source data, we found that the driving processes for diapycnal diffusivities mainly included the near-inertial waves and internal tides. Furthermore, we provide evidence that the mesoscale environment in the Philippine Sea played a significant role in regulating the intensity and shaping the spatial inhomogeneity of the internal tidal mixing.

OS03-A001
Tidal and Wind Mixing in the Arafura Sea

Robin ROBERTSON1#+, Zhenhua XU2
1Xiamen University Malaysia, Malaysia, 2IOCAS, China


Time series of shipboard observations in the southern Arafura Sea near the Tiwi Islands indicated that the water column dynamics differed between the east and west sides of the islands.  On the west side, the water column, defined as temperature, salinity, and velocity, was barotropic and tidal advection dominated.  On the east side, the water column was baroclinic and internal tides were present along with tidal advection.  These conditions affected the distribution of the turbidity and fluorescence in the water column with the fluorescence distributed throughout the water column on the western side, but concentrated in the lower layer on the eastern side.  Likewise, the influence of the daily solar radiation cycle reached the bottom on the western side, but was limited to the upper layer above the thermocline on the eastern side.  The fluorescence peaks also differed between the east and west sides, with the eastern side dominated by the semidiurnal tides and the western side by the daily solar cycle.  Also on the eastern side, fluorescence was limited to the lower layer, while on the western side, it encompassed the entire water column at times and peaked below the warmer, higher oxygen water generated by solar radiation.  These dynamics have distinct implications for biological productivity and also may affect a proposed tidal power system in the region.  

OS03-A012
The Scale and Activity of Symmetric Instability Estimated from a Global Submesoscale-permitting Ocean Model

Jihai DONG1#+, Baylor FOX-KEMPER2, Hong ZHANG3, Changming DONG1,4
1Nanjing University of Information Science & Technology, China, 2Brown University, United States, 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, United States, 4Southern Marine Science and Engineering Guangdong Laboratory, China


Symmetric instability (SI) extracts kinetic energy from fronts in the surface mixed layer (SML), potentially affecting the SML structure and dynamics. Here, a global submesoscale-permitting ocean model named MITgcm LLC4320 simulation is used to examine the Stone (1966) linear prediction of the maximum SI scale to estimate grid spacings needed to begin resolving SI. Furthermore, potential effects of SI on the usable wind-work are estimated roughly: this estimate of SI "activity" is useful for assessing if these modes should be resolved or parameterized. The maximum SI scale varies by latitude with median values of 568 m to 23 m. Strong seasonality is observed in the SI scale and activity. The median scale in winter is 188 m globally, 2.5 times of that of summer (75 m). SI is more active in winter: 15% of the time compared with 6% in summer. The strongest SI activity is found in the western Pacific, western Atlantic, and Southern Oceans. The required grid spacings for a global model to begin resolving SI eddies in the SML are 24 m (50% of regions resolved) and 7.9 m (90%) in winter, decreasing to 9.4 m (50%) and 3.6 m (90%) in summer. It is also estimated that SI may reduce usable wind-work by an upper bound of 0.83 mW m−2 globally, or 5% of the global magnitude.