Weak Thermocline Mixing in the North Pacific Low‐Latitude Western Boundary Current System

Liu, Zhiyu; Lian, Qiang; Zhang, Fangtao; Wang, Lei; Li, Mingming; Bai, Xiaolin; Wang, Jianing; Wang, Fan

doi:10.1002/2017gl075210

Cited by 41 publications

(56 citation statements)

References 27 publications

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“…From the spectrum levels at the internal wave subrange (Figure b), it is also expected that the internal wave energy is released at the frontal zone (higher level) and then spreads outward (lower level). However, the general pattern with lower diffusivity in the eddy center and higher diffusivity around the eddy is similar to the turbulent microstructure measurements (Liu et al, ; Yang et al, ).…”

Section: Resultssupporting

confidence: 79%

Submesoscale Features and Turbulent Mixing of an Oblique Anticyclonic Eddy in the Gulf of Alaska Investigated by Marine Seismic Survey Data

et al. 2020

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Mesoscale eddies are ubiquitous in the global ocean and can be captured by marine multichannel seismic surveys. In this study, a short-lived anticyclonic eddy is characterized along the seismic line STEEP11 acquired on 30 September 2008 in northern Gulf of Alaska. Fine-scale eddy stratification with alternative strong striae and weak layers is regarded as a typical eddy structure of the study region. The eddy center dips along a NW tilted axis of 1.9 ± 0.2°from the horizontal. Submesoscale structures including fronts and filaments coexist around the eddy periphery and dominate~70% of the eddy volume. The estimated geostrophic current is asymmetric and ageostrophic components must play a significant role in balancing the eddy system. Nonlinearity of the eddy is strong as its rotation speed is much higher than its translation speed. We suggest that Ekman transport is probably responsible for the skewed shape with regards to the asymmetric geostrophic current and asymmetric submesoscale processes. Detailed turbulent diffusivities in and around the eddy are quantified, with an average level of 6.5 × 10 −5 m 2 s −1 . The diapycnal diffusivities show an increasing pattern from the eddy center to the surrounding water. With the pervasive submesoscale processes as the transitional dynamics, a forward cascade of energy could be expected from the mesoscale eddy to the fine-scale wave-breaking and turbulence. The irregular vortex structure may reduce the structural stability, facilitate the energy conversion, and accelerate the eddy dissipation.Plain Language Summary Mesoscale eddies are the weather of the global ocean and can be captured by marine multichannel seismic surveys. Vertical axes of mesoscale eddies could be tilt rather than canonically straight, and therefore, their detailed internal structures may also be asymmetric. Here we use high-resolution seismic methods to characterize a skewed, short-lived (eight weeks) anticyclonic eddy in northern Gulf of Alaska for the first time. The eddy center is strongly displaced at depth with a NW tilted axis of~2°from the horizontal. Geostrophic currents, submesoscale structures, and turbulent diffusivities are asymmetrically distributed in and around the eddy. Except for the standard description/interpretation of a seismic imaged eddy, this study strives to derive numerous and detailed physical oceanographic characteristics from seismic data and stems from a close collaboration between seismologists and oceanographers.

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Section: Resultssupporting

confidence: 79%

Submesoscale Features and Turbulent Mixing of an Oblique Anticyclonic Eddy in the Gulf of Alaska Investigated by Marine Seismic Survey Data

et al. 2020

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“…To the best of our knowledge, this study reports the first observation of this sandwich structure. Diapycnal diffusivities in the upper weak diapycnal diffusivity layer were O (10 −6 ) m 2 s −1 , which is consistent with previous observations and predictions from the wave‐wave interaction theory (Gregg et al, ; Liu et al, ). This layer occupies almost the entire thermocline.…”

Section: Discussion and Summarysupporting

confidence: 91%

“…Most of these studies indicated that strong diapycnal mixing mainly occurs within the surface mixed layer and near the top of the thermocline, while the main thermocline is dominated by weak diapycnal mixing. Weak thermocline diapycnal mixing is consistent with predictions from the wave‐wave interaction theory that diapycnal mixing in equatorial waters is significantly weak due to the latitude effect (Gregg et al, ; Liu et al, ). Diapycnal diffusivities of O (10 −6 ) m 2 s −1 are typically adopted in ocean models, although the equatorial ocean is sensitive to the value of diapycnal diffusivity (Bryan & Lewis, ; Jochum, ; Large et al, ).…”

Section: Introductionsupporting

confidence: 83%

“…The shear was not strong enough to overcome the stratification and induce shear instability. Diapycnal mixing in the WDL is likely to be sustained by the energy cascade through weak wave‐wave interactions, which, as predicted by theory, are rather weak in equatorial waters (Gregg et al, ; Liu et al, ). Unstable Richardson numbers were also rarely found in the MDL, although the stratification in the MDL was comparable to that of the EDL.…”

Section: Resultsmentioning

confidence: 93%

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Enhanced Diapycnal Mixing Between Water Masses in the Western Equatorial Pacific

Liang

Shang

et al. 2019

JGR Oceans

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Diapycnal mixing of momentum and heat is crucially important to ocean dynamics, as it affects the state of the ocean and its interactions with the atmosphere. For realistic climate and circulation predictions, the magnitude and distribution of diapycnal mixing must be accurately represented in ocean models. Based on full-depth microstructure data obtained in the western equatorial Pacific, we demonstrate that there is an enhanced diapycnal diffusivity layer nested below the thermocline of the western equatorial Pacific. Diapycnal diffusivities in this layer range from 5×10 −6 to 5× 10 −4 m 2 s −1 , one to two orders of magnitude higher than those predicted by the wave-wave interaction theory (5 ×10 −7 −5×10 −6 m 2 s −1 ). These enhanced diapycnal mixings are strongly related to the South Pacific Tropical Water intrusion. Stratification is largely weakened by the South Pacific Tropical Water intrusion; thus, the shear can easily trigger shear instabilities and induce strong diapycnal mixing. This enhanced diapycnal diffusivity layer occupies the water column between~250 and 750 m and spans from the equator to 10°N. Such widespread enhanced diapycnal mixing related to water mass intrusions may play an important role in the dynamics of the equatorial ocean, and it should be taken into account in ocean models. Plain Language SummaryThe Pacific is the largest ocean in the world and plays an important role in the global climate change. For example, it affects the typhoons, rainfall, and monsoon, which clearly affects human activities. To accurately predict the ocean circulation and climate, it is needed to understand the dynamic mechanism of the ocean, especially the turbulent mixing that changes the properties of sea water and drives the ocean circulation. In this study, we investigated the turbulent mixing between water masses with observation. We found that there is a layer of enhanced turbulent mixing with a thickness of approximately 500 m in the western equatorial Pacific. This strong turbulent mixing layer was strongly related to the water mass intrusion. Understanding the impact of the water mass on the turbulent mixing can provide references for ocean models and improve their prediction of the climate and circulation.

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“…Due to the lack of long‐term continuous high vertical resolution observations, nevertheless, the relative contributions of these mechanisms to the overall mixing remain unclear. The more recent microstructure measurements in the western boundary of the WEP suggest that thermocline mixing was generally weak with the diapycnal diffusivity κ ρ ~ O (10 −6 ) m 2 /s, and elevated mixing was only found where geostrophic shear was significant (Liu et al, ). In addition, based on long‐term ADCP (acoustic Doppler current profiler) measurements from several subsurface moorings, Zhang et al () pointed out that thermocline mixing in the WEP may be to a large degree associated with the strong subinertial velocity shear between the EUC and Equatorial Intermediate Current.…”

Section: Introductionmentioning

confidence: 99%

Elevated Diapycnal Mixing by a Subthermocline Eddy in the Western Equatorial Pacific

Zhang

Liu

Richards

et al. 2019

Geophysical Research Letters

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Diapycnal mixing plays an important role in modulating upper‐ocean heat content of the western equatorial Pacific (WEP) that profoundly impacts the global climate. Given the sparsity of long‐term in situ observations, the mechanisms driving thermocline mixing in the WEP remain poorly understood. Here, based on yearlong mooring measurements in the WEP, we first report that the occurrence of shear instability was significantly enhanced by an anticyclonic subthermocline eddy (STE). As a result of strong subinertial velocity shear and weakened stratification associated with the STE, the Richardson number was decreased below 1/4 and the estimated diapycnal diffusivity was increased by 400%. Moreover, contrary to surface‐intensified anticyclonic eddies, the STE appeared to act as a dynamic barrier for downward penetration of wind‐generated near‐inertial energy, which may also fertilize mixing near the upper thermocline. Given the frequent occurrence of STEs, they may play a pivotal role in driving thermocline mixing in the WEP.

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Weak Thermocline Mixing in the North Pacific Low‐Latitude Western Boundary Current System

Cited by 41 publications

References 27 publications

Submesoscale Features and Turbulent Mixing of an Oblique Anticyclonic Eddy in the Gulf of Alaska Investigated by Marine Seismic Survey Data

Submesoscale Features and Turbulent Mixing of an Oblique Anticyclonic Eddy in the Gulf of Alaska Investigated by Marine Seismic Survey Data

Enhanced Diapycnal Mixing Between Water Masses in the Western Equatorial Pacific

Elevated Diapycnal Mixing by a Subthermocline Eddy in the Western Equatorial Pacific

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