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Liang Gui Chen

Abstract

An analysis of the Levitus data is employed to examine Stommel's mixed layer density ratio regulator hypothesis. Three different methods of computing the lateral density ratio (R l ≡αΔT/βΔS) are used and the least squares method was found to have the least variance in the density ratio over the temperature range of 7°–17°C. Seasonal mean and annual mean density ratios in the North Pacific Ocean are calculated. The spring season has the highest density ratio of 2.1, and the fall season has the lowest of 1.64. The vertical variation in the lateral density ratio is small, in the upper 50 m, especially during the winter season. Overall, in the world's ocean, the mixed layer annual mean density ratios in the 7°–17°C range show remarkably consistent values, in support of Stommel's hypothesis. However, our estimate shows a systematically lower density ratio than that of Stommel in each ocean. Only the South Pacific and South Atlantic have density ratios equal to or slightly over 2. The North Pacific, North Atlantic, and Indian Oceans have density ratios around 1.7.

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Liang Gui Chen
and
William K. Dewar

Abstract

A three-layer, wind-driven, general circulation model involving both subtropical and subpolar gyres has been developed to study intergyre exchange. Following some early studies, the present work allows flow to baroclinically cross the intergyre boundary. This model differs from past work by examining a three-layer fluid. Solutions with both southward and northward subsurface flows are obtained. The two principal objectives of this work are to clarify the structure and maintenance of the permanent thermocline and to aid in understanding the distribution of deep water masses.

A class of thermocline structures at the zero Ekman pumping line has been constructed that permits intergyre exchange, or communication. The zones of exchange are called windows. In this study, the windows have several unique properties relative to those computed elsewhere, and exhibit relatively rich structure. Principally, the addition of an active third layer allows a new second baroclinic window to open. This new window is physically and dynamically distinct from the first window (found in previous studies), and most of the intergyre baroclinic transport can occur through it. Its appearance also supports the conjecture that the number of communication windows increases with the number of active layers.

In addition to the model development, observed potential vorticity distributions have been reexamined within the context of this model. Possible explanations for deep potential vorticity contours in the North Atlantic and North Pacific oceans are proposed.

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Yang Yang
,
X. San Liang
,
Bo Qiu
, and
Shuiming Chen

Abstract

Previous studies have found that the decadal variability of eddy kinetic energy (EKE) in the upstream Kuroshio Extension is negatively correlated with the jet strength, which seems counterintuitive at first glance because linear stability analysis usually suggests that a stronger jet would favor baroclinic instability and thus lead to stronger eddy activities. Using a time-varying energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), and the MS-EVA-based nonlinear instability theory, this study investigates the physical mechanism responsible for such variations with the state estimate from the Estimating the Circulation and Climate of the Ocean (ECCO), Phase II. For the first time, it is found that the decadal modulation of EKE is mainly controlled by the barotropic instability of the background flow. During the high-EKE state, violent meanderings efficiently induce strong barotropic energy transfer from mean kinetic energy (MKE) to EKE despite the rather weak jet strength. The reverse is true in the low-EKE state. Although the enhanced meander in the high-EKE state also transfers a significant portion of energy from mean available potential energy (MAPE) to eddy available potential energy (EAPE) through baroclinic instability, the EAPE is not efficiently converted to EKE as the two processes are not well correlated at low frequencies revealed in the time-varying energetics. The decadal modulation of barotropic instability is found to be in pace with the North Pacific Gyre Oscillation but with a time lag of approximately 2 years.

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Chengyan Liu
,
Zhaomin Wang
,
Xi Liang
,
Xiang Li
,
Xichen Li
,
Chen Cheng
, and
Di Qi

Abstract

Warm deep water intrusion over the Antarctic continental shelves threatens the Antarctic ice sheet stability by enhancing the basal melting of ice shelves. In East Antarctica, the Antarctic Slope Current (ASC), along with the Antarctic Slope Front (ASF), acts as a potential vorticity barrier to prevent the warm modified Circumpolar Deep Water (mCDW) from ventilating the cold and fresh shelf. However, mCDW onshore transport is still observed within certain shelf regions, such as submarine troughs running perpendicular to the continental shelf. This study focuses on the dynamic mechanisms governing mCDW intrusion within a submarine trough over the fresh shelf regions, East Antarctica. Based on an idealized eddy-resolving coupled ocean–ice shelf model, two high-resolution process-oriented numerical experiments are conducted to reveal the mechanisms responsible for the mCDW onshore transport. Three dynamic mechanisms governing cross-slope mCDW intrusion are identified: 1) the bottom pressure torque, 2) the topography beta spiral, and 3) the topography Rossby waves. These three mechanisms simultaneously govern the mCDW intrusion together. The bottom pressure torque plays a leading role in driving the time-mean onshore flow whose vertical structure is determined by the topography beta spiral, while the topography Rossby waves contribute to the high-frequency oscillations in the onshore volume and heat transport. The simulated spatial distribution and seasonality of mCDW intrusion qualitatively coincide with the observed mCDW intrusion over fresh shelf regions, East Antarctica. Both the topography beta spiral and the ASC play an important role in governing the seasonality of mCDW intrusion.

Open access
Chang-Rong Liang
,
Xiao-Dong Shang
,
Yong-Feng Qi
,
Gui-Ying Chen
, and
Ling-Hui Yu

Abstract

Finescale parameterizations are of great importance to explore the turbulent mixing in the open ocean due to the difficulty of microstructure measurements. Studies based on finescale parameterizations have greatly aided our knowledge of the turbulent mixing in the open ocean. In this study, we introduce a modified finescale parameterization (MMG) based on shear/strain variance ratio R ω and compare it with three existing parameterizations, namely, the MacKinnon–Gregg (MG) parameterization, the Gregg–Henyey–Polzin (GHP) parameterization based on shear and strain variances, and the GHP parameterization based on strain variance. The result indicates that the prediction of MG parameterization is the best, followed by the MMG parameterization, then the shear-and-strain-based GHP parameterization, and finally the strain-based GHP parameterization. The strain-based GHP parameterization is less effective than the shear-and-strain-based GHP parameterization, which is mainly due to its excessive dependence on stratification. The predictions of the strain-based MMG parameterization can be comparable to that of the MG parameterization and better than that of the shear-and-strain-based GHP parameterization. Most importantly, MMG parameterization is even effective over rough topography where the GHP parameterization fails. This modified MMG parameterization with prescribed R ω can be applied to extensive CTD data. It would be a useful tool for researchers to explore the turbulent mixing in the open ocean.

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Gengxin Chen
,
Weiqing Han
,
Xiaolin Zhang
,
Linlin Liang
,
Huijie Xue
,
Ke Huang
,
Yunkai He
,
Jian Li
, and
Dongxiao Wang

Abstract

Using 4-yr mooring observations and ocean circulation model experiments, this study characterizes the spatial and temporal variability of the Equatorial Intermediate Current (EIC; 200–1200 m) in the Indian Ocean and investigates the causes. The EIC is dominated by seasonal and intraseasonal variability, with interannual variability being weak. The seasonal component dominates the midbasin with a predominant semiannual period of ~166 days but weakens toward east and west where the EIC generally exhibits large intraseasonal variations. The resonant second and fourth baroclinic modes at the semiannual period make the largest contribution to the EIC, determining the overall EIC structures. The higher baroclinic modes, however, modify the EIC’s vertical structures, forming multiple cores during some time periods. The EIC intensity has an abrupt change near 73°E, which is strong to the east and weak to the west. Model simulation suggests that the abrupt change is caused primarily by the Maldives, which block the propagation of equatorial waves. The Maldives impede the equatorial Rossby waves, reducing the EIC’s standard deviation associated with reflected Rossby waves by ~48% and directly forced waves by 20%. Mode decomposition further demonstrates that the semiannual resonance amplitude of the second baroclinic mode reduces by 39% because of the Maldives. However, resonance amplitude of the four baroclinic mode is less affected, because the Maldives fall in the node region of mode 4’s resonance. The research reveals the spatiotemporal variability of the poorly understood EIC, contributing to our understanding of equatorial wave–current dynamics.

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Jinliang Liu
,
Jun-Hong Liang
,
James C. McWilliams
,
Peter P. Sullivan
,
Yalin Fan
, and
Qin Chen

Abstract

A large-eddy simulation (LES) model is configured to investigate the effect of the horizontal (northward) component of Earth’s rotation on upper-ocean turbulence. The focus is on the variability of the effect with latitude/hemisphere in the presence of surface gravity waves and when capped by a stable stratification beneath the surface layer. When is included, the mean flow, turbulence, and vertical mixing depend on the wind direction. The value and effect of are the largest in the tropics and decrease with increasing latitudes. The variability in turbulent flows to wind direction is different at different latitudes and in opposite hemispheres. When limited by stable stratification, the variability in turbulence intensity to wind direction reduces, but the entrainment rate changes with wind direction. In wave-driven Langmuir turbulence, the variability in mean current to wind direction is reduced, but the variability of turbulence to wind direction is evident. When there is wind-following swell, the variability in the mean current to wind direction is further reduced. When there is strong wind-opposing swell so that the total wave forcing is opposite to the wind, the variability in the mean current to wind direction is reduced, but the variability of turbulence to wind direction is enhanced, compared to in Ekman turbulence. The profiles of eddy viscosity, including its shape and its value, show a strong wind direction dependence for both stratified wind-driven and wave-driven Langmuir turbulence. Our study demonstrates that wind direction is an important parameter to upper-ocean mixing, though it is overlooked in existing ocean models.

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