Browse

You are looking at 41 - 50 of 8,285 items for :

  • Journal of Physical Oceanography x
  • Refine by Access: All Content x
Clear All
Guang-Bing Yang, Changshui Xia, Xia Ju, Quanan Zheng, Yeli Yuan, Xue-Jun Xiong, and Fangli Qiao

Abstract

Previous in-situ observations have suggested that bottom water temperature variations in shelf seas can drive significant ocean bottom heat flux (BHF) by heat conduction. The BHF-driven bottom water temperature variations, however, have been overlooked in ocean general circulation models. In this study, we established a sea-sediment fully coupled model through incorporating the BHF. The coupled model included a sediment temperature module/model, and the BHF was calculated based on the sediment heat content variations. Meanwhile, we applied temporally varying BHF in the calculation of the bottom water temperature, which further determined the sediment temperature. The two-way coupled BHF process presents a more complete and physically reasonable heat budget in the ocean model and a synchronously varying sediment temperature profile. The coupled model was validated using a one-dimensional test case, and then it was applied in a domain covering the Bohai and Yellow Seas. The results suggest that when a strong thermocline exists, the BHF can change the bottom water temperature by more than 1°C because the effects of the BHF are limited to within a shallow bottom layer. However, when the water column is well mixed, the BHF changes the temperature of the entire water column, and the heat transported across the bottom boundary is ventilated to the atmosphere. Thus, the BHF has less effect on water temperature and may directly affect air-sea heat flux. The sea-sediment interactions dampen the amplitude of the bottom water temperature variations, which we propose calling the seabed dampening ocean heat content variation mechanism (SDH).

Restricted access
Xueli Yin, Dongliang Yuan, Xiang Li, Zheng Wang, Yao Li, Corry Corvianawatie, Adhitya Kusuma Wardana, Dewi Surinati, Adi Purwandana, Mochamad Furqon Azis Ismail, Asep Sandra Budiman, Ahmad Bayhaqi, Praditya Avianto, Edi Kusmanto, Priyadi Dwi Santoso, Dirhamsyah, and Zainal Arifin

Abstract

The mean circulation and volume budgets in the upper 1200 m of the Maluku Sea are studied using multi-year current meter measurements of four moorings in the Maluku Channel and of one synchronous mooring in the Lifamatola Passage. The measurements show that the mean current in the depth range of 60 m - 450 m is northward towards the Pacific Ocean with a mean transport of 2.07 Sv - 2.60 Sv. In the depth range of 450 m - 1200 m, a mean western boundary current (WBC) flows southward through the western Maluku Sea and connects with the southward flow in the Lifamatola Passage. The mean currents in the central-eastern Maluku Channel are found to flow northward at this depth range, suggesting an anti-clockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83 Sv - 2.25 Sv, which is balanced by three transports: (1) 0.62 Sv - 0.93 Sv southward transport into the Seram-Banda Seas through the Lifamatola Passage, (2) 0.97 Sv-1.01 Sv returning to the western Pacific Ocean through the central-eastern Maluku Channel, and (3) a residual transport surplus, suggested to upwell to the upper layer joining the northward transport into the Pacific Ocean. The dynamics of the intermediate gyre circulation are explained by the potential vorticity (PV) integral constraint of a semi-enclosed basin.

Restricted access
L. Mahrt, Erik Nilsson, and Anna Rutgersson

Abstract

We analyze approximately four years of heat-flux measurements at two levels, profiles of air temperature, and multiple measurements of the water temperature collected at a coastal zone site. Our analysis considers underestimation of the sea-surface flux due to vertical divergence of the heat flux between the surface and the lowest flux level. We examine simple relationships of the heat flux to the wind speed and stratification and the potential influence of fetch and temperature advection. The fetch ranges from about 4 km to near 400 km. For a given wind-direction sector, the transfer coefficient varies only slowly with increasing instability, but decreases significantly with increasing stability. The intention here is not to recommend a new parameterization but rather to establish relationships that underly the bulk formula that could lead to assessments of uncertainty and improvement of the bulk formula.

Restricted access
Louise Rousselet and Paola Cessi

Abstract

The diabatic transformations of the mid-depth meridional overturning circulation (MOC) as it exits and reenters the South Atlantic to close the AMOC are studied using a state estimate assimilating data into a dynamically consistent ocean model. Virtual Lagrangian parcels in the lower branch of the MOC are followed in their global tour as they return to the upper branch of the MOC. Three return pathways are identified. The first pathway enters the abyssal Indo-Pacific as Circumpolar Deep Water, directly from the northern Antarctic Circumpolar Current (ACC), and before sampling the Antarctic margin. The second pathway sinks to abyssal densities exclusively in the Southern Ocean, then upwells while circulating within the ACC and eventually enters the Indo-Pacific or Atlantic at mid-to-upper-depths. The third pathway never reaches densities in the abyssal range. Parcels sinking in the Antarctic Bottom Water range upwell to mid-to-upper depths south of 55°S. Parcels in all three pathways experience additional diabatic transformations after upwelling in the Southern Ocean, with more diabatic changes north of about 30°S than elsewhere. Diabatic changes are predominantly in the mixed layer of the tropical and subpolar gyres, enabled by Ekman suction. A simple model of the wind-driven flow illustrates that parcels always reach the surface in the tropical and subpolar gyres, regardless of their initial condition, because of coupling among gyres, the Ekman transport and its return.

Restricted access
Henri F. Drake, Xiaozhou Ruan, and Raffaele Ferrari

Abstract

Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Indirect microstructure measurements of in-situ turbulence suggest that mixing is bottom-enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. Tracer Release Experiments (TREs), in which inert tracers are purposefully released and their dispersion is surveyed over time, have been used to independently infer turbulent diffusivities—but typically provide estimates in excess of microstructure ones. In an attempt to reconcile these differences, Ruan and Ferrari (2021) derived exact tracer-weighted buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. A tracer’s diapycnal displacement rate is exactly twice the tracer-averaged buoyancy velocity, itself a convolution of an asymmetric upwelling/downwelling dipole. The tracer’s diapycnal spreading rate, however, involves both the expected positive contribution from the tracer-averaged in-situ diffusion as well as an additional non-linear diapycnal distortion term, which is caused by correlations between buoyancy and the buoyancy velocity, and can be of either sign. Distortion is generally positive (stretching) due to bottom-enhanced mixing in the stratified interior but negative (contraction) near the bottom. Our simulations suggest that these two effects coincidentally cancel for the Brazil Basin Tracer Release Experiment, resulting in negligible net distortion. By contrast, near-bottom tracers experience leading-order distortion that varies in time. Errors in tracer moments due to realistically sparse sampling are generally small (< 20%), especially compared to the O(1) structural errors due to the omission of distortion effects in inverse models. These results suggest that TREs, although indispensable, should not be treated as “unambiguous” constraints on diapycnal mixing.

Restricted access
Hendrik Jongbloed, Henk M. Schuttelaars, Yoeri M. Dijkstra, Paul B. Donkers, and Antonius J.F. Hoitink

Abstract

An idealized width-averaged model is employed to study the influence of wind stress on subtidal salt intrusion and stratification in well-mixed and partially stratified estuaries. We show that even in mild conditions, wind forcing can influence the estuarine salinity structure in a substantial way. By studying the role of wind forcing on dominant salt transport balances and associated salt transport regimes, we unify and clarify ambiguous observations from previous authors regarding the influence of wind stress: The response of the estuarine salinity structure to wind forcing is different depending on the underlying dominant salt transport balance, which in turn was found to determine whether wind-induced salinity shear, wind-induced modulation of the longitudinal salt distribution or wind-induced mixing dominates.

Restricted access
Ming-Huei Chang, Yu-Hsin Cheng, Yu-Yu Yeh, Yiing Jang Yang, Sen Jan, Chih-Lun Liu, Takeshi Matsuno, Takahiro Endoh, Eisuke Tsutsumi, Jia-Lin Chen, and Xinyu Guo

Abstract

Complex small-scale processes and energetic turbulence are observed at a sill located on the I-Lan Ridge that spans across the strong Kuroshio off Taiwan. The current speed above the sill is strong (1.5 m s−1) and unsteady (±0.5 m s−1) due to the Kuroshio modulated by the semidiurnal tide. Above the sill crest, isothermal domes, with vertical scales of ∼20 m and ∼50 m during the low and high tides, respectively, are generated by turbulent mixing as a result of shear instability in the bottom boundary layer. Tidally modulated hydraulic character modifies the small-scale processes occurring on the leeward side of the sill. Criticality analysis, performed by solving the Taylor-Goldstein equation, suggests that the observed lee waves and intermediate layer sandwiched by two free shear layers are related to the mode-1 and mode-2 critical control between the sill crest and immediate lee, respectively. Around high tide, lee waves are advected further downstream, and only mode-1 critical control can occur, leading to a warm water depression. The shear instabilities ensuing from the hydraulic transition processes continuously mediate flow kinetic energy to turbulence such that the status of marginal instability where Richardson number converges at approximately 0.25 is reached. The resultant eddy diffusivity Kp is concentrated at O(10−4) to O(10−3) m2 s−1 and has a maximum value of 10 m2 s−1. The sill on the western flank of the Kuroshio is a hot spot for energetic mixing of Kuroshio waters and South China Sea waters.

Open access
Georgy E. Manucharyan and Andrew L. Stewart

Abstract

The Beaufort Gyre (BG) is hypothesized to be partially equilibrated by those mesoscale eddies that form via baroclinic instabilities of its currents. However, our understanding of the eddy field’s dependence on the mean BG currents and the role of sea ice remains incomplete. This theoretical study explores the scales and vertical structures of eddies forming specifically due to baroclinic instabilities of interior BG flows. An idealized quasi-geostrophic model is used to show that flows driven only by the Ekman pumping contain no interior potential vorticity (PV) gradients and generate weak and large eddies, ℴ(200km) in size, with predominantly barotropic and first baroclinic mode energy. However, flows containing realistic interior PV gradients in the Pacific halocline layer generate significantly smaller eddies of about 50 km in size, with a distinct second baroclinic mode structure and a subsurface kinetic energy maximum. The dramatic change in eddy characteristics is shown to be caused by the stirring of interior PV gradients by large-scale barotropic eddies. The sea ice-ocean drag is identified as the dominant eddy dissipation mechanism, leading to realistic sub-surface maxima of eddy kinetic energy for drag coefficients higher than about 2×10−3. A scaling law is developed for the eddy potential enstrophy, demonstrating that it is directly proportional to the interior PV gradient and the square root of the barotropic eddy kinetic energy. This study proposes a possible formation mechanism of large BG eddies and points to the importance of accurate representation of the interior PV gradients and eddy dissipation by ice-ocean drag in BG simulations and theory.

Restricted access
Yueyang Lu, Igor Kamenkovich, and Pavel Berloff

Abstract

Lateral mesoscale eddy-induced tracer transport is traditionally represented in coarse-resolution models by the flux-gradient relation. In its most complete form, the relation assumes the eddy tracer flux as a product of the large-scale tracer concentration gradient and an eddy transport coefficient tensor. However, several recent studies reported that the tensor has significant spatio-temporal complexity and is not uniquely defined, that is, it is sensitive to the tracer distributions and to the presence of non-divergent (“rotational”) component of the eddy flux. These issues could lead to significant biases in the representation of the eddy-induced transport. Using a high-resolution tracer model of the Gulf Stream region, we examine the diffusive and advective properties of lateral eddy-induced transport of dynamically passive tracers, re-evaluate the utility of the flux-gradient relation, and propose an alternative approach based on modeling the local eddy forcing by a combination of diffusion and generalized eddy-induced advection. Mesoscale eddies are defined by a scale-based spatial filtering, which leads to the importance of new eddy-induced terms, including eddy-mean covariances in the eddy fluxes. The results show that the biases in representing these terms are noticeably reduced by the new approach. A series of targeted simulations in the high-resolution model further demonstrates that the approach outperforms the flux-gradient model in reproducing the stirring and dispersing effect of eddies. Our study indicates potential to upgrade the traditional flux-gradient relation for representing the eddy-induced tracer transport.

Restricted access
Renjian Li and Ming Li

Abstract

Using an idealized channel representative of a coastal plain estuary, we conducted numerical simulations to investigate the generation of internal lee waves by lateral circulation. It is shown that the lee waves can be generated across all salinity regimes in an estuary. Since the lateral currents are usually subcritical with respect to the lowest mode, mode-2 lee waves are most prevalent but a hydraulic jump may develop during the transition to subcritical flows in the deep channel, producing high energy dissipation and strong mixing. Unlike flows over a sill, stratified water in the deep channel may become stagnant such that a mode-1 depression wave can form higher up in the water column. With the lee wave Froude number above 1 and the intrinsic wave frequency between the inertial and buoyancy frequency, the lee waves generated in coastal plain estuaries are nonlinear waves with the wave amplitude scaling approximately with 𝑉/ N¯ where V is the maximum lateral flow velocity and N¯ is the buoyancy frequency. The model results are summarized using the estuarine classification diagram based on the freshwater Froude number Frf and the mixing parameter M. Δh decreases with increasing Frf as stronger stratification suppresses waves, and no internal waves are generated at large Frf. Δh initially increases with increasing M as the lateral flows become stronger with stronger tidal currents, but decreases or saturates to a certain amplitude as M further increases. This modeling study suggests that lee waves can be generated over a wide range of estuarine conditions.

Restricted access