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Hans Burchard
,
Karsten Bolding
,
Xaver Lange
, and
Alexander Osadchiev

Abstract

For Arctic estuaries that are characterized by landfast sea ice cover during the winter season, processes generating estuarine circulation and residual stratification have not yet been investigated, although some of the largest estuaries in the world belong to this class. Landfast sea ice provides a no-slip surface boundary condition in addition to the bottom boundary, such that frictional effects are expected to be increased. For this study of estuarine circulation and residual stratification under landfast sea ice, first, a simple linear analytical model is used. To include tidally varying scenarios, a water-column model is applied with a second-moment turbulence closure to juxtapose free-surface and ice-covered estuaries. Well-mixed and strongly stratified tidally periodic scenarios are analyzed by means of a decomposition of estuarine circulation into contributions from gravitational circulation, eddy viscosity–shear covariance (ESCO), surface stress, and river runoff. A new method is developed to also decompose tidal residual salinity anomaly profiles. Estuarine circulation intensity and tidally residual potential energy anomaly are studied for a parameter space spanned by the Simpson number and the unsteadiness number. These are the major results of this study that will support future scenario studies in Arctic estuaries under conditions of accelerated warming: (i) residual surface drag under ice opposes estuarine circulation; (ii) residual differential advection under ice destabilizes the near-surface flow; (iii) reversal of ESCO during strong stratification does not occur under landfast sea ice; (iv) tidal pumping (s-ESCO) contributes dominantly to residual stratification also with sea ice cover.

Significance Statement

Our work gives a first qualitative and quantitative understanding of how landfast sea ice cover on tidal estuaries impacts on the generation of estuarine circulation and residual stratification. Along the Arctic coasts, where some of the world’s largest estuaries are located, these processes play a significant role for the economy and ecology by means of transports of sediments, nutrients and pollutants. Due to Arctic amplification, the conditions for ice-covered estuaries are strongly changing in a way that the ice-covered periods may be shorter in the future. Our results intend to motivate field observations and realistic model studies to allow for better predicting the consequences of these changes.

Open 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 multiyear 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 60–450 m is northward toward the Pacific Ocean with a mean transport of 2.07–2.60 Sv (1 Sv ≡ 106 m3 s−1). In the depth range of 450–1200 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 anticlockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83–2.25 Sv, which is balanced by three transports: 1) 0.62–0.93 Sv southward transport into the Seram–Banda Seas through the Lifamatola Passage, 2) 0.97–1.01 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 semienclosed basin.

Significance Statement

The Indonesian Throughflow plays an important role in the global ocean circulation and climate variations. Existing studies of the Indonesian Throughflow have focused on the upper thermocline currents. Here we identify, using mooring observations, an intermediate western boundary current with the core at 800–1000-m depth in the Maluku Sea, transporting intermediate waters from the Pacific into the Seram–Banda Seas through the Lifamatola Passage. Potential vorticity balance suggests an anticlockwise gyre circulation in the intermediate Maluku Sea, which is evidenced by the mooring and model data. Transport estimates suggest northward countercurrent in the upper Maluku Sea toward the Pacific, supplied by the Lifamatola Passage transport and upwelling from the intermediate layer in the Maluku Sea. Our results suggest the importance of the intermediate Indonesian Throughflow in global ocean circulation and overturn. More extensive investigations of the Indo-Pacific intermediate ocean circulation should be conducted to improve our understanding of global ocean overturn and heat and CO2 storages.

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Kewei Lyu
,
Xuebin Zhang
,
John A. Church
,
Quran Wu
,
Russell Fiedler
, and
Fabio Boeira Dias

Abstract

A rapid warming and freshening of the Southern Ocean have been observed over the past several decades and are attributed to anthropogenic climate change. In this study, ocean model perturbation experiments are conducted to separate roles of individual surface forcing in the Southern Ocean temperature and salinity changes. Model-based findings are compared with results from a theoretical framework including three idealized processes defined on the θS diagram. Under the future scenario of CO2 doubling, the heat flux forcing dominates the large-scale warming, deepening of isopycnals, and spiciness changes along isopycnals, which can be captured by an idealized pure warming process to represent the subduction of surface heat uptake. The poleward-intensifying westerly winds account for 24% of the enhanced warming between 35° and 50°S and would have comparable contribution as the heat flux forcing after removing the global ocean warming effect. In contrast, the widespread freshening in the Southern Ocean driven by increased surface freshwater input is largely compensated by the wind-driven saltening. The response to freshwater forcing could not be approximated as a similar pure freshening process as the induced cooling and freshening have comparable effects on density. The wind-driven changes are primarily through the local heave of isopycnals, thus resembling an idealized pure heave process, but contain considerable spiciness signals especially in the midlatitude Southern Ocean, resulting from anomalous northward transport and subduction of heat and salt that are largely density-compensating. These distinct signatures of individual surface forcing help us to better understand observed and projected changes in the Southern Ocean.

Significance Statement

Considerable changes including a rapid warming and freshening have been observed in the Southern Ocean as it absorbs most of the extra heat from the anthropogenic climate change, receives increased surface freshwater input, and experiences a poleward shift and intensification of the westerly winds. The purpose of this study is to distinguish different contributions from surface heat flux, freshwater flux, and wind forcing to the Southern Ocean temperature and salinity changes, based on ocean model experiments and three idealized processes from a theoretical framework. Our study reveals distinct signatures of individual surface forcing that help us to understand linkages between changes seen at the surface and in the interior Southern Ocean.

Restricted 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 quasigeostrophic 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 ( 200 ) km 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 subsurface 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.

Open access
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).

Open access
Ke Huang
,
Ming Feng
,
Ying Wu
,
Dongxiao Wang
,
Wen Zhou
,
Tingting Zu
,
Weiqiang Wang
,
Qiang Xie
,
Lei Yang
,
Jinglong Yao
, and
Wei Zhou

Abstract

Leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO) and their connections were studied based on 17 years (2002–18) of oceanic historical and reanalysis data. Empirical orthogonal function (EOF) analysis depicted the dominant roles of the first two leading modes in salinity variability in the TIO over a wide range of interannual time scales. Among the rich oscillations of the leading EOF modes, a coherent near-biennial band was identified with basinwide loading of sea surface salinity anomalies (SSSa) (EOF1) leading/lagging the northeast–southwest dipolar mode of SSSa (EOF2) by around 4 months across the TIO, with southwestward migration of SSSa center. The spatial loadings of the SSSa leading modes in the TIO were strongly shaped by sea surface temperature–related freshwater fluxes and wind-driven regional ocean circulation on a near-biennial time scale. Composite analysis of the mixed layer salinity budget reflected characteristic features of basin-scale ocean–atmosphere coupling, both temporally and regionally during the life cycle of the near-biennial fluctuation in anomalous salinity in the TIO. Consistent with the intrinsic oscillation paradigm in the observed Indian Ocean dipole (IOD) variation, the dynamic and thermodynamic feedbacks associated with switches from the positive to negative IOD modes provided the phase-connection mechanisms for the SSSa leading-mode displacement over the TIO.

Significance Statement

This study investigates the leading modes of interannual variability in upper-ocean salinity in the tropical Indian Ocean (TIO). The intrinsic oscillation and associated dynamic and thermodynamic feedbacks over the TIO drive the basinwide connections of upper-ocean salinity variability. Our results show that a coherent near-biennial band is identifiable within the leading modes of sea surface salinity anomalies (SSSa), in which the wind-induced horizontal advections and evaporation-minus-precipitation anomalies associated with the switches from positive to negative Indian Ocean dipole modes mainly provide the phase-transition mechanism of SSSa. This research illustrates substantial evidence for the displacement of basin-scale sea surface temperature anomalies modulating the structures of SSSa and inducing the dynamical connections of leading modes of SSSa on the near-biennial time scale.

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 spatiotemporal complexity and is not uniquely defined, that is, it is sensitive to the tracer distributions and to the presence of nondivergent (“rotational”) components 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, reevaluate 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
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 resulting from 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 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 underlie the bulk formula that could lead to assessments of uncertainty and improvement of the bulk formula.

Significance Statement

The behavior of surface heat fluxes in the coastal zone is normally more complex than over the open ocean but has a large impact on human activity. Our study examines extensive flux measurements on a tower in the Baltic Sea that allows partitioning of the fluxes according to wind direction without seriously depleting the data for a given wind-direction sector. Because some of the normal assumptions for the usual parameterization are not met, our study examines relationships behind the parameterization of the surface fluxes.

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Amy F. Waterhouse
,
Tyler Hennon
,
Eric Kunze
,
Jennifer A. MacKinnon
,
Matthew H. Alford
,
Robert Pinkel
,
Harper Simmons
,
Caitlin B. Whalen
,
Elizabeth C. Fine
,
Jody Klymak
, and
Julia M. Hummon

Abstract

Internal waves are predominantly generated by winds, tide–topography interactions, and balanced flow–topography interactions. Observations of vertical shear of horizontal velocity (uz , υz ) from lowered acoustic Doppler current profilers (LADCP) profiles conducted during GO-SHIP hydrographic surveys, as well as vessel-mounted sonars, are used to interpret these signals. Vertical directionality of intermediate-wavenumber [ λ z O ( 100 )  m ] internal waves is inferred in this study from rotary-with-depth shears. Total shear variance and vertical asymmetry ratio (Ω), i.e., the normalized difference between downward- and upward-propagating intermediate wavenumber shear variance, where Ω > 0 (<0) indicates excess downgoing (upgoing) shear variance, are calculated for three depth ranges: 200–600 m, 600 m–1000 mab (meters above bottom), and below 1000 mab. Globally, downgoing (clockwise-with-depth in the Northern Hemisphere) exceeds upgoing (counterclockwise-with-depth in the Northern Hemisphere) shear variance by 30% in the upper 600 m of the water column (corresponding to the globally averaged asymmetry ratio of Ω ¯ = 0.13 ), with a near-equal distribution below 600-m depth ( Ω ¯ 0 ). Downgoing shear variance in the upper water column dominates at all latitudes. There is no statistically significant correlation between the global distribution of Ω and internal wave generation, pointing to an important role for processes that redistribute energy within the internal wave continuum on wavelengths of O ( 100 )  m .

Open access
Zhiwu Chen
,
Gengbin Liu
,
Zhiyu Liu
,
Shaomin Chen
,
Huaihao Lu
,
Jiexin Xu
,
Yankun Gong
,
Jieshuo Xie
,
Yinghui He
,
Ju Chen
,
Yunkai He
, and
Shuqun Cai

Abstract

Tide-induced near-inertial internal waves (NIWs) are generated by tide–topography interaction and are energized by internal tides through triadic resonant interaction of internal waves. They are located above topography and could be in close contact with wind-induced NIWs when the topography is a tall ridge, like in the Luzon Strait of the northern South China Sea (SCS). A natural question arises as to whether there is significant interaction between wind- and tide-induced NIWs. By using moored velocity observations, a satellite-tracked surface drifter dataset, and idealized numerical simulations, we find that in the presence of tide-induced NIWs, the wind can inject slightly more near-inertial energy (NIE), while in the presence of wind-induced NIWs, significantly more tidal energy is transferred to NIWs. Thus, wind- and tide-induced NIWs can mutually enhance each other, producing more NIE than a linear superposition of that generated by wind and tide forcing alone. Increasing wind intensity and tidal excursion lead to saturation of NIE enhancement, while a taller ridge leads to stronger enhancement. The high mixed layer NIE near Luzon Strait is mostly generated by the wind, while the mutual enhancement between wind- and tide-induced NIWs can further enhance this pattern. The interaction between wind- and tide-induced NIWs leads to an enhancement of 25% more NIE. If tide-induced NIWs are neglected, as is usually the case in the estimation of NIE, the total NIE will be underestimated by almost 50%. This might imply that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait.

Significance Statement

Near-inertial internal waves (NIWs) usually occupy the most kinetic energy of internal waves and contribute significantly to ocean mixing. Near the surface they are usually generated by wind forcing, but near the bottom they can be generated by geostrophic or tidal flow interacting with topography. Above the tall ridge in Luzon Strait, wind- and tide-induced NIWs are in close contact, leading to potential interactions. It is found that these NIWs can mutually enhance each other, with most of the additional near-inertial energy (NIE) coming from the tides. If tide-induced NIWs are neglected, the total NIE will be underestimated by almost 50%. This suggests that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait.

Restricted access