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The impact of a large shallow semi‐enclosed lagoon on freshwater exchange across an inlet channel is investigated using an idealized numerical model. Lagoons are often found between a river mouth and the ocean; we focus on those where the river discharge rate is small and the inlet channel is narrower and deeper than the lagoon. Tides generate freshwater and oceanic‐water plumes across the channel; a stratified freshwater plume forms in the ocean from the late ebb to early flood phase, while a vertically well‐mixed oceanic‐water plume forms in the lagoon from the late flood to early ebb phase. The shallow depth of the lagoon increases the flow speed of the oceanic‐water plume, which results in the formation of a sharp and vertically well‐mixed salinity front within the lagoon. When this front moves toward the ocean during the ebb phase, vertical mixing increases where the bathymetry deepens and freshwater encounters oceanic water below. Without a dredged bottom slope, the impact of mixing would be greatly reduced within the shallow lagoon and channel, as the shallow depth would limit the subsurface intrusion of oceanic water. The narrow channel further causes the flow to converge and accelerate, enhancing both internal shear‐driven and bottom boundary‐layer mixing at the channel and increasing freshwater plume thickness where it enters the ocean. Sensitivity experiments showed that the role of tidal pumping in freshwater exchange across the channel increases when the lagoon area and tidal mixing increase and when the estuarine Richardson number decreases. Plain Language Summary: Freshwater outflows from estuaries play an important role in supporting coastal marine environments. Large shallow lagoons are often found at river mouths with an inlet channel; this study uses idealized numerical model experiments to investigate how lagoons affect freshwater exchange across an inlet channel where the river discharge rate is low. Freshwater exchange often results in estuarine circulation wherein freshwater enters the ocean near the surface, while oceanic water intrudes into the estuary near the bottom, with weaker circulation expected for a shallow environment. We find that large lagoons force strong tidal flows across the channel, enhancing mixing between freshwater and oceanic water, especially during the ebb phase. Due to the fast flow generated over the shallow lagoon, mixing occurs when freshwater in the lagoon moves across a sloping bottom and interacts with oceanic water below. Freshwater outflow becomes thicker and tidally pulsed plumes are generated. Key Points: A large lagoon with a narrow inlet induces strong tidal pumping across the channel, in turn generating plumes in the ocean and lagoonMixing is enhanced as the sharp salinity front formed within the shallow lagoon moves across the sloping bottom during the ebb phaseTidal pumping induces more freshwater exchange than estuarine circulation when the estuarine Richardson number is below unity [ABSTRACT FROM AUTHOR]

Previous studies have shown the presence of strong mesoscale eddy activities in the Indonesian Seas and their influence on the transport and water mass properties of the Indonesian Throughflow (ITF), a mean flow from the Pacific Ocean to the Indian Ocean through the Indonesian Archipelago. This study explores the effects of these eddy activities, or high‐frequency flow variability (HFFV), on residence time and pathway of the ITF by conducting Lagrangian particle tracking experiments using a velocity field from an eddy‐resolving ocean general circulation model. Particles are released at key locations in the western and eastern routes of the ITF and tracked both backward and forward in time. To assess the effects of flow variability that has a time scale longer than a day but shorter than a month, the definition of HFFV in this study, we conduct parallel experiments using daily and monthly averaged velocity fields. Particle trajectories reveal the contrasting circulation characteristics of the Sulawesi and Banda Seas. HFFV in the Sulawesi Sea (in the western route) is high, causing water to circulate longer over a broader area. The longer residence time in the Sulawesi Sea helps the upwelling of the inflowing Pacific waters, especially the intermediate water masses, to rise above 300 m at the Makassar Strait, and also has the potential to allow mixing processes to modify the water mass properties of the ITF. In contrast, HFFV is much lower in the Banda Sea and has minimal effects on the ITF. Plain Language Summary: As part of the world ocean general circulation, warm water in the upper layer flows from the Pacific Ocean to the Indian Ocean through the Indonesian Archipelago. The flow through the archipelago, known as the Indonesian Throughflow (ITF), is neither a steady one nor a single stream. The ITF experiences fluctuation and turbulence while navigating through the various seas, straits, and passages. In this study, using the velocity field generated by an ocean circulation model, we explore the roles that the flow variability plays in the ITF pathway and its transit time through the major seas. We find that flow variability in the Sulawesi Sea is high, causing water to circulate longer over a broader area and to rise from the intermediate depth to near the surface, thus providing the potential for turbulence to modify the water properties of the ITF. On the other hand, flow variability is low in the Banda Sea and has minimal effects on the ITF. Key Points: Residence time and pathways of the Indonesian Throughflow are examined using virtual particles advected in an eddy‐resolving modeled velocity fieldHigh‐frequency flow variability increases residence time and supports the upwelling of the inflowing Pacific water, especially in the Sulawesi SeaThere is a need for adequate representation of mixing processes (including tidal and shear‐induced) occurring in the Indonesian Seas in ocean models [ABSTRACT FROM AUTHOR]

A quasi-global eddying ocean hindcast simulation using a new version of our model, called OFES2 (Ocean General Circulation Model for the Earth Simulator version 2), was conducted to overcome several issues with unrealistic properties in its previous version, OFES. This paper describes the model and the simulated oceanic fields in OFES2 compared with OFES and also observed data. OFES2 includes a sea-ice model and a tidal mixing scheme, is forced by a newly created surface atmospheric dataset called JRA55-do, and simulated the oceanic fields from 1958 to 2016. We found several improvements in OFES2 over OFES: smaller biases in the global sea surface temperature and sea surface salinity as well as the water mass properties in the Indonesian and Arabian seas. The time series of the Niño3.4 and Indian Ocean Dipole (IOD) indexes are somewhat better in OFES2 than in OFES. Unlike the previous version, OFES2 reproduces more realistic anomalously low sea surface temperatures during a positive IOD event. One possible cause of these improvements in El Niño and IOD events is the replacement of the atmospheric dataset. On the other hand, several issues remained unrealistic, such as the pathways of the Kuroshio and Gulf Stream and the unrealistic spreading of salty Mediterranean overflow. Given the worldwide use of the previous version and the improvements presented here, the output from OFES2 will be useful in studying various oceanic phenomena with broad spatiotemporal scales. [ABSTRACT FROM AUTHOR]

The Ganges‐Brahmaputra‐Meghna (GBM) delta is a major source of freshwater for the Bay of Bengal. The flow through this megadelta is complex because of its large size and numerous river networks, making the region a challenging area for river‐routing models. This study investigates the dynamics of the riverine outflow across the GBM delta and its interaction with the ocean on monthly to seasonal time scales, using a two‐layer model that represents the riverine water and the oceanic water. The model simulates a seasonal increase and decrease of discharge and sea surface height (SSH) at both the main stem and the distributaries. Although discharge through the main stem is driven by that propagating from the upstream, model experiments show that the distributaries are an active region of river‐ocean interactions as a result of a river plume established on the oceanic side of the river mouth. This river plume is induced by river discharge in the presence of the Coriolis force and is associated with an increase in SSH along the coast. Backwater is induced at the river mouth, resulting in convergence and higher SSH in distributaries. The narrow, meandering, and shallow paths of the distributaries enhance the role of friction on the flow from the upstream, making their river mouths more sensitive to oceanic variability. The results of our study demonstrate that river plumes may play a central role in the dynamics of megadelta flow by connecting the dynamics at the main stem and the distributaries with ocean dynamics. Key Points: The dynamics of river discharge through the Ganges‐Brahmaputra‐Meghna delta to the Bay of Bengal were investigated using a two‐layer modelActive river‐ocean interactions occur within the distributaries due to sea surface perturbation induced by a river plumeIncreased river outflow at the main stem can cause backwater at the mouths of the distributaries, inducing upstream flow [ABSTRACT FROM AUTHOR]

The Indonesian Seas receive one of the largest amounts of rainfall around the globe. Part of this freshwater disperses to the Indian Ocean through the Indonesian Throughflow (ITF), the Pacific and Indian interocean exchange flow, making the Indonesian Seas a major source of freshwater, and plays an important part of the global hydrological cycle. By using a Lagrangian particle tracking model, we examine the pathways behind the dispersion of freshwater that the Indonesian Seas receive through precipitation. The model suggests that the dispersion from the near‐surface water of the Indonesian Seas occurs in about 6 months, primarily through advection to the surrounding seas, followed by evaporation, entrainment, and vertical mixing. The Lombok Strait and the Timor Strait are the major outflowing straits, and the freshwater exiting through these straits are found to originate from limited areas and seasons. The sources for the Lombok Strait outflow are the Java Sea precipitated freshwater during boreal fall and winter, while the sources for the Timor Strait outflow are the Flores‐Banda Seas and Arafura Sea precipitated freshwater during winter and spring. Mixing with the thermocline water is found to occur when the monsoonal winds induce upwelling events in winter and summer, along the shelf breaks and steep coastlines surrounding the Flores‐Banda Seas. Vertical mixing provides a pathway for the surface freshwater to enter the ITF thermocline, and our model suggests that it is the Java Sea precipitated freshwater during winter that is entering the ITF thermocline along its main pathway. Key Points: The fate of precipitated origin freshwater of the Indonesian Seas is examined using a Lagrangian particle tracking modelFreshwater outflows from the Indonesian Seas to the Indian Ocean near the surface occur mostly through the Lombok and Timor StraitsVertical mixing to the Indonesian Throughflow thermocline water occurs along shelf breaks and coastlines during the upwelling seasons [ABSTRACT FROM AUTHOR]

The dynamics responsible for the East/Japan Sea throughflow are examined using the Finite Volume Coastal Ocean Model. Existing ocean data assimilation models (Δx ~10 km) present overestimates of the volume transport through the Tsugaru Strait, and the outflow partitioning between the Tsugaru and Soya/La Perouse Straits also differs depending on the models. We find that this overestimate occurs when the form drag that is induced by detailed bottom topographic features in the Tsugaru Strait is not adequately resolved. A range of experiments with different horizontal resolutions in the Korea/Tsushima, Tsugaru, and Soya/La Perouse Straits were conducted to examine how detailed bottom topographic features in these three straits may affect the throughflow. We find that the volume transport is highly sensitive to the spatial resolution at the Tsugaru Strait but not at the Korea/Tsushima and Soya/La Perouse Straits. The high‐resolution experiment showed a decreased outflow through the Tsugaru Strait and a reduction in the model bias. The outflow through the Soya/La Perouse Strait increased, while the inflow through the Korea/Tsushima Strait remained relatively unchanged. Additional experiments with modified topography further indicated that the abrupt change of the topography located at the upstream side of the Tsugaru Strait plays an important role in the outflow partitioning of the East/Japan Sea throughflow. Key Points: The dynamics responsible for the East/Japan Sea throughflow are examined using the Finite Volume Coastal Ocean ModelTopographically induced form drag in the Tsugaru Strait plays an important role in the outflow partitioning in the East/Japan SeaThe enhancement of form drag at the Tsugaru Strait is created by the rugged and sharp topographical features in the upstream/downstream region [ABSTRACT FROM AUTHOR]

The mechanisms of spring phytoplankton blooms are investigated from a Lagrangian framework by using a Lagrangian NPZD model that can track the movement and transfers of nutrient parcels in a turbulent environment. The model reveals that the onset of spring blooms depends on the cumulative euphotic age, which is the total time that inorganic nutrient is exposed to light before the photosynthetic conversion to phytoplankton biomass. A spring bloom, defined as a tenfold increase of near-surface phytoplankton, occurs when this cumulative euphotic age is approximately [ABSTRACT FROM AUTHOR]

The mechanism responsible for the annual cycle of the flow through the straits of the Japan Sea is investigated using a two-layer model. Observations show maximum throughflow from summer to fall and minimum in winter, occurring synchronously at the three major straits: Tsushima, Tsugaru, and Soya Straits. This study finds the subpolar winds located to the north of Japan as the leading forcing agent, which first affects the Soya Strait rather than the Tsushima or Tsugaru Straits. The subpolar winds generate baroclinic Kelvin waves along the coastlines of the subpolar gyre, affect the sea surface height at the Soya Strait, and modify the flow through the strait. This causes barotropic adjustment to occur inside the Japan Sea and thus affect the flow at the Tsugaru and Tsushima Straits almost synchronously. The barotropic adjustment mechanism explains well why the observations show a similar annual cycle at the three straits. The annual cycle at the Tsugaru Strait is further shown to be weaker than that in the other two straits based on frictional balance around islands, that is, frictional stresses exerted around an island integrate to zero. In the Tsugaru Strait, the flows induced by the frictional integrals around the northern (Hokkaido) and southern (Honshu) islands are in opposite directions and tend to cancel out. Frictional balance also suggests that the annual cycle at the Tsugaru Strait is likely in phase with that at the Soya Strait because the length scale of the northern island is much shorter than that of the southern island. [ABSTRACT FROM AUTHOR]

The impact of open oceanic processes on the Antarctic Bottom Water (AABW) outflows is investigated using a numerical model with a focus on outflows that occur through deep channels. A major branch of the AABW outflow is known to occur as an overflow from the Filchner Depression to the Weddell Sea through a deep channel a few hundred kilometers wide and a sill roughly 500 m deep. When this overflow enters the Weddell Sea, it encounters the Antarctic Slope Front (ASF) at the shelf break, a density front commonly found along the Antarctic continental shelf break. The presence of an AABW outflow and the ASF create a v-shaped isopycnal structure across the shelf break, indicating an interaction between the overflow and oceanic processes. Model experiments show the overflow transport to increase significantly when an oceanic wind stress increases the depth of the ASF. This enhancement of overflow transport occurs because the channel walls allow a pressure gradient in the along-slope direction to exist and the overflow transport is geostrophically controlled with its ambient oceanic water at the shelf break. Because the ASF is associated with a lighter water mass that reaches the depth close to that of the channel, an increase in its depth increases the density gradient across the shelf break and therefore the geostrophic overflow transport. The enhancement of overflow transport is also likely to result in a lighter overflow water mass, although such an adjustment of density likely occurs on a much longer time scale than the adjustment of transport. [ABSTRACT FROM AUTHOR]

Marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms—by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability. Because both mechanisms tend to be localized in space, the resulting upper ocean circulation can be characterized as a β plume for which the relevant background potential vorticity is set by the slope of the topography, that is, a topographic β plume. The entrainment-driven topographic β plume consists of a single gyre that is aligned along isobaths. The circulation is cyclonic within the upper ocean (water columns are stretched). The transport within one branch of the topographic β plume may exceed the entrainment flux by a factor of 2 or more. Overflows are likely to be baroclinically unstable, especially near the strait. This creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and energy in the vertical. In the time mean, the eddies accompanying baroclinic instability set up a double-gyre circulation in the upper ocean, an eddy-driven topographic β plume. In regions where baroclinic instability is growing, the momentumflux from the overflow into the upper ocean acts as a drag on the overflow and causes the overflow to descend the slope at a steeper angle than what would arise from bottom friction alone. Numerical model experiments suggest that the Faroe Bank Channel overflow should be the most prominent example of an eddy-driven topographic β plume and that the resulting upper-layer transport should be comparable to that of the overflow. The overflow-layer eddies that accompany baroclinic instability are analogous to those observed in moored array data. In contrast, the upper layer of the Mediterranean overflow is likely to be dominated more by an entrainment-driven topographic β plume. The difference arises because entrainment occurs at a much shallower location for the Mediterranean case and the background potential vorticity gradient of the upper ocean is much larger. [ABSTRACT FROM AUTHOR]

The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows enter the open ocean as dense gravity currents that flow along and down the continental slope. While descending the slope, overflows typically double their volume transport by entraining upper oceanic water. The upper oceanic layer must balance this loss of mass, and the resulting convergent flow produces significant vortex stretching. Overflows thus represent an intense and localized mass and vorticity forcing for the upper ocean. In this study, simulations show that the upper ocean responds to the overflow-induced forcing by establishing topographic β plumes that are aligned more or less along isobaths and that have a transport that is typically a few times larger than that of the overflows. For the topographic β plume driven by the Mediterranean overflow, the occurrence of eddies near Cape St. Vincent, Portugal, allows the topographic β plume to flow across isobaths. The modeled topographic β-plume circulation forms two transatlantic zonal jets that are analogous to the Azores Current and the Azores Countercurrent. In other cases (e.g., the Denmark Strait overflow), the same kind of circulation remains trapped along the western boundary and hence would not be readily detected. [ABSTRACT FROM AUTHOR]

Spatial and temporal information on oceanic flow is fundamental to oceanography and crucial for marine-related social activities. This study attempts to describe the short-term surface flow variation in the area south of the Lombok Strait in the northern summer using the hourly Himawari-8 sea surface temperature (SST). Although the uncertainty of this temperature is relatively high (about 0.6 ∘ C), it could be used to discuss the flow variation with high spatial resolution because sufficient SST differences are found between the areas north and south of the strait. The maximum cross-correlation (MCC) method is used to estimate the surface velocity. The Himawari-8 SST clearly shows Flores Sea water intruding into the Indian Ocean with the high-SST water forming a warm thermal plume on a tidal cycle. This thermal plume flows southward at a speed of about 2 m / s . The Himawari-8 SST indicates a southward flow from the Lombok Strait to the Indian Ocean, which blocks the South Java Current flowing eastward along the southern coast of Nusa Tenggara. Although the satellite data is limited to the surface, we found it useful for understanding the spatial and temporal variations in the surface flow field. [ABSTRACT FROM AUTHOR]