Browse
Abstract
To close the overturning circulation, dense bottom water must upwell via turbulent mixing. Recent studies have identified thin bottom boundary layers (BLs) as locations of intense upwelling, yet it remains unclear how they interact with and shape the large-scale circulation of the abyssal ocean. The current understanding of this BL–interior coupling is shaped by 1D theory, suggesting that variations in locally produced BL transport generate exchange with the interior and thus a global circulation. Until now, however, this picture has been based on a 1D theory that fails to capture the local evolution in even highly idealized 2D geometries. The present work applies BL theory to revised 1D dynamics, which more naturally generalizes to two and three dimensions. The BL is assumed to be in quasi-equilibrium between the upwelling of dense water and the convergence of downward buoyancy fluxes. The BL transport, for which explicit formulas are presented, exerts an influence on the interior by modifying the bottom boundary condition. In 1D, this BL transport is independent of the interior evolution, but in 2D the BL and interior are fully coupled. Once interior variables and the bottom slope are allowed to vary in the horizontal, the resulting convergences and divergences in the BL transport exchange mass with the interior. This framework allows for the analysis of previously inaccessible problems such as the BL–interior coupling in the presence of an exponential interior stratification, laying the foundation for developing a full theory for the abyssal circulation.
Abstract
To close the overturning circulation, dense bottom water must upwell via turbulent mixing. Recent studies have identified thin bottom boundary layers (BLs) as locations of intense upwelling, yet it remains unclear how they interact with and shape the large-scale circulation of the abyssal ocean. The current understanding of this BL–interior coupling is shaped by 1D theory, suggesting that variations in locally produced BL transport generate exchange with the interior and thus a global circulation. Until now, however, this picture has been based on a 1D theory that fails to capture the local evolution in even highly idealized 2D geometries. The present work applies BL theory to revised 1D dynamics, which more naturally generalizes to two and three dimensions. The BL is assumed to be in quasi-equilibrium between the upwelling of dense water and the convergence of downward buoyancy fluxes. The BL transport, for which explicit formulas are presented, exerts an influence on the interior by modifying the bottom boundary condition. In 1D, this BL transport is independent of the interior evolution, but in 2D the BL and interior are fully coupled. Once interior variables and the bottom slope are allowed to vary in the horizontal, the resulting convergences and divergences in the BL transport exchange mass with the interior. This framework allows for the analysis of previously inaccessible problems such as the BL–interior coupling in the presence of an exponential interior stratification, laying the foundation for developing a full theory for the abyssal circulation.
Abstract
Submesoscale currents, comprising fronts and mixed-layer eddies, exhibit a dual cascade of kinetic energy: a forward cascade to dissipation scales at fronts and an inverse cascade from mixed-layer eddies to mesoscale eddies. Within a coarse-graining framework using both spatial and temporal filters, we show that this dual cascade can be captured in simple mathematical form obtained by writing the cross-scale energy flux in the local principal strain coordinate system, wherein the flux reduces to the sum of two terms, one proportional to the convergence and the other proportional to the strain. The strain term is found to cause the inverse energy flux to larger scales while an approximate equipartition of the convergent and strain terms captures the forward energy flux, demonstrated through model-based analysis and asymptotic theory. A consequence of this equipartition is that the frontal forward energy flux is simply proportional to the frontal convergence. In a recent study, it was shown that the Lagrangian rate of change of quantities like the divergence, vorticity, and horizontal buoyancy gradient are proportional to convergence at fronts, implying that horizontal convergence drives frontogenesis. We show that these two results imply that the primary mechanism for the forward energy flux at fronts is frontogenesis. We also analyze the energy flux through a Helmholtz decomposition and show that the rotational components are primarily responsible for the inverse cascade while a mix of the divergent and rotational components cause the forward cascade, consistent with our asymptotic analysis based on the principal strain framework.
Abstract
Submesoscale currents, comprising fronts and mixed-layer eddies, exhibit a dual cascade of kinetic energy: a forward cascade to dissipation scales at fronts and an inverse cascade from mixed-layer eddies to mesoscale eddies. Within a coarse-graining framework using both spatial and temporal filters, we show that this dual cascade can be captured in simple mathematical form obtained by writing the cross-scale energy flux in the local principal strain coordinate system, wherein the flux reduces to the sum of two terms, one proportional to the convergence and the other proportional to the strain. The strain term is found to cause the inverse energy flux to larger scales while an approximate equipartition of the convergent and strain terms captures the forward energy flux, demonstrated through model-based analysis and asymptotic theory. A consequence of this equipartition is that the frontal forward energy flux is simply proportional to the frontal convergence. In a recent study, it was shown that the Lagrangian rate of change of quantities like the divergence, vorticity, and horizontal buoyancy gradient are proportional to convergence at fronts, implying that horizontal convergence drives frontogenesis. We show that these two results imply that the primary mechanism for the forward energy flux at fronts is frontogenesis. We also analyze the energy flux through a Helmholtz decomposition and show that the rotational components are primarily responsible for the inverse cascade while a mix of the divergent and rotational components cause the forward cascade, consistent with our asymptotic analysis based on the principal strain framework.
Abstract
The role of the modulational instability for rogue wave generation in the ocean is still under debate. We investigated a continuous dataset, consisting of buoy and radar wave elevation data of different frequency resolutions, from eight measurement stations in the southern North Sea. For periods with rogue waves, we evaluated the presence of conditions for the modulational instability to work, that is, a narrow-banded wave spectrum in both frequency and angular direction. We found rogue waves that exceed twice the significant wave height indeed occur at slightly lower frequency bandwidths than usual. For rogue waves that are defined only by high crests, this was, however, not the case. The results were dependent on the measurement frequency. The directional spreading of the buoy spectra yielded no information on the presence of a rogue wave. In general, all spectra estimated from the dataset were found to be broad in frequency and angular direction, while the Benjamin–Feir index yielded no indication on a high nonlinearity of the sea states. These are unfavorable conditions for the evolution of a rogue wave through modulational instability. We conclude that the modulational instability did not play a substantial role in the formation of the rogue waves identified in our dataset from the southern North Sea.
Significance Statement
This work investigates whether rogue waves measured in the southern North Sea may have been generated by a modulational instability. The latter is a nonlinear mechanism of wave energy focusing that has been proven mathematically and confirmed in laboratory experiments. However, it is still unclear whether this mechanism is responsible for rogue wave generation under realistic ocean conditions. The modulational instability primarily arises when waves have similar frequencies and directions. In our data, these conditions were not satisfied. This finding leads to the insight that the modulational instability is not the most probable mechanism to generate rogue waves in our dataset.
Abstract
The role of the modulational instability for rogue wave generation in the ocean is still under debate. We investigated a continuous dataset, consisting of buoy and radar wave elevation data of different frequency resolutions, from eight measurement stations in the southern North Sea. For periods with rogue waves, we evaluated the presence of conditions for the modulational instability to work, that is, a narrow-banded wave spectrum in both frequency and angular direction. We found rogue waves that exceed twice the significant wave height indeed occur at slightly lower frequency bandwidths than usual. For rogue waves that are defined only by high crests, this was, however, not the case. The results were dependent on the measurement frequency. The directional spreading of the buoy spectra yielded no information on the presence of a rogue wave. In general, all spectra estimated from the dataset were found to be broad in frequency and angular direction, while the Benjamin–Feir index yielded no indication on a high nonlinearity of the sea states. These are unfavorable conditions for the evolution of a rogue wave through modulational instability. We conclude that the modulational instability did not play a substantial role in the formation of the rogue waves identified in our dataset from the southern North Sea.
Significance Statement
This work investigates whether rogue waves measured in the southern North Sea may have been generated by a modulational instability. The latter is a nonlinear mechanism of wave energy focusing that has been proven mathematically and confirmed in laboratory experiments. However, it is still unclear whether this mechanism is responsible for rogue wave generation under realistic ocean conditions. The modulational instability primarily arises when waves have similar frequencies and directions. In our data, these conditions were not satisfied. This finding leads to the insight that the modulational instability is not the most probable mechanism to generate rogue waves in our dataset.
Abstract
Surface gravity wave effects on currents (WEC) cause the emergence of Langmuir cells (LCs) in a suite of high horizontal resolution (Δx = 30 m), realistic oceanic simulations in the open ocean of central California. During large wave events, LCs develop widely but inhomogeneously, with larger vertical velocities in a deeper mixed layer. They interact with extant submesoscale currents. A 550-m horizontal spatial filter separates the signals of LCs and of submesoscale and larger-scale currents. The LCs have a strong velocity variance with small density gradient variance, while submesoscale currents are large in both. Using coarse graining, we show that WEC induces a forward cascade of kinetic energy in the upper ocean up to at least a 5-km scale. This is due to strong positive vertical Reynolds stress (in both the Eulerian and the Stokes drift energy production terms) at all resolved scales in the WEC solutions, associated with large vertical velocities. The spatial filter elucidates the role of LCs in generating the shear production on the vertical scale of Stokes drift (10 m), while submesoscale currents affect both the horizontal and vertical energy fluxes throughout the mixed layer (50–80 m). There is a slightly weaker forward cascade associated with nonhydrostatic LCs (by 13% in average) than in the hydrostatic case, but overall the simulation differences are small. A vertical mixing scheme K-profile parameterization (KPP) partially augmented by Langmuir turbulence yields wider LCs, which can lead to lower surface velocity gradients compared to solutions using the standard KPP scheme.
Abstract
Surface gravity wave effects on currents (WEC) cause the emergence of Langmuir cells (LCs) in a suite of high horizontal resolution (Δx = 30 m), realistic oceanic simulations in the open ocean of central California. During large wave events, LCs develop widely but inhomogeneously, with larger vertical velocities in a deeper mixed layer. They interact with extant submesoscale currents. A 550-m horizontal spatial filter separates the signals of LCs and of submesoscale and larger-scale currents. The LCs have a strong velocity variance with small density gradient variance, while submesoscale currents are large in both. Using coarse graining, we show that WEC induces a forward cascade of kinetic energy in the upper ocean up to at least a 5-km scale. This is due to strong positive vertical Reynolds stress (in both the Eulerian and the Stokes drift energy production terms) at all resolved scales in the WEC solutions, associated with large vertical velocities. The spatial filter elucidates the role of LCs in generating the shear production on the vertical scale of Stokes drift (10 m), while submesoscale currents affect both the horizontal and vertical energy fluxes throughout the mixed layer (50–80 m). There is a slightly weaker forward cascade associated with nonhydrostatic LCs (by 13% in average) than in the hydrostatic case, but overall the simulation differences are small. A vertical mixing scheme K-profile parameterization (KPP) partially augmented by Langmuir turbulence yields wider LCs, which can lead to lower surface velocity gradients compared to solutions using the standard KPP scheme.
Abstract
Internal waves close to the seafloor of abyssal oceans are the key energy suppliers driving near-bottom mixing and the upwelling branches of meridional overturning circulation, but their spatiotemporal variability and intrinsic mechanisms remain largely unclear. In this study, measurements from 10 long-term moorings were used to investigate the internal wave activities in the abyssal South China Sea, which is an important upwelling zone. Strong near-inertial internal waves (NIWs) with current velocity pulses exceeding 5 cm s−1 were observed to dominate the near-bottom internal wave field at approximately 14°N. These abyssal NIWs were phase-coupled with diurnal internal tides (D1), and both displayed common seasonal variations that were larger in winter and summer, providing evidence of diurnal parametric subharmonic instability (PSI) near its critical latitudes (CLs). Emitted from the bottom, near-inertial kinetic energy rapidly decreased by one order of magnitude from depths of ∼120 to ∼620 m above the bottom. Near rough topographies, the abyssal PSI was shifted poleward to approximately 14.8°N by negative relative vorticities of passing anticyclonic eddies or topographic Rossby waves. Compared with flat topography, PSI near rough topography was significantly promoted by topographic-localized strong D1 with high-mode structures, creating abyssal NIW bursts. Bottom-reaching shipboard conductivity–temperature–depth profiles revealed that the bottom mixed layers became much thicker when approaching CLs, suggesting that abyssal PSI potentially accelerates the ventilation and upwelling of bottom water. The observational results presented here illustrate notable spatiotemporal variations in abyssal NIWs regulated by PSI and call for consideration of PSI to better understand near-bottom mixing and upwelling.
Abstract
Internal waves close to the seafloor of abyssal oceans are the key energy suppliers driving near-bottom mixing and the upwelling branches of meridional overturning circulation, but their spatiotemporal variability and intrinsic mechanisms remain largely unclear. In this study, measurements from 10 long-term moorings were used to investigate the internal wave activities in the abyssal South China Sea, which is an important upwelling zone. Strong near-inertial internal waves (NIWs) with current velocity pulses exceeding 5 cm s−1 were observed to dominate the near-bottom internal wave field at approximately 14°N. These abyssal NIWs were phase-coupled with diurnal internal tides (D1), and both displayed common seasonal variations that were larger in winter and summer, providing evidence of diurnal parametric subharmonic instability (PSI) near its critical latitudes (CLs). Emitted from the bottom, near-inertial kinetic energy rapidly decreased by one order of magnitude from depths of ∼120 to ∼620 m above the bottom. Near rough topographies, the abyssal PSI was shifted poleward to approximately 14.8°N by negative relative vorticities of passing anticyclonic eddies or topographic Rossby waves. Compared with flat topography, PSI near rough topography was significantly promoted by topographic-localized strong D1 with high-mode structures, creating abyssal NIW bursts. Bottom-reaching shipboard conductivity–temperature–depth profiles revealed that the bottom mixed layers became much thicker when approaching CLs, suggesting that abyssal PSI potentially accelerates the ventilation and upwelling of bottom water. The observational results presented here illustrate notable spatiotemporal variations in abyssal NIWs regulated by PSI and call for consideration of PSI to better understand near-bottom mixing and upwelling.
Abstract
Standing meanders of the Antarctic Circumpolar Current (ACC) and associated eddy hotspots play an important role for the meridional heat flux and downward momentum transfer in the Southern Ocean. Previous modeling studies show that the vorticity balance characterizing standing meanders in the upper ocean is dominated by advection of relative vorticity and stretching. Through the adjustment of this vorticity balance, standing meanders have been suggested to provide a pathway for the transfer of the momentum input by the wind from the surface to the bottom, leading to stronger bottom flows and energy dissipation. However, the dynamics governing the meander formation and its adjustment to wind remain unclear. Here we develop a quasigeostrophic theory and combine it with a regional model of the Macquarie Ridge region and an idealized channel model to explore the dynamics and vertical structure of standing meanders of the ACC. The results show that the entire vertical structure of the meander, including its dynamics in the upper ocean, is controlled by the bottom flow interacting with topography. Based on our results, we suggest a novel mechanism for the response of the ACC to wind in which “flexing” of the meander, or change in its curvature, is a response to changes in the bottom (barotropic) flow. Stronger bottom flow in response to stronger wind interacts with topography and generates a larger-amplitude Rossby wave propagating into the upper ocean. The ACC mean shear aloft amplifies the Rossby wave and leads to a larger-amplitude meander in the upper ocean dominated by advection of relative vorticity and stretching.
Abstract
Standing meanders of the Antarctic Circumpolar Current (ACC) and associated eddy hotspots play an important role for the meridional heat flux and downward momentum transfer in the Southern Ocean. Previous modeling studies show that the vorticity balance characterizing standing meanders in the upper ocean is dominated by advection of relative vorticity and stretching. Through the adjustment of this vorticity balance, standing meanders have been suggested to provide a pathway for the transfer of the momentum input by the wind from the surface to the bottom, leading to stronger bottom flows and energy dissipation. However, the dynamics governing the meander formation and its adjustment to wind remain unclear. Here we develop a quasigeostrophic theory and combine it with a regional model of the Macquarie Ridge region and an idealized channel model to explore the dynamics and vertical structure of standing meanders of the ACC. The results show that the entire vertical structure of the meander, including its dynamics in the upper ocean, is controlled by the bottom flow interacting with topography. Based on our results, we suggest a novel mechanism for the response of the ACC to wind in which “flexing” of the meander, or change in its curvature, is a response to changes in the bottom (barotropic) flow. Stronger bottom flow in response to stronger wind interacts with topography and generates a larger-amplitude Rossby wave propagating into the upper ocean. The ACC mean shear aloft amplifies the Rossby wave and leads to a larger-amplitude meander in the upper ocean dominated by advection of relative vorticity and stretching.
Abstract
The origins of the upper limb of the Atlantic meridional overturning circulation and the partition among different routes has been quantified with models at eddy-permitting and one eddy-resolving model or with low-resolution models assimilating observations. Here, a step toward bridging this gap is taken by using the Southern Ocean State Estimate (SOSE) at the eddy-permitting 1/6° horizontal resolution to compute Lagrangian diagnostics from virtual particle trajectories advected between 6.7°S and two meridional sections: one at Drake Passage (cold route) and the other from South Africa to Antarctica (warm route). Our results agree with the prevailing concept attributing the largest transport contribution to the warm route with 12.3 Sv (88%) (1 Sv ≡ 106 m3 s−1) compared with 1.7 Sv (12%) for the cold route. These results are compared with a similar Lagrangian experiment performed with the lower-resolution state estimate from Estimating the Circulation and Climate of the Ocean. Eulerian and Lagrangian means highlight an overall increase in the transport of the major South Atlantic currents with finer resolution, resulting in a relatively larger contribution from the cold route. In particular, the Malvinas Current to Antarctic Circumpolar Current (MC/ACC) ratio plays a more important role on the routes partition than the increased Agulhas Leakage. The relative influence of the mean flow versus the eddy flow on the routes partition is investigated by computing the mean and eddy kinetic energies and the Lagrangian-based eddy diffusivity. Lagrangian diffusivity estimates are largest in the Agulhas and Malvinas regions but advection by the mean flow dominates everywhere.
Abstract
The origins of the upper limb of the Atlantic meridional overturning circulation and the partition among different routes has been quantified with models at eddy-permitting and one eddy-resolving model or with low-resolution models assimilating observations. Here, a step toward bridging this gap is taken by using the Southern Ocean State Estimate (SOSE) at the eddy-permitting 1/6° horizontal resolution to compute Lagrangian diagnostics from virtual particle trajectories advected between 6.7°S and two meridional sections: one at Drake Passage (cold route) and the other from South Africa to Antarctica (warm route). Our results agree with the prevailing concept attributing the largest transport contribution to the warm route with 12.3 Sv (88%) (1 Sv ≡ 106 m3 s−1) compared with 1.7 Sv (12%) for the cold route. These results are compared with a similar Lagrangian experiment performed with the lower-resolution state estimate from Estimating the Circulation and Climate of the Ocean. Eulerian and Lagrangian means highlight an overall increase in the transport of the major South Atlantic currents with finer resolution, resulting in a relatively larger contribution from the cold route. In particular, the Malvinas Current to Antarctic Circumpolar Current (MC/ACC) ratio plays a more important role on the routes partition than the increased Agulhas Leakage. The relative influence of the mean flow versus the eddy flow on the routes partition is investigated by computing the mean and eddy kinetic energies and the Lagrangian-based eddy diffusivity. Lagrangian diffusivity estimates are largest in the Agulhas and Malvinas regions but advection by the mean flow dominates everywhere.
Abstract
Energy exchanges between large-scale ocean currents and mesoscale eddies play an important role in setting the large-scale ocean circulation but are not fully captured in models. To better understand and quantify the ocean energy cycle, we apply along-isopycnal spatial filtering to output from an isopycnal 1/32° primitive equation model with idealized Atlantic and Southern Ocean geometry and topography. We diagnose the energy cycle in two frameworks: 1) a non-thickness-weighted framework, resulting in a Lorenz-like energy cycle, and 2) a thickness-weighted framework, resulting in the Bleck energy cycle. This paper shows that framework 2 is more useful for studying energy pathways when an isopycnal average is used. Next, we investigate the Bleck cycle as a function of filter scale. Baroclinic conversion generates mesoscale eddy kinetic energy over a wide range of scales and peaks near the deformation scale at high latitudes but below the deformation scale at low latitudes. Away from topography, an inverse cascade transfers kinetic energy from the mesoscales to larger scales. The upscale energy transfer peaks near the energy-containing scale at high latitudes but below the deformation scale at low latitudes. Regions downstream of topography are characterized by a downscale kinetic energy transfer, in which mesoscale eddies are generated through barotropic instability. The scale- and flow-dependent energy pathways diagnosed in this paper provide a basis for evaluating and developing scale- and flow-aware mesoscale eddy parameterizations.
Significance Statement
Blowing winds provide a major energy source for the large-scale ocean circulation. A substantial fraction of this energy is converted to smaller-scale eddies, which swirl through the ocean as sea cyclones. Ocean turbulence causes these eddies to transfer part of their energy back to the large-scale ocean currents. This ocean energy cycle is not fully simulated in numerical models, but it plays an important role in transporting heat, carbon, and nutrients throughout the world’s oceans. The purpose of this study is to quantify the ocean energy cycle by using fine-scale idealized numerical simulations of the Atlantic and Southern Oceans. Our results provide a basis for how to include unrepresented energy exchanges in coarse global climate models.
Abstract
Energy exchanges between large-scale ocean currents and mesoscale eddies play an important role in setting the large-scale ocean circulation but are not fully captured in models. To better understand and quantify the ocean energy cycle, we apply along-isopycnal spatial filtering to output from an isopycnal 1/32° primitive equation model with idealized Atlantic and Southern Ocean geometry and topography. We diagnose the energy cycle in two frameworks: 1) a non-thickness-weighted framework, resulting in a Lorenz-like energy cycle, and 2) a thickness-weighted framework, resulting in the Bleck energy cycle. This paper shows that framework 2 is more useful for studying energy pathways when an isopycnal average is used. Next, we investigate the Bleck cycle as a function of filter scale. Baroclinic conversion generates mesoscale eddy kinetic energy over a wide range of scales and peaks near the deformation scale at high latitudes but below the deformation scale at low latitudes. Away from topography, an inverse cascade transfers kinetic energy from the mesoscales to larger scales. The upscale energy transfer peaks near the energy-containing scale at high latitudes but below the deformation scale at low latitudes. Regions downstream of topography are characterized by a downscale kinetic energy transfer, in which mesoscale eddies are generated through barotropic instability. The scale- and flow-dependent energy pathways diagnosed in this paper provide a basis for evaluating and developing scale- and flow-aware mesoscale eddy parameterizations.
Significance Statement
Blowing winds provide a major energy source for the large-scale ocean circulation. A substantial fraction of this energy is converted to smaller-scale eddies, which swirl through the ocean as sea cyclones. Ocean turbulence causes these eddies to transfer part of their energy back to the large-scale ocean currents. This ocean energy cycle is not fully simulated in numerical models, but it plays an important role in transporting heat, carbon, and nutrients throughout the world’s oceans. The purpose of this study is to quantify the ocean energy cycle by using fine-scale idealized numerical simulations of the Atlantic and Southern Oceans. Our results provide a basis for how to include unrepresented energy exchanges in coarse global climate models.
Abstract
Moored observations and a realistic, tidally forced 3D model are presented of flow and internal-tide-driven turbulence over a supercritical 3D fan in southeastern Luzon Strait. Two stacked moored profilers, an acoustic Doppler current profiler, and a thermistor string measured horizontal velocity, density, and salinity over nearly the entire water column every 1.5 h for 50 days. Observed dissipation rate computed from Thorpe scales decays away from the bottom and shows a strong spring–neap cycle; observed depth-integrated dissipation rate scales as
Significance Statement
This paper describes deep ocean turbulence caused by strong tidal and low-frequency meandering flows over and around a three-dimensional bump, using moored observations and a computer simulation. Such information is important for accurately including these effects in climate simulations. The observations and model agree well enough to be able to use both to synthesize a coherent picture. The observed and modeled turbulence scale as the cube of the tidal speed as expected from theory, but low-frequency flows complicate the picture. We also demonstrate the underestimation of the turbulence that can result when vertical profiling rates are comparable to the internal wave velocities.
Abstract
Moored observations and a realistic, tidally forced 3D model are presented of flow and internal-tide-driven turbulence over a supercritical 3D fan in southeastern Luzon Strait. Two stacked moored profilers, an acoustic Doppler current profiler, and a thermistor string measured horizontal velocity, density, and salinity over nearly the entire water column every 1.5 h for 50 days. Observed dissipation rate computed from Thorpe scales decays away from the bottom and shows a strong spring–neap cycle; observed depth-integrated dissipation rate scales as
Significance Statement
This paper describes deep ocean turbulence caused by strong tidal and low-frequency meandering flows over and around a three-dimensional bump, using moored observations and a computer simulation. Such information is important for accurately including these effects in climate simulations. The observations and model agree well enough to be able to use both to synthesize a coherent picture. The observed and modeled turbulence scale as the cube of the tidal speed as expected from theory, but low-frequency flows complicate the picture. We also demonstrate the underestimation of the turbulence that can result when vertical profiling rates are comparable to the internal wave velocities.
Abstract
From 12 to 16 October 2016, a series of three major low pressure systems, including the tail end of Typhoon Songda, crossed the coasts of British Columbia (BC) and the state of Washington (WA). Songda was generated on 2 October and, after traveling northward along the coast of Japan, turned eastward toward North America. Once there, it merged with two extratropical cyclones moving along the coast of Vancouver Island. The combined lows generated pronounced storm surges, seiches, and infragravity waves off southern BC and northern WA. Here, we examine the event in terms of sea levels measured by tide gauges and offshore bottom pressure recorders, together with reanalysis data, and high-resolution air pressure and wind measurements from 182 meteorological stations. Surge heights during the event typically exceeded 80 cm, with maximum heights of over 100 cm observed at La Push (WA) and New Westminster (BC). At Tofino, on the west coast of Vancouver Island, there was a sharp 40-cm increase in sea level on 14 October in response to a marked air pressure disturbance; slightly lower sea level peaks were also observed at other outer coast locations. In all cases, the sea level response was 1.5–2.5 times as great as that expected from the inverted barometer effect, consistent with local topographic amplification. The sea level oscillations at Tofino had the form of a forced solitary wave (“meteorological tsunami,” or meteotsunami), whereas those on the southwestern shelf off Vancouver Island are well described by classical standing-wave theory. A numerical model closely reproduces the observed meteotsunami peaks and standing-wave oscillations.
Abstract
From 12 to 16 October 2016, a series of three major low pressure systems, including the tail end of Typhoon Songda, crossed the coasts of British Columbia (BC) and the state of Washington (WA). Songda was generated on 2 October and, after traveling northward along the coast of Japan, turned eastward toward North America. Once there, it merged with two extratropical cyclones moving along the coast of Vancouver Island. The combined lows generated pronounced storm surges, seiches, and infragravity waves off southern BC and northern WA. Here, we examine the event in terms of sea levels measured by tide gauges and offshore bottom pressure recorders, together with reanalysis data, and high-resolution air pressure and wind measurements from 182 meteorological stations. Surge heights during the event typically exceeded 80 cm, with maximum heights of over 100 cm observed at La Push (WA) and New Westminster (BC). At Tofino, on the west coast of Vancouver Island, there was a sharp 40-cm increase in sea level on 14 October in response to a marked air pressure disturbance; slightly lower sea level peaks were also observed at other outer coast locations. In all cases, the sea level response was 1.5–2.5 times as great as that expected from the inverted barometer effect, consistent with local topographic amplification. The sea level oscillations at Tofino had the form of a forced solitary wave (“meteorological tsunami,” or meteotsunami), whereas those on the southwestern shelf off Vancouver Island are well described by classical standing-wave theory. A numerical model closely reproduces the observed meteotsunami peaks and standing-wave oscillations.
