Browse
Abstract
The subpolar North Atlantic is known to have rich mesoscale and submesoscale variations, however, their spectral characteristics have not been documented in observations. This study documents the Kinetic Energy (KE) spectra using Acoustic Doppler Current Profiler measurements that cover both the Iceland Basin and the Irminger Sea. The KE spectrum is partitioned into geostrophically balanced motions and unbalanced motions. The results reveal that balanced motions dominate the KE spectra. The unbalanced motions enhance in spring and fall to flatten the spectra and dominate small scale (<50km) energy, though uncertainty is high due to measurement noise and method assumptions. In addition, the dynamical framework that drives the balanced motions undergoes distinct seasonal shifts. In the spring and summer seasons of the Iceland Basin, as well as the summer season of the Irminger Sea, the wavenumber spectra of balanced motions exhibit a slope of approximately −3, consistent with the internal quasi-geostrophic turbulence theory. Conversely, in the fall season of the Iceland Basin and the spring and fall seasons of the Irminger Sea, the wavenumber spectra of geostrophic balanced motions have a slope close to −2, consistent with surface quasi-geostrophic turbulence theory. Additionally, we have found that the intensity of mesoscale eddies in the spring season can modulate both the slope and intensity of the wavenumber spectra of geostrophic balanced flows.
Abstract
The subpolar North Atlantic is known to have rich mesoscale and submesoscale variations, however, their spectral characteristics have not been documented in observations. This study documents the Kinetic Energy (KE) spectra using Acoustic Doppler Current Profiler measurements that cover both the Iceland Basin and the Irminger Sea. The KE spectrum is partitioned into geostrophically balanced motions and unbalanced motions. The results reveal that balanced motions dominate the KE spectra. The unbalanced motions enhance in spring and fall to flatten the spectra and dominate small scale (<50km) energy, though uncertainty is high due to measurement noise and method assumptions. In addition, the dynamical framework that drives the balanced motions undergoes distinct seasonal shifts. In the spring and summer seasons of the Iceland Basin, as well as the summer season of the Irminger Sea, the wavenumber spectra of balanced motions exhibit a slope of approximately −3, consistent with the internal quasi-geostrophic turbulence theory. Conversely, in the fall season of the Iceland Basin and the spring and fall seasons of the Irminger Sea, the wavenumber spectra of geostrophic balanced motions have a slope close to −2, consistent with surface quasi-geostrophic turbulence theory. Additionally, we have found that the intensity of mesoscale eddies in the spring season can modulate both the slope and intensity of the wavenumber spectra of geostrophic balanced flows.
Abstract
Enhanced diapycnal mixing induced by the near-bottom breaking of internal waves is an essential component of the lower meridional overturning circulation. Despite its crucial role in the ocean circulation, tidally driven internal wave breaking is challenging to observe due to its inherently short spatial and temporal scales. We present detailed moored and shipboard observations that resolve the spatio-temporal variability of the tidal response over a small-scale bump embedded in the continental slope of Tasmania. Cross-shore tidal currents drive a nonlinear trapped response over the steep bottom around the bump. The observations are roughly consistent with two-dimensional high-mode tidal lee-wave theory. However, the alongshore tidal velocities are large, suggesting that the alongshore bathymetric variability modulates the tidal response driven by the cross-shore tidal flow. The semidiurnal tide and energy dissipation rate are correlated at subtidal timescales, but with complex temporal variability. Energy dissipation from a simple scattering model shows that the elevated near-bottom turbulence can be sustained by the impinging mode-1 internal tide, where the dissipation over the bump is O(1%) of the incident depth-integrated energy flux. Despite this small fraction, tidal dissipation is enhanced over the bump due to steep topography at O(1) km horizontal scale and may locally drive significant diapycnal mixing.
Abstract
Enhanced diapycnal mixing induced by the near-bottom breaking of internal waves is an essential component of the lower meridional overturning circulation. Despite its crucial role in the ocean circulation, tidally driven internal wave breaking is challenging to observe due to its inherently short spatial and temporal scales. We present detailed moored and shipboard observations that resolve the spatio-temporal variability of the tidal response over a small-scale bump embedded in the continental slope of Tasmania. Cross-shore tidal currents drive a nonlinear trapped response over the steep bottom around the bump. The observations are roughly consistent with two-dimensional high-mode tidal lee-wave theory. However, the alongshore tidal velocities are large, suggesting that the alongshore bathymetric variability modulates the tidal response driven by the cross-shore tidal flow. The semidiurnal tide and energy dissipation rate are correlated at subtidal timescales, but with complex temporal variability. Energy dissipation from a simple scattering model shows that the elevated near-bottom turbulence can be sustained by the impinging mode-1 internal tide, where the dissipation over the bump is O(1%) of the incident depth-integrated energy flux. Despite this small fraction, tidal dissipation is enhanced over the bump due to steep topography at O(1) km horizontal scale and may locally drive significant diapycnal mixing.
Abstract
A variety of submesoscale coherent vortices (SCVs) in the Kuroshio Extension region have been reported by recent observational studies, and the preliminary understanding of their properties, spatial distribution and possible origins has progressively improved. However, due to relatively sparse in situ observations, the generation mechanisms of these SCVs and associated dynamic processes remain unclear. In this study, we use high-resolution model simulations to fill the gaps of the in situ observations in terms of the three-dimensional structures and life cycles of SCVs. Vortex detection and tracking algorithms are adopted and the characteristics of warm-core and cold-core SCVs are revealed. These vortices have finite Rossby numbers (0.25-0.4) and their horizontal structures can be well described by the Tayler vortex model in terms of the gradient wind balance. The vertical velocity field is characterized by a distinct dipole pattern with upwelling and downwelling cells at the vortex edge. It is very likely that both types of SCVs are generated along the eastern Japan coast through flow–topography interactions, and the Izu–Ogasawara Ridge and Hokkaido slope are found to be two important generation sites where topography friction produces extremely low potential vorticity. After leaving the boundary, SCVs can propagate over long distances and trap a water volume of ~1011 m3.
Abstract
A variety of submesoscale coherent vortices (SCVs) in the Kuroshio Extension region have been reported by recent observational studies, and the preliminary understanding of their properties, spatial distribution and possible origins has progressively improved. However, due to relatively sparse in situ observations, the generation mechanisms of these SCVs and associated dynamic processes remain unclear. In this study, we use high-resolution model simulations to fill the gaps of the in situ observations in terms of the three-dimensional structures and life cycles of SCVs. Vortex detection and tracking algorithms are adopted and the characteristics of warm-core and cold-core SCVs are revealed. These vortices have finite Rossby numbers (0.25-0.4) and their horizontal structures can be well described by the Tayler vortex model in terms of the gradient wind balance. The vertical velocity field is characterized by a distinct dipole pattern with upwelling and downwelling cells at the vortex edge. It is very likely that both types of SCVs are generated along the eastern Japan coast through flow–topography interactions, and the Izu–Ogasawara Ridge and Hokkaido slope are found to be two important generation sites where topography friction produces extremely low potential vorticity. After leaving the boundary, SCVs can propagate over long distances and trap a water volume of ~1011 m3.
Abstract
Through an expansive series of simulations, we investigate the effects of spatially uniform shear on the transport, structure, and dynamics of salt fingers. The simulations reveal that shear adversely affects the heat and salt fluxes of the system, reducing them by up to an order of magnitude. We characterize this in detail across a broad range of Richardson numbers and density ratios. We demonstrate that the density ratio is strongly related to the amount of shear required to disrupt fingers with larger density ratio systems being more susceptible to disruption. An empirical relationship is proposed that captures this behavior that could be implemented into global ocean models. The results of these simulations accurately reproduce the microstructure measurements from NATRE observations. This work suggests that typical salt finger fluxes in the ocean will likely be a factor of 2–3 less than predicted by models not taking the effects of shear on double-diffusive systems into account.
Abstract
Through an expansive series of simulations, we investigate the effects of spatially uniform shear on the transport, structure, and dynamics of salt fingers. The simulations reveal that shear adversely affects the heat and salt fluxes of the system, reducing them by up to an order of magnitude. We characterize this in detail across a broad range of Richardson numbers and density ratios. We demonstrate that the density ratio is strongly related to the amount of shear required to disrupt fingers with larger density ratio systems being more susceptible to disruption. An empirical relationship is proposed that captures this behavior that could be implemented into global ocean models. The results of these simulations accurately reproduce the microstructure measurements from NATRE observations. This work suggests that typical salt finger fluxes in the ocean will likely be a factor of 2–3 less than predicted by models not taking the effects of shear on double-diffusive systems into account.
Abstract
Full-depth ocean zonal currents in the tropical and extratropical northwestern Pacific (TNWP) are studied using current measurements from 17 deep-ocean moorings deployed along the 143°E meridian from the equator to 22°N during January 2016–February 2017. Mean transports of the North Equatorial Current and North Equatorial Countercurrent are estimated to be 42.7 ± 7.1 Sv (1 Sv ≡ 106 m3 s−1) and 10.5 ± 5.3 Sv, respectively, both of which exhibit prominent annual cycles with opposite phases in this year. The observations suggest much larger vertical extents of several of the major subsurface currents than previously reported, including the Lower Equatorial Intermediate Current, Northern Intermediate Countercurrent, North Equatorial Subsurface Current, and North Equatorial Undercurrent (NEUC) from south to north. The Northern Subsurface Countercurrent and NEUC are found to be less steady than the other currents. Seasonal variations of these currents are also revealed in the study. In the deep ocean, the currents below 2000 m are reported for the first time. The observations confirm the striation patterns of meridionally alternating zonal currents in the intermediate and deep layers. Further analyses suggest a superposition of at least the first four and two baroclinic modes to represent the mean equatorial and off-equatorial currents, respectively. Meanwhile, seasonal variations of the currents are generally dominated by the first baroclinic mode associated with the low-mode Rossby waves. Overall, the above observational results not only enhance the knowledge of full-depth current system in the TNWP but also provide a basis for future model validation and skill improvement.
Abstract
Full-depth ocean zonal currents in the tropical and extratropical northwestern Pacific (TNWP) are studied using current measurements from 17 deep-ocean moorings deployed along the 143°E meridian from the equator to 22°N during January 2016–February 2017. Mean transports of the North Equatorial Current and North Equatorial Countercurrent are estimated to be 42.7 ± 7.1 Sv (1 Sv ≡ 106 m3 s−1) and 10.5 ± 5.3 Sv, respectively, both of which exhibit prominent annual cycles with opposite phases in this year. The observations suggest much larger vertical extents of several of the major subsurface currents than previously reported, including the Lower Equatorial Intermediate Current, Northern Intermediate Countercurrent, North Equatorial Subsurface Current, and North Equatorial Undercurrent (NEUC) from south to north. The Northern Subsurface Countercurrent and NEUC are found to be less steady than the other currents. Seasonal variations of these currents are also revealed in the study. In the deep ocean, the currents below 2000 m are reported for the first time. The observations confirm the striation patterns of meridionally alternating zonal currents in the intermediate and deep layers. Further analyses suggest a superposition of at least the first four and two baroclinic modes to represent the mean equatorial and off-equatorial currents, respectively. Meanwhile, seasonal variations of the currents are generally dominated by the first baroclinic mode associated with the low-mode Rossby waves. Overall, the above observational results not only enhance the knowledge of full-depth current system in the TNWP but also provide a basis for future model validation and skill improvement.
Abstract
Eddies generated off the west Greenland coast modulate the deep convection in the Labrador Sea, while there are still open questions related to their formation mechanisms. Using 11 years (2008–18) of output from a NEMO model configured with a 1/60° nest in the Labrador Sea, we present the patterns of baroclinic and barotropic instability off the west Greenland coast. We highlight the generation of Irminger Rings at Cape Desolation and boundary current eddies at the location of the Overturning in the Subpolar North Atlantic Program (OSNAP) West section. In between these formation sites, eddy energy attenuation occurs along the West Greenland Current (WGC). Overall, baroclinic instability dominates in the upper 1000 m and is twice as strong as the barotropic instability. Seasonally, the instabilities are generally twice as strong in winter compared to summer. Interannually from 2008 to 2018, the instabilities generally show a strengthening trend, with values in 2018 two to three times as strong as those in 2008. We found that on an interannual time scale, the strengthening of WGC and the steepening of its velocity contours enhance the barotropic instability, and the intrusion of the upper Irminger Sea Intermediate Water (uISIW) on the Irminger Water enhances the baroclinic instability by increasing the horizontal density gradient. On a seasonal time scale, variability of the eddy momentum and density fluxes modulate the barotropic and baroclinic instability, respectively. From observation-based datasets, we also found that the downstream eddy kinetic energy is highly correlated with the uISIW transports, suggesting that the amount of uISIW affects the eddy formation. Using a very high-resolution numerical model, our study provides new insight into the variability and mechanisms of eddy formation along the west Greenland coast.
Significance Statement
Vigorous eddy activity exists off the west Greenland coast. The eddies flux buoyancy to the interior Labrador Sea and thus weaken the convection, which feeds the lower limb of the Atlantic meridional overturning circulation. Given uncertainties in the eddy formation mechanisms, by using an ocean model with very high resolution that resolves those eddies, we show the factors that control the production and variability of the eddy formation off the western coast of Greenland. The eddy formation generally strengthens over the years 2008–18, which is a result of the intrusion of intermediate water on the continental slope and a stronger boundary current. The eddy formation shows a seasonal cycle—it is generally the strongest in winter and weakest in summer, which is modulated by the seasonal variability of eddy momentum and density fluxes.
Abstract
Eddies generated off the west Greenland coast modulate the deep convection in the Labrador Sea, while there are still open questions related to their formation mechanisms. Using 11 years (2008–18) of output from a NEMO model configured with a 1/60° nest in the Labrador Sea, we present the patterns of baroclinic and barotropic instability off the west Greenland coast. We highlight the generation of Irminger Rings at Cape Desolation and boundary current eddies at the location of the Overturning in the Subpolar North Atlantic Program (OSNAP) West section. In between these formation sites, eddy energy attenuation occurs along the West Greenland Current (WGC). Overall, baroclinic instability dominates in the upper 1000 m and is twice as strong as the barotropic instability. Seasonally, the instabilities are generally twice as strong in winter compared to summer. Interannually from 2008 to 2018, the instabilities generally show a strengthening trend, with values in 2018 two to three times as strong as those in 2008. We found that on an interannual time scale, the strengthening of WGC and the steepening of its velocity contours enhance the barotropic instability, and the intrusion of the upper Irminger Sea Intermediate Water (uISIW) on the Irminger Water enhances the baroclinic instability by increasing the horizontal density gradient. On a seasonal time scale, variability of the eddy momentum and density fluxes modulate the barotropic and baroclinic instability, respectively. From observation-based datasets, we also found that the downstream eddy kinetic energy is highly correlated with the uISIW transports, suggesting that the amount of uISIW affects the eddy formation. Using a very high-resolution numerical model, our study provides new insight into the variability and mechanisms of eddy formation along the west Greenland coast.
Significance Statement
Vigorous eddy activity exists off the west Greenland coast. The eddies flux buoyancy to the interior Labrador Sea and thus weaken the convection, which feeds the lower limb of the Atlantic meridional overturning circulation. Given uncertainties in the eddy formation mechanisms, by using an ocean model with very high resolution that resolves those eddies, we show the factors that control the production and variability of the eddy formation off the western coast of Greenland. The eddy formation generally strengthens over the years 2008–18, which is a result of the intrusion of intermediate water on the continental slope and a stronger boundary current. The eddy formation shows a seasonal cycle—it is generally the strongest in winter and weakest in summer, which is modulated by the seasonal variability of eddy momentum and density fluxes.
Abstract
The vertical structure of subinertial variability is examined using full-depth horizontal velocity and vertical isopycnal displacement observations derived from the Ocean Observatory Initiative (OOI). Vertical profiles on time scales between 100 hours and 1 year or longer are characterized through Empirical Orthogonal Function decomposition and qualitatively compared to theoretical modal predictions for the cases of flat, sloping and rough bathymetry. OOI observations were obtained from mooring clusters at four deep-ocean sites: Argentine Basin, Southern Ocean, Station Papa, and Irminger Sea. As no single OOI mooring in these arrays provides temperature, salinity and horizontal velocity information over the full water column, sensor observations from two or more moorings are combined. Depths greater than ~150-300 m were sampled by McLane Moored Profilers; in three of the four cases, two Profilers were utilized on the moorings. Owing to instrument failures on the deployments examined here, only about two years of full-ocean-depth observations are available from three of the four sites and some three+ years from the other. Results from the OOI Global sites are contrasted with a parallel analysis of three and one half years of observations about the axis of the Gulf Stream where much of the subinertial variability is associated with Stream meandering past the moorings. Looking across the observations, no universal vertical structure is found that characterizes the subinertial variability at the five sites examined; regional bathymetry, stratification, baroclinicity, nonlinearity and the forcing (both local and remote) likely all play a role in shaping the vertical structure of the subinertial variability in individual ocean regions.
Abstract
The vertical structure of subinertial variability is examined using full-depth horizontal velocity and vertical isopycnal displacement observations derived from the Ocean Observatory Initiative (OOI). Vertical profiles on time scales between 100 hours and 1 year or longer are characterized through Empirical Orthogonal Function decomposition and qualitatively compared to theoretical modal predictions for the cases of flat, sloping and rough bathymetry. OOI observations were obtained from mooring clusters at four deep-ocean sites: Argentine Basin, Southern Ocean, Station Papa, and Irminger Sea. As no single OOI mooring in these arrays provides temperature, salinity and horizontal velocity information over the full water column, sensor observations from two or more moorings are combined. Depths greater than ~150-300 m were sampled by McLane Moored Profilers; in three of the four cases, two Profilers were utilized on the moorings. Owing to instrument failures on the deployments examined here, only about two years of full-ocean-depth observations are available from three of the four sites and some three+ years from the other. Results from the OOI Global sites are contrasted with a parallel analysis of three and one half years of observations about the axis of the Gulf Stream where much of the subinertial variability is associated with Stream meandering past the moorings. Looking across the observations, no universal vertical structure is found that characterizes the subinertial variability at the five sites examined; regional bathymetry, stratification, baroclinicity, nonlinearity and the forcing (both local and remote) likely all play a role in shaping the vertical structure of the subinertial variability in individual ocean regions.
Abstract
We investigate the mechanisms with which the sea surface temperature (SST) in the tropical Pacific responds to the perturbation of an exponential form to the background vertical mixing of the upper ocean. For a surface value of 0.005 m2 s−1 and a scale depth of 10 m (as typically used in the so-called nonbreaking wave parameterization), it is found that only ocean temperature within the equatorial eastern Pacific (EEP) is directly impacted; surface cooling and thermocline warming anomalies are produced. These signals propagate poleward as coastal Kelvin waves and then westward as equatorial Rossby waves. The surface cooling is severely damped while the thermocline warming is able to reach the western coast. This warm anomaly is brought up to the surface by equatorial upwelling more strongly around 110°W than at other places. In the coupled model, such equatorial warming induces an El Niño–like large-scale warming through Bjerknes feedback. Increasing the surface value of vertical mixing by a factor of 10 does not increase the equatorial surface warming while increasing the scale depth to 20 m does. Increasing the scale depth generates thermocline warming also in the subtropical region, which then propagates to the equatorial thermocline and enhances the warming there. Moreover, the off-equatorial cooling is enhanced, which makes the final warming anomaly narrower meridionally compared to an El Niño pattern.
Abstract
We investigate the mechanisms with which the sea surface temperature (SST) in the tropical Pacific responds to the perturbation of an exponential form to the background vertical mixing of the upper ocean. For a surface value of 0.005 m2 s−1 and a scale depth of 10 m (as typically used in the so-called nonbreaking wave parameterization), it is found that only ocean temperature within the equatorial eastern Pacific (EEP) is directly impacted; surface cooling and thermocline warming anomalies are produced. These signals propagate poleward as coastal Kelvin waves and then westward as equatorial Rossby waves. The surface cooling is severely damped while the thermocline warming is able to reach the western coast. This warm anomaly is brought up to the surface by equatorial upwelling more strongly around 110°W than at other places. In the coupled model, such equatorial warming induces an El Niño–like large-scale warming through Bjerknes feedback. Increasing the surface value of vertical mixing by a factor of 10 does not increase the equatorial surface warming while increasing the scale depth to 20 m does. Increasing the scale depth generates thermocline warming also in the subtropical region, which then propagates to the equatorial thermocline and enhances the warming there. Moreover, the off-equatorial cooling is enhanced, which makes the final warming anomaly narrower meridionally compared to an El Niño pattern.
Abstract
Typhoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm s−1. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ∼3.9 and ∼2.5 m s−1 north and south of the typhoon’s track, respectively, while those of mode 3 were ∼2.1 and ∼1.7 m s−1, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode-2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ∼300 m and lengthened the duration of energetic NIWs observed in the north.
Significance Statement
Near-inertial waves (NIWs), generally caused by wind (e.g., typhoons and monsoons) in the upper ocean, are one of the two types of energetic internal waves widely observed in the ocean. After their generation near the surface, energetic NIWs propagate downward and equatorward, thereby significantly contributing to turbulent mixing in the upper and deep ocean and acting as a mechanism of energy transfer from the surface to the deep ocean. The unprecedented NIW observations in the South China Sea describe the generation, propagation, and vertical normal modes of typhoon-induced NIWs in the upper and deep oceans, and contribute to knowledge regarding the dynamic responses of abyssal processes to typhoons.
Abstract
Typhoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm s−1. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ∼3.9 and ∼2.5 m s−1 north and south of the typhoon’s track, respectively, while those of mode 3 were ∼2.1 and ∼1.7 m s−1, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode-2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ∼300 m and lengthened the duration of energetic NIWs observed in the north.
Significance Statement
Near-inertial waves (NIWs), generally caused by wind (e.g., typhoons and monsoons) in the upper ocean, are one of the two types of energetic internal waves widely observed in the ocean. After their generation near the surface, energetic NIWs propagate downward and equatorward, thereby significantly contributing to turbulent mixing in the upper and deep ocean and acting as a mechanism of energy transfer from the surface to the deep ocean. The unprecedented NIW observations in the South China Sea describe the generation, propagation, and vertical normal modes of typhoon-induced NIWs in the upper and deep oceans, and contribute to knowledge regarding the dynamic responses of abyssal processes to typhoons.
