Abstract
The surface and air temperature gradient (T S00-T air) drives the development of the convective boundary layer and the occurrence of clouds and precipitation. However, its variability is still poorly understood due to the lack of high-quality observations. This study fills in this gap by investigating the diurnal to decadal variability in T S00-T air from 2002 to 2022 based on hourly observations collected at over 100 stations of the U.S. Climate Reference Network. It is found that T S00-T air reaches its maximum at noon with an average of 6.85°C over the Contiguous United States, which decreases to 4.28°C when the soil moisture exceeds 30%. The daily minimum of T S00-T air has an average of −2.08°C, which generally occurs in the early evening but is postponed as the cloud fraction decreases. Moreover, while existing studies have used the near-surface soil temperature, such as the 5-cm soil temperature (T S05), to calculate T S05-T air, we find that T S00-T air and T S05-T air have opposite diurnal cycles, and their amplitudes differed drastically. The daily minimum of T S00-T air has a significant decreasing trend (−0.50±0.007°C/decade) from 2002 to 2022 due to T air increasing at a higher rate than T S00 during the nighttime. The occurrence frequency of near surface stable condition (T S00-T air<0) increases significantly, and the frequency of unstable condition (T S00-T air>0) decreases notably throughout the year except for winter. When it is stable, the magnitude of T S00-T air tends to decrease while the T S00-T air tends to increase when it is unstable, which is consistent with the drying condition caused by precipitation deficit. This study provides the first observational evidence on how T S00-T air responds to warming.
Abstract
The surface and air temperature gradient (T S00-T air) drives the development of the convective boundary layer and the occurrence of clouds and precipitation. However, its variability is still poorly understood due to the lack of high-quality observations. This study fills in this gap by investigating the diurnal to decadal variability in T S00-T air from 2002 to 2022 based on hourly observations collected at over 100 stations of the U.S. Climate Reference Network. It is found that T S00-T air reaches its maximum at noon with an average of 6.85°C over the Contiguous United States, which decreases to 4.28°C when the soil moisture exceeds 30%. The daily minimum of T S00-T air has an average of −2.08°C, which generally occurs in the early evening but is postponed as the cloud fraction decreases. Moreover, while existing studies have used the near-surface soil temperature, such as the 5-cm soil temperature (T S05), to calculate T S05-T air, we find that T S00-T air and T S05-T air have opposite diurnal cycles, and their amplitudes differed drastically. The daily minimum of T S00-T air has a significant decreasing trend (−0.50±0.007°C/decade) from 2002 to 2022 due to T air increasing at a higher rate than T S00 during the nighttime. The occurrence frequency of near surface stable condition (T S00-T air<0) increases significantly, and the frequency of unstable condition (T S00-T air>0) decreases notably throughout the year except for winter. When it is stable, the magnitude of T S00-T air tends to decrease while the T S00-T air tends to increase when it is unstable, which is consistent with the drying condition caused by precipitation deficit. This study provides the first observational evidence on how T S00-T air responds to warming.
Abstract
Water vapor supersaturation in clouds is a random variable that drives activation and growth of cloud droplets. The Pi Convection-Cloud Chamber generates a turbulent cloud with a microphysical steady-state that can be varied from clean to polluted by adjusting the aerosol injection rate. The supersaturation distribution and its moments, e.g., mean and variance, are investigated for varying cloud microphysical conditions. High-speed and co-located Eulerian measurements of temperature and water vapor concentration are combined to obtain the temporally resolved supersaturation distribution. This allows quantification of the contributions of variances and covariances between water vapor and temperature. Results are consistent with expectations for a convection chamber, with strong correlation between water vapor and temperature; departures from ideal behavior can be explained as resulting from dry regions on the warm boundary, analogous to entrainment. The saturation ratio distribution is measured under conditions that show monotonic increase of liquid water content and decrease of mean droplet diameter with increasing aerosol injection rate. The change in liquid water content is proportional to the change in water vapor concentration between no-cloud and cloudy conditions. Variability in the supersaturation remains even after cloud droplets are formed, and no significant buffering is observed. Results are interpreted in terms of a cloud microphysical Damköhler number (Da), under conditions corresponding to Da ≲ 1, i.e., the slow-microphysics regime. This implies that clouds with very clean regions, such that Da ≲ 1 is satisfied, will experience supersaturation fluctuations without them being buffered by cloud droplet growth.
Abstract
Water vapor supersaturation in clouds is a random variable that drives activation and growth of cloud droplets. The Pi Convection-Cloud Chamber generates a turbulent cloud with a microphysical steady-state that can be varied from clean to polluted by adjusting the aerosol injection rate. The supersaturation distribution and its moments, e.g., mean and variance, are investigated for varying cloud microphysical conditions. High-speed and co-located Eulerian measurements of temperature and water vapor concentration are combined to obtain the temporally resolved supersaturation distribution. This allows quantification of the contributions of variances and covariances between water vapor and temperature. Results are consistent with expectations for a convection chamber, with strong correlation between water vapor and temperature; departures from ideal behavior can be explained as resulting from dry regions on the warm boundary, analogous to entrainment. The saturation ratio distribution is measured under conditions that show monotonic increase of liquid water content and decrease of mean droplet diameter with increasing aerosol injection rate. The change in liquid water content is proportional to the change in water vapor concentration between no-cloud and cloudy conditions. Variability in the supersaturation remains even after cloud droplets are formed, and no significant buffering is observed. Results are interpreted in terms of a cloud microphysical Damköhler number (Da), under conditions corresponding to Da ≲ 1, i.e., the slow-microphysics regime. This implies that clouds with very clean regions, such that Da ≲ 1 is satisfied, will experience supersaturation fluctuations without them being buffered by cloud droplet growth.
Abstract
The Turkana Jet is an equatorial low-level jet (LLJ) in East Africa. The jet influences both flooding and droughts, and powers Africa’s largest wind farm. Much of what we know about the jet, including the characteristics of its diurnal cycle, derives from reanalysis simulations which are not constrained by radiosonde observations in the region. Here, we report the characteristics of the Turkana Jet with data from a field campaign during March-April 2021 - The Radiosonde Investigation For the Turkana Jet (RIFTJet). The southeasterly jet forms on average at 380 m above the surface, with mean speeds of 15.0 m.s−1. The strongest low-level winds are during the night and early morning from 0300 LT to 0600 LT (>16 m.s−1). The average wind profile retains a characteristic low-level jet structure throughout the day, with the low-level wind maximum weakening to a minimum of 10.9 m.s−1 at 1500 LT. There is significant shear, of up to 1.5 m.s−1 per 100 m maintained through the 1000 m above the wind maximum. The diurnal cycle of the jet is associated with the nocturnal strengthening and lowering of elevated subsidence inversions, which form above the jet. Reanalysis simulations (ERA5 and MERRA2) do not capture the daytime persistence of the jet and underestimate the speed of the jet throughout the diurnal cycle. The largest absolute errors of over 4.5 m.s−1 (−35%) occur at 0900 LT. The reanalyses also fail to simulate the elevated subsidence inversions above the jet and associated dry layer in the lower troposphere.
Abstract
The Turkana Jet is an equatorial low-level jet (LLJ) in East Africa. The jet influences both flooding and droughts, and powers Africa’s largest wind farm. Much of what we know about the jet, including the characteristics of its diurnal cycle, derives from reanalysis simulations which are not constrained by radiosonde observations in the region. Here, we report the characteristics of the Turkana Jet with data from a field campaign during March-April 2021 - The Radiosonde Investigation For the Turkana Jet (RIFTJet). The southeasterly jet forms on average at 380 m above the surface, with mean speeds of 15.0 m.s−1. The strongest low-level winds are during the night and early morning from 0300 LT to 0600 LT (>16 m.s−1). The average wind profile retains a characteristic low-level jet structure throughout the day, with the low-level wind maximum weakening to a minimum of 10.9 m.s−1 at 1500 LT. There is significant shear, of up to 1.5 m.s−1 per 100 m maintained through the 1000 m above the wind maximum. The diurnal cycle of the jet is associated with the nocturnal strengthening and lowering of elevated subsidence inversions, which form above the jet. Reanalysis simulations (ERA5 and MERRA2) do not capture the daytime persistence of the jet and underestimate the speed of the jet throughout the diurnal cycle. The largest absolute errors of over 4.5 m.s−1 (−35%) occur at 0900 LT. The reanalyses also fail to simulate the elevated subsidence inversions above the jet and associated dry layer in the lower troposphere.
Abstract
An idealized large eddy simulation of a tropical marine cloud population was performed. At any time, it contained hundreds of clouds, and updraft width in shallow convection emerging from a sub-cloud layer appeared to be an important indicator of whether specific convective elements deepened. In an environment with 80–90% relative humidity below the 0°C level, updrafts that penetrated the 0°C level were larger at and above cloud base, which occurred at the lifting condensation level near 600 m. Parcels rising in these updrafts appeared to emerge from boundary layer eddies that averaged ∼200 m wider than those in clouds that only reached 1.5–3 km height. The deeply ascending parcels (growers) possessed statistically similar values of effective buoyancy below the level of free convection (LFC) as parcels that began to ascend in a cloud but stopped before reaching 3000 m (non-growers). The growers also experienced less dilution above the LFC. Non-growers were characterized by negative effective buoyancy and rapid deceleration above the LFC, while growers continued to accelerate well above the LFC. Growers occurred in areas with greater magnitude of background convergence (or weaker divergence) in the sub-cloud layer, especially between 300 m and cloud base, but whether the convergence actually led to eddy widening is unclear.
Abstract
An idealized large eddy simulation of a tropical marine cloud population was performed. At any time, it contained hundreds of clouds, and updraft width in shallow convection emerging from a sub-cloud layer appeared to be an important indicator of whether specific convective elements deepened. In an environment with 80–90% relative humidity below the 0°C level, updrafts that penetrated the 0°C level were larger at and above cloud base, which occurred at the lifting condensation level near 600 m. Parcels rising in these updrafts appeared to emerge from boundary layer eddies that averaged ∼200 m wider than those in clouds that only reached 1.5–3 km height. The deeply ascending parcels (growers) possessed statistically similar values of effective buoyancy below the level of free convection (LFC) as parcels that began to ascend in a cloud but stopped before reaching 3000 m (non-growers). The growers also experienced less dilution above the LFC. Non-growers were characterized by negative effective buoyancy and rapid deceleration above the LFC, while growers continued to accelerate well above the LFC. Growers occurred in areas with greater magnitude of background convergence (or weaker divergence) in the sub-cloud layer, especially between 300 m and cloud base, but whether the convergence actually led to eddy widening is unclear.
Abstract
Supercell thunderstorms develop low-level rotation via tilting of environmental horizontal vorticity ( ω h ) by the updraft. This rotation induces dynamic lifting that can stretch near-surface vertical vorticity into a tornado. Low-level updraft rotation is generally thought to scale with 0–500 m storm-relative helicity (SRH): the combination of storm-relative flow, |SRF|, | ω h |, and cosϕ (where ϕ is the angle between SRF and ω h ). It is unclear how much influence each component of SRH has in intensifying the low-level mesocyclone. This study surveys these three components using self-organizing maps (SOMs) to distill 15 906 proximity soundings for observed right-moving supercells. Statistical analyses reveal the component most highly correlated to SRH and to streamwise vorticity (ωs ) in the observed profiles is | ω h |. Furthermore, | ω h | and |SRF| are themselves highly correlated due to their shared dependence on the hodograph length. The representative profiles produced by the SOMs were combined with a common thermodynamic profile to initialize quasi-realistic supercells in a cloud model. The simulations reveal that, across a range of real-world profiles, intense low-level mesocyclones are most closely linked to ω h and SRF, while the angle between them appears to be mostly inconsequential.
Significance Statement
About three-fourths of all tornadoes are produced by rotating thunderstorms (supercells). When the part of the storm near cloud base (approximately 1 km above the ground) rotates more strongly, the chance of a tornado dramatically increases. The goal of this study is to identify the simplest characteristic(s) of the environmental wind profile that can be used to forecast the likelihood of strong cloud-base rotation. This study concludes that the most important ingredients for storm rotation are the magnitudes of the horizontal vertical wind shear between the surface and 500 m and the storm inflow wind, irrespective of their relative directions. This finding may lead to improved operational identification of environments favoring tornado formation.
Abstract
Supercell thunderstorms develop low-level rotation via tilting of environmental horizontal vorticity ( ω h ) by the updraft. This rotation induces dynamic lifting that can stretch near-surface vertical vorticity into a tornado. Low-level updraft rotation is generally thought to scale with 0–500 m storm-relative helicity (SRH): the combination of storm-relative flow, |SRF|, | ω h |, and cosϕ (where ϕ is the angle between SRF and ω h ). It is unclear how much influence each component of SRH has in intensifying the low-level mesocyclone. This study surveys these three components using self-organizing maps (SOMs) to distill 15 906 proximity soundings for observed right-moving supercells. Statistical analyses reveal the component most highly correlated to SRH and to streamwise vorticity (ωs ) in the observed profiles is | ω h |. Furthermore, | ω h | and |SRF| are themselves highly correlated due to their shared dependence on the hodograph length. The representative profiles produced by the SOMs were combined with a common thermodynamic profile to initialize quasi-realistic supercells in a cloud model. The simulations reveal that, across a range of real-world profiles, intense low-level mesocyclones are most closely linked to ω h and SRF, while the angle between them appears to be mostly inconsequential.
Significance Statement
About three-fourths of all tornadoes are produced by rotating thunderstorms (supercells). When the part of the storm near cloud base (approximately 1 km above the ground) rotates more strongly, the chance of a tornado dramatically increases. The goal of this study is to identify the simplest characteristic(s) of the environmental wind profile that can be used to forecast the likelihood of strong cloud-base rotation. This study concludes that the most important ingredients for storm rotation are the magnitudes of the horizontal vertical wind shear between the surface and 500 m and the storm inflow wind, irrespective of their relative directions. This finding may lead to improved operational identification of environments favoring tornado formation.
Comparison of ADCP and ECCOv4r4 Currents in the Pacific Equatorial UndercurrentAbstract
The Pacific Equatorial Undercurrent (EUC) flows eastward across the Pacific at the equator in the thermocline. Its variability is related to El Niño. Moored acoustic Doppler current profiler (ADCP) measurements recorded at four widely separated sites along the equator in the EUC were compared to currents generated by version 4 release 4 of the Estimating the Circulation and Climate of the Ocean (ECCOv4r4) global model–data synthesis product. We are interested to learn how well ECCOv4r4 currents could complement sparse in situ current measurements. ADCP measurements were not assimilated in ECCOv4r4. Comparisons occurred at 5-m depth intervals at 165°E, 170°W, 140°W, and 110°W over time intervals of 10–14 years from 1995 to 2010. Hourly values of ECCOv4r4 and ADCP EUC core speeds were strongly correlated, similar for the EUC transport per unit width (TPUW). Correlations were substantially weaker at 110°W. Although we expected means and standard deviations of ECCOv4r4 currents to be smaller than ADCP values because of ECCOv4r4’s grid representation error, the large differences were unforeseen. The appearance of ECCOv4r4 diurnal-period current oscillations was surprising. As the EUC moved eastward from 170° to 140°W, the ECCOv4r4 TPUW exhibited a much smaller increase compared to the ADCP TPUW. A consequence of smaller ECCOv4r4 EUC core speeds was significantly fewer instances of gradient Richardson number (Ri) less than 1/4 above and below the depth of the core speed compared to Ri computed with ADCP observations. We present linear regression analyses to use monthly-mean ECCOv4r4 EUC core speeds and TPUWs as proxies for ADCP measurements.
Significance Statement
Hundreds of scientific papers have used ECCO data products generated with a continually evolving state-of-the-art ocean-model–data synthesis system. We ask, How representative is the latest version of ECCO equatorial ocean currents? We use independent in situ current measurements as the reference dataset to establish the accuracy of ECCO currents in the tropical Pacific. Attention is focused on the Pacific Equatorial Undercurrent (EUC) because it contributes to the formation of El Niño and La Niña events. ECCO EUC core speeds were smaller in magnitude and less variable in time compared to observations. As a consequence, ECCO currents generated smaller vertical mixing in the EUC compared to that inferred from current measurements. We developed a linear regression model to improve representation of monthly-mean ECCO currents.
Abstract
The Pacific Equatorial Undercurrent (EUC) flows eastward across the Pacific at the equator in the thermocline. Its variability is related to El Niño. Moored acoustic Doppler current profiler (ADCP) measurements recorded at four widely separated sites along the equator in the EUC were compared to currents generated by version 4 release 4 of the Estimating the Circulation and Climate of the Ocean (ECCOv4r4) global model–data synthesis product. We are interested to learn how well ECCOv4r4 currents could complement sparse in situ current measurements. ADCP measurements were not assimilated in ECCOv4r4. Comparisons occurred at 5-m depth intervals at 165°E, 170°W, 140°W, and 110°W over time intervals of 10–14 years from 1995 to 2010. Hourly values of ECCOv4r4 and ADCP EUC core speeds were strongly correlated, similar for the EUC transport per unit width (TPUW). Correlations were substantially weaker at 110°W. Although we expected means and standard deviations of ECCOv4r4 currents to be smaller than ADCP values because of ECCOv4r4’s grid representation error, the large differences were unforeseen. The appearance of ECCOv4r4 diurnal-period current oscillations was surprising. As the EUC moved eastward from 170° to 140°W, the ECCOv4r4 TPUW exhibited a much smaller increase compared to the ADCP TPUW. A consequence of smaller ECCOv4r4 EUC core speeds was significantly fewer instances of gradient Richardson number (Ri) less than 1/4 above and below the depth of the core speed compared to Ri computed with ADCP observations. We present linear regression analyses to use monthly-mean ECCOv4r4 EUC core speeds and TPUWs as proxies for ADCP measurements.
Significance Statement
Hundreds of scientific papers have used ECCO data products generated with a continually evolving state-of-the-art ocean-model–data synthesis system. We ask, How representative is the latest version of ECCO equatorial ocean currents? We use independent in situ current measurements as the reference dataset to establish the accuracy of ECCO currents in the tropical Pacific. Attention is focused on the Pacific Equatorial Undercurrent (EUC) because it contributes to the formation of El Niño and La Niña events. ECCO EUC core speeds were smaller in magnitude and less variable in time compared to observations. As a consequence, ECCO currents generated smaller vertical mixing in the EUC compared to that inferred from current measurements. We developed a linear regression model to improve representation of monthly-mean ECCO currents.
Abstract
This study investigates the connection between the arrival of dry stratospheric air with the Soberanes Fire (2016). The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) and Goddard Earth Observing System Forward Processing model (GEOS-FP) are used for back-trajectories and offshore deep stratospheric intrusion (SI) in conjunction with the ignition and outbreak of the fire. The back-trajectory analysis indicates most air reaching the vertical column was critically dry, exhibiting relative humidity values below 10%. As the fire ignited, dry air arrived from due west at heights of 1-3 km about 24 hours prior. During the overnight fire growth, dry air arrived from the northwest to north-northwest at heights of 3.5-5.5 km 48-72 hours prior. The synoptic and the GEOS-FP analysis demonstrate offshore mid-to-low stratospheric intrusion. On July 21, 2016, an enclosed upper-level low approached the California/Oregon border along the northwesterly subtropical jet stream hours before the fire outbreak. The GEOS-FP results of potential vorticity, specific humidity, and ozone along the back-trajectories to the west and northwest of the fire suggest a stratospheric intrusion event into the mid-to-low troposphere at the back-trajectory start points, and vertical velocity indicates sinking motion. The specific humidity analyzed at the arrival time shows the transport of the abnormally dry air to the Soberanes Fire. Results suggest a connection between dry stratospheric air transported to the Soberanes Fire at ignition and overnight accelerated growth, supported by a dark bank in satellite water vapor imagery. The prediction of low-level transport of dry stratospheric air to the coastal communities could help predict the occurrence of wildfire outbreaks, or periods of accelerated fire growth.
Abstract
This study investigates the connection between the arrival of dry stratospheric air with the Soberanes Fire (2016). The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) and Goddard Earth Observing System Forward Processing model (GEOS-FP) are used for back-trajectories and offshore deep stratospheric intrusion (SI) in conjunction with the ignition and outbreak of the fire. The back-trajectory analysis indicates most air reaching the vertical column was critically dry, exhibiting relative humidity values below 10%. As the fire ignited, dry air arrived from due west at heights of 1-3 km about 24 hours prior. During the overnight fire growth, dry air arrived from the northwest to north-northwest at heights of 3.5-5.5 km 48-72 hours prior. The synoptic and the GEOS-FP analysis demonstrate offshore mid-to-low stratospheric intrusion. On July 21, 2016, an enclosed upper-level low approached the California/Oregon border along the northwesterly subtropical jet stream hours before the fire outbreak. The GEOS-FP results of potential vorticity, specific humidity, and ozone along the back-trajectories to the west and northwest of the fire suggest a stratospheric intrusion event into the mid-to-low troposphere at the back-trajectory start points, and vertical velocity indicates sinking motion. The specific humidity analyzed at the arrival time shows the transport of the abnormally dry air to the Soberanes Fire. Results suggest a connection between dry stratospheric air transported to the Soberanes Fire at ignition and overnight accelerated growth, supported by a dark bank in satellite water vapor imagery. The prediction of low-level transport of dry stratospheric air to the coastal communities could help predict the occurrence of wildfire outbreaks, or periods of accelerated fire growth.
Abstract
The influence of meridional shift of the oceanic subtropical front (STF) on the Agulhas Current (AC) regime shifts is studied using satellite altimeter data and a 1.5-layer ocean model. The satellite observations suggest the northward shift of the STF leads to the AC leaping across the gap with little Agulhas leakage, and the southward shift of the STF mainly results in the AC intruding into the Atlantic Ocean in the forms of a loop current and an eddy-shedding path, while there are three flow patterns of AC for moderate latitude of the STF. The ocean model results suggest no hysteresis (associated with multiple equilibrium states) exists in the AC system. The model reproduces similar AC regimes depending on different gap widths as in the observations, and model results can be used to explain the observed Agulhas leakage well. We also present the parameter space of the critical AC strength that results in different AC flow patterns as a function of the gap width. The vorticity dynamics of the AC regime shift suggests that the β term is mainly balanced by the viscosity term for the AC in the leaping and loop current paths, while the β and instantaneous vorticity terms are mainly balanced by the advection and viscosity terms for the AC in the eddy-shedding path. These findings help explain the dynamics of the AC flowing across the gateway beyond the tip of Africa affected by the north–south shift of the STF in the leaping regime or penetrating regime.
Abstract
The influence of meridional shift of the oceanic subtropical front (STF) on the Agulhas Current (AC) regime shifts is studied using satellite altimeter data and a 1.5-layer ocean model. The satellite observations suggest the northward shift of the STF leads to the AC leaping across the gap with little Agulhas leakage, and the southward shift of the STF mainly results in the AC intruding into the Atlantic Ocean in the forms of a loop current and an eddy-shedding path, while there are three flow patterns of AC for moderate latitude of the STF. The ocean model results suggest no hysteresis (associated with multiple equilibrium states) exists in the AC system. The model reproduces similar AC regimes depending on different gap widths as in the observations, and model results can be used to explain the observed Agulhas leakage well. We also present the parameter space of the critical AC strength that results in different AC flow patterns as a function of the gap width. The vorticity dynamics of the AC regime shift suggests that the β term is mainly balanced by the viscosity term for the AC in the leaping and loop current paths, while the β and instantaneous vorticity terms are mainly balanced by the advection and viscosity terms for the AC in the eddy-shedding path. These findings help explain the dynamics of the AC flowing across the gateway beyond the tip of Africa affected by the north–south shift of the STF in the leaping regime or penetrating regime.
Abstract
Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity and microscale thermal-variance dissipation rate up- and downstream of the ∼ 10-km wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates ε ∼ О(10−7 – 10−6 W kg−1) and diapycnal diffusivities K ∼ О(10−2 m2 s−1) above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence which is expected to decay in less than 100mbased on turbulence decay time of N −1 ∼ 100 s and the 0.5m s−1 Kuroshio flowspeed. Thus, the turbulentwake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortexwake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy and (iii) reduced-shear, are tested. Average ε vertical profiles are well-reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent of ε and spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.
Abstract
Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity and microscale thermal-variance dissipation rate up- and downstream of the ∼ 10-km wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates ε ∼ О(10−7 – 10−6 W kg−1) and diapycnal diffusivities K ∼ О(10−2 m2 s−1) above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence which is expected to decay in less than 100mbased on turbulence decay time of N −1 ∼ 100 s and the 0.5m s−1 Kuroshio flowspeed. Thus, the turbulentwake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortexwake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy and (iii) reduced-shear, are tested. Average ε vertical profiles are well-reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent of ε and spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.
