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Abstract
An earlier observation of about 50 cm2 s−2 for the maximum abyssal (∼4000 m depth) eddy kinetic energy (KE ) in the western North Pacific along 152°E, occurring in the vicinity of the Kuroshio Extension, is now supported by nearly two years of observation with moored instruments deployed from 28 to 41°N. The maximum occurred at the same site each year (nominal, from one array setting to the next), near 35°N, and the composite or two-year average is 45 cm2 s−2. At analogous longitudes in the western North Atlantic, one finds abyssal kinetic energies as high as 100–150 cm2 s−2 in the vicinity of the Gulf Stream. However, the abyssal western North Pacific could be more energetic at longitudes other than 152°E. Eddy intensities at abyssal depths near the Gulf Stream do drop off to smaller values approaching either Cape Hatteras or longitudes in the vicinity of the Grand Banks of Newfoundland.
Abyssal KE values at specific sites along 152°E were observed to vary by as low as a few percent and as high as a factor of 2 from year to year. A prominent secondary maximum in KE , primarily due to zonal variability, was observed to exist north of the Kuroshio Extension for one year, but not the next. However, the maximum KE at 4000 m depth, its location, and the general shape of the latitudinal distribution of eddy intensity are relatively stable. Averages over all moorings where these data are available were 22.5 and 18 cm2 s−2 for the two respective deployments. The mesoscale contribution (periods of 30 to 150 days) to abyssal KE along 152°E was typically 50–60%.
Abstract
An earlier observation of about 50 cm2 s−2 for the maximum abyssal (∼4000 m depth) eddy kinetic energy (KE ) in the western North Pacific along 152°E, occurring in the vicinity of the Kuroshio Extension, is now supported by nearly two years of observation with moored instruments deployed from 28 to 41°N. The maximum occurred at the same site each year (nominal, from one array setting to the next), near 35°N, and the composite or two-year average is 45 cm2 s−2. At analogous longitudes in the western North Atlantic, one finds abyssal kinetic energies as high as 100–150 cm2 s−2 in the vicinity of the Gulf Stream. However, the abyssal western North Pacific could be more energetic at longitudes other than 152°E. Eddy intensities at abyssal depths near the Gulf Stream do drop off to smaller values approaching either Cape Hatteras or longitudes in the vicinity of the Grand Banks of Newfoundland.
Abyssal KE values at specific sites along 152°E were observed to vary by as low as a few percent and as high as a factor of 2 from year to year. A prominent secondary maximum in KE , primarily due to zonal variability, was observed to exist north of the Kuroshio Extension for one year, but not the next. However, the maximum KE at 4000 m depth, its location, and the general shape of the latitudinal distribution of eddy intensity are relatively stable. Averages over all moorings where these data are available were 22.5 and 18 cm2 s−2 for the two respective deployments. The mesoscale contribution (periods of 30 to 150 days) to abyssal KE along 152°E was typically 50–60%.
Abstract
Latitudinal distributions of estimates of eddy kinetic energy and off-diagonal component of the horizontal Reynolds stresses are demonstrated to have similar shapes at corresponding (generally western) locations, but at much different depths, in the mid-latitude North Atlantic and North Pacific Oceans. This comparison is based on direct current measurements at abyssal depths in the North Atlantic along 55°W, and on indirect estimates of upper-level velocity differences in the vertical for the North Pacific between the Izu (or Izu-Ogasawara or South Honshu) Ridge and the Shatsky Rise. These sections span the more energetic segments of the Kuroshio Extension and Gulf Stream systems.
Abstract
Latitudinal distributions of estimates of eddy kinetic energy and off-diagonal component of the horizontal Reynolds stresses are demonstrated to have similar shapes at corresponding (generally western) locations, but at much different depths, in the mid-latitude North Atlantic and North Pacific Oceans. This comparison is based on direct current measurements at abyssal depths in the North Atlantic along 55°W, and on indirect estimates of upper-level velocity differences in the vertical for the North Pacific between the Izu (or Izu-Ogasawara or South Honshu) Ridge and the Shatsky Rise. These sections span the more energetic segments of the Kuroshio Extension and Gulf Stream systems.
Abstract
Fourteen moorings were deployed across the midlatitude North Pacific 165°E to 152°W, for approximately 2 years during 1983–85. Ten mooring sites had previously been occupied at similar latitudes (30°–40°N nominal) for roughly two years (1980–82) along 152°E. Taken together, these observations form the basis for the first systematic basinwide zonal exploration of the eddy field based on moored instrument techniques in the midlatitude North Pacific along the Kuroshio Extension System and North Pacific drift. Eddy kinetic energy (KE ) at abyssal depths decays sharply moving east from 152°E, and has decreased by a factor of 4 by 165°E. There is a plateau in abyssal KE of about 10 cm2 s−2 across the Emperor Seamounts from 165° to 175°E. Abyssal KE drops to roughly 5 cm2 s−2 at 175°W and 1 cm2 s−2 at 152°W, for a total decay of a factor of about 50 across the midlatitude North Pacific. Upper level KE decreases by a total of roughly two orders of magnitude (approximately 103 to 101) from 152°E to 152°W.
The most energetic sites at 152° and 165°E have essentially the same vertical structure (shape), with the deep and near surface amplitudes at 152°E being 4 and 3 times higher, respectively. In fact, the same type of vertical profile for KE is appropriate as a first approximation across the entire midlatitude North Pacific, with amplitudes generally decreasing eastward and away from the Kuroshio Extension. Distributions of KE with frequency are typically peaked somewhat at the mesoscale near the Kuroshio Extension, and generally become more “red” proceeding east and/or toward lower energy areas, although examples of essentially every type of partitioning are available. The KE values at 165°E are generally the most stable from year-to-year that have ever been measured in energetic regions of the open ocean, at all depths.
Abstract
Fourteen moorings were deployed across the midlatitude North Pacific 165°E to 152°W, for approximately 2 years during 1983–85. Ten mooring sites had previously been occupied at similar latitudes (30°–40°N nominal) for roughly two years (1980–82) along 152°E. Taken together, these observations form the basis for the first systematic basinwide zonal exploration of the eddy field based on moored instrument techniques in the midlatitude North Pacific along the Kuroshio Extension System and North Pacific drift. Eddy kinetic energy (KE ) at abyssal depths decays sharply moving east from 152°E, and has decreased by a factor of 4 by 165°E. There is a plateau in abyssal KE of about 10 cm2 s−2 across the Emperor Seamounts from 165° to 175°E. Abyssal KE drops to roughly 5 cm2 s−2 at 175°W and 1 cm2 s−2 at 152°W, for a total decay of a factor of about 50 across the midlatitude North Pacific. Upper level KE decreases by a total of roughly two orders of magnitude (approximately 103 to 101) from 152°E to 152°W.
The most energetic sites at 152° and 165°E have essentially the same vertical structure (shape), with the deep and near surface amplitudes at 152°E being 4 and 3 times higher, respectively. In fact, the same type of vertical profile for KE is appropriate as a first approximation across the entire midlatitude North Pacific, with amplitudes generally decreasing eastward and away from the Kuroshio Extension. Distributions of KE with frequency are typically peaked somewhat at the mesoscale near the Kuroshio Extension, and generally become more “red” proceeding east and/or toward lower energy areas, although examples of essentially every type of partitioning are available. The KE values at 165°E are generally the most stable from year-to-year that have ever been measured in energetic regions of the open ocean, at all depths.
Abstract
Unexpectedly large and stable abyssal mean flow have recently been observed along 165°E, between 31° and 41°N. These results are based on two deployments of a moored array for approximately a year each. Time-averaged currents at 4000 m are about 5 ± 0.5 cm s−1 to the southwest near 41°N and 6 ± 0.9 cm s−1 to the northwest near 33°N, on opposite of the Kuroshio Extension. These mean flows were reproduced between array deployments to within a fraction of a cm s−1 and few degrees (True). At 41°N the abyssal mean kinetic energy is several times larger than eddy kinetic energy so that the flow visually does not reverse. At 33°N, near the southern edge of the Kuroshio Extension, the abyssal mean and eddy fields are of roughly the same amplitude. These result along 165°E are in contrast to similar observations along 152°E, where the abyssal zonal mean flows are notably less stable than at 165°E. However, estimates of the latitudinal and two-years averaged zonal flow components are approximately the same at both longitudes, typically in the range of 1.4 to 1.9 cm s−1 to the west. The new currents described here have not be observed previously and are not a feature of any proposed circulation scheme with which the author is familiar.
Abstract
Unexpectedly large and stable abyssal mean flow have recently been observed along 165°E, between 31° and 41°N. These results are based on two deployments of a moored array for approximately a year each. Time-averaged currents at 4000 m are about 5 ± 0.5 cm s−1 to the southwest near 41°N and 6 ± 0.9 cm s−1 to the northwest near 33°N, on opposite of the Kuroshio Extension. These mean flows were reproduced between array deployments to within a fraction of a cm s−1 and few degrees (True). At 41°N the abyssal mean kinetic energy is several times larger than eddy kinetic energy so that the flow visually does not reverse. At 33°N, near the southern edge of the Kuroshio Extension, the abyssal mean and eddy fields are of roughly the same amplitude. These result along 165°E are in contrast to similar observations along 152°E, where the abyssal zonal mean flows are notably less stable than at 165°E. However, estimates of the latitudinal and two-years averaged zonal flow components are approximately the same at both longitudes, typically in the range of 1.4 to 1.9 cm s−1 to the west. The new currents described here have not be observed previously and are not a feature of any proposed circulation scheme with which the author is familiar.
Abstract
All available observations indicate that the most energetic time-dependent currents are located in the vicinity of intense large-scale oceanic current systems. This characteristic is also a basic property of eddy-resolving gyre-scale numerical models. An initial detailed intercomparison of two-layer eddy-resolving numerical experiments with observation focused on the largest scales of horizontal structure in patterns of abyssal eddy kinetic energy, and on time scales. The numerical experiments examined generally had relevant temporal and meridional scales, but not necessarily realistic zonal scales. The model eddy field did not penetrate as far from the western boundary as observed distributions, by a factor of 2 to 3.
The present study examines the physical processes that govern the model zonal penetration scale and suggests reasons for the previous discrepancy. It is demonstrated that a subtle balance exists between the complex instability processes that tend to tear the jet apart (restricting its zonal penetration) and the tendency for inertial processes to carry the intense current right across the basin. It would seem that any factor that changes the nature of the instability of the thin Gulf Stream jet will alter the penetration scale. In these models this means not only changing physical parameters and including different physics, but also changing such model dependent factors as vertical resolution. Earlier work suggested the need for enhanced vertical resolution to give realistic zonal penetration, but it is now clear that all stabilizing/destabilizing effects conspire together to give a particular penetration scale.
Abstract
All available observations indicate that the most energetic time-dependent currents are located in the vicinity of intense large-scale oceanic current systems. This characteristic is also a basic property of eddy-resolving gyre-scale numerical models. An initial detailed intercomparison of two-layer eddy-resolving numerical experiments with observation focused on the largest scales of horizontal structure in patterns of abyssal eddy kinetic energy, and on time scales. The numerical experiments examined generally had relevant temporal and meridional scales, but not necessarily realistic zonal scales. The model eddy field did not penetrate as far from the western boundary as observed distributions, by a factor of 2 to 3.
The present study examines the physical processes that govern the model zonal penetration scale and suggests reasons for the previous discrepancy. It is demonstrated that a subtle balance exists between the complex instability processes that tend to tear the jet apart (restricting its zonal penetration) and the tendency for inertial processes to carry the intense current right across the basin. It would seem that any factor that changes the nature of the instability of the thin Gulf Stream jet will alter the penetration scale. In these models this means not only changing physical parameters and including different physics, but also changing such model dependent factors as vertical resolution. Earlier work suggested the need for enhanced vertical resolution to give realistic zonal penetration, but it is now clear that all stabilizing/destabilizing effects conspire together to give a particular penetration scale.
Abstract
The meridional structure of the zonal flow in the Kuroshio Extension is investigated using a combination of data from hydrographic sections and moored current meter arrays. We emphasize 165°E, between 30° and 42°N, where high quality and very stable current measurements at 150 and 4000 m extend over a two-year period from October 1983 to October 1985. Hydrographic (CTD/O2) sections were occupied during the initial deployment and a second time when the array of six moorings was reset in 1984. The deep currents were extremely reproducible from one year to the next and revealed a pattern of weak eastward flow at 4000 m under the axis of the Kuroshio with strong westward flow on either flank. When combined with the hydrographic data, the total transport of the eastward flowing Kuroshio Extension was estimated to be 57.0 ± 3.7 Sv (Sv = 106 m3 s−1), essentially the same as when referenced to the broom (57.0 ± 2.0 Sv). South of 34°N, the velocities were westward at all levels, with a net transport of −85.1 Sv; north of 37°N the flow in the upper kilometer was eastward (22 Sv) near the axis of the Oyashio, or subarctic front, and westward elsewhere, yielding a net transport of −34.6 Sv. The net transport across the entire section from 30° to 42°N was westward and equal to −62.7 ± 12.3 Sv.
New methods of estimating transport when combining direct current and hydrographic data are illustrated where compatibility with dynamic height estimates is required. Observations of dynamic height variability across the 165°E array using the current meters suggested that the mean currents at 4000 m were consistent with the dynamic height range observed hydrograpically. However, the yearly averaged velocities al 150 m under-sampled the eastward upper level flow. Results are also compared to previously published work at 152°E and with the new data at 175°W. At 152°E, previous estimates of zonal transport over a similar latitude range yield −31 ± 16 Sv when current meter and hydrographic date were combined; our study suggests −31 ± 31 Sv. The section-averaged zonal transport changes sign across the Emperor Seamounts, becoming positive at 175°W, where the hydrographic and yearly averaged array data are totally consistent.
Abstract
The meridional structure of the zonal flow in the Kuroshio Extension is investigated using a combination of data from hydrographic sections and moored current meter arrays. We emphasize 165°E, between 30° and 42°N, where high quality and very stable current measurements at 150 and 4000 m extend over a two-year period from October 1983 to October 1985. Hydrographic (CTD/O2) sections were occupied during the initial deployment and a second time when the array of six moorings was reset in 1984. The deep currents were extremely reproducible from one year to the next and revealed a pattern of weak eastward flow at 4000 m under the axis of the Kuroshio with strong westward flow on either flank. When combined with the hydrographic data, the total transport of the eastward flowing Kuroshio Extension was estimated to be 57.0 ± 3.7 Sv (Sv = 106 m3 s−1), essentially the same as when referenced to the broom (57.0 ± 2.0 Sv). South of 34°N, the velocities were westward at all levels, with a net transport of −85.1 Sv; north of 37°N the flow in the upper kilometer was eastward (22 Sv) near the axis of the Oyashio, or subarctic front, and westward elsewhere, yielding a net transport of −34.6 Sv. The net transport across the entire section from 30° to 42°N was westward and equal to −62.7 ± 12.3 Sv.
New methods of estimating transport when combining direct current and hydrographic data are illustrated where compatibility with dynamic height estimates is required. Observations of dynamic height variability across the 165°E array using the current meters suggested that the mean currents at 4000 m were consistent with the dynamic height range observed hydrograpically. However, the yearly averaged velocities al 150 m under-sampled the eastward upper level flow. Results are also compared to previously published work at 152°E and with the new data at 175°W. At 152°E, previous estimates of zonal transport over a similar latitude range yield −31 ± 16 Sv when current meter and hydrographic date were combined; our study suggests −31 ± 31 Sv. The section-averaged zonal transport changes sign across the Emperor Seamounts, becoming positive at 175°W, where the hydrographic and yearly averaged array data are totally consistent.
Abstract
An adiabatic, primitive equation, eddy-resolving circulation model has been applied to the Gulf Stream System from Cape Hatteras to east of the Grand Banks (30°–48°N, 78°–45°W). A two-layer version of the model was driven both by direct wind forcing and by transport prescribed at inflow ports south of Cape Hatteras for the Gulf Stream and near the Grand Banks of Newfoundland for the deep western boundary current. The mean upper-layer thickness was sufficiently large for interface outcropping not to occur. Numerical experiments previously run at 0.2° horizontal resolution (∼20 km) had some realistic features, but a key unresolved deficiency was that the highest eddy kinetic energies obtained near the Gulf Stream were too low relative to data by a factor of about 2, with inadequate eastward penetration.
A unique set of new numerical experiments has extended previous results to higher horizontal resolution, all other conditions being held fixed. At 0.1° horizontal resolution, eddy kinetic energies in the vicinity of the Gulf Stream realistically increase by a factor of roughly 2 relative to 0.2°. The increase in eddy activity is a result of enhanced energy conversion from mean flow to fluctuations due to barotropic and baroclinic instabilities, with the nature of the instability mixture as well as eddy energy changing with increased resolution. One experiment at 0.05° horizontal resolution (∼5 km) yielded kinetic energies and key energy transfer terms that are within 10% of the equivalent 0.1° case, suggesting that convergence of the numerical solutions has nearly been reached.
Abstract
An adiabatic, primitive equation, eddy-resolving circulation model has been applied to the Gulf Stream System from Cape Hatteras to east of the Grand Banks (30°–48°N, 78°–45°W). A two-layer version of the model was driven both by direct wind forcing and by transport prescribed at inflow ports south of Cape Hatteras for the Gulf Stream and near the Grand Banks of Newfoundland for the deep western boundary current. The mean upper-layer thickness was sufficiently large for interface outcropping not to occur. Numerical experiments previously run at 0.2° horizontal resolution (∼20 km) had some realistic features, but a key unresolved deficiency was that the highest eddy kinetic energies obtained near the Gulf Stream were too low relative to data by a factor of about 2, with inadequate eastward penetration.
A unique set of new numerical experiments has extended previous results to higher horizontal resolution, all other conditions being held fixed. At 0.1° horizontal resolution, eddy kinetic energies in the vicinity of the Gulf Stream realistically increase by a factor of roughly 2 relative to 0.2°. The increase in eddy activity is a result of enhanced energy conversion from mean flow to fluctuations due to barotropic and baroclinic instabilities, with the nature of the instability mixture as well as eddy energy changing with increased resolution. One experiment at 0.05° horizontal resolution (∼5 km) yielded kinetic energies and key energy transfer terms that are within 10% of the equivalent 0.1° case, suggesting that convergence of the numerical solutions has nearly been reached.
Abstract
A primitive-equation, n-layer, eddy-resolving creation model has been applied to the Gulf Stream System from Cape Hatteras to cast of the Grand Banks (78°–45°W, 30°–48°N). Within the limitations of the model, realistic coastlines, bottom topography, and forcing functions have been used. A two-layer version of the model was driven by observed mean climatological wind forcing and mass transport prescribed at inflow. Outflow was determined by a radiation boundary condition and an integral constraint on the mass field in each layer. Specification of a Deep Western Boundary Current (DWBC) was included in some model runs.
Six numerical experiments, from a series of over fifty integrated to statistical equilibrium, were selected for detailed description and intercomparison with observations. The basic case consisted of a flat bottom regime driven by wind forcing only. Realistic inflow transport in the upper layer was then prescribed and two different outflow specifications at the eastern boundary were studied in experiments 2 and 3. Three additional experiments involved (4) adding bottom topography (including the New England Seamount Chain), (5) adding a DWBC to experiment 4 with 20 Sv (Sv ≡ 106 m3 s−1) total transports and (6) increasing the DWBC, to 40 Sv. A brief discussion of the influence of parameter variations includes modifications of dissipation (lateral eddy diffusion and bottom friction) and stratification.
Results from the sequence of experiments suggest an important role for the DWBC in determining the mean path of the Gulf Stream and consequently the distribution of eddy kinetic energy, and the character of the deep mean flow. The most realistic experiment compares to within a factor of two or better with observations of the amplitude of eddy kinetic energy and rms fluctuations of the thermocline and sea surface height. Abyssal eddy kinetic energy was smaller than observed. The mean flow is characterized by recirculations to the north and south of the Gulf Stream and a deep cyclonic gyre just east of the northern portion of the New England Seamount Chain, as found in the data.
Abstract
A primitive-equation, n-layer, eddy-resolving creation model has been applied to the Gulf Stream System from Cape Hatteras to cast of the Grand Banks (78°–45°W, 30°–48°N). Within the limitations of the model, realistic coastlines, bottom topography, and forcing functions have been used. A two-layer version of the model was driven by observed mean climatological wind forcing and mass transport prescribed at inflow. Outflow was determined by a radiation boundary condition and an integral constraint on the mass field in each layer. Specification of a Deep Western Boundary Current (DWBC) was included in some model runs.
Six numerical experiments, from a series of over fifty integrated to statistical equilibrium, were selected for detailed description and intercomparison with observations. The basic case consisted of a flat bottom regime driven by wind forcing only. Realistic inflow transport in the upper layer was then prescribed and two different outflow specifications at the eastern boundary were studied in experiments 2 and 3. Three additional experiments involved (4) adding bottom topography (including the New England Seamount Chain), (5) adding a DWBC to experiment 4 with 20 Sv (Sv ≡ 106 m3 s−1) total transports and (6) increasing the DWBC, to 40 Sv. A brief discussion of the influence of parameter variations includes modifications of dissipation (lateral eddy diffusion and bottom friction) and stratification.
Results from the sequence of experiments suggest an important role for the DWBC in determining the mean path of the Gulf Stream and consequently the distribution of eddy kinetic energy, and the character of the deep mean flow. The most realistic experiment compares to within a factor of two or better with observations of the amplitude of eddy kinetic energy and rms fluctuations of the thermocline and sea surface height. Abyssal eddy kinetic energy was smaller than observed. The mean flow is characterized by recirculations to the north and south of the Gulf Stream and a deep cyclonic gyre just east of the northern portion of the New England Seamount Chain, as found in the data.
Abstract
Since 1988, what appears to be an abnormal number of maximum temperature records has been set at the National Weather Service Office in Tucson, Arizona (TUS). We present several analyses that indicate that the current measurement system at TUS is indicating daytime temperatures that are 2 to 3 degrees too high. It appears that the instrument is not appropriately aspirated so that, during the day, temperature readings are significantly warmer than ambient air temperatures, while at night they are slightly cooler. The system at TUS is similar to one that has been installed at many National Weather Service sites around the country. We speculate on the impact this system may have on the climate record if the errors noted at Tucson are similar at the other sites.
Abstract
Since 1988, what appears to be an abnormal number of maximum temperature records has been set at the National Weather Service Office in Tucson, Arizona (TUS). We present several analyses that indicate that the current measurement system at TUS is indicating daytime temperatures that are 2 to 3 degrees too high. It appears that the instrument is not appropriately aspirated so that, during the day, temperature readings are significantly warmer than ambient air temperatures, while at night they are slightly cooler. The system at TUS is similar to one that has been installed at many National Weather Service sites around the country. We speculate on the impact this system may have on the climate record if the errors noted at Tucson are similar at the other sites.
Abstract
It is demonstrated that the outcome of an intercomparison between data and the vertical distribution of eddy kinetic energy predicted by a previously developed numerical model of the MODE area is frequency dependent. In the range of periods from 50 to 150 or even to 400 days (one definition of the temporal mesoscale, the scale that the model was designed to simulate), the comparison is quite good. For periods in the range of 5 to 50 days, the agreement is poor. For periods longer than 400 days, the comparison is indeterminate. Earlier conclusions concerning the relation of model results to the MODE data should be qualified by stipulating frequency range, and future intercomparisons for any model in all regions should be conscious of the desirability of doing so across common frequencies.
Abstract
It is demonstrated that the outcome of an intercomparison between data and the vertical distribution of eddy kinetic energy predicted by a previously developed numerical model of the MODE area is frequency dependent. In the range of periods from 50 to 150 or even to 400 days (one definition of the temporal mesoscale, the scale that the model was designed to simulate), the comparison is quite good. For periods in the range of 5 to 50 days, the agreement is poor. For periods longer than 400 days, the comparison is indeterminate. Earlier conclusions concerning the relation of model results to the MODE data should be qualified by stipulating frequency range, and future intercomparisons for any model in all regions should be conscious of the desirability of doing so across common frequencies.
