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- Author or Editor: Christopher N. K. Mooers x
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Abstract
Kinematic aspects of winter cold fronts in Florida are examined for a subsynoptic scale representing the frontal wind shift at the earth's surface. Fifteen selected cold fronts are studied by using time series of surface wind observations for Miami, Fla. With the aid of frontal movements estimated from six-hourly synoptic charts, the spatial wind distribution near the frontal wind shift is obtained by transforming time series observations into space series. The study of steady wind regimes ahead and behind the front indicates that the major wind change across the front occurs only in the v-component. As measured along a direction perpendicular to the front (136°T), this major change occurs over a distance of about 140 km, corresponding to a time scale of about 4.5 h. Divergence and vorticity estimates are based upon the subsynoptic wind distribution. These estimates are similar to those found previously for non-frontal subsynoptic studies on moist convection in Florida. It is found that our estimates can be roughly approximated by the spatial derivative of the v-component.
Abstract
Kinematic aspects of winter cold fronts in Florida are examined for a subsynoptic scale representing the frontal wind shift at the earth's surface. Fifteen selected cold fronts are studied by using time series of surface wind observations for Miami, Fla. With the aid of frontal movements estimated from six-hourly synoptic charts, the spatial wind distribution near the frontal wind shift is obtained by transforming time series observations into space series. The study of steady wind regimes ahead and behind the front indicates that the major wind change across the front occurs only in the v-component. As measured along a direction perpendicular to the front (136°T), this major change occurs over a distance of about 140 km, corresponding to a time scale of about 4.5 h. Divergence and vorticity estimates are based upon the subsynoptic wind distribution. These estimates are similar to those found previously for non-frontal subsynoptic studies on moist convection in Florida. It is found that our estimates can be roughly approximated by the spatial derivative of the v-component.
Abstract
The dispersion characteristics of stable, discrete, barotropic, continental shelf wave (CSW) modes propagating in a barotropic boundary current are strongly modified by the dynamical effects of nonuniform horizontal shear. For example, the CSW's propagate cum sole with no mean current, but their direction of propagation can be reversed by an opposing uniform mean current. In contrast, an opposing sheared mean current increases the tendency for cum sole propagation relative to an opposing uniform mean current, and produces a high-wavenumber cutoff, at least for modes higher than the first. If the sheared mean flow vanishes somewhere, the discrete CSW modes all propagate cum sole once again. For the mean current profiles considered, the high-frequency cutoff is lowered in the nonuniform shear case compared to the zero current case.
In a simple geometry motivated by the Florida Current and Florida Straits, southward CSW propagation can occur, in opposition to the Current, primarily because the cyclonic shear of the Current is similar in magnitude to the local Coriolis parameter. The short-period cutoff (zero group speed) for the first mode CSW is about 12 days; this CSW has a wavelength of about 190 km, corresponding to a southward phase speed of about 17 cm s−1. Within the limitations of the model, the results indicate that the Florida Straits–Florida Current system can accumulate energy at time scales of 10–14 days, corresponding to those of atmospheric cold front forcing.
Abstract
The dispersion characteristics of stable, discrete, barotropic, continental shelf wave (CSW) modes propagating in a barotropic boundary current are strongly modified by the dynamical effects of nonuniform horizontal shear. For example, the CSW's propagate cum sole with no mean current, but their direction of propagation can be reversed by an opposing uniform mean current. In contrast, an opposing sheared mean current increases the tendency for cum sole propagation relative to an opposing uniform mean current, and produces a high-wavenumber cutoff, at least for modes higher than the first. If the sheared mean flow vanishes somewhere, the discrete CSW modes all propagate cum sole once again. For the mean current profiles considered, the high-frequency cutoff is lowered in the nonuniform shear case compared to the zero current case.
In a simple geometry motivated by the Florida Current and Florida Straits, southward CSW propagation can occur, in opposition to the Current, primarily because the cyclonic shear of the Current is similar in magnitude to the local Coriolis parameter. The short-period cutoff (zero group speed) for the first mode CSW is about 12 days; this CSW has a wavelength of about 190 km, corresponding to a southward phase speed of about 17 cm s−1. Within the limitations of the model, the results indicate that the Florida Straits–Florida Current system can accumulate energy at time scales of 10–14 days, corresponding to those of atmospheric cold front forcing.
Abstract
Evidence for long coastal-trapped waves off the west coast of the United States is obtained from sea level, surface atmospheric pressure and wind records over a 1500 km alongshore separation for two months in the summer of 1973. Corresponding evidence is obtained from current measurements off the northern Oregon coast.
The dominant low-frequency motion occurred at a period of 10 days. Consistent with the theory of coastal-trapped waves, the observations indicate that 1) the alongshore current fluctuations were mainly barotropic, coastally trapped, and were related geostrophically to the adjusted sea level fluctuations; and 2) the adjusted sea level fluctuations propagated northward with a phase speed which depended upon the local shelf geometry.
The 10-day fluctuations in the coastal ocean were driven by the northward traveling, large-scale winds; the response was nonlocal. There were also 4-day fluctuations for which the coastal water response was essentially local. The difference in response characteristics for the two periods can be explained with simple forced wave theory.
Abstract
Evidence for long coastal-trapped waves off the west coast of the United States is obtained from sea level, surface atmospheric pressure and wind records over a 1500 km alongshore separation for two months in the summer of 1973. Corresponding evidence is obtained from current measurements off the northern Oregon coast.
The dominant low-frequency motion occurred at a period of 10 days. Consistent with the theory of coastal-trapped waves, the observations indicate that 1) the alongshore current fluctuations were mainly barotropic, coastally trapped, and were related geostrophically to the adjusted sea level fluctuations; and 2) the adjusted sea level fluctuations propagated northward with a phase speed which depended upon the local shelf geometry.
The 10-day fluctuations in the coastal ocean were driven by the northward traveling, large-scale winds; the response was nonlocal. There were also 4-day fluctuations for which the coastal water response was essentially local. The difference in response characteristics for the two periods can be explained with simple forced wave theory.
Abstract
Interactions between the circulation of Prince William Sound (PWS), Alaska, and that of the continental shelf region of the northern Gulf of Alaska are studied numerically. The focus is on the flow structure at Hinchinbrook Entrance (HE) and Montague Strait (MS) and the associated PWS interior circulation under various initial state and forcing configurations. Bottom topography inhibits flow into PWS under barotropic conditions, while stratification very much facilitates flow into PWS. Layered flow develops at HE and MS under baroclinic conditions, with two (four) layers under March (September) stratification conditions. In both baroclinic cases, the top layer transports inflow and a cyclonic eddy develops over the central basin of PWS. Westward wind (the predominant direction) enhances this upper-layer inflow and the lower-layer outflow. Eastward wind completely reverses the flow direction not only at HE but also inside PWS. Remarkably, the total outflow through MS is consistently less than the total inflow through HE; thus, there is a significant outflow through HE. Therefore, the simple throughflow-driven circulation (inflow at HE and outflow at MS) previously thought dominant needs to be re-examined.
Abstract
Interactions between the circulation of Prince William Sound (PWS), Alaska, and that of the continental shelf region of the northern Gulf of Alaska are studied numerically. The focus is on the flow structure at Hinchinbrook Entrance (HE) and Montague Strait (MS) and the associated PWS interior circulation under various initial state and forcing configurations. Bottom topography inhibits flow into PWS under barotropic conditions, while stratification very much facilitates flow into PWS. Layered flow develops at HE and MS under baroclinic conditions, with two (four) layers under March (September) stratification conditions. In both baroclinic cases, the top layer transports inflow and a cyclonic eddy develops over the central basin of PWS. Westward wind (the predominant direction) enhances this upper-layer inflow and the lower-layer outflow. Eastward wind completely reverses the flow direction not only at HE but also inside PWS. Remarkably, the total outflow through MS is consistently less than the total inflow through HE; thus, there is a significant outflow through HE. Therefore, the simple throughflow-driven circulation (inflow at HE and outflow at MS) previously thought dominant needs to be re-examined.
Abstract
The theory of coastal-trapped waves is extended to include the general features of continuous density stratification, variable bottom topography, and a finite coastal wall. In the limit of a vanishing coastal wall, topographic Rossby waves are the only class of sub-inertial frequency, trapped wave motion. The stratification effect on topographic Rossby waves depends on both the local baroclinic radius of deformation and the characteristic offshore length scale of the wave motion. For intermediate density stratification, long waves are nearly depth-independent in the shelf region, and are bottom-trapped in the slope region. The topographic Rossby waves reduce to the barotropic shelf waves and the bottom-trapped waves in the limits of small and large density stratification, respectively.
In the general case of comparable influences from the coastal wall and bottom slope effects, baroclinic Kelvin waves and topographic Rossby waves are eigenmodes of the system. The eigenfunctions are modified from the elementary cases, which can be discerned by their structures along the coastal and bottom bound-aries. In particular, a resonance condition is suggested, i.e., the properties of a wavemode vary with the wavenumber and stratification. For example, mode 1 is a topographic Rossby wave for small wavenumbers and it is a baroclinic Kelvin wave for large wavenumbers. Also, the high-frequency cutoff found in the barotropic theories is lost.
Abstract
The theory of coastal-trapped waves is extended to include the general features of continuous density stratification, variable bottom topography, and a finite coastal wall. In the limit of a vanishing coastal wall, topographic Rossby waves are the only class of sub-inertial frequency, trapped wave motion. The stratification effect on topographic Rossby waves depends on both the local baroclinic radius of deformation and the characteristic offshore length scale of the wave motion. For intermediate density stratification, long waves are nearly depth-independent in the shelf region, and are bottom-trapped in the slope region. The topographic Rossby waves reduce to the barotropic shelf waves and the bottom-trapped waves in the limits of small and large density stratification, respectively.
In the general case of comparable influences from the coastal wall and bottom slope effects, baroclinic Kelvin waves and topographic Rossby waves are eigenmodes of the system. The eigenfunctions are modified from the elementary cases, which can be discerned by their structures along the coastal and bottom bound-aries. In particular, a resonance condition is suggested, i.e., the properties of a wavemode vary with the wavenumber and stratification. For example, mode 1 is a topographic Rossby wave for small wavenumbers and it is a baroclinic Kelvin wave for large wavenumbers. Also, the high-frequency cutoff found in the barotropic theories is lost.
Abstract
The skill with which amplitudes of quasi-geostrophic modes can be estimated is important in the analysis and modeling of data from mixed CTD/XBT surveys. Here, several methods for estimation of quasi-geostrophic vertical mode amplitudes (QGMs) are compared, both in the context of idealized estimation and (especially) in application to some recent CTD and XBT data from the California Current Systems (CCS). The methods compared are: 1) direct least-squares fitting by QGMs (LSF); 2) projection of “empirical orthogonal function” amplitudes onto QGM amplitudes at each station (EOF); 3) ridge regression (RR); 4) an “optimal estimate” using covariances between QGM amplitudes (OE); and 5) another optimal estimate using covariances between EOF amplitudes and QGM amplitudes (CEOF). For deep CTD casts (>1500 m), all methods perform well. For shallow CTD and XBT casts (<750 m), method five (CEOF) is recommended, using EOFs and amplitude covariances derived from just the deeper CTD casts. Since low-frequency internal waves have the same modal structure for density as the QGMS, they are not distinguishable from the QGMs in the present analysis. The analysis is applied to a recent survey to produce amplitude maps for the first few baroclinic modes. Comparisons with another survey indicate that the density analysis is transportable, but the T-S characteristics are so variable that the temperature analysis is not (the surveys are approximately three months apart).
Abstract
The skill with which amplitudes of quasi-geostrophic modes can be estimated is important in the analysis and modeling of data from mixed CTD/XBT surveys. Here, several methods for estimation of quasi-geostrophic vertical mode amplitudes (QGMs) are compared, both in the context of idealized estimation and (especially) in application to some recent CTD and XBT data from the California Current Systems (CCS). The methods compared are: 1) direct least-squares fitting by QGMs (LSF); 2) projection of “empirical orthogonal function” amplitudes onto QGM amplitudes at each station (EOF); 3) ridge regression (RR); 4) an “optimal estimate” using covariances between QGM amplitudes (OE); and 5) another optimal estimate using covariances between EOF amplitudes and QGM amplitudes (CEOF). For deep CTD casts (>1500 m), all methods perform well. For shallow CTD and XBT casts (<750 m), method five (CEOF) is recommended, using EOFs and amplitude covariances derived from just the deeper CTD casts. Since low-frequency internal waves have the same modal structure for density as the QGMS, they are not distinguishable from the QGMs in the present analysis. The analysis is applied to a recent survey to produce amplitude maps for the first few baroclinic modes. Comparisons with another survey indicate that the density analysis is transportable, but the T-S characteristics are so variable that the temperature analysis is not (the surveys are approximately three months apart).
Abstract
The series of cruises off Northern California comprising OPTOMA11, during two months in summer 1984, were specifically designed as an ocean prediction experiment. In addition to a regional survey from Cape Mendocino to Monterey, six surveys were made of a (150 km)2 domain offshore of Pt. Arena/Pt. Arena/Pt. Reyes. During the initial phase (over about ten days) of OPTOMA11, an intense (speeds up to 50 cm s−1, relative to 450 m) jet/cyclone system propagated offshore at about 5 km day−1. The subsequent evolution (over about 40 days) of the streamfunction field was governed by the meandering of the jet and the associated changes in the intensity of the anticyclonic region to the north of the jet and the cyclonic region to the south. From quasi-geostrophic (QG) model hindcast experiments using the streamfunction data, wind stress curl was an important forcing mechanism in the later phase of the experiment. Forecast in a domain extending over the continental slope were in agreement with objective analyses (OA) in the upper water column when the local topographic slope was used in the model. Asynopticity in initialization data (in this case, data acquired over eight days) did not seriously degree forecasts, although forecasts which used synoptic estimates (via a time-dependent objective analysis) of initial and boundary data were more accurate. The repetition in sampling allowed estimation of a space-time covariance function which was used for statistical forecasts. Quasi-geostrophic dynamical forecasts, generated using statistically forecast boundary data, evolved consistent with the OA in the interior of the forecast domain (rms difference 56% after 16 days). Assimilation of truly synoptic data, in the interior of the forecast domain as well as on the boundaries, improved the forecast so that it gave a better estimate of the streamfunction field than the OA (rms difference from the best field estimate was 20% after 16 days). Energetics analyses, based on-best estimates of the streamfunction and vorticity fields obtained by dynamical interpolation, indicate that the cyclonic region to the south of the jet grew due to baroclinic instability. The inclusion of wind stress curl forcing was essential to the interpretation of the energetics.
Abstract
The series of cruises off Northern California comprising OPTOMA11, during two months in summer 1984, were specifically designed as an ocean prediction experiment. In addition to a regional survey from Cape Mendocino to Monterey, six surveys were made of a (150 km)2 domain offshore of Pt. Arena/Pt. Arena/Pt. Reyes. During the initial phase (over about ten days) of OPTOMA11, an intense (speeds up to 50 cm s−1, relative to 450 m) jet/cyclone system propagated offshore at about 5 km day−1. The subsequent evolution (over about 40 days) of the streamfunction field was governed by the meandering of the jet and the associated changes in the intensity of the anticyclonic region to the north of the jet and the cyclonic region to the south. From quasi-geostrophic (QG) model hindcast experiments using the streamfunction data, wind stress curl was an important forcing mechanism in the later phase of the experiment. Forecast in a domain extending over the continental slope were in agreement with objective analyses (OA) in the upper water column when the local topographic slope was used in the model. Asynopticity in initialization data (in this case, data acquired over eight days) did not seriously degree forecasts, although forecasts which used synoptic estimates (via a time-dependent objective analysis) of initial and boundary data were more accurate. The repetition in sampling allowed estimation of a space-time covariance function which was used for statistical forecasts. Quasi-geostrophic dynamical forecasts, generated using statistically forecast boundary data, evolved consistent with the OA in the interior of the forecast domain (rms difference 56% after 16 days). Assimilation of truly synoptic data, in the interior of the forecast domain as well as on the boundaries, improved the forecast so that it gave a better estimate of the streamfunction field than the OA (rms difference from the best field estimate was 20% after 16 days). Energetics analyses, based on-best estimates of the streamfunction and vorticity fields obtained by dynamical interpolation, indicate that the cyclonic region to the south of the jet grew due to baroclinic instability. The inclusion of wind stress curl forcing was essential to the interpretation of the energetics.
Abstract
Weekly Gulf Stream paths within 1000 km downstream of Cape Hatteras were obtained for 1975–78 from the Navy's weekly EOFA charts based on satellite IR imagery. They displayed two dominant meander modes: first, a standing meander energetic over periods between 4 months and at least 4 years; and second. down-stream-propagating meanders that were most energetic at periods of several weeks. The long-period standing meander was confined between nodes located at the separation point near Cape Hatteras (i.e., where the Stream's mean path turns seaward) and at a point about 600 km farther downstream. The rms amplitude was 36 km at the antinode. The amplitude of propagating meanders increased rapidly in the first 200 km downstream of the separation point, where the capture of warm-core eddies was common. Farther down-stream, the predominant meanders had a wavelength averaging 330 km, a period averaging 1.5 month, a phase speed averaging 8 cm s−1, a downstream group speed averaging 17 cm s−1, and downstream exponential spatial growth rate averaging 3.2 × 10−3 km−1. They were energetic over a broad wavenumber-frequency band (periods of 1–6 months and wavelengths of 200 to more than 800 km) due to variable wavelengths, propagation speeds, and inter-meander space and time scales. The energetic wavenumber band was broadest near 4 cpy; it narrowed and shifted to larger wavenumbers with increasing frequency. The amplitude and frequency of occurrence of propagating meanders had large variability over time scales of a few months and longer.
Abstract
Weekly Gulf Stream paths within 1000 km downstream of Cape Hatteras were obtained for 1975–78 from the Navy's weekly EOFA charts based on satellite IR imagery. They displayed two dominant meander modes: first, a standing meander energetic over periods between 4 months and at least 4 years; and second. down-stream-propagating meanders that were most energetic at periods of several weeks. The long-period standing meander was confined between nodes located at the separation point near Cape Hatteras (i.e., where the Stream's mean path turns seaward) and at a point about 600 km farther downstream. The rms amplitude was 36 km at the antinode. The amplitude of propagating meanders increased rapidly in the first 200 km downstream of the separation point, where the capture of warm-core eddies was common. Farther down-stream, the predominant meanders had a wavelength averaging 330 km, a period averaging 1.5 month, a phase speed averaging 8 cm s−1, a downstream group speed averaging 17 cm s−1, and downstream exponential spatial growth rate averaging 3.2 × 10−3 km−1. They were energetic over a broad wavenumber-frequency band (periods of 1–6 months and wavelengths of 200 to more than 800 km) due to variable wavelengths, propagation speeds, and inter-meander space and time scales. The energetic wavenumber band was broadest near 4 cpy; it narrowed and shifted to larger wavenumbers with increasing frequency. The amplitude and frequency of occurrence of propagating meanders had large variability over time scales of a few months and longer.
Abstract
We studied the frontal zone of the coastal upwelling region off Oregon, from observations made in two successive years. The measurements were made between July and September in 1965 and 1966. The alongshore flow field was determined by combining direct measurements and geostrophic calculations. A near-surface southward jet and a subsurface northward undercurrent existed in the frontal zone. They were separated by an inclined frontal layer (permanent pycnocline). The frontal layer tended to intersect the sea surface about 10 km offshore, where a surface front was formed. Through a combination of direct current measurement and water mass analysis, the cross-stream flow was estimated to be seaward near the surface, shoreward at the top of the inclined frontal layer, but seaward at the bottom of the inclined frontal layer and shoreward below that. During a 25 h anchor station, a high degree of correlation existed between the vertical structure of the alongshore and cross-stream flows. An anomalously warm water mass occurred at the base of the frontal layer. We believe it was formed near the surface front and that it sank and flowed seaward along the base of the inclined frontal layer. Vertical shears in the horizontal velocity were caused by the mean baroclinic flow and the tidal and longer period baroclinic oscillations. A zone of low dynamic stability was produced near the base of the inclined frontal layer, coincident with the warm anomaly, providing a mixing mechanism for the erosion of the warm anomaly and the broadening of the frontal layer offshore. Estimates of temporal and spatial scales and of horizontal eddy viscosity coefficients are given. Internal tidal motions provided an energy flux to the mean motion. A conceptual model is presented for the mean state (averaged over a fortnight or, equivalently, over one or more upwelling “wind event cycles”) of coastal upwelling.
Abstract
We studied the frontal zone of the coastal upwelling region off Oregon, from observations made in two successive years. The measurements were made between July and September in 1965 and 1966. The alongshore flow field was determined by combining direct measurements and geostrophic calculations. A near-surface southward jet and a subsurface northward undercurrent existed in the frontal zone. They were separated by an inclined frontal layer (permanent pycnocline). The frontal layer tended to intersect the sea surface about 10 km offshore, where a surface front was formed. Through a combination of direct current measurement and water mass analysis, the cross-stream flow was estimated to be seaward near the surface, shoreward at the top of the inclined frontal layer, but seaward at the bottom of the inclined frontal layer and shoreward below that. During a 25 h anchor station, a high degree of correlation existed between the vertical structure of the alongshore and cross-stream flows. An anomalously warm water mass occurred at the base of the frontal layer. We believe it was formed near the surface front and that it sank and flowed seaward along the base of the inclined frontal layer. Vertical shears in the horizontal velocity were caused by the mean baroclinic flow and the tidal and longer period baroclinic oscillations. A zone of low dynamic stability was produced near the base of the inclined frontal layer, coincident with the warm anomaly, providing a mixing mechanism for the erosion of the warm anomaly and the broadening of the frontal layer offshore. Estimates of temporal and spatial scales and of horizontal eddy viscosity coefficients are given. Internal tidal motions provided an energy flux to the mean motion. A conceptual model is presented for the mean state (averaged over a fortnight or, equivalently, over one or more upwelling “wind event cycles”) of coastal upwelling.
