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- Author or Editor: Warren B. White x
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Abstract
Year-to-year changes are found in Australian precipitation (APP) covarying with those in sea surface temperature (SST) and troposphere moisture flux (MF) over the three oceans surrounding Australia for 40 yr from 1958 to 1997. Australia’s wet (dry) years are associated with warm (cool) SST anomalies surrounding Australia and convergent (divergent) MF anomalies directly overhead. Differences in APP (SST) between wet and dry years can reach 0.75 m (1.2°C) in northeast Australia (subtropical Indian Ocean). Wet (dry) years often occur during La Niña (El Niño), but significant differences in covarying SST, MF, and APP anomalies from one El Niño to the next are found, indicating that regional climate changes also influence APP. Covarying SST and MF anomalies on basin space scales and interannual timescales are found to take 2–3 yr to propagate eastward from Africa to Australia. This propagation occurs in association with the Antarctic Circumpolar Wave (ACW) in the Southern Ocean, the north branch of the ACW in the Indian Ocean, and the global El Niño–Southern Oscillation wave in the tropical ocean. A statistical climate prediction system based upon the slow eastward propagation of SST anomalies and their nearly one-to-one relationship with APP anomalies is constructed, yielding significant hindcast skill for predicting interannual APP anomalies at lead times of 1 and 2 yr. Best hindcast skill for the extratropical portion of Australia derives from the ACW south of Australia and the north branch of the ACW west of Australia. Eastward propagation of SST anomalies in these two oceanic domains is capable of predicting more than 50% of the total interannual variance over Victoria and New South Wales and over Western Australia poleward of 20°S over the 40-yr record. This percentage is much better than expected from chance or persistence, demonstrating the importance of the ACW upon year-to-year changes in APP at these latitudes.
Abstract
Year-to-year changes are found in Australian precipitation (APP) covarying with those in sea surface temperature (SST) and troposphere moisture flux (MF) over the three oceans surrounding Australia for 40 yr from 1958 to 1997. Australia’s wet (dry) years are associated with warm (cool) SST anomalies surrounding Australia and convergent (divergent) MF anomalies directly overhead. Differences in APP (SST) between wet and dry years can reach 0.75 m (1.2°C) in northeast Australia (subtropical Indian Ocean). Wet (dry) years often occur during La Niña (El Niño), but significant differences in covarying SST, MF, and APP anomalies from one El Niño to the next are found, indicating that regional climate changes also influence APP. Covarying SST and MF anomalies on basin space scales and interannual timescales are found to take 2–3 yr to propagate eastward from Africa to Australia. This propagation occurs in association with the Antarctic Circumpolar Wave (ACW) in the Southern Ocean, the north branch of the ACW in the Indian Ocean, and the global El Niño–Southern Oscillation wave in the tropical ocean. A statistical climate prediction system based upon the slow eastward propagation of SST anomalies and their nearly one-to-one relationship with APP anomalies is constructed, yielding significant hindcast skill for predicting interannual APP anomalies at lead times of 1 and 2 yr. Best hindcast skill for the extratropical portion of Australia derives from the ACW south of Australia and the north branch of the ACW west of Australia. Eastward propagation of SST anomalies in these two oceanic domains is capable of predicting more than 50% of the total interannual variance over Victoria and New South Wales and over Western Australia poleward of 20°S over the 40-yr record. This percentage is much better than expected from chance or persistence, demonstrating the importance of the ACW upon year-to-year changes in APP at these latitudes.
Abstract
During the four year period 1976–80, mesoscale anomalies of 300 m temperature in the eastern midlatitude North Pacific were observed propagating westward from the coast of North America to 165°W at approximately 2 cm s−1, with characteristics similar to baroclinic, nondispersive Rossby waves. These mesoscale anomalies had a dominant period scale of approximately two years and a dominant wavelength scale of approximately 1000 km. In this study, a baroclinic, nondispersive Rossby wave model is driven by the observed wind stress curl in an attempt to simulate the amplitude and phase of these observed mesoscale anomalies over this period of time. Model/data intercomparison finds the frequency/zonal wavenumber spectra of model 300 m temperature to be similar in pattern to that observed for waves of periods greater than one year, with peak spectral energy density in both occurring at zonal wavelengths of 1000–1200 km and periods of 2–3 years. These spectral peaks occur on the Rossby wave dispersion curve at the frequency/wavenumber location where peak spectral energy density occurs in the wind-stress curl spectrum. Coherence between model and observed mesoscale anomalies is maximum at the frequency/wavenumber location of these spectral peaks, significant at the 70% confidence level, with approximately 0° phase difference. The response functions in frequency/wavenumber space of both model ocean and real ocean are nearly identical in pattern for periods greater than one year, with maximum values located along the Rossby wave dispersion curve. Therefore, a resonant response of the baroclinic, nondispersive, Rossby waves to the wind stress curl is indicated. These resonant waves begin at the coast of North America and propagate westward with amplitude increasing linearly. The zonal profile of rms differences of observed mesoscale anomalies from 125–165°W is very similar to the model profile, the latter dominated by the resonant response which explains 60–90% of the total model response. The off-resonant model response is small, even though the wind strew curl variability is largest at off-resonant frequencies and wavenumbers. In the real ocean the off-resonant response is much larger than in the model, with the largest off-resonant response occurring in the dispersive portion of the spectral domain where the model is not applicable. This, together with white noise in both model and observed data, accounts for the model's inability to explain more than 20% of the total observed mesoscale variance.
Abstract
During the four year period 1976–80, mesoscale anomalies of 300 m temperature in the eastern midlatitude North Pacific were observed propagating westward from the coast of North America to 165°W at approximately 2 cm s−1, with characteristics similar to baroclinic, nondispersive Rossby waves. These mesoscale anomalies had a dominant period scale of approximately two years and a dominant wavelength scale of approximately 1000 km. In this study, a baroclinic, nondispersive Rossby wave model is driven by the observed wind stress curl in an attempt to simulate the amplitude and phase of these observed mesoscale anomalies over this period of time. Model/data intercomparison finds the frequency/zonal wavenumber spectra of model 300 m temperature to be similar in pattern to that observed for waves of periods greater than one year, with peak spectral energy density in both occurring at zonal wavelengths of 1000–1200 km and periods of 2–3 years. These spectral peaks occur on the Rossby wave dispersion curve at the frequency/wavenumber location where peak spectral energy density occurs in the wind-stress curl spectrum. Coherence between model and observed mesoscale anomalies is maximum at the frequency/wavenumber location of these spectral peaks, significant at the 70% confidence level, with approximately 0° phase difference. The response functions in frequency/wavenumber space of both model ocean and real ocean are nearly identical in pattern for periods greater than one year, with maximum values located along the Rossby wave dispersion curve. Therefore, a resonant response of the baroclinic, nondispersive, Rossby waves to the wind stress curl is indicated. These resonant waves begin at the coast of North America and propagate westward with amplitude increasing linearly. The zonal profile of rms differences of observed mesoscale anomalies from 125–165°W is very similar to the model profile, the latter dominated by the resonant response which explains 60–90% of the total model response. The off-resonant model response is small, even though the wind strew curl variability is largest at off-resonant frequencies and wavenumbers. In the real ocean the off-resonant response is much larger than in the model, with the largest off-resonant response occurring in the dispersive portion of the spectral domain where the model is not applicable. This, together with white noise in both model and observed data, accounts for the model's inability to explain more than 20% of the total observed mesoscale variance.
Abstract
Individual seasonal mean maps of temperature at 300 m in the North Pacific Current cast of 180° from 1976 to 1980 were constructed from TRANSPAC XBT data. The long-term annual mean map is relatively smooth, with some weak quasi-stationary meander activity. Most of the total variance was due to large-scale interannual variability (i.e., ∼60%), loss to the mesoscale perturbations (i.e., ∼30%), and least to the annual cycle (i.e., ∼10%). However, individual mesoscale perturbations were significant, clearly wave-like with a wavelength scale of 500–1000 km and a period scale of 1–2 years, and generally coherent in phase over 10° of latitude. These wave-like mesoscale perturbations emanated from the eastern boundary and propagated westward as coherent features at the phase speed of linear, non-dispersive, baroclinic long-waves. The latitudinal reduction in phase speed from 2.7 cm s−1 at 35°N to 1.4 cm s−1 at 45°N was consistent with baroclinic long-wave theory. An increase in time scale of these wave-like perturbations with latitude was consistent with the “critical latitude” concept.
Abstract
Individual seasonal mean maps of temperature at 300 m in the North Pacific Current cast of 180° from 1976 to 1980 were constructed from TRANSPAC XBT data. The long-term annual mean map is relatively smooth, with some weak quasi-stationary meander activity. Most of the total variance was due to large-scale interannual variability (i.e., ∼60%), loss to the mesoscale perturbations (i.e., ∼30%), and least to the annual cycle (i.e., ∼10%). However, individual mesoscale perturbations were significant, clearly wave-like with a wavelength scale of 500–1000 km and a period scale of 1–2 years, and generally coherent in phase over 10° of latitude. These wave-like mesoscale perturbations emanated from the eastern boundary and propagated westward as coherent features at the phase speed of linear, non-dispersive, baroclinic long-waves. The latitudinal reduction in phase speed from 2.7 cm s−1 at 35°N to 1.4 cm s−1 at 45°N was consistent with baroclinic long-wave theory. An increase in time scale of these wave-like perturbations with latitude was consistent with the “critical latitude” concept.
Abstract
A mathematical barotropic model based upon the conservation of absolute vorticity is used to determine the effect of the spherical shape of the rotating earth [approximated by the beta (β) effect] on a steady uniform eastward current streaming past a cylindrical island in an unbounded ocean of uniform depth. Upstream far-field conditions are introduced that confine the disturbance pattern produced by the island to the region downstream from the island.
For an initially uniform eastward flow of velocity u 0 streaming past a cylindrical island of radius a, the downstream disturbance consists of a trail of meanders and eddies. The amplitude of these features depends upon the magnitude of the Island number [Is=(βa 2/u 0)½ and the radial wavenumber equals (β/u 0)½, which is, the Rossby wavenumber for stationary planetary waves.
In order to confirm the theoretical results of the beta-plane wake for an eastward flow situation, appeal is made to a laboratory model, consisting of a rotating annulus with a sloping bottom to simulate the beta effect. Dynamic similarity is achieved through the nondimensional Island number. The resulting flow pattern reveals a uniform flow field upstream from the island with the formation of a stationary disturbance downstream that agrees qualitatively with the theoretical results.
Abstract
A mathematical barotropic model based upon the conservation of absolute vorticity is used to determine the effect of the spherical shape of the rotating earth [approximated by the beta (β) effect] on a steady uniform eastward current streaming past a cylindrical island in an unbounded ocean of uniform depth. Upstream far-field conditions are introduced that confine the disturbance pattern produced by the island to the region downstream from the island.
For an initially uniform eastward flow of velocity u 0 streaming past a cylindrical island of radius a, the downstream disturbance consists of a trail of meanders and eddies. The amplitude of these features depends upon the magnitude of the Island number [Is=(βa 2/u 0)½ and the radial wavenumber equals (β/u 0)½, which is, the Rossby wavenumber for stationary planetary waves.
In order to confirm the theoretical results of the beta-plane wake for an eastward flow situation, appeal is made to a laboratory model, consisting of a rotating annulus with a sloping bottom to simulate the beta effect. Dynamic similarity is achieved through the nondimensional Island number. The resulting flow pattern reveals a uniform flow field upstream from the island with the formation of a stationary disturbance downstream that agrees qualitatively with the theoretical results.
Abstract
Study of a schematic linear, wind-driven, inertio–gravity wave model in the upper ocean finds resonant, near-inertial waves of finite horizontal wavelength to have both horizontal and vertical motions. The mean product of these horizontal motions with the vertical motion yields an internal vertical Reynolds stress that is zero at the sea surface and nonzero at the base of the mixed layer and in the main pycnocline below. Assuming near-inertial waves of finite wavelength to be generated by finite wavelength disturbances in the atmosphere, advected by the mean wind, the internal vertical Reynolds stress at the base of the mixed layer is directed 45° to the right of the mean wind direction. Below the mixed layer, near-inertial waves in the mixed layer. This generation mechanism produces an internal vertical stress throughout the main pycnocline, directed initially 45° to the right of the mean wind direction, but rotating into it with increasing depth. The divergence of this internal vertical Reynolds stress produces an Eulerian mean flow. In the mixed layer, the Eulerian mean flow (vertically averaged) is directed 45° to the left of the mean with direction. Below the mixed layer, the Eulerian mean flow is directed principally upwind over the upper portion of the main pycnocline. For wavelength scales of O(100 km), The Eulerian mean flow is of the same order of magnitude as the Ekman mean flow driven by the synoptic mean wind. This means that Eulerian measurements (e.g., current meter observations) of wind-driven currents in the upper ocean may have to consider the Eulerian mean motion from this source. In this linear situation, that Eulerian mean flow is equal, but opposite to the Stokes drift associated with these near-inertial waves; consequently, the net Lagrangian particle motion is zero everywhere in the column.
Analysis of the Lagrangian mean motions in this schematic model establishes that measurement systems which are traditionally considered Lagrangian (e.g., drogued drifters) and Eulerian (e.g., moored current meters) may in fact be mixed. Drogued drifters are found to act like Lagrangian tracers with respect to horizontal displacements, but not with respect to vertical displacements. In the context of the present model, this inability to follow the particle vertically yields considerable downwind mean motion of the drogued drifter over the lower half of the mixed layer. This motion has not dynamical significance (i.e., it is not related to the transport of volume, heat, salt, etc.), but it is larger than the crosswind Ekman flow and may explain why drogued drifters prefer downwind motion. Moored current meters may taken on quasi-Lagrangian character by being displaced vertically in response to mooring motion. This can have an effect upon the measured mean motion that is as large as the purely Eulerian mean flow past the mooring.
Abstract
Study of a schematic linear, wind-driven, inertio–gravity wave model in the upper ocean finds resonant, near-inertial waves of finite horizontal wavelength to have both horizontal and vertical motions. The mean product of these horizontal motions with the vertical motion yields an internal vertical Reynolds stress that is zero at the sea surface and nonzero at the base of the mixed layer and in the main pycnocline below. Assuming near-inertial waves of finite wavelength to be generated by finite wavelength disturbances in the atmosphere, advected by the mean wind, the internal vertical Reynolds stress at the base of the mixed layer is directed 45° to the right of the mean wind direction. Below the mixed layer, near-inertial waves in the mixed layer. This generation mechanism produces an internal vertical stress throughout the main pycnocline, directed initially 45° to the right of the mean wind direction, but rotating into it with increasing depth. The divergence of this internal vertical Reynolds stress produces an Eulerian mean flow. In the mixed layer, the Eulerian mean flow (vertically averaged) is directed 45° to the left of the mean with direction. Below the mixed layer, the Eulerian mean flow is directed principally upwind over the upper portion of the main pycnocline. For wavelength scales of O(100 km), The Eulerian mean flow is of the same order of magnitude as the Ekman mean flow driven by the synoptic mean wind. This means that Eulerian measurements (e.g., current meter observations) of wind-driven currents in the upper ocean may have to consider the Eulerian mean motion from this source. In this linear situation, that Eulerian mean flow is equal, but opposite to the Stokes drift associated with these near-inertial waves; consequently, the net Lagrangian particle motion is zero everywhere in the column.
Analysis of the Lagrangian mean motions in this schematic model establishes that measurement systems which are traditionally considered Lagrangian (e.g., drogued drifters) and Eulerian (e.g., moored current meters) may in fact be mixed. Drogued drifters are found to act like Lagrangian tracers with respect to horizontal displacements, but not with respect to vertical displacements. In the context of the present model, this inability to follow the particle vertically yields considerable downwind mean motion of the drogued drifter over the lower half of the mixed layer. This motion has not dynamical significance (i.e., it is not related to the transport of volume, heat, salt, etc.), but it is larger than the crosswind Ekman flow and may explain why drogued drifters prefer downwind motion. Moored current meters may taken on quasi-Lagrangian character by being displaced vertically in response to mooring motion. This can have an effect upon the measured mean motion that is as large as the purely Eulerian mean flow past the mooring.
Abstract
In the Pacific Equatorial Undercurrent downstream (east) from the Galapagos archipelago, an unusual meander pattern was observed in the spring of 1967. Two separate hypotheses present themselves as explanations for the observed wake phenomenon. The wake may have been a variation of the familiar von Kármán wake, or it may have been a form of the Rossby wake, only recently discussed by White. Through a scale analysis, both hypotheses are found to be reasonable, and both give characteristic length scales (500 km) that agree well with the observed wavelengths. A fundamental difference between the two hypotheses is that the Rossby wake is stationary, while the von Kármán wake is time-dependent. However, the time scale for eddy shedding in a von Kármán wake is found to be on the same scale (2 months) as the length of the cruise that observed the wake phenomenon. Therefore, it appears that the observed oceanic wake may have had characteristics of both the von Kármán and Rossby wakes.
Abstract
In the Pacific Equatorial Undercurrent downstream (east) from the Galapagos archipelago, an unusual meander pattern was observed in the spring of 1967. Two separate hypotheses present themselves as explanations for the observed wake phenomenon. The wake may have been a variation of the familiar von Kármán wake, or it may have been a form of the Rossby wake, only recently discussed by White. Through a scale analysis, both hypotheses are found to be reasonable, and both give characteristic length scales (500 km) that agree well with the observed wavelengths. A fundamental difference between the two hypotheses is that the Rossby wake is stationary, while the von Kármán wake is time-dependent. However, the time scale for eddy shedding in a von Kármán wake is found to be on the same scale (2 months) as the length of the cruise that observed the wake phenomenon. Therefore, it appears that the observed oceanic wake may have had characteristics of both the von Kármán and Rossby wakes.
Abstract
The annual cycle in TOPEX/Poseidon sea level height (SLH) and National Centers for Environmental Prediction (NCEP) sea surface temperature (SST) and meridional surface wind (MSW) from 1993 to 1998 in the trade wind zone over the Indo-Pacific Ocean from 5° to 25° latitude in both hemispheres is examined. Zonal wavenumber–frequency spectra of monthly mean SLH, SST, and MSW residuals about the annual mean find peak spectral energy density for westward-propagating waves of annual period and basinscale wavelengths in all three variables. Moreover, spectral coherence and phase between the three variables are significant, with warm SST residuals displaced 0°–90° of phase to the west of high SLH residuals, and poleward MSW residuals displaced 0°–90° of phase to the east of warm SST residuals, similar to those observed for coupled Rossby waves on interannual period scales. Removing zonal means at each latitude from the annual cycle in SLH, SST, and MSW allows westward phase propagation to be observed in each variable, together with poleward refraction stemming from the reduction in westward phase speed with latitude. The fact that the latter occurs in both oceanic and atmospheric variables indicates that annual Rossby waves in the ocean are coupled to the overlying atmosphere. The reduction in westward phase speed with latitude (and, hence, the poleward refraction) is less than expected for free Rossby waves, with the apparent coupled Rossby waves traveling faster (slower) poleward (equatorward) of ∼12° latitude.
Abstract
The annual cycle in TOPEX/Poseidon sea level height (SLH) and National Centers for Environmental Prediction (NCEP) sea surface temperature (SST) and meridional surface wind (MSW) from 1993 to 1998 in the trade wind zone over the Indo-Pacific Ocean from 5° to 25° latitude in both hemispheres is examined. Zonal wavenumber–frequency spectra of monthly mean SLH, SST, and MSW residuals about the annual mean find peak spectral energy density for westward-propagating waves of annual period and basinscale wavelengths in all three variables. Moreover, spectral coherence and phase between the three variables are significant, with warm SST residuals displaced 0°–90° of phase to the west of high SLH residuals, and poleward MSW residuals displaced 0°–90° of phase to the east of warm SST residuals, similar to those observed for coupled Rossby waves on interannual period scales. Removing zonal means at each latitude from the annual cycle in SLH, SST, and MSW allows westward phase propagation to be observed in each variable, together with poleward refraction stemming from the reduction in westward phase speed with latitude. The fact that the latter occurs in both oceanic and atmospheric variables indicates that annual Rossby waves in the ocean are coupled to the overlying atmosphere. The reduction in westward phase speed with latitude (and, hence, the poleward refraction) is less than expected for free Rossby waves, with the apparent coupled Rossby waves traveling faster (slower) poleward (equatorward) of ∼12° latitude.
