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- Author or Editor: Richard D. Romea x
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Abstract
A linear and finite-amplitude analysis of the baroclinic instability of a zonal current is presented. Both the β effect and viscosity are included. A small but finite amount of dissipation destabilizes the system, lowering the curve of marginal stability by an O(1) amount. In the limit of vanishing viscosity, steady unstable waves of finite amplitude are discovered for shears which are subcritical with respect to the inviscid criterion.
Abstract
A linear and finite-amplitude analysis of the baroclinic instability of a zonal current is presented. Both the β effect and viscosity are included. A small but finite amount of dissipation destabilizes the system, lowering the curve of marginal stability by an O(1) amount. In the limit of vanishing viscosity, steady unstable waves of finite amplitude are discovered for shears which are subcritical with respect to the inviscid criterion.
Abstract
Analyses of low-pass filtered coastal sea-level data and geostrophic winds derived from surface pressure charts for the period of 1 December 1980 through 31 March 1981 indicate that the wintertime dynamics in the Northeast China Sea is strongly influenced by the passage of rapidly moving cold fronts from northwest to southeast across the region. Along the west coast of Korea, sea-level fluctuations are highly coherent with the north–south wind in two bands centered, respectively, at about 0.17 and 0.36 cpd, and propagate to the south for the low-frequency band. Removal of the wind-forced component of the sea-level signal yields freely propagating fluctuations in bands centered at about 0.20, 0.34 and 0.50 cpd, that travel northward at approximately the phase speed expected for barotropic Kelvin waves, indicative of a relaxation in the sea-level field following the successive passages of cold fronts. Coherence studies of winds and sea-level differences in the Tsushima Strait indicate a correlation, at low frequencies (ω < 0.25 cpd), between the along-strait sea-level slope and winds oriented north–south, indicative perhaps of the difference in sign in offshore Ekman transports, under a broad-scale north–south wind, between the east and west coasts of Korea. For 0.5 > ω > 0.08 cpd, fluctuations in sea-level records from stations in the Ryukyu island chain are wind-forced and propagate eastward with similar speeds as the wind forcing, while for ω < 0.08 cpd the island sea levels are not well correlated with the winds. These latter fluctuations propagate eastward with much smaller phase speeds and may be associated with fluctuations in the Kuroshio.
Abstract
Analyses of low-pass filtered coastal sea-level data and geostrophic winds derived from surface pressure charts for the period of 1 December 1980 through 31 March 1981 indicate that the wintertime dynamics in the Northeast China Sea is strongly influenced by the passage of rapidly moving cold fronts from northwest to southeast across the region. Along the west coast of Korea, sea-level fluctuations are highly coherent with the north–south wind in two bands centered, respectively, at about 0.17 and 0.36 cpd, and propagate to the south for the low-frequency band. Removal of the wind-forced component of the sea-level signal yields freely propagating fluctuations in bands centered at about 0.20, 0.34 and 0.50 cpd, that travel northward at approximately the phase speed expected for barotropic Kelvin waves, indicative of a relaxation in the sea-level field following the successive passages of cold fronts. Coherence studies of winds and sea-level differences in the Tsushima Strait indicate a correlation, at low frequencies (ω < 0.25 cpd), between the along-strait sea-level slope and winds oriented north–south, indicative perhaps of the difference in sign in offshore Ekman transports, under a broad-scale north–south wind, between the east and west coasts of Korea. For 0.5 > ω > 0.08 cpd, fluctuations in sea-level records from stations in the Ryukyu island chain are wind-forced and propagate eastward with similar speeds as the wind forcing, while for ω < 0.08 cpd the island sea levels are not well correlated with the winds. These latter fluctuations propagate eastward with much smaller phase speeds and may be associated with fluctuations in the Kuroshio.
Abstract
Time series of coastal sea level during 1976–77, from 2°12′S to 17°S along the west coast of South America, show that low-frequency, ω < 0.25 cycles per day (cpd), fluctuations propagate poleward with the phase speed of baroclinic Kelvin waves (2–3 m s−1). The alongshore coherence is highest in the frequency band 0.1–0.2 cpd. Computing the frequency-domain empirical orthogonal functions (EOF) for alongshore current, from an army of current meters extending from 5°S to 15°S during March-May 1977, gives 70% of the variance in the 0.1–0.2 cpd frequency band to an EOF mode with poleward phase propagation at 2.75 m s−1. The vertical structure of the alongshore current fluctuations (0.1–0.2 cpd) over the continental slope at 5°S and 15°S is consistent with a first-mode baroclinic Kelvin wave. The current and sea-level fluctuations are coherent and propagate poleward through latitudes where their frequency equals the local inertial frequency. The fluctuations are not significantly coherent with coastal winds from 4°S to 15°S and am therefore presumed to have an equatorial origin. Intermittent sea-level data at the Galapagos Islands during the period provide tenuous evidence that these fluctuations, propagating poleward as coastally trapped waves, previously traveled in the equatorial wave guide.
Abstract
Time series of coastal sea level during 1976–77, from 2°12′S to 17°S along the west coast of South America, show that low-frequency, ω < 0.25 cycles per day (cpd), fluctuations propagate poleward with the phase speed of baroclinic Kelvin waves (2–3 m s−1). The alongshore coherence is highest in the frequency band 0.1–0.2 cpd. Computing the frequency-domain empirical orthogonal functions (EOF) for alongshore current, from an army of current meters extending from 5°S to 15°S during March-May 1977, gives 70% of the variance in the 0.1–0.2 cpd frequency band to an EOF mode with poleward phase propagation at 2.75 m s−1. The vertical structure of the alongshore current fluctuations (0.1–0.2 cpd) over the continental slope at 5°S and 15°S is consistent with a first-mode baroclinic Kelvin wave. The current and sea-level fluctuations are coherent and propagate poleward through latitudes where their frequency equals the local inertial frequency. The fluctuations are not significantly coherent with coastal winds from 4°S to 15°S and am therefore presumed to have an equatorial origin. Intermittent sea-level data at the Galapagos Islands during the period provide tenuous evidence that these fluctuations, propagating poleward as coastally trapped waves, previously traveled in the equatorial wave guide.
Abstract
As a sequel to data analysis reported in Part I, a numerical model of the wintertime wind-driven circulation of the northeast Chine Sea is developed with the primary purpose of achieving an understanding of the underlying dynamics to several observed features of the coastal sea-level behavior. The model is linear and pertains to the vertically integrated balance of momentum and mass in a well mixed shallow ocean. The model bathymetry incorporates a realistic shelf embayment and an open-ocean region of a depth so limited that observed sea-level differences across open boundaries in the deep western Pacific do not result in unreasonably large model transports.
From results of model runs for the 1980/81 winter, it is found that 1) along the west coast of South Korea, sea-level setup to the south is a consequence chiefly of the presence, at the end of the Korean Peninsula, of relatively deep offshore water that is less responsive to wind forcing than the shallow shelf water, 2) pressure gradient force dominates in deep waters along the axis of the Yellow Sea embayment and contributes to an upwind (northward) flow that is particularly noticeable during relaxation when the north wind abates; 3) free, coastally trapped waves that travel counterclockwise around the basin dominate the relaxation response and are generated partly from the relaxation of coastal sea-level setups; 4) part of the free oscillations around 0.33 cpd detected in sea-level records along the west coast of South Korea appear to have originated from sea-level fluctuations generated along the western portion of the southern coast of the Korean Peninsula by the east-west wind component which exhibits a distinct spectral peak at that frequency; 5) the superposition of the mean of the wind-driven model circulation and the Kuroshio-driven steady state yields a northward flowing current similar to the observed Yellow Sea Warm Current and cyclonic gyres in the Yellow Sea and the East China Sea region southwest of Cheju that are long suspected from various oceanographic measurements.
Abstract
As a sequel to data analysis reported in Part I, a numerical model of the wintertime wind-driven circulation of the northeast Chine Sea is developed with the primary purpose of achieving an understanding of the underlying dynamics to several observed features of the coastal sea-level behavior. The model is linear and pertains to the vertically integrated balance of momentum and mass in a well mixed shallow ocean. The model bathymetry incorporates a realistic shelf embayment and an open-ocean region of a depth so limited that observed sea-level differences across open boundaries in the deep western Pacific do not result in unreasonably large model transports.
From results of model runs for the 1980/81 winter, it is found that 1) along the west coast of South Korea, sea-level setup to the south is a consequence chiefly of the presence, at the end of the Korean Peninsula, of relatively deep offshore water that is less responsive to wind forcing than the shallow shelf water, 2) pressure gradient force dominates in deep waters along the axis of the Yellow Sea embayment and contributes to an upwind (northward) flow that is particularly noticeable during relaxation when the north wind abates; 3) free, coastally trapped waves that travel counterclockwise around the basin dominate the relaxation response and are generated partly from the relaxation of coastal sea-level setups; 4) part of the free oscillations around 0.33 cpd detected in sea-level records along the west coast of South Korea appear to have originated from sea-level fluctuations generated along the western portion of the southern coast of the Korean Peninsula by the east-west wind component which exhibits a distinct spectral peak at that frequency; 5) the superposition of the mean of the wind-driven model circulation and the Kuroshio-driven steady state yields a northward flowing current similar to the observed Yellow Sea Warm Current and cyclonic gyres in the Yellow Sea and the East China Sea region southwest of Cheju that are long suspected from various oceanographic measurements.
