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Topographically generated eddies
Oceanographic Abstracts
275
the Tsushima Current and the Korean coastal waters was routinely detected and displayed on the electro-optically contoured thermal imagery. The contour intervals are 1.6°C, and noise-induced effects give a thermal resolution of 0.8°C. Temperature differences across the front of 3.29°C ( a = 0.42°C) were measured be seven ship crossings, and temperature differences of 3.3°C ( a = 0.4°C) were estimated from five satellite overpasses. Thermal patterns of a one-sided divergence and a cyclonic eddy were detected in the coastal waters at the flow separation where the western edge of the Tsushima Current curves away from the coast into the Sea of Japan. A critique of the oceanographic capabilities and limitations of the system is provided as a guide to potential users. HUPPERT H. E. and KIRK BRYAN, 1976. Topographically generated eddies. Deep-Sea Res., 23 (8): 655-679. The interaction between temporally varying currents and the bottom topography of the ocean is investigated by the numerical and analytic examination of the following simple model. The flow of an inviscid, stratified fluid is initiated from relative rest in a uniformly rotating system containing an isolated topographic feature. The evolution of the flow redistributes vorticity and temperature in such a way that relatively cold water with anticyclonic vorticity exists over the topographic feature, while water shed from above the topographic feature sinks, thereby inducing a warm anomaly with cyclonic vorticity. For sufficiently strong oncoming flows, the shed fluid continually drifts downstream in the form of a relatively warm eddy. If the oncoming flow is relatively weak, the interaction between the anticyclonic and cyclonic vorticity distributions traps the warm eddy and it remains in the vicinity of the topographic feature. We suggest that recent observations of an eddy in the vicinity of the Atlantis 1I Seamount and the existence of the large amount of high frequency energy near the bottom of the ocean measured by the MODE experiment may be partly explained in terms of the above mechanism. We conclude by speculating that vorticity redistribution by topography may be a contributing factor to cyclogenesis in the atmosphere. HURLBURT H . E . , J. C. KINDLE and J. J. O'BRIEN, 1976. A numerical simulation of the onset of El Nifio. ,L Phys.
Oceanogr., 6 (5): 621-631. El Nifio may be defined oceanographically as a massive influx of.warm water into the coastal region of Ecuador and Peru. We have tested the hypothesis that these rare events occur after a substantial reduction of the atmospheric trade winds over the central Pacific Ocean. An idealized nonlinear, two-layer, equatorial beta-plane ocean is spun-up with easterly winds for 50 days after which the wind is relaxed over several days. The relaxation of the wind initiates internal Kelvin wave fronts at both sides of the ocean at the equator. The eastern wave fronts propagate poleward and the western ones eastward. Internal Rossby waves are generated which propagate westward from the eastern boundary. As the Kelvin wave fronts move poleward along the eastern boundary, strong downwelling occurs and the coastal currel,ts reverse direction and become poleward. The rapid downwelling and the sudden reversal of the coastal current are consistent with observations during El Nifio. This downwelling is much more rapid than the upwelling which occurred during spin-up due to nonlinear Kelvin wave dispersion. The dispersion results in the development of a frontal character at the leading edge of the waves. When the western Kelvin wave fronts reach the eastern boundary, the downwelling ceases and the poleward currents separate from the coast, propagating westward as Rossby waves. Thus we suggest the pulsating nature of E1 Nifio is related to the occurrence of major equatorial wind changes and the dynamics of internal Kelvin waves whose nonlinear attributes may greatly sharpen the pulses. HURLBURT H. E. and J. D. THOMPSON, 1976. A numerical model of the Somali Current. J. phys. Oceanogr., 6 (5t: 646-664. We have sought to simulate and understand consistently observed features of the Somali Current system during the southwest monsoon using a two-layer, nonlinear numerical ocean model driven from rest by a uniform south wind in a flat bottom, rectangular geometry. High spatial resolution in both equatorial and coastal boundary regions was retained in this free-surface model. The model Somali Current is best classed as a time-dependent, barocfinic inertial boundary current. Analytical solutions to a quasi-steady linear model of the Somali Culrent are shown to be self-inconsistent with the linear approximation. While linear theory predicts perfect symmetry about the equator, the nonlinear numerical solutions exhibit marked asymmetries in less than a month as the model Somali Current becomes strongly inertial. By day 30 the current has attained its maximum value (140 cm s -t ) within a few degrees of the equator, in accord with observations. In this time-dependent case, boundary layer separation occurs at the northern end of the inertial current as the northward advection of the current precedes the adjustment of the mass field. Associated with the northward movement e f the baroclinic inertial boundary current is a "great whirl" similar in scale and intensity tQ that observed. This remarkable whirl is characterized by anticyclonic inflow in the upper layer, cyclonic outlfi)w in the lowe~ layer, and a northward translation speed of about 27 cm s -~ . At the coasl~ west of the whirl, is an upwelling maximum also found in the observations. A consideration of the eastern and equatorial solution shows that the south wind case excites the n 0 mode for equatorially trapped inertia-gravity oscillations. These oscillations are strongly coupled to the eastern boundary layer and excite a poleward propagating train of internal Kelvin waves. Prior to the arrival of the leading edge of the wave train, upwelling (downwelling) occurs south (north) of the equator at the eastern boundary. Due to the symmetry properties of the solution, no internal Kelvin wave of significant amplitude is excited anywhere along the western boundary. The trapped inertia-gravity oscillations are damped as a Yanai wave propagates away from the western boundary. Significantly, in the eastern equatorial ocean the time scale for cessation of vertical motion driven by a meridional wind is the same as that for onset for a zonal wind.
275
the Tsushima Current and the Korean coastal waters was routinely detected and displayed on the electro-optically contoured thermal imagery. The contour intervals are 1.6°C, and noise-induced effects give a thermal resolution of 0.8°C. Temperature differences across the front of 3.29°C ( a = 0.42°C) were measured be seven ship crossings, and temperature differences of 3.3°C ( a = 0.4°C) were estimated from five satellite overpasses. Thermal patterns of a one-sided divergence and a cyclonic eddy were detected in the coastal waters at the flow separation where the western edge of the Tsushima Current curves away from the coast into the Sea of Japan. A critique of the oceanographic capabilities and limitations of the system is provided as a guide to potential users. HUPPERT H. E. and KIRK BRYAN, 1976. Topographically generated eddies. Deep-Sea Res., 23 (8): 655-679. The interaction between temporally varying currents and the bottom topography of the ocean is investigated by the numerical and analytic examination of the following simple model. The flow of an inviscid, stratified fluid is initiated from relative rest in a uniformly rotating system containing an isolated topographic feature. The evolution of the flow redistributes vorticity and temperature in such a way that relatively cold water with anticyclonic vorticity exists over the topographic feature, while water shed from above the topographic feature sinks, thereby inducing a warm anomaly with cyclonic vorticity. For sufficiently strong oncoming flows, the shed fluid continually drifts downstream in the form of a relatively warm eddy. If the oncoming flow is relatively weak, the interaction between the anticyclonic and cyclonic vorticity distributions traps the warm eddy and it remains in the vicinity of the topographic feature. We suggest that recent observations of an eddy in the vicinity of the Atlantis 1I Seamount and the existence of the large amount of high frequency energy near the bottom of the ocean measured by the MODE experiment may be partly explained in terms of the above mechanism. We conclude by speculating that vorticity redistribution by topography may be a contributing factor to cyclogenesis in the atmosphere. HURLBURT H . E . , J. C. KINDLE and J. J. O'BRIEN, 1976. A numerical simulation of the onset of El Nifio. ,L Phys.
Oceanogr., 6 (5): 621-631. El Nifio may be defined oceanographically as a massive influx of.warm water into the coastal region of Ecuador and Peru. We have tested the hypothesis that these rare events occur after a substantial reduction of the atmospheric trade winds over the central Pacific Ocean. An idealized nonlinear, two-layer, equatorial beta-plane ocean is spun-up with easterly winds for 50 days after which the wind is relaxed over several days. The relaxation of the wind initiates internal Kelvin wave fronts at both sides of the ocean at the equator. The eastern wave fronts propagate poleward and the western ones eastward. Internal Rossby waves are generated which propagate westward from the eastern boundary. As the Kelvin wave fronts move poleward along the eastern boundary, strong downwelling occurs and the coastal currel,ts reverse direction and become poleward. The rapid downwelling and the sudden reversal of the coastal current are consistent with observations during El Nifio. This downwelling is much more rapid than the upwelling which occurred during spin-up due to nonlinear Kelvin wave dispersion. The dispersion results in the development of a frontal character at the leading edge of the waves. When the western Kelvin wave fronts reach the eastern boundary, the downwelling ceases and the poleward currents separate from the coast, propagating westward as Rossby waves. Thus we suggest the pulsating nature of E1 Nifio is related to the occurrence of major equatorial wind changes and the dynamics of internal Kelvin waves whose nonlinear attributes may greatly sharpen the pulses. HURLBURT H. E. and J. D. THOMPSON, 1976. A numerical model of the Somali Current. J. phys. Oceanogr., 6 (5t: 646-664. We have sought to simulate and understand consistently observed features of the Somali Current system during the southwest monsoon using a two-layer, nonlinear numerical ocean model driven from rest by a uniform south wind in a flat bottom, rectangular geometry. High spatial resolution in both equatorial and coastal boundary regions was retained in this free-surface model. The model Somali Current is best classed as a time-dependent, barocfinic inertial boundary current. Analytical solutions to a quasi-steady linear model of the Somali Culrent are shown to be self-inconsistent with the linear approximation. While linear theory predicts perfect symmetry about the equator, the nonlinear numerical solutions exhibit marked asymmetries in less than a month as the model Somali Current becomes strongly inertial. By day 30 the current has attained its maximum value (140 cm s -t ) within a few degrees of the equator, in accord with observations. In this time-dependent case, boundary layer separation occurs at the northern end of the inertial current as the northward advection of the current precedes the adjustment of the mass field. Associated with the northward movement e f the baroclinic inertial boundary current is a "great whirl" similar in scale and intensity tQ that observed. This remarkable whirl is characterized by anticyclonic inflow in the upper layer, cyclonic outlfi)w in the lowe~ layer, and a northward translation speed of about 27 cm s -~ . At the coasl~ west of the whirl, is an upwelling maximum also found in the observations. A consideration of the eastern and equatorial solution shows that the south wind case excites the n 0 mode for equatorially trapped inertia-gravity oscillations. These oscillations are strongly coupled to the eastern boundary layer and excite a poleward propagating train of internal Kelvin waves. Prior to the arrival of the leading edge of the wave train, upwelling (downwelling) occurs south (north) of the equator at the eastern boundary. Due to the symmetry properties of the solution, no internal Kelvin wave of significant amplitude is excited anywhere along the western boundary. The trapped inertia-gravity oscillations are damped as a Yanai wave propagates away from the western boundary. Significantly, in the eastern equatorial ocean the time scale for cessation of vertical motion driven by a meridional wind is the same as that for onset for a zonal wind.