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S. G. H. Philander

Abstract

Nonlinearities weaken westward equatorial jets and cause them to be shallower and broader than their linear counterparts. Nonlinear eastward equatorial jets, on the other hand, are more intense, deeper and narrower than linear jets. Since nonlinear effects are important on time scales longer than about one week, winds that fluctuate on such time scales introduce hysteresis effects and can generate flow with a complicated vertical structure in the surface layers of the equatorial oceans. Coastal jets differ from equatorial jets in that they are only weakly influenced by nonlinearities; this result could change if alongshore pressure forces are taken into account.

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S. G. H. Philander

Abstract

The trade winds over the central Pacific are observed to weaken several months after the appearance of anomalously warm surface waters in the eastern equatorial Pacific Ocean. The following results obtained with a numerical model indicate how this relaxation of the winds affect the later stages of El Niño. A weakening of the westward trade winds causes a zonal redistribution of heat in the equatorial oceans and a warming of the eastern part of the basin. The warming depends on the zonal extent of the region over which the winds relax, and on the length of time T for. which the winds relax. As T increases the warming in the east increases until it asymtotes to a maximum value when T exceeds the adjustment time of the basin (which is ∼400 days in the case of the Pacific Ocean). Maximum heating is associated with a permanent weakening of the winds, unless the winds reverse direction and become eastward. Even weak eastward winds for a short period can cause disproportionately large temperature increases (because of nonlinear mechanisms).

In the region where the winds relax, the heating is due to convergence of surface waters on the equator, and advection by accelerating eastward surface currents. As the time scale T increases, the acceleration becomes less pronounced. East of the region where the winds relax, Kelvin waves suppress the thermocline but leave the sea surface temperature unchanged in linear models. In nonlinear models advection by eastward currents in the wake of Kelvin waves can cause a warming, even at the surface. For winds with a realistic spatial and temporal structure the identification of these waves is difficult.

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S. G. H. Philander

Abstract

The Southern Oscillation, an irregular interannual fluctuation between warm El Niño and cold La Niña conditions that has its largest amplitude in the tropical Pacific, is attributable to interactions between the ocean and atmosphere and corresponds to a natural mode of the coupled ocean-atmosphere system (somewhat analogous to the way in which weather corresponds to an unstable mode of the atmosphere). Stability analyses reveal that a variety of unstable modes are possible. Coupled ocean-atmosphere models that march forward in time (and can be used for predictions) capture some of these modes. The differences between the various models and their relevance to the observed phenomenon are discussed.

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S. G. H. Philander

Abstract

Because of the narrow region over which it has high speeds, the Equatorial Undercurrent has little effect on long waves with large phase speeds, such as long Kelvin and equatorially trapped inertia-gravity modes with equivalent depths ≳30 cm. The Rossby branch of the Rossby-gravity family, and the gravest Rossby modes, have phase velocities comparable to the maximum speed of the Undercurrent and are significantly modified by this current. Meanders of the Undercurrent that are due to superimposed neutral (non-amplifying) waves must have westward phase propagation; standing or eastward traveling meanders are possible only if the Equatorial Undercurrent is unstable.

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S. G. H. Philander

Abstract

El Niño and La Niña are the two complementary phase of the Southern Oscillation. During E1 Niña, the area of high sea surface temperatures increases, while the atmospheric convection zones of the tropical Pacific expand and merge so that there is a tendency toward spatially homogeneous conditions. La Niña is associated with low sea surface temperatures near the equator, with atmospheric convergence zones that are isolated from each other, and with spatial wales smaller than those of El Niña. It is proposed that both phases of the Southern Oscillation can be attributed to unstable interactions between the tropical ocean and atmosphere. During El Niña, the increase release of latent heat to the atmosphere drives the instability. During La Niña, when the heating of the atmosphere decreases, the compression of the convection into smaller and smaller areas permits an instability that intensifies the trade winds and the oceanic currents. The unstable air-sea interactions are modulated by the seasonal movements of the atmospheric convergence zones, and this determines some of the characteristics of the perturbations that can be amplified. The zonal integral of winds along the equator, rather than winds over a relatively small part of the Pacific such as the region west of the date line, is identified as a useful indicator of subsequent developments in the Pacific.

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S. G. H. Philander and J-H. Yoon

Abstract

The adjustment of the eastern coastal zone of an inviscid ocean with vertical walls to a change in wind conditions occurs in two stages. After the propagation of a Kelvin wave across the forced region in a time Tk which is of the order of a day or two, the coastal upwelling zone is temporarily in equilibrium with the wind. Further adjustment occurs after a time TR, which is of the order of a few mouths, when westward Rossby dispersion of the coastal jet becomes important. These time scales define three frequency ranges that characterize the response to fluctuating winds with period P. 1) At high frequencies (PTk) short Kelvin waves can destroy coherence between the forcing and response, alonshore coherence of oceanic variables is small, and the spectrum of the response is red even if that of the forcing is white. The offshore wale of the response is the radius of deformation. Poleward phase propagation at Kelvin wave speed c in unforced regions and at speed 2c in the forced region is prominent in this frequency range and at an lower frequencies. 2) At intermediate frequencies (TkP TR) long Kelvin waves from the boundary of the forced region establish an equilibrium response so that the ocean and atmosphere are practically in phase, but Kelvin waves excited by remote winds could destroy this coherence. Alongshore correlations are high and the spectrum of the response is much less red than at higher frequencies. The offshore male exceeds the radius of deformation and increases with decreasing frequency. 3) At low frequencies (PTR) the offshore scale is the distance Rossby waves travel in time P. A complex system of northward and southward currents appears near the eastern boundary of the basin. It is proposed that the California Current system is generated in this manner.

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J. A. Carton and S. G. H. Philander

Abstract

Four years of winds from the northeastern Pacific are used to drive a reduced-gravity ocean model which includes a high-resolution eastern coastal zone that span 17° longitude and 30° latitude. Spectra of the alongshore velocity and interface height, measured in the coastal zone, are red to 100-day periods. At periods less than 50 days, 1) the circulation is strongly trapped within a radius of deformation of the coast and 2) the alongshore current is well correlated with the alongshore wind stress. At periods longer than 50 days, wind-stress curl becomes important. The alongshore pressure gradient becomes well correlated with the alongshore wind stress. Much of the ocean variability is at periods longer than 10 days. At periods longer than 100 days the alongshore currents begin to weaken and disperse away from the eastern boundary in a series of jets alternating northward and southward.

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Yi Chao and S. G. H. Philander

Abstract

A realistic oceanic general circulation model is forced with winds observed over the tropical Pacific between1967 and 1979. The structure of the simulated Southern Oscillation is strikingly different in the western andeastern sides of the basin, because the principal interannual zonal-wind fluctuations are confined to the westand are in the form of an equatorial jet. This causes thermocline displacements to have maxima offthe equatorin the west (where the curl of the wind is large) but on the equator in the east. Zonal phase propagation, bothon and offthe equator, is at different speeds in the west and east. The phase pattern is complex, and there is,on interannual time scale, no explicit evidence ofindividuai equatorial waves. These results lead to a modificationof the "delayed oscillator" mechanism originally proposed by Schopfand Suarez to explain a continual SouthernOscillation. The results also permit an evaluation of the various coupled ocean-atmosphere models that simulatethe Southern Oscillation and indicate which measurements are necessary to determine which models are most- relevant to reality.

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S. G. H. Philander and W. J. Hurlin

Abstract

The heat budget of a model that realistically simulate the 1982–83 El Niño indicates that the enormous changes in the winds during that event failed to disrupt the usual seasonal variations in meridional heat transport. Cross-equatorial transport towards the winter hemisphere continued as in a regular seasonal cycle. The key factor was the continued seasonal migrations of the ITCZ during El Niño. In early 1983 the ITCZ strayed farther south than usual and remained near the equator longer than usual thus causing an increase in the northward heat transport. This, together with an increase in the evaporative heat loss because of higher sea surface temperature, resulted in a large loss of heat from the band of latitudes approximately 12°N–12°S during El Niño.

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R. C. Pacanowski and S. G. H. Philander

Abstract

Measurements indicate that mixing processes are intense in the surface layers of the ocean but weak below the thermocline, except for the region below the core of the Equatorial Undercurrent where vertical temperature gradients are small and the shear is large. Parameterization of these mixing processes by means of coefficients of eddy mixing that are Richardson-number dependent, leads to realistic simulations of the response of the equatorial oceans to different windstress patterns. In the case of eastward winds results agree well with measurements in the Indian Ocean. In the case of westward winds it is of paramount importance that the nonzero heat flux into the ocean be taken into account. This beat flux stabilizes the upper layers and reduces the intensity of the mixing, especially in the cast. With an appropriate surface boundary condition, the results are relatively insensitive to values assigned to constants in the parameterization formula.

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