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Tianming Li
and
S. G. H. Philander

Abstract

Although the seasonal cycle of the equatorial Atlantic and Pacific Oceans have many similarities, for example, an annual signal is dominant at the equator even though the sun “crosses” the equator twice a year, different processes determine the seasonal cycles of the two oceans and in the Atlantic different processes are important in the east and west. In the Gulf of Guinea in the eastern equatorial Atlantic, the seasonal cycle of surface winds is primarily in response to seasonal variations in land temperatures so that annual changes in sea surface temperatures are, to a first approximation, the passive response of the ocean to the winds. The seasonal cycle of the western equatorial Atlantic has similarities with that of the equatorial Pacific—both are strongly influenced by ocean–atmosphere interactions in which the surface winds and sea surface temperature patterns depend on each other—but only in the western equatorial Atlantic are the seasonal variations in sea surface temperature influenced by vertical excursions of the thermocline. These results are obtained by means of a general circulation model of the atmosphere and a relatively simple coupled ocean–atmosphere model.

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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

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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Daifang Gu
and
S. G. H. Philander

Abstract

Wavelet transforms, which can unfold signals in both time and frequency domains, are used to analyze the Comprehensive Ocean and Atmospheric Data Sets for the period 1870–1988. The focus is on secular changes in the interannual variability and the annual cycle of selected equatorial regions. The amplitude of El Niño/Southern Oscillation (ENSO) is found to be large from 1885 to 1915, to be small during the period 1915–1950, and to increase rapidly after about 1960. Surprisingly, the decadal variations in the amplitude of ENSO are not matched by similar decadal variations in the amplitude of the annual cycle.

On short timescales of 2–5 years, ENSO strongly influences the annual cycle in certain parts of the central and eastern tropical Pacific where the thermocline is shallow. The annual cycle is weak in warm El Niño years and is strong in cold La Niña years. This result suggests that the amplitude of the seasonal cycle is affected by interannual variations in the depth of the thermocline and in the intensity of the trade winds.

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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
Yi Chao

Abstract

Although the winds on the equator at 28°W in the Atlantic and 140°W in the Pacific have similar seasonal variations, the current fluctuations have pronounced differences. In the Pacific the maximum speed of the Equatorial Undercurrent, attained in the northern spring, can exceed 140 cm s−1, while the minimum speed, in the autumn, is less than 80 cm s−1. In the Atlantic the maximum speed of 80 cm s−1 hardly varies seasonally, although it tends to be largest in the autumn. Analyses of results from a realistic simulation of the equatorial currents indicate that the larger zonal extent of the Pacific, and the seasonal variations of the winds over the western Pacific, which can be out of phase with those in the east, are the principal reasons for the differences between the Atlantic and Pacific.

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Zhengyu Liu
and
S. G. H. Philander

Abstract

An oceanic GCM is used to investigate the response of the tropical and subtropical thermocline circulation and structure to different wind stress patterns. Although the subtropical winds do not affect the transport or the speed of the Equatorial Undercurrent significantly, they do change the equatorial temperature field in the lower part of the equatorial thermocline significantly. A weaker subtropical wind curl causes a cooling of the subsurface equatorial region and, hence, an intensification of the equatorial thermocline. A weakening of the subtropical wind curl by a factor of 2 cools the equatorial lower thermocline water by 2°C.

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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

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

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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