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R. C. Pacanowski

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

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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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S. G. H. Philander, T. Yamagata, and R. C. Pacanowski

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During El Niño Southern Oscillation events modest anomalies amplify spatially and temporally until the entire tropical Pacific Ocean and the global atmospheric circulation are affected. Unstable interactions between the ocean and atmosphere could cause this amplification when the release of latent heat by the ocean affects the atmosphere in such a manner that the altered surface winds induce the further release of latent heat. Coupled shallow water models are used to simulate this instability which is modulated by the seasonal movements of the atmospheric convergence zones.

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J. Willebrand, S. G. H. Philander, and R. C. Pacanowski

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This paper is an analytical and numerical study of the response of the ocean to the fluctuating component of the wind stress as computed from twice-daily weather maps for the period 1973 to 1976. The results are described in terms of (time) mean and rms fields, frequency spectra and horizontal cross spectra, and local cross spectra between oceanic and atmospheric variables.

A forcing function with scales strictly larger than O(100 km) induces oceanic motion that is depth independent at periods between the inertial period and ∼300 days. The dynamics is essentially linear so that rectified currents are small, the associated rectified transport amounts to at most 1–2 Sv in the western boundary layer. Root-mean-square currents are typically a few centimeters per second and are most intense in the western part of the basin, and near major topographic features. Fluctuations in the transport of the western boundary layer can be as large as 20–30 Sv. Three distinct frequency bands characterize the wind-induced barotropic fluctuations: 1) At periods between the inertial period and about one week the energy density increases steeply with decreasing frequency. Current spectra have a slope between −2 and −4. These forced waves can show an (imperfect) coherence between wind stress and the corresponding current components, and between atmospheric pressure and subsurface pressure. But spatial inhomogeneities in the wind field or bottom topography can destroy this coherence. 2) At periods between a week and a month planetary (or topographic) Rossby waves are dominant so that westward phase propagation is prominent. 3) At longer periods westward phase propagation is less evident and there is a time-dependent Sverdrup balance between meridional (cross-isobath) currents and wind stress curl. The spectra at these long periods are frequency independent (white) and the zonal (along-isobath) velocity component is more energetic than the meridional (cross-isobath) component.

Despite the high degree of idealization in the models, local coherence between oceanic and atmospheric variables is virtually nonexistent (except possibly at periods between 1 and 10 days) because of the wavelike structure of the oceanic response, the broadband stochastic character of the atmospheric variability, and inhomogeneities in the wind field and bottom topography.

It is proposed that fluctuations observed at site D north of the Gulf Stream are primarily atmospherically forced. At the MODE central mooring, however, there must be an additional energy source.

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S. G. H. Philander, W. J. Hurlin, and R. C. Pacanowski

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A general circulation model of the tropical Pacific Ocean, which realistically simulates El Niño of 1982–83, has been used to determine how different initial conditions affect the model. Given arbitrary initial conditions (not in equilibrium with the wind) the model takes almost a year to return to a state in which the currents and density gradients are in equilibrium with the winds. Errors in the absolute value of the temperature persist far longer, however, indicating that accurate density data are essential initial conditions. If the correct density field is specified initially, but no information is provided about the currents, then the model recovers the currents within an inertial period, except for the eastern equatorial region. That region is affected by equatorial Kelvin waves which are excited because the model is initially in an unbalanced state. The currents associated with these waves are relatively modest and do not affect the density field significantly. Because of the large zonal scale of the thermal field in the tropical Pacific, three or four high resolution meridional density sections appear adequate for the initialization of the model. This result, however, takes into account neither the energetic waves, with a scale of 1000 km, that are associated with instabilities of the equatorial currents nor other high frequency fluctuations in the ocean.

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Zhengyu Liu, S. G. H. Philander, and R. C. Pacanowski

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Experiments with an oceanic general circulation model indicate that the tropical and subtropical oceanic circulations are linked in three ways. Far from coast in the oceanic interior, equatorial surface waters flow poleward to the southern part of the subtropical gyre, and then are subducted and returned in the thermocline to the upper part of the core of the Equatorial Undercurrent. There is, in addition, a surface western boundary current that carries waters from the equatorial region to the northern part of the subtropical gyre. After subduction, that water reaches the equator by means of a subsurface western boundary current and provides a substantial part (2/3 approximately) of the initial transport of the Equatorial Undercurrent. The eastward flow in the Equatorial Undercurrent is part of an intense equatorial cell in which water rises to the surface at the equator, drifts westward and poleward, then sinks near 3° latitude to flow equatorward where it rejoins the undercurrent.

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S. G. H. Philander, R. C. Pacanowski, N-C. Lau, and M. J. Nath

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A global atmospheric general circulation model (GCM) coupled to an oceanic GCM that is dynamically active only in the tropical Pacific simulates variability over a broad spectrum of frequencies even though the forcing, the annual mean incoming solar radiation, is steady. Of special interest is the simulation of a realistically irregular Southern Oscillation between warm El Niño and cold La Niña states. Its time scale is on the order of 5 years. The spatial structure is strikingly different in the eastern and western halves of the ocean basin. Sea surface temperature changes have their largest amplitude in the central and eastern tropical Pacific, but the low-frequency zonal wind fluctuations are displaced westward and are large over the western half of the basin. These zonal wind anomalies are essentially confined to the band of latitudes 10° to 10°S so that they form a jet and have considerable latitudinal shear. During El Niño the associated curl contributes to a pair of pronounced minima in thermocline depth, symmetrically about the equator in the west, near 8°N and 8°S. In the east, where the low-frequency wind forcing is at a minimum, the deepening of the thermocline in response to the winds in the west has a very different shape—an approximate Gaussian shape centered on the equator.

The low-frequency sea surface temperature and zonal wind anomalies wax and wane practically in place and in phase without significant zonal phase propagation. Thermocline depth variations have phase propagation; it is eastward at a speed near 15 cm s−1 along the equator in the western half of the basin and is westward off the equator. This phase propagation, a property of the oceanic response to the quasi-periodic winds that force currents and excite a host of waves with periods near 5 years, indicates that the ocean is not in equilibrium with the forcing. In other words, the ocean-atmosphere interactions that cause El Niño to develop at a certain time are countered and, in due course, reversed by the delayed response of the ocean to earlier winds. This “delayed oscillator” mechanism that sustains interannual oscillations in the model differ in its details from that prevoiusly discussed by Schopf and Suarez and others. The latter investigators invoke an explicit role for Kelvin and Rossby waves. These waves cannot be identified in the low-frequency fluctuations of this model, but they are energetic at relatively short periods and are of vital importance to a quasi-resonant oceanic mode with a period near 7 months that is excited in the model. The similarities and differences between the results of this simulation and those with other models, especially the one described in a companion paper, are discussed.

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A. V. Fedorov, R. C. Pacanowski, S. G. Philander, and G. Boccaletti

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Studies of the effect of a freshening of the surface waters in high latitudes on the oceanic circulation have thus far focused almost entirely on the deep thermohaline circulation and its poleward heat transport. Here it is demonstrated, by means of an idealized general circulation model, that a similar freshening can also affect the shallow, wind-driven circulation of the ventilated thermocline and its heat transport from regions of gain (mainly in the upwelling zones of low latitudes) to regions of loss in higher latitudes. A freshening that decreases the surface density gradient between low and high latitudes reduces this poleward heat transport, thus forcing the ocean to gain less heat in order to maintain a balanced heat budget. The result is a deepening of the equatorial thermocline. (The deeper the thermocline in equatorial upwelling zones is, the less heat the ocean gains.) For a sufficiently strong freshwater forcing, the poleward heat transport all but vanishes, and permanently warm conditions prevail in the Tropics. The approach to warm oceanic conditions is shown to introduce a bifurcation mechanism for the north–south asymmetry of the thermal and salinity structure of the upper ocean.

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S. G. H. Philander, D. Gu, G. Lambert, T. Li, D. Halpern, N-C. Lau, and R. C. Pacanowski

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Although the distribution of sunshine is symmetrical about the equator, the earth's climate is not. Climatic asymmetries are prominent in the eastern tropical Pacific and Atlantic Oceans where the regions of maximum sea surface temperature, convective cloud cover, and rainfall are north of the equator. This is the result of two sets of factors: interactions between the ocean and atmosphere that are capable of converting symmetry into asymmetry, and the geometries of the continents that determine in which longitudes the interactions are effective and in which hemisphere the warmest waters and the intertropical convergence zone are located. 'The Ocean-atmosphere interactions are most effective where the thermocline is shallow because the winds can readily affect sea surface temperatures in such regions. The thermocline happens to shoal in the eastern equatorial Pacific and Atlantic, but not in the eastern Indian Ocean, because easterly trade winds prevail over the tropical Atlantic and Pacific whereas monsoons, with a far larger meridional component are dominant over the Indian Ocean. That is how the global distribution of the continents, by determining the large-scale wind patterns, causes climatic asymmetries to be prominent in some bands of longitude but not others. The explanation for asymmetries that favor the Northern rather than Southern Hemisphere with the warmest waters and the ITCZ- involves the details of the local coastal geometries: the bulge of western Africa to the north of the Gulf of Guinea and the slope of the western coast of the Americas relative to meridians. low-level stratus clouds over cold waters are crucial to the maintenance of the asymmetries.

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