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Mark A. Cane

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Mark A. Cane

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Mark A. Cane

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We test the hypothesis that sea level variations associated with EL Niño events are a response to wind changes in the central Pacific and that the signal is transmitted to the coast of South America by packets of equatorial Kelvin waves. A linear model is forced by wind stress anomalies composited from six EL Niños occurring between 1951 and 1972 (Rasmusson and Carpenter). Model sea level is compared with similar composites of tidegage measurements from selected equatorial Pacific stations.

Although several vertical modes are included in the calculation, only the two gravest baroclinic modes make significant contributions to the sea level signal. Model results duplicate the pattern and timing of the observed sea level anomalies but amplitudes are systematically low. This is especially true at central and western Pacific stations, and it is suggested that the poor quality of the forcing data may be at fault there. Results at the east provide better support for the theory: in particular, the twin-peaked signal characteristic of El Niño sea-level anomalies is reproduced. The second peak is shown to be a response to the massive collapse of the trades occurring in the middle of the El Niño year and its amplitude is correctly hindcast by the model. The first peak is a response to the weaker wind changes occurring in the boreal fall preceding El Niño; its calculated amplitude is too small. The implication of this discrepancy is that the linear Kelvin wave theory will have to be modified if it is to account for the initial El Niño warning.

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Nandini Ramesh and Mark A. Cane

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Tropical Pacific decadal variability (TPDV), though not the totality of Pacific decadal variability, has wide-ranging climatic impacts. It is currently unclear whether this phenomenon is predictable. In this study, we reconstruct the attractor of the tropical Pacific system in long, unforced simulations from an intermediate-complexity model, two general circulation models (GCMs), and the observations with the aim of assessing the predictability of TPDV in these systems. We find that in the intermediate-complexity model, positive (high variance, El Niño–like) and negative (low variance, La Niña–like) phases of TPDV emerge as a pair of regime-like states. The observed system bears resemblance to this behavior, as does one GCM, while the other GCM does not display this structure. However, these last three time series are too short to confidently characterize the full distribution of interdecadal variability. The intermediate-complexity model is shown to lie in highly predictable parts of its attractor 37% of the time, during which most transitions between TPDV regimes occur. The similarities between the observations and this system suggest that the tropical Pacific may be somewhat predictable on interdecadal time scales.

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Daiwei Wang and Mark A. Cane

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By analyzing a set of the Coupled Model Intercomparison Project phase 3 (CMIP3) climate model projections of the twenty-first century, it is found that the shallow meridional overturning of the Pacific subtropical cells (STCs) show contrasting trends between two hemispheres in a warming climate. The strength of STCs and equivalently the STC surface-layer transport tend to be weakening (strengthening) in the Northern (Southern) Hemisphere as a response to large-scale surface wind changes over the tropical Pacific. The STC pycnocline transport convergence into the equatorial Pacific Ocean from higher latitudes shows a robust weakening in the twenty-first century. This weakening is mainly through interior pathways consistent with the relaxation of the zonal pycnocline tilt, whereas the transport change through western boundary pathways is small and not consistent across models. It is found that the change of the western boundary pycnocline transport is strongly affected by the shoaling of the pycnocline base. In addition, there is a robust weakening of the Indonesian Throughflow (ITF) transport in a warming climate. In the multimodel ensemble mean, the response to greenhouse warming of the upper-ocean mass balance associated with the STCs is such that the weakening of the equatorward pycnocline transport convergence is balanced by a weakening of the poleward surface-layer transport divergence and the ITF transport of similar amounts.

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Mark A. Cane and Vladimir Kamenkovich

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Alexander Krupitsky and Mark A. Cane

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The behavior of the solution to a two-layer wind-driven model in a multiply connected domain with bottom topography imitating the Southern Ocean is described. The abyssal layer of the model is forced by interfacial friction, crudely simulating the effect of eddies. The analysis of the low friction regime is based on the method of characteristics. It is found that characteristics in the upper layer are closed around Antarctica, while those in the lower layer are blocked by solid boundaries. The momentum input from wind in the upper layer is balanced by lateral and interfacial friction and by interfacial pressure drag. In the lower layer the momentum input from interfacial friction and interfacial pressure drag is balanced by topographic pressure drag. Thus, the total momentum input by the wind is balanced by upper-layer lateral friction and by topographic pressure drag.

In most of the numerical experiments the circulations in the two layers appear to be decoupled. The decoupling can be explained by the JEBAR term, whose magnitude decreases as interfacial friction increases. The solution tends toward the barotropic one if the interfacial friction is large enough to render the JEBAR term to be no larger than the wind stress curl term in the potential vorticity equation. The change of regimes occurs when the value of the interfacial friction coefficient κ equals κ 0 = H 1f0(L y/L x)(A/H 0), where f 0 is the mean value of the Coriolis parameter; L y and L x are the meridional and zonal domain dimensions; H 0 and H 1 are the mean depths of the ocean and of the upper layer; and A is the amplitude of topographic perturbations. Note that κ 0 does not depend on the strength of the wind stress.

The magnitude of the total transport is found to depend crucially on the efficiency of the momentum transfer from the upper to the lower layer, that is, on the ratio κ/ε, where ε is the lateral friction coefficient. If ε and κ are assumed to be proportional, the upper-layer transport and total transport vary as ε −5/6.

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Mark A. Cane and E. S. Sarachik

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Seasonal heat transport is examined in a simple, linear shallow-water model on the equatorial beta plane. It is found in this model that meridional transport by the seasonally varying western boundary current is of the same magnitude but opposite phase to the seasonally varying interior transport and therefore tends to cancel.

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Julien Emile-Geay and Mark A. Cane

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It has recently been proposed, within the framework of the linear shallow-water equations, that tropical Pacific decadal variability (PDV) can be accounted for by basin modes with eigenperiods of 10 to 20 yr, amplifying a midlatitude wind forcing with an essentially white spectrum. Here the authors use a different formalism of linear equatorial wave theory. The Green’s function is computed for the wind-forced response of a linear equatorial shallow-water ocean and use the earlier results of Cane and Moore to obtain a compact, closed form expression for the motion of the equatorial thermocline, which applies to all frequencies lower than seasonal. This expression is new and allows a systematic comparison of the effect of low- and high-latitude winds on the equatorial thermocline. At very low frequencies (decadal time scales), the planetary geostrophic solution used by Cessi and Louazel is recovered, as well as the equatorial wave solution of Liu, and a formal explanation for this convergence is given. Nonetheless, this more general solution leads one to a different interpretation of the results. In contrast to the aforementioned studies, the authors find that the equatorial thermocline is inherently more sensitive to local than to remote wind forcing and that planetary Rossby modes only weakly alter the spectral characteristics of the response. Tropical winds are able to generate a strong equatorial response with periods of 10 to 20 yr, while midlatitude winds can only do so for periods longer than about 50 yr. The results suggest that ocean basin modes are an unlikely explanation of decadal fluctuations in tropical Pacific sea surface temperature.

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Antonio J. Busalacchi and Mark A. Cane

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A formalism is developed to examine the effect of zonally varying stratification on equatorial wave phenomena; an effect present in the real ocean but neglected from standard linear theory. The approach utilized involves the application of a matching condition to equatorial waves incident on a single zonal discontinuity in the density field of a shallow water system. Transmission and reflection coefficients are sought for the projection of an incoming wave onto the entire set of resultant vertical and horizontal wave modes of a general continuously stratified fluid. The limiting case of a meridional density front is extended, in a manner analogous to radiative transfer problems, to a series of discrete density intervals. These techniques are applied to specific choices of stratification ranging from a zonal jump discontinuity in the density field to density changes with zonal scales large with respect to the waves in question, i.e., a WKB limit. The results demonstrate that zonally varying stratification does not produce substantial changes in the energy flux of propagating equatorial waves. However, as a result of changes to the equatorial radius of deformation, the amplification of equatorial zonal velocity can be appreciable. A corresponding decrease in pressure, albeit smaller, may also be non-negligible.

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