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The most prominent feature of the time-mean global meridional overturning circulation (MOC) is the Atlantic MOC (AMOC). However, interannual variability of the global MOC is shown here to be dominated by Pacific MOC (PMOC) variability over the full depth of the ocean at most latitudes. This dominance of interannual PMOC variability is robust across modern climate models and an observational state estimate. PMOC interannual variability has large-scale organization, its most prominent feature being a cross-equatorial cell spanning the tropics. Idealized experiments show that this variability is almost entirely wind driven. Interannual anomalies of zonal mean zonal wind stress produce zonally integrated Ekman transport anomalies that are larger in the Pacific Ocean than in the Atlantic Ocean, simply because the Pacific is wider than the Atlantic at most latitudes. This contrast in Ekman transport variability implies greater variability in the near-surface branch of the PMOC when compared with the near-surface branch of the AMOC. These near-surface variations in turn drive compensating flow anomalies below the Ekman layer. Because the baroclinic adjustment time is longer than a year at most latitudes, these compensating flow anomalies have baroclinic structure spanning the full depth of the ocean. Additional analysis reveals that interannual PMOC variations are the dominant contribution to interannual variations of the global meridional heat transport. There is also evidence of interaction between interannual PMOC variability and El Niño–Southern Oscillation.

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.

A time-dependent trajectory algorithm is used to determine the sources of the Pacific Ocean Equatorial Undercurrent (EUC) in a global climate model with ¼° (eddy permitting) resolution and forced with realistic winds. The primary sources and pathways are identified, and the transformation of properties in temperature/salinity space is explored. An estimate for the quantity of recirculation, a notoriously difficult property to estimate from observational data, is given. Over two-thirds of the water in the Pacific EUC at 140°W originates south of the equator; 70% of the EUC is ventilated outside of the Tropics (poleward of 13°S or 10°N): three-quarters of these extratropical trajectories travel through the western boundary currents between their subduction and incorporation into the EUC, and one-fifth of the extratropical trajectories enter and leave the tropical band at least once before entering the EUC.

Pacific Ocean oceanic heat transport is studied in an ocean model coupled to an atmospheric mixed-layer model. The shallow meridional overturning circulation cells in the Tropics and subtropics transport heat away from the equator. The heat transport by the horizontal gyre circulation in the Tropics is smaller and directed toward the equator. The response of the Pacific oceanic heat transport to El Nino-like winds, extratropical winds, and variations in the Indonesian Throughflow is studied. Large, opposing changes are found in the heat transport by the meridional overturning and the horizontal gyres in response to El Nino-like winds. Consequently, the change in total heat transport is relatively small. The overturning transport decreases and the gyres spin down when the winds decrease in the Tropics. This compensation breaks down when the Indonesian Throughflow is allowed to vary in the model. A reduced Indonesian Throughflow, as observed during El Nino-like conditions, causes a large reduction of poleward heat transport in the South Pacific and affects the ocean heat transport in the southern tropical Pacific. Extratropical atmospheric anomalies can affect tropical ocean heat transport as the tropical thermocline is ventilated from the extratropics. The authors find that changes in the heat loss in the midlatitudes affect tropical ocean heat transport by driving an enhanced buoyancy-driven overturning that reaches into the Tropics. The results are related to observed changes in the overturning circulation in the Pacific in the 1990s, sea surface temperarture changes, and changes in atmospheric circulation. The results imply that the ratio of heat transport in the ocean to that in the atmosphere can change.

A number of studies have attempted to explain the cause of decadal variability in the tropical Pacific and explore its possible link to decadal variability in the midlatitude Pacific. To investigate some of the current theories of Pacific decadal variability, a linear, wind-driven model, designed to simulate only baroclinic wave dynamics, was forced with wind stress anomalies in the Pacific Ocean basin from 1945 through 1992. An analysis technique designed to isolate the decadal/interdecadal scale variability from interannual ENSO variability was performed on the model’s thermocline depth anomaly (TDA). It was found that the temporal and spatial patterns of the observed tropical decadal sea surface temperatures are consistent with our modeled TDA. Furthermore, restricting the wind forcing to within 58 of the equator does not substantially alter the decadal/interdecadal variability of the equatorial region. The authors conclude that the observed decadal variability in the low-latitude Pacific is primarily a linear dynamical response to tropical wind forcing and does not directly require an oceanic link to the midlatitudes. The question of how tropical wind anomalies are generated is not addressed. In addition, it is shown that in model scenarios where the wind forcing is restricted to the western equatorial Pacific, the 1976‐77 climate shift is still clearly visible as a dominant feature of tropical decadal variability. The temporal decadal signal of the model-generated TDA is more pronounced during the eastern equatorial upwelling season (July‐September) than in the boreal winter. This is consistent with the observed seasonal bias in tracer and SST data from the eastern equatorial Pacific.

The observed subtropical gyre in the North Pacific shows a shift in meridional location with depth. At shallow levels the density deviation peaks at around 15°N while at deep levels the peak is more like 30°N. It is argued here using analytical solutions to the beta-plane shallow-water equations that such a shift can be explained by the effects of oceanic dissipation processes. These solutions show that the highly damped solution is approximately proportional to Ekman pumping whereas the lightly damped case tends toward the classical Sverdrup solution. In the North Pacific, Ekman pumping peaks near 15°N while the Sverdrup solution peaks at 30°N. It is further demonstrated that 1) density deviations in the upper ocean are more highly influenced by higher order baroclinic modes than those in the deep, which are influenced by the lower modes, and 2) constant dissipation effectively acts much more strongly on the higher order baroclinic modes because of their slower speeds and smaller Rossby radii. T...

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 b...

A linear equivalent barotropic (EB) model is applied to study the effects of the bottom topography H and baroclinicity on the total transport and the position of the Antarctic Circumpolar Current (ACC). The model is based on the observation of Killworth that the time mean velocity field of the FRAM Model is self-similar in the vertical. A realistic large-scale topography H is constructed by filtering 5-minute resolution data with an appropriate smoothing kernel. It is shown that the asymptotic behavior of the solution of the barotropic model (a particular case of the EB model) in the limit of very small bottom friction depends on subtle details of topography and basin geometry. Given the uncertainties of the smoothing procedure the authors conclude that the barotropic model is not robust with respect to possible variations of model topography. The authors found that the EB model with a vertical profile function similar to that of Killworth reproduces the major features of the time- and depth-ave...

Recent modeling studies of thermohaline variability have imposed rapid damping of modeled sea surface temperature (SST) anomalies equivalent to assuming the atmosphere has an infinite heat capacity. Such surface heat flux parameterizations effectively exclude the possibility of SST playing an active role in the thermohaline circulation. The authors present results of simple thermodynamic modeling of the lower atmosphere that suggest the sensitivity of the surface heat fluxes to variations in SST is much smaller than often assumed. It is found that the flux response is strongly dependent on the scale of the SST anomaly. For the very largest scales the fluxes increase by only a few watts per square meter per kelvin change of SST. For the scales typical of observed anomalies the nonlocality of the response enhances the sensitivity, which may reach up to ∼15 W m−2 K−1. This extreme is still less than half of the values typically assumed in ocean models. The small sensitivity arises from the adjustmen...

To determine the value of the adjustable parameters of an ocean model required to optimally fit the observations, an adaptive inverse method is developed and applied to a sea surface temperature (SST) model of the tropical Atlantic. The best-fit calculation is performed by minimizing a misfit between observed and simulated data, which depends on the observational and the modeling errors. An adaptive procedure is designed in which the model being tuned is also used to construct a model of the observational errors. This is done by performing the optimization on the mean seasonal cycle and using the SST anomalies obtained for different years and plausible forcing fields as additional information to construct a sample estimate of the observational error covariance matrix. Assuming idealized modeling errors, the procedure is applied to the SST model of Blumenthal and Cane, yielding refined estimates for several models and heat flux parameters. The simulation of the mean annual SST is improved but not ...

A fast, efficient numerical procedure for modeling the linear low-frequency motions on an equatorial beta plane is developed. The model is capable of simulating the seasonal and interannual variability in realistically shaped ocean basins forced by realistic winds. The timestep allowed is the order of days rather than hours as allowed by more conventional schemes. The numerical method is designed around the special characteristics of low-frequency equatorial waves. A crucial element is the formulation of proper boundary conditions, including those for a partial boundary such as the western end of the Gulf of Guinea. The response of an Atlantic-shaped basin to a periodic wind is compared with the analytic results of Cane and Sarachik for a meridionally unbounded ocean. The response along the equator is essentially the same. A narrow boundary layer forms along the Guinea coast, too narrow to be a single coastally trapped wave: it is the sum of many modes. The near lack of phase variation along the coast and the smallness of the phase difference between the equator and the coast are additional contrasts to the results for an initial value problem studied by Moore and others. The implication is that the annually recurring upwelling in the Gulf of Guinea cannot be adequately modeled as the response to an impulsively applied wind stress.

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 strati...

A model testing procedure based on multivariate statistical analysis has been developed to provide an objective measure of the fit between ocean model simulations and observations, taking into account the uncertainties in the atmospheric forcing and the inaccuracies in the oceanic data. The method is applied to the seasonal variations in an ocean model of the tropical Atlantic. A wind-driven linear multimode model with a simple mixed-layer is tested against surface currents estimated from ship drift. The uncertainties in the observations, and in the model response due to random errors in the wind stress and its interannual variability create a substantial indeterminacy in the available sample, but it is shown that they do not explain the large discrepancies that are found between observed and modeled seasonal surface currents. Nor are the uncertainties in the wind stress bulk formulation sufficient to account for the model-data differences. These can then be attributed unambiguously to the oversi...

A simplified model originally developed to simulate sea surface temperature (SST) in the tropical Pacific is extended in two directions. First, the model is formulated and run for the tropical Atlantic. Second, optimal values are determined for a number of model parameters, especially those determining surface heat flux. In the Pacific, the difference between model SST and data SST is the size of the probable error due to uncertainties in the heat flux, 0 (35 W m−2). in the equatorial Atlantic the discrepancies may also be attributed to heat flux errors but model-data differences in the Atlantic near coastal regions north of 10°N and south of 10°S are too large to be explained by heat flux errors: model problems involving other physical processes must be involved. By giving the best result possible in the face of our uncertain knowledge of the parameters and observations, the optimal-fit procedure unambiguously indicates which oceanic features require model changes in order to be adequately chara...

We describe a new numerical model designed to study the interactions between hydrodynamics and thermodynamics in the upper ocean. The model incorporates both primitive equation dynamics and a parameterization of mixed layer physics. There is a consistent treatment of mixed layer structure for all physical processes. In order to study interplay between dynamics and mixed layer physics in the equatorial ocean, we carried out a series of numerical experiments with simple patterns of wind stress and surface heating. In some cases stratification and/or mixed layer physics were suppressed. On the basis of these experiments we reached the following conclusions: The vertical circulation at the equator is so vigorous that surface heating is essential if stratification is to be maintained for periods longer than a few months. Without stratification to inhibit mixed layer deepening momentum will be mixed uniformly to the main thermocline and the equatorial undercurrent will disappear. Vertical transfers of momentum due to vertical advection and mixed layer entrainment are essential features of equatorial dynamics. These process influence currents, SST and upwelling rates more than changes in sea surface elevation. Consequently, the overall mass field adjustments of equatorial oceans are more nearly linear than are the currents or SST variations. The connection between changes in SST and dynamical quantities such as sea surface topography need not be straightforward. For example, increased upwelling will make the mixed layer shallower but will not reduce SST unless it induces increased entrainment of colder water. The influences of upwelling and down-welling on SST are highly asymmetric so that the influence of perturbations cannot be predicted without considering the mean vertical velocity. The asymmetry in the interaction between vertical velocity and mixed layer physics can result in the formation of surface fronts. On the upwelling side of a w = 0 line the surface layer is cold and shallow while on the downwelling side it is warm and deep. Differential advection creates a temperature discontinuity at the depth discontinuity. It is suggested that the Galapagos Front has this character.

The Kalman filter is implemented and tested for a simple model of sea level anomalies in the tropical Pacific, using tide gauge data from six selected island stations to update the model. The Kalman filter requires detailed statistical assumptions about the errors in the model and the data. In this study, it is assumed that the model errors are dominated by the errors in the wind stress analysis. The error model is a simple covariance function with parameters fit from the observed differences between the tide gauge data and the model output. The fitted parameters are consistent with independent estimates of the errors in the wind stress analysis. The calibrated error model is used in a Kalman filtering scheme to generate monthly sea level height anomaly maps for the tropical Pacific. The filtered maps, i.e., those which result from data assimilation, exhibit fine structure that is absent from the unfiltered model output, even in regions removed from the data insertion points. Error estimates, an ...

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.