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R. Kleeman

Abstract

A low-order tropical atmospheric model is developed that gives a reasonable account of the tropical precipitation and circulation anomalies observed during various El Niño and La Niña events. The dynamical part of the model is a linearization about a state of rest with the usual tropical vertical mode retained. The heating is obtained from latent and nonlatent sources with the former obtained from perturbing a steady-state moisture equation about climatology and confining the heating to high SST regions. A thorough sensitivity analysis is undertaken and the model tendency to place major anomalies on the western and northern side of SST anomalies is examined.

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S. B. Power and R. Kleeman

Abstract

A global ocean general circulation model is forced using various upper boundary conditions (BCs) on temperature and salinity. Solutions are obtained under restoring and mixed BCs (i.e., a restoring condition on the upper-level temperature but using a fixed, specified surface salt flux).

Salt flux anomalies are temporarily applied under mixed BCs, and solutions are obtained in which overturning associated with deep-water formation is either present or absent in the North Atlantic and either vigorous or weak in the North Pacific. A comparison between these solutions helps to clarify the role North Atlantic deep-water formation plays in maintaining the current climate.

The surface heat fluxes differ substantially between the solutions, and their very existence is dependent upon these differences. As a result they are not multiple equilibria of the ocean model alone. Instead, they should be regarded as multiple equilibria of a very crude coupled atmosphere-ocean system. As the global ocean is substantially altered between the equilibria, it is unreasonable to expect that the parameters in the heat flux formulation will remain unchanged. Consequently, the calculated heat flux anomaly may be in error and there is, therefore, no guarantee that the additional equilibria will exist in more sophisticated models. If multiple equilibria do actually exist in such models, they could be quite different from those obtained under the restorative condition.

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M. Latif, R. Kleeman, and C. Eckert

Abstract

The dominant variability modes in the Tropics are investigated and contrasted with the anomalous situation observed during the last few years. The prime quantity analyzed is anomalous sea surface temperature (SST) in the region 30°S–60°N. Additionally, observed tropical surface wind stress fields were investigated. Further tropical atmospheric information was derived from a multidecadal run with an atmospheric general circulation model that was forced by the same SSTs. The tropical SST variability can be characterized by three modes: an interannual mode [the El Niño–Southern Oscillation (ENSO)], a decadal mode, and a trend or unresolved ultra-low-frequency variability.

The dominant mode of SST variability is the ENSO mode. It is strongest in the eastern equatorial Pacific, but influences also the SSTs in other regions through atmospheric teleconnections, such as the Indian and North Pacific Oceans. The ENSO mode was strong during the 1980s, but it existed with very weak amplitude and short period after 1991. The second most energetic mode is characterized by considerable decadal variability. This decadal mode is connected with SST anomalies of the same sign in all three tropical oceans. The tropical Pacific signature of the decadal mode resembles closely that observed during the last few years and can be characterized by a horseshoe pattern, with strongest SST anomalies in the western equatorial Pacific, extending to the northeast and southeast into the subtropics. It is distinct from the ENSO mode, since it is not connected with any significant SST anomalies in the eastern equatorial Pacific, which is the ENSO key region. However, the impact of the decadal mode on the tropical climate resembles in many respects that of ENSO. In particular, the decadal mode is strongly linked to decadal rainfall fluctuations over northeastern Australia in the observations. It is shown that the anomalous 1990s were dominated by the decadal mode.

Considerable SST variability can be attributed also to a linear trend or unresolved ultra-low-frequency variability. This trend that might be related to greenhouse warming is rather strong and positive in the Indian Ocean and western equatorial Pacific where it accounts for up to 30% of the total SST variability. Consistent with the increase of SST in the warm pool region, the trends over the tropical Pacific derived from both the observations and the model indicate a strengthening of the trade winds. This is inconsistent with the conditions observed during the 1990s. If the wind trends reflect greenhouse warming, it must be concluded that the anomalous 1990s are not caused by greenhouse warming.

Finally, hybrid coupled ocean–atmosphere model experiments were conducted in order to investigate the sensistivity of ENSO to the low-frequency changes induced by the decadal mode and the trend. The results indicate that ENSO is rather sensitive to these changes in the background conditions.

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Richard Kleeman, Andrew M. Moore, and Neville R. Smith

Abstract

An adjoint variational assimilation technique is used to assimilate observations of both the oceanic state and wind stress data into an intermediate coupled ENSO prediction model. This method of initialization is contrasted with the more usual method, which uses only wind stress data to establish the initial state of the ocean. It is shown that ocean temperature data has a positive impact on the prediction skill in such models. On the basis of hindcasts for the period 1982–91, it is shown that NIN03 SST anomaly correlations greater than 0.7 can be obtained for hindcasts of duration up to 13 months and greater than 0.6 up to 16 months. There are also clear indications of skill at two years.

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J. Zavala-Garay, C. Zhang, A. M. Moore, and R. Kleeman

Abstract

The possibility that the tropical Pacific coupled system linearly amplifies perturbations produced by the Madden–Julian oscillation (MJO) is explored. This requires an estimate of the low-frequency tail of the MJO. Using 23 yr of NCEP–NCAR reanalyses of surface wind and Reynolds SST, we show that the spatial structure that dominates the intraseasonal band (i.e., the MJO) also dominates the low-frequency band once the anomalies directly related to ENSO have been removed. This low-frequency contribution of the intraseasonal variability is not included in most ENSO coupled models used to date. Its effect in a coupled model of intermediate complexity has, therefore, been studied. It is found that this “MJO forcing” (τ MJO) can explain a large fraction of the interannual variability in an asymptotically stable version of the model. This interaction is achieved via linear dynamics. That is, it is the cumulative effect of individual events that maintains ENSOs in this model. The largest coupled wind anomalies are initiated after a sequence of several downwelling Kelvin waves of the same sign have been forced by τ MJO . The cumulative effect of the forced Kelvin waves is to persist the (small) SST anomalies in the eastern Pacific just enough for the coupled ocean–atmosphere dynamics to amplify the anomalies into a mature ENSO event. Even though τ MJO explains just a small fraction of the energy contained in the stress not associated with ENSO, a large fraction of the modeled ENSO variability is excited by this forcing. The characteristics that make τ MJO an optimal stochastic forcing for the model are discussed. The large zonal extent is an important factor that differentiates the MJO from other sources of stochastic forcing.

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S. B. Power, R. Kleeman, R. A. Colman, and B. J. McAvaney

Abstract

An atmospheric general circulation model (AGCM), a simplified atmospheric model (SAM) of surface heat flux, and various idealized analytic models have been used to investigate the atmospheric response over the North Atlantic to SST anomalies including a general cooling associated with a weakened thermohaline circulation. Latent heating dominates the surface heat flux response, while sensible heating plays an important secondary role. The total heat flux response is weaker than presumed in recent studies using ocean models under highly idealized surface boundary conditions. This implies that stability of the thermohaline circulation to high-latitude freshening in more sophisticated coupled systems (that incorporate either AGCMs or models like SAM) will be increased.

All three kinds of atmospheric models exhibit nonrestorative behavior away from the anomaly peak that is primarily associated with the advection of cooled air eastward. This simple picture is complicated in the AGCM by the fact that the winds weaken over the SST anomaly, which helps to moderate the response.

Analytic models for atmospheric temperature forced using imposed surface temperature anomalies highlight conditions under which a nonrestorative response can arise. Previous work has shown that the length scale of spatially periodic anomalies partially determines the magnitude of the response in a diffusive atmosphere. Here the authors show that this scale dependence has much wider applicability by considering more localized anomalies and by the inclusion of advective transport processes.

The modification of the response by sea ice changes and the absence of any statistically significant change in the basin-averaged hydrological cycle are also discussed.

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S. B. Power, N. R. Smith, R. Kleeman, A. M. Moore, and D. A. Post

Abstract

A global ocean general circulation model is forced using mixes boundary conditions (i.e., a restoring condition on the upper-level temperature but using a fixed, specified surface salt flux). Freshwater flux anomalies lasting 5 years are then applied over the western half of the subpolar gyre in the northern North Atlantic.

The current climate is found to be stable to anomalies that have salt deficits equivalent to about seven times that estimated for the “great salinity anomaly” of 1968–1982, although this value is a function of the duration over which the anomaly is imposed. Above this level the thermohaline circulation collapses to a state in which the zonally averaged overturning associated with North Atlantic Deep Water formation is only about half its original value, the sea surface temperatures over the North Atlantic are lowered, and both the subpolar and subtropical gyres have weakened horizontal transports. Various atmospheric feedbacks on the momentum and salt flux are then applied under a restorative condition on temperature. The feedbacks on the momentum flux do not have a significant impact on the overturning, other than to increase the Ekman flow, while a modest recovery is possible if the salt flux feedback includes an enhanced divergence of freshwater out of the Atlantic basin.

In contrast, the collapse is critically dependent upon the restorative condition on temperature. This central role suggests that the heat flux feedback maintains the stability exhibited by the collapsed state modeled by Manabe and Stouffer.

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J. Zavala-Garay, A. M. Moore, C. L. Perez, and R. Kleeman

Abstract

In this work the role that observed intraseasonal atmospheric variability may play in controlling and maintaining ENSO variability is examined. To this end, an asymptotically stable intermediate coupled model of El Niño–Southern Oscillation (ENSO) is forced with observed estimates of stochastic forcing, which are defined to be the part of the atmospheric variability that is apparently independent of the ocean circulation. The stochastic forcing (SF) was estimated from 51 yr (1950–2000) of NCEP–NCAR reanalyses of surface winds and net surface heat flux, 32 yr (1950–81) of reconstructed sea surface temperatures (SST), and 19 yr (1982–2000) of Reynolds SST in the tropical Pacific. The deterministic component of the surface wind and heat flux anomalies that can be linearly related to SST anomalies was estimated using the singular value decomposition of the covariance between the anomaly fields, and was then removed from the atmospheric anomaly fields to recover the stochastic component of the ocean surface forcing. Principal component analysis reveals that the stochastic component has no preferred mode of variability, exhibits decorrelation times of a few days, and has a spectrum that is indistinguishable from red noise. A 19-yr stochastically forced coupled model integration qualitatively shows some similarities with the observed equatorial SST. The robustness of this result is checked by performing different sensitivity experiments. The model mostly exhibits a linear (and nonnormal) response to the low-frequency tail of SF. Using the ideas of generalized linear stability theory, the dynamically important contributions of the SF are isolated, and it is shown that most of the variability in the stochastically forced model solution is produced by stochastically induced Kelvin waves forced in the western and central Pacific. Moreover, the two most dynamically important patterns of stochastic forcing (which account for 71% of the expected variance in the model response) describe eastward propagation of the forcing similar to the MJO. The results of this study support the hypothesis that a significant fraction of ENSO variability may be due to SF, and suggest that a better understanding of the influence of SF on the ocean surface in the western/central Pacific may be required in order to understand the predictability of ENSO.

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Andrew M. Moore, Jérôme Vialard, Anthony T. Weaver, David L. T. Anderson, Richard Kleeman, and Jolie R. Johnson

Abstract

In this paper the structure and dynamics of the optimal perturbations of tropical low-frequency coupled ocean–atmosphere oscillations relevant to El Niño–Southern Oscillation (ENSO) are explored. These optimal perturbations yield information about potential precursors for ENSO events, and about the fundamental dynamical processes that may control perturbation growth and limit the predictability of interannual variability. The present study uses a hierarchy of hybrid coupled models. Each model is configured for the tropical Pacific Ocean and shares a common ocean general circulation model. Three different atmospheric models are used: a statistical model, a dynamical model, and a combination of a dynamical model and boundary layer model. Each coupled model possesses a coupled ocean–atmosphere eigenmode oscillation with a period of the order of several years. The properties of these various eigenmodes and their corresponding adjoint eigenmodes are explored.

The optimal perturbations of each coupled model for two different perturbation growth norms are also examined, and their behavior can be understood in terms of the properties of the aforementioned eigenmode oscillations. It is found that the optimal perturbation spectrum of each coupled model is primarily dominated by one member. The dominant optimal perturbation evolves into the most unstable eigenmode of the system. The structure of the optimal perturbations of each model is found to be controlled by the dynamics of the atmospheric model and air–sea interaction processes. For the coupled model with a statistical atmosphere, the optimal perturbation center of action is spread across the entire tropical Pacific in the form of a dipole. For the coupled models that include deep atmospheric convection, the optimal perturbation center of action is primarily confined to the western Pacific warm pool. In addition, the degree of nonnormality of the eigenmodes is controlled by the atmospheric model dynamics. These findings are in general agreement with the results obtained from intermediate coupled models. In particular, the atmospheric models used here have also been used in intermediate coupled models that have been employed extensively in previous studies of the optimal perturbations of El Niño–Southern Oscillation. Thus, a direct comparison of the optimal perturbation behavior of those intermediate models and the optimal perturbations of the hybrid models used here can be made.

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J. Zavala-Garay, C. Zhang, A. M. Moore, A. T. Wittenberg, M. J. Harrison, A. Rosati, Jérôme Vialard, and R. Kleeman

Abstract

A common practice in the design of forecast models for ENSO is to couple ocean general circulation models to simple atmospheric models. Therefore, by construction these models (known as hybrid ENSO models) do not resolve various kinds of atmospheric variability [e.g., the Madden–Julian oscillation (MJO) and westerly wind bursts] that are often regarded as “unwanted noise.” In this work the sensitivity of three hybrid ENSO models to this unresolved atmospheric variability is studied. The hybrid coupled models were tuned to be asymptotically stable and the magnitude, and spatial and temporal structure of the unresolved variability was extracted from observations. The results suggest that this neglected variability can add an important piece of realism and forecast skill to the hybrid models. The models were found to respond linearly to the low-frequency part of the neglected atmospheric variability, in agreement with previous findings with intermediate models. While the wind anomalies associated with the MJO typically explain a small fraction of the unresolved variability, a large fraction of the interannual variability can be excited by this forcing. A large correlation was found between interannual anomalies of Kelvin waves forced by the intraseasonal MJO and the Kelvin waves forced by the low-frequency part of the MJO. That is, in years when the MJO tends to be more active it also produces a larger low-frequency contribution, which can then resonate with the large-scale coupled system. Other kinds of atmospheric variability not related to the MJO can also produce interannual anomalies in the hybrid models. However, when projected on the characteristics of Kelvin waves, no clear correlation between its low-frequency content and its intraseasonal activity was found. This suggests that understanding the mechanisms by which the intraseasonal MJO interacts with the ocean to modulate its low-frequency content may help to better to predict ENSO variability.

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