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R. Saravanan

Abstract

The characteristics of atmospheric low-frequency variability and midlatitude SST variability as simulated by the National Center for Atmospheric Research’s Climate System Model are analyzed in the vicinity of the North Pacific and North Atlantic basins. The simulated spatial patterns of variability correspond quite well to those seen in observational datasets, although there are some differences in the amplitudes of variability. Companion uncoupled integrations using the atmospheric component of the coupled model are also analyzed to identify the mechanisms of midlatitude SST variability on interannual timescales. These integrations are subject to a hierarchy of SST boundary conditions, ranging from the climatological annual cycle to global monthly mean observed SST. Even uncoupled atmospheric model integrations forced by climatological SST boundary conditions are capable of simulating the spatial patterns of atmospheric variability fairly well, although coupling to an interactive ocean does produce some improvements in the spatial patterns. However, the presence of realistic SST variability, especially in the Tropics, is necessary to obtain the right variance amplitudes for the different modes of variability. It appears that coupling to an interactive ocean essentially reorders, rather than reshapes, the dominant modes of atmospheric low-frequency variability. The results indicate that the dominant modes of SST variability in each ocean basin are forced by the respective dominant modes of atmospheric low-frequency variability in the vicinity of the ocean basin. The relationship between atmospheric variability and the surface heat flux is also analyzed. Evidence is found for a local thermal feedback in the coupled integration, associated with the finite heat capacity of the ocean, that acts to damp surface heat flux variability. It is also shown that the relationship between midlatitude SST anomalies and the surface heat flux in the Atmospheric Model Intercomparison Project–type of atmospheric model integrations is quite different from that in the coupled model integration.

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R. Saravanan

Abstract

Suarez and Duffy have noted an interesting bifurcation in a two-level gridpoint general circulation model when strong tropical heating is imposed. This bifurcation results in a model climatology with strong upper-level westerlies in the tropics. In this paper, it is argued that this bifurcation is essentially due to the dominant role played by extratropical baroclinic transients in the tropical angular momentum budget. A series of numerical experiments is analyzed with a global two-level primitive equation model, using spectral truncation in the horizontal. The model climatologies in these experiments fall into two categories: 1) conventional, that is, weak upper-level easterlies/westerlies in the tropics; and 2) superrotating, that is, strong upper-level westerlies in the tropics. An attempt is made to explain the maintenance of the general circulation in these two radically different climatologies by studying the properties of unstable normal modes for the two different time-mean states. The spectral characteristics of angular momentum transport due to transient eddies in these two climatologies are also discussed. To understand the meridional propagation of transient eddies, the notion of a “modal” refractive index in the quasigeostrophic approximation is introduced. From this analysis it is concluded that the conventional climatology is stable to weak perturbations, with the “restoring” force being provided primarily by extratropical baroclinic eddies. Strong perturbations completely change the propagation characteristics of these eddies, leading to a bifurcation of the general circulation. This has interesting implications for the range of validity of two-level models and the transitivity of tropospheric general circulation.

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R. Saravanan

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A simple one-dimensional model of the quasi-biennial oscillation is discussed. Our model is essentially a generalization of the Holton–Lindzen models. We consider a large number of vertically propagating internal waves interacting simultaneously with the mean flow. Effects of both wave damping and critical level absorption are included, but wave–wave interaction is neglected. The effects of momentum advection due to the Hadley circulation are also parameterized. This model is used to study how the mean flow in the equatorial lower stratosphere would respond to forcing by a tropospheric wave spectrum with a significant amount of momentum flux at slow horizontal phase speeds. We find that a “continuous” wave spectrum forces mean flow oscillations in a manner quite similar to a “discrete” two-wave spectrum. But the factors that control the period and amplitude of the oscillations, in the case of a continuous spectrum, seem to be quite different. Our results also suggest that mean rising motion in the tropics may play an important role in determining the vertical structure of the QBO near the tropopause.

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A. Capotondi and R. Saravanan

Abstract

The performance of thermal surface boundary conditions based on energy balance models for the atmosphere is tested using a two-dimensional (meridional plane) ocean model. The results are compared to those from an idealized ocean – atmosphere coupled system. The latter consists of a two-dimensional Boussinesq ocean model coupled to a two-layer global atmospheric model. The various thermal boundary conditions are applied to the same ocean model used in the coupled system, and their ability to capture the essential atmospheric feedbacks is investigated. Some of the effects associated with the atmospheric eddy moisture transport are also incorporated by empirically relating variations in the surface freshwater flux to variations in the surface heat flux based on the coupled model results. Comparisons with the coupled results show a considerable improvement in the characteristics of the equilibria of the ocean thermohaline circulation when the alternative thermohaline boundary conditions are used instead of the so-called “mixed boundary conditions” commonly used in ocean-only integrations. Furthermore, the response of the pole-to-pole equilibrium to a freshening of the high northern latitudes is in remarkably good agreement with the one observed in the coupled model. However, a tendency for the “energy balance” boundary conditions to overstabilize the circulation is detected, and limitations in the present treatment of the eddy moisture transport effects are found, especially in the presence of convective adjustment.

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R. Saravanan and Ping Chang

Abstract

The interaction between tropical Atlantic variability and El Niño–Southern Oscillation (ENSO) is investigated using three ensembles of atmospheric general circulation model integrations. The integrations are forced by specifying observed sea surface temperature (SST) variability over a forcing domain. The forcing domain is the global ocean for the first ensemble, limited to the tropical ocean for the second ensemble, and further limited to the tropical Atlantic region for the third ensemble. The ensemble integrations show that extratropical SST anomalies have little impact on tropical variability, but the effect of ENSO is pervasive in the Tropics. Consistent with previous studies, the most significant influence of ENSO is found during the boreal spring season and is associated with an anomalous Walker circulation. Two important aspects of ENSO’s influence on tropical Atlantic variability are noted. First, the ENSO signal contributes significantly to the “dipole” correlation structure between tropical Atlantic SST and rainfall in the Nordeste Brazil region. In the absence of the ENSO signal, the correlations are dominated by SST variability in the southern tropical Atlantic, resulting in less of a dipole structure. Second, the remote influence of ENSO also contributes to positive correlations between SST anomalies and downward surface heat flux in the tropical Atlantic during the boreal spring season. However, even when ENSO forcing is absent, the model integrations provide evidence for a positive surface heat flux feedback in the deep Tropics, which is analyzed in a companion study by Chang et al. The analysis of model simulations shows that interannual atmospheric variability in the tropical Pacific–Atlantic system is dominated by the interaction between two distinct sources of tropical heating: (i) an equatorial heat source in the eastern Pacific associated with ENSO and (ii) an off-equatorial heat source associated with SST anomalies near the Caribbean. Modeling this Caribbean heat source accurately could be very important for seasonal forecasting in the Central American–Caribbean region.

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R. Saravanan and James C. McWilliams

Abstract

Ocean–atmosphere interaction plays a key role in climate fluctuations on interdecadal timescales. In this study, different aspects of this interaction are investigated using an idealized ocean–atmosphere model, and a hierarchy of uncoupled and stochastic models derived from it. The atmospheric component is an eddy-resolving two-level global primitive equation model with simplified physical parameterizations. The oceanic component is a zonally averaged sector model of the thermohaline circulation. The coupled model exhibits spontaneous oscillations of the thermohaline circulation on interdecadal timescales. The interdecadal oscillation has qualitatively realistic features, such as dipolar sea surface temperature anomalies in the extratropics. Atmospheric forcing of the ocean plays a dominant role in exciting this oscillation. Although the coupled model is in itself deterministic, it is convenient to conceptualize the atmospheric forcing arising from weather excitation as having stochastic time dependence. Spatial correlations inherent in the atmospheric low-frequency variability play a crucial role in determining the oceanic interdecadal variability, through a form of spatial resonance. Local feedback from the ocean affects the amplitude of the interdecadal variability. The spatial patterns of correlations between the atmospheric flow and the oceanic variability fall into two categories: (i) upstream forcing patterns, and (ii) downstream response patterns. Both categories of patterns are expressible as linear combinations of the dominant modes of variability associated with the uncoupled atmosphere.

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L. M. Polvani and R. Saravanan

Abstract

The three-dimensional nature of breaking Rossby waves in the polar wintertime stratosphere is studied using an idealized global primitive equation model. The model is initialized with a well-formed polar vortex, characterized by a latitudinal band of steep potential vorticity (PV) gradients. Planetary-scale Rossby waves are generated by varying the topography of the bottom boundary, corresponding to undulations of the tropopause. Such topographically forced Rossby waves then propagate up the edge of the vortex, and their amplification with height leads to irreversible wave breaking.

These numerical experiments highlight several nonlinear aspects of stratospheric dynamics that are beyond the reach of both isentropic two-dimensional models and fully realistic GCM simulations. They also show that the polar vortex is contorted by the breaking Rossby waves in a surprisingly wide range of shapes.

With zonal wavenumber-1 forcing, wave breaking usually initiates as a deep helical tongue of PV that is extruded from the polar vortex. This tongue is often observed to roll up into deep isolated columns, which, in turn, may be stretched and tilted by horizontal and vertical shears. The wave amplitude directly controls the depth of the wave breaking region and the amount of vortex erosion. At large forcing amplitudes, the wave breaking in the middle/lower portions of the vortex destroys the PV gradients essential for vertical propagation, thus shielding the top of the vortex from further wave breaking.

The initial vertical structure of the polar vortex is shown to play an important role in determining the characteristics of the wave breaking. Perhaps surprisingly, initially steeper PV gradients allow for stronger vertical wave propagation and thus lead to stronger erosion. Vertical wind shear has the notable effect of tilting and stretching PV structures, and thus dramatically accelerating the downscale stirring. An initial decrease in vortex area with increasing height (i.e., a conical shape) leads to focusing of wave activity, which amplifies the wave breaking. This effect provides a geometric interpretation of the “preconditioning” that often precedes a stratospheric sudden warming event. The implications for stratospheric dynamics of these and other three-dimensional vortex properties are discussed.

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R. Saravanan and James C. McWilliams

Abstract

Atmospheric variability on timescales of a month or longer is dominated by a small number of large-scale spatial patterns (“teleconnections”), whose time evolution has a significant stochastic component because of weather excitation. One may expect these patterns to play an important role in ocean–atmosphere interaction. On interannual and longer timescales, horizontal advection in the ocean can also play an important role in such interaction. The authors develop a simple one-dimensional stochastic model of the interaction between spatially coherent atmospheric forcing patterns and an advective ocean. The model may be considered a generalization of the zero-dimensional stochastic climate model proposed by Hasselmann. The model equations are simple enough that they can be solved analytically, allowing one to fully explore the parameter space. The authors find that the solutions fall into two regimes: (i) a slowshallow regime where local damping effects dominate and (ii) a fastdeep regime where nonlocal advective effects dominate. An interesting feature of the fast–deep regime is that the ocean–atmosphere system shows preferred timescales, although there is no underlying oscillatory mechanism in the uncoupled ocean or in the uncoupled atmosphere. Furthermore, the existence of the preferred timescale in the ocean does not depend upon a strong atmospheric response to SST anomalies. The timescale is determined by the advective velocity scale associated with the upper ocean and the length scale associated with low-frequency atmospheric variability. For the extratropical North Atlantic basin, this timescale would be of the order of a decade, indicating that advective ocean–atmosphere interaction could play an important role in decadal climate variability. The solutions also highlight the differences between local thermodynamic feedbacks associated with changes in the air–sea temperature difference and nonlocal dynamic feedbacks associated with horizontal ocean advection.

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Ping Chang, Link Ji, and R. Saravanan

Abstract

A hybrid coupled model (HCM) is used to explore the underlying dynamics governing tropical Atlantic variability (TAV) and the dynamic regime that may be most relevant to TAV. By coupling an empirical atmospheric feedback model to an ocean GCM, the authors have conducted a detailed investigation on the potential importance of an unstable ocean–atmosphere interaction between wind-induced heat flux and sea surface temperature (SST) in driving decadal climate variability in the tropical Atlantic basin. The investigation consists of a systematic parameter sensitivity study of the hybrid coupled model. It is shown that in a strong coupling regime the local air–sea feedbacks can support a self-sustained decadal oscillation that exhibits strong cross-equatorial SST gradient and meridional wind variability. An upper-ocean heat budget analysis suggests that the oscillation results from an imbalance between the positive and negative feedbacks in the model. The dominant negative feedback that counteracts the positive feedback between surface heat flux and SST appears to be the advection of heat by ocean currents. The major imbalance in the model occurs in the north tropical Atlantic between 5° and 15°N, caused by a phase delay between the surface heat flux forcing and horizontal heat advection. It is suggested that this may be one of the crucial regions of ocean–atmosphere interactions for TAV.

Based on the HCM results, a simple 1D model is derived to further elucidate key coupled dynamics. The model assumes that air–sea coupling takes place in a limited area within the deep Tropics of the Atlantic sector and the change of upper-ocean heat transport is regulated by the advection of anomalous temperatures by the mean meridional current and equatorial upwelling. The analysis shows that the simple model captures many of the salient features of the decadal SST cycle in the HCM, suggesting that the decadal oscillations simulated by the HCM are primarily controlled by the coupled dynamics local to the deep Tropics.

The parameter sensitivity study further suggests that in reality the local air–sea coupling in the tropical Atlantic is most likely to be too weak to maintain a self-sustained oscillation, and stochastic forcing may be necessary to excite the coupled variability. Using a realistic representation of external “noise” derived from a 145-yr simulation of the National Center for Atmospheric Research atmospheric GCM (CCM3) forced with the observed SST annual cycle, the effect of stochastic forcing on TAV when the coupled system resides in a stable dynamical regime is examined. It is found that the local air–sea feedback and the North Atlantic oscillation–(NAO) dominated “noise” forcing are both required to simulate a realistic TAV. In the absence of the local air–sea feedback, the “noise” forcing can produce substantial SST anomalies in the subtropical Atlantic up to about 15°N, particularly off the coast of North Africa. The local air–sea feedback appears to be particularly important for generating the covarying pattern of interhemipheric SST gradient and cross-equatorial atmospheric flow within the deep Tropics. However, too-strong local coupling can lead to an exaggerated tropical response. It is therefore conjectured that TAV may best fit into a weakly coupled scenario in which at minimum the air–sea feedback plays a role in enhancing the persistence of the cross-equatorial gradient of SST and the circulation anomalies, while the NAO provides an important source of external forcing to excite the coupled variability in the Tropics. Furthermore, it is argued that the“noise” forcing can significantly weaken the correlation between the SST variability on either side of the equator, thus hiding any underlying weak “dipole” structure in the SST.

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Christina M. Patricola, R. Saravanan, and Ping Chang

Abstract

Atlantic tropical cyclone (TC) activity is influenced by interannual tropical Pacific sea surface temperature (SST) variability characterized by the El Niño–Southern Oscillation (ENSO), as well as interannual-to-decadal variability in the interhemispheric gradient in tropical Atlantic SST characterized by the Atlantic meridional mode (AMM). Individually, the negative AMM phase (cool northern and warm southern tropical Atlantic SST anomalies) and El Niño each inhibit Atlantic TCs, and vice versa. The impact of concurrent strong phases of the ENSO and AMM on Atlantic TC activity is investigated. The response of the atmospheric environment relevant for TCs is evaluated with a genesis potential index.

Composites of observed accumulated cyclone energy (ACE) suggest that ENSO and AMM can amplify or dampen the influence of one another on Atlantic TCs. To support the observational analysis, numerical simulations are performed using a 27-km resolution regional climate model. The control simulation uses observed SST and lateral boundary conditions (LBCs) of 1980–2000, and perturbed experiments are forced with ENSO phases through LBCs and eastern tropical Pacific SST and AMM phases through Atlantic SST.

Simultaneous strong El Niño and strongly positive AMM, as well as strong concurrent La Niña and negative AMM, produce near-average Atlantic ACE suggesting compensation between the two influences, consistent with the observational analysis. Strong La Niña and strongly positive AMM together produce extremely intense Atlantic TC activity, supported largely by above average midtropospheric humidity, while strong El Niño and negative AMM together are not necessary conditions for significantly reduced Atlantic tropical cyclone activity.

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