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Daniel J. Vimont

Abstract

Predictability and variability of the tropical Atlantic Meridional Mode (AMM) is investigated using linear inverse modeling (LIM). Analysis of the LIM using an “energy” norm identifies two optimal structures that experience some transient growth, one related to El Niño–Southern Oscillation (ENSO) and the other to the Atlantic multidecadal oscillation (AMO)/AMM patterns. Analysis of the LIM using an AMM-norm identifies an “AMM optimal” with similar structure to the second energy optima (OPT2). Both the AMM-optimal and OPT2 exhibit two bands of SST anomalies in the mid- to high-latitude Atlantic. The AMM-optimal also contains some elements of the first energy optimal (ENSO), indicating that the LIM captures the well-known relationship between ENSO and the AMM.

Seasonal correlations of LIM predictions with the observed AMM show enhanced AMM predictability during boreal spring and for long-lead (around 11–15 months) forecasts initialized around September. Regional LIMs were constructed to determine the influence of tropical Pacific and mid- to high-latitude Atlantic SST on the AMM. Analysis of the regional LIMs indicates that the tropical Pacific is responsible for the AMM predictability during boreal spring. Mid- to high-latitude SST anomalies contribute to boreal summer and fall AMM predictability, and are responsible for the enhanced predictability from September initial conditions. Analysis of the empirical normal modes of the full LIM confirms these physical relationships. Results indicate a potentially important role for mid- to high-latitude Atlantic SST anomalies in generating AMM (and tropical Atlantic SST) variations, though it is not clear whether those anomalies provide any societally useful predictive skill.

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Daniel J. Vimont

Abstract

A defining feature of Pacific decadal ENSO-like variability is the similarity between its spatial expression in sea surface temperature (SST) and the spatial structure of interannual ENSO variability. This similarity may indicate that the decadal variability is merely a long-term average over interannual ENSO variability. In contrast, subtle differences (namely the meridionally broadened tropical SST signature and emphasized midlatitude SST anomalies for the decadal ENSO-like pattern) may indicate that fundamentally different processes are responsible for generating variability on the decadal to interdecadal time scale. The present study attempts to reconcile the subtly different spatial structures of interannual ENSO and decadal ENSO-like variability by relating the decadal pattern to various SST patterns associated with the development of the interannual ENSO cycle. First, a statistical analysis is used to reconstruct the decadal ENSO-like SST pattern as a linear combination of interannual SST patterns. It is shown that the decadal ENSO-like pattern is well reconstructed in the absence of decadal spatial information. Next, these interannual patterns are physically interpreted in relation to the interannual ENSO cycle. The analysis reveals that the decadal ENSO-like SST pattern is obtained by averaging over three SST patterns associated with ENSO precursors, the peak of an ENSO event, and ENSO “leftovers.”

The study provides a plausible physical explanation for the spatial structure of ENSO-like decadal variability as an average over variations in the interannual ENSO cycle. The results indicate that the prominent spatial features of decadal ENSO-like variability are generated by physical mechanisms that operate through the interannual ENSO cycle. This does not imply, however, that decadal processes are unimportant in altering the decadal properties of ENSO. Results may provide a framework for interpreting modeled decadal ENSO-like variability and for constraining plausible mechanisms of tropical decadal variability.

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Daniel J. Vimont

Abstract

The dynamics of thermodynamically coupled disturbances in the tropics that bear a strong resemblance to observed meridional mode variations are investigated using two simple linear coupled models. Both models involve an ocean equation coupled to the atmosphere via the linearized effect of zonal wind variations on the surface bulk latent heat flux. The two models differ in their atmospheric components, which consist of (i) a Gill–Matsuno style model of the free troposphere in which atmospheric heating is parameterized to be linearly proportional to sea surface temperature and (ii) a reduced-gravity model of the tropical boundary layer in which SST anomalies are associated with hydrostatic pressure perturbations throughout the boundary layer. Both atmospheric models follow the standard shallow-water equations on an equatorial beta plane.

Growth rates and propagation of coupled disturbances are calculated and diagnosed via eigenanalysis of the linear models and singular value decomposition of the Green’s function for each model. It is found that the eigenvectors of either model are all damped, not orthogonal, and not particularly meaningful in understanding observed tropical coupled variability. The nonnormality of the system, however, leads to transient growth over a time period of about 100 days (based on the choice of parameters in this study). The idealized initial and final conditions that experience this transient growth resemble observed tropical meridional mode variations and tend to propagate equatorward and westward in accord with findings from previous theoretical and modeling studies. Instantaneous growth rates and propagation characteristics of idealized transient disturbances are diagnosed via the linearized atmospheric potential vorticity equation and via propagation characteristics of atmospheric equatorial Rossby waves.

Constraints on the poleward extent of initial conditions or imposed steady forcing that can lead to tropical meridional mode variations are identified through analysis of the steady coupled equations. Three constraints limit the poleward extent of forcing that can generate tropical meridional mode variations: (i) a dynamical constraint imposed by the damping rate of the temperature equation as well as the propagation speed of the mode along its wave characteristic; (ii) a constraint imposed by the effectiveness of zonal wind variations in generating surface latent heat flux anomalies; and (iii) the surface moisture convergence, which limits the poleward extent and strength of ocean to atmosphere coupling.

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Dimitry Smirnov and Daniel J. Vimont

Abstract

An observational and modeling study is conducted to investigate the structure of the Atlantic Meridional Mode (AMM) during the Atlantic hurricane season, and the relationship between AMM-related SST anomalies and environmental conditions that influence seasonal tropical cyclone activity. The observational analysis shows that during the Atlantic hurricane season the AMM exhibits a similar SST and low-level wind structure as during boreal spring (when the AMM is most active). Observed AMM SST variations are accompanied by air temperature and moisture anomalies that are limited to the boundary layer and an anomalous baroclinic circulation structure in the northern subtropical Atlantic with an anomalous lower-level cyclonic circulation residing under an anomalous upper-level anticyclone during a warm phase. This baroclinic structure contributes to a reduction in vertical wind shear over the tropical Atlantic that is dominated by changes in the upper-level flow.

Two sets of model experiments were conducted, in which the NCAR Community Atmospheric Model version 3.1 (CAM3.1) was coupled to a slab ocean model or a data ocean model. In each experiment, the model was either initialized with or forced by AMM-like SST anomalies during boreal summer. The simulations yielded a similar spatial structure to that in the observations, including the baroclinic atmospheric circulation and associated reduction in vertical wind shear. The similarity between the modeled and observed AMM structures strongly suggests a causal relationship in which the AMM-like SST anomalies are responsible for generating environmental conditions that can strongly influence seasonal tropical cyclone variability.

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Dimitry Smirnov and Daniel J. Vimont

Abstract

The connection between midlatitude Atlantic sea surface temperature (SST) anomalies and tropical SST variations during boreal summer and fall are investigated using a coupled general circulation model (GCM). This research follows on an observational study that finds that, using linear inverse modeling (LIM), predictions of boreal summer tropical Atlantic Meridional Mode (AMM) variations can be made with skill exceeding persistence with lead times of about one year. The LIM framework identified extratropical Atlantic SST anomalies as important precursors to the AMM variations.

The authors have corroborated this finding using a general circulation model coupled to a slab ocean, which represents a completely different physical basis from the LIM. Initializing the GCM with the LIM-derived “optimal” SST anomaly in November results in a steady equatorward propagation of SST anomalies into the subtropics during the following boreal spring. Thereafter, the GCM suggests that two possible feedbacks propagate the SST anomalies farther equatorward and westward with minimal loss of amplitude: the dominant wind–evaporation–SST (WES) thermodynamic feedback and a secondary low-cloud–SST radiative feedback. This study shows that this result has strong seasonal dependence and consists of nonlinear interactions when considering warm and cold “optimal” conditions separately. One main finding is that oceanic dynamics are not essential to understanding extratropical–tropical interaction in the Atlantic basin. The authors also discuss the results of the study in context with previous studies investigating the extratropical forcing of tropical air–sea variability.

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Cristian Martinez-Villalobos and Daniel J. Vimont

Abstract

This study uses a simple linear coupled model to investigate the role of the WES feedback and ITCZ mean states in meridional mode variability. Optimal structures that maximize transient growth are calculated for mean states characteristic of boreal spring and boreal fall in the tropical Atlantic. During boreal spring the leading optimal structure is a zonal mode that propagates westward and does not resemble the observed meridional mode. In contrast, the leading optimal structure during fall is a sea surface temperature (SST) monopole over the Northern Hemisphere (NH) that propagates equatorward and westward and that closely matches meridional mode variability during this season. It is found that the boreal fall optimal growth greatly exceeds growth of the corresponding optimal during boreal spring, despite the observed boreal spring peak in Atlantic meridional mode variance.

Sensitivity studies are used to explore the role of Northern or Southern Hemisphere initial conditions, ITCZ width, and ITCZ location in meridional mode growth and structure. It is found that growth is favored (i) for optimal structures that originate in the Northern Hemisphere, especially for boreal fall mean states; (ii) for symmetric mean states, equatorially symmetric structures maximize growth under narrow ITCZ configurations, and antisymmetric structures maximize growth under wider ITCZ configurations; and (iii) for antisymmetric mean states (and realistic ITCZ width), growth is maximized when the ITCZ is located off of the equator. The implications of these findings are discussed.

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Erin E. Thomas and Daniel J. Vimont

Abstract

Interactions between the Pacific meridional mode (PMM) and El Niño–Southern Oscillation (ENSO) are investigated using the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) and an intermediate coupled model (ICM). The two models are configured so that the CESM simulates the PMM but not ENSO, and the ICM simulates ENSO but not the PMM, allowing for a clean separation between the PMM evolution and the subsequent ENSO response. An ensemble of CESM simulations is run with an imposed surface heat flux associated with the North Pacific Oscillation (NPO) generating a sea surface temperature (SST) and wind response representative of the PMM. The PMM wind is then applied as a forcing to the ICM to simulate the ENSO response. The positive (negative) ensemble-mean PMM wind forcing results in a warm (cold) ENSO event although the responses are not symmetric (warm ENSO events are larger in amplitude than cold ENSO events), and large variability between ensemble members suggests that any individual ENSO event is strongly influenced by natural variability contained within the CESM simulations. Sensitivity experiments show that 1) direct forcing of Kelvin waves by PMM winds dominates the ENSO response, 2) seasonality of PMM forcing and ENSO growth rates influences the resulting ENSO amplitude, 3) ocean dynamics within the ICM dominate the ENSO asymmetry, and 4) the nonlinear relationship between PMM wind anomalies and surface wind stress may enhance the La Niña response to negative PMM variations. Implications for ENSO variability are discussed.

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Cristian Martinez-Villalobos and Daniel J. Vimont

Abstract

A theoretical framework is developed for understanding the transient growth and propagation characteristics of thermodynamically coupled, meridional mode–like structures in the tropics. The model consists of a Gill–Matsuno-type steady atmosphere under the long-wave approximation coupled via a wind–evaporation–sea surface temperature (WES) feedback to a “slab” ocean model. When projected onto meridional basis functions for the atmosphere the system simplifies to a nonnormal set of equations that describes the evolution of individual sea surface temperature (SST) modes, with clean separation between equatorially symmetric and antisymmetric modes. The following major findings result from analysis of the system: 1) a transient growth process exists whereby specific SST modes propagate toward lower-order modes at the expense of the higher-order modes; 2) the same dynamical mechanisms govern the evolution of symmetric and antisymmetric SST modes except for the lowest-order wavenumber, where for symmetric structures the atmospheric Kelvin wave plays a critically different role in enhancing decay; and 3) the WES feedback is positive for all modes (with a maximum for the most equatorially confined antisymmetric structure) except for the most equatorially confined symmetric mode where the Kelvin wave generates a negative WES feedback. Taken together, these findings explain why equatorially antisymmetric “dipole”-like structures may dominate thermodynamically coupled ocean–atmosphere variability in the tropics. The role of nonnormality and the role of realistic mean states in meridional mode variability are discussed.

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John C. H. Chiang and Daniel J. Vimont

Abstract

From observational analysis a Pacific mode of variability in the intertropical convergence zone (ITCZ)/cold tongue region is identified that possesses characteristics and interpretation similar to the dominant “meridional” mode of interannual–decadal variability in the tropical Atlantic. The Pacific and Atlantic meridional modes are characterized by an anomalous sea surface temperature (SST) gradient across the mean latitude of the ITCZ coupled to an anomalous displacement of the ITCZ toward the warmer hemisphere. Both are forced by trade wind variations in their respective northern subtropical oceans. The Pacific meridional mode exists independently of ENSO, although ENSO nonlinearity projects strongly on it during the peak anomaly season of boreal spring. It is suggested that the Pacific and Atlantic modes are analogous, governed by physics intrinsic to the ITCZ/ cold tongue complex.

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Daniel J. Vimont, Michael Alexander, and Abigail Fontaine

Abstract

A set of ensemble model experiments using the National Center for Atmospheric Research Community Atmospheric Model version 3.0 (CAM3) is run to investigate the tropical Pacific response to midlatitude atmospheric variability associated with the atmospheric North Pacific Oscillation (NPO). Heat flux anomalies associated with the NPO are used to force a set of model simulations during boreal winter (when the NPO is most energetic), after which the forcing is switched off and the coupled model evolves on its own. Sea surface temperature (SST) and wind anomalies continue to amplify in the tropical Pacific after the imposed forcing has been shut off, indicating that coupled ocean–atmosphere interactions in the tropical Pacific alter the spatial and temporal structure of variability associated with midlatitude forcing. The tropical circulation evolves through feedbacks between the surface wind, evaporation, and SST (the WES feedback), as well as through changes in the shortwave radiative heat flux (caused by changes in convection).

Sensitivity experiments are run to investigate how thermodynamic coupling and seasonality affect the tropical response to NPO-related forcing. Seasonality is found to affect the WES feedback through (i) altering the sensitivity of surface evaporation to changes in the low-level wind field and (ii) altering the structure and strength of the lower-level wind response to SST anomalies. Thermodynamic coupling causes an equatorward and westward development of SST anomalies and an associated equatorward shift in the lower-level zonal wind anomalies.

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