Interdecadal Mode Research Articles

Sensitivity of North Atlantic Multidecadal Variability to Freshwater Flux Forcing

Abstract In this paper, an explanation is proposed for the changes in the amplitude of multidecadal variability found in the GFDL climate model when different restoring salinity fields in the flux adjustments were considered. This explanation arises from a study of the stability of three-dimensional thermohaline flows in an idealized coupled ocean–atmosphere model. The shape of the freshwater flux affects the stability properties of the thermohaline flows, in particular the growth rate of a stable interdecadal mode. The physics of this change in decay rate is explained by analyzing the energy conversions in the flows. Under a stronger freshening of the northern North Atlantic, the interdecadal mode destabilizes, which can result in an increase of the amplitude of the multidecadal variability.

Midlatitude ocean-atmosphere interaction in an idealized coupled model

Interannual-to-interdecadal ocean–atmosphere interaction in midlatitudes is studied using an idealized coupled model consisting of eddy resolving two-layer quasi-geostrophic oceanic and atmospheric components with a simple diagnostic oceanic mixed layer. The model solutions exhibit structure and variability that resemble qualitatively some aspects of the observed climate variability over the North Atlantic. The atmospheric climatology is characterized by a zonally modulated jet. The single-basin ocean climatology consists of a midlatitude double jet that represents the Gulf Stream and Labrador currents, which are parts of the subtropical and subpolar gyres, respectively. The leading mode of the atmospheric low-frequency variability consists predominantly of meridional displacements of the zonal jet, with a local maximum over the ocean. The first basin-scale mode of sea-surface temperature has a red power spectrum, is largely of one polarity and bears qualitative similarities with the observed interdecadal mode identified by Kushnir. A warm sea-surface temperature anomaly is accompanied by anomalously low atmospheric pressure, an intensified model Gulf Stream and a weakened Labrador current. This mode is found not to be affected significantly by oceanic coupling. In the western part of the basin, this sea-surface temperature pattern is shown to be forced by the slowest components of the surface-wind anomaly through a delayed modulation of the baroclinic time-dependent boundary currents which advect mean SST, with synchronous variations in the two oceanic jets. The response in the east is found to be dominated by local atmospheric forcing. Basin-scale intrinsic oceanic variability consists of a damped oceanic oscillatory mode in the baroclinic flow field that is excited by the atmospheric noise. Its period is around 5.5 years, but it has a negligible influence on the evolution of sea-surface temperature. Important for this mode's excitation is the meridional position of the atmospheric center of action relative to the ocean gyres.

Sea surface temperature and sea surface height variability in the North Pacific Ocean from 1993 to 1999

Correlations between advanced very high resolution radiometer Pathfinder sea surface temperature (SST) and TOPEX sea surface height (SSH) anomalies are calculated for the 7 year period 1993–1999 in the equatorial and North Pacific. These correlations provide measures of SSH and SST covariability and are obtained using an empirical orthogonal function analysis separately on the two fields and a singular value decomposition (SVD) method on both fields simultaneously. These correlations reveal distinct SST‐SSH states: a warm, elevated period from January 1993 to December 1994; a cooler, lower phase spanning January 1995 through December 1996; and the strongly variable El Niño‐Southern Oscillation (ENSO) period spanning 1997–1999. The relationships between these SST‐SSH states and the dominant interannual and interdecadal modes of Pacific variability are investigated through correlations with indices for the Southern Oscillation, Pacific‐North American (PNA) pattern, and Pacific Decadal Oscillation (PDO). When including the tropics, significant correlations between the first SVD mode of the SST‐SSH covariability and the interannual ENSO pattern and interdecadal PDO pattern are evident. Restricting the focus to the extratropical North Pacific reveals significant correlations with only the interdecadal PDO pattern. While the first mode reveals connections to the ENSO and PDO patterns, the second SVD mode indicates significant correlations to the PNA pattern for both the tropical and extratropical Pacific.

A model of decadal ocean‐atmosphere interaction in the North Pacific Basin

From a simple and a global coupled model, we showed that the tropical and extratropical ocean‐atmosphere interactions and the teleconnection between the tropics and extratropics in these interactions are responsible for the co‐variations of the decadal to interdecadal climate fluctuations in the tropical and extratropical Pacific. The subtropical oceanic Rossby waves are found to play an important role in forming the coupled decadal to interdecadal mode. They are responsible for slow exchanges of mass and heat between the tropical and subtropical ocean gyres that serve as an important oceanic linking bridge of the tropical and extratropical decadal to interdecadal ocean‐atmosphere interactions in the Pacific basin.

Impact of global warming on the interannual and interdecadal climate modes in a coupled GCM

In this study, we investigated the impact of global warming on the variabilities of large-scale interannual and interdecadal climate modes and teleconnection patterns with two long-term integrations of the coupled general circulation model of ECHAM4/OPYC3 at the Max-Planck-Institute for Meteorology, Hamburg. One is the control (CTRL) run with fixed present-day concentrations of greenhouse gases. The other experiment is a simulation of transient greenhouse warming, named GHG run. In the GHG run the averaged geopotential height at 500 hPa is increased significantly, and a negative phase of the Pacific/North American (PNA) teleconnection-like distribution pattern is intensified. The standard deviation over the tropics (high latitudes) is enhanced (reduced) on the interdecadal time scales and reduced (enhanced) on the interannual time scales in the GHG run. Except for an interdecadal mode related to the Southern Oscillation (SO) in the GHG run, the spatial variation patterns are similar for different (interannual + interdecadal, interannual, and interdecadal) time scales in the GHG and CTRL runs. Spatial distributions of the teleconnection patterns on the interannual and interdecadal time scales in the GHG run are also similar to those in the CTRL run. But some teleconnection patterns show linear trends and changes of variances and frequencies in the GHG run. Apart from the positive linear trend of the SO, the interdecadal modulation to the El Nino/SO cycle is enhanced during the GHG 2040 ∼ 2099. This is the result of an enhancement of the Walker circulation during that period. La Nina events intensify and El Nino events relatively weaken during the GHG 2070 ∼ 2090. It is interesting to note that with increasing greenhouse gas concentrations the relation between the SO and the PNA pattern is reversed significantly from a negative to a positive correlation on the interdecadal time scales and weakened on the interannual time scales. This suggests that the increase of the greenhouse gas concentrations will trigger the nonstationary correlation between the SO and the PNA pattern both on the interdecadal and interannual time scales.

ENSO‐like interdecadal variability in the Pacific Ocean as simulated in a coupled general circulation model

Spatial and temporal structures of interdecadal variability in the Pacific Ocean are investigated using results from an atmosphere‐ocean coupled general circulation model (AOGCM). The model shows a basin‐wide spatial pattern of the principal sea surface temperature (SST) variability similar to the observed one. Both interdecadal and interannual temporal structures of the SST variability agree well between the observation and the AOGCM. On the other hand, a slab ocean model coupled to the same atmospheric model as the AOGCM fails to simulate the observed temporal structure. Therefore the timescale of the coupled variability is associated with dynamical processes in the ocean. A distinct interdecadal mode of the coupled atmosphere‐upper ocean temperature variability is found in the AOGCM, with a spatiotemporal structure coherent with the SST variability. The mode accompanies an El Niño‐Southern Oscillation (ENSO)‐like spatial pattern of SST and the surface wind and behaves like a delayed oscillator in ENSO. A wedge‐shaped anomaly pattern of the upper thermocline temperature is formed in the eastern Pacific, and its northern subtropical signal propagates westward, enhanced by a subtropical wind forcing at the central basin. Arrival of the subtropical signal at the western Pacific around 20°N switches the anomaly of subsurface temperature in the equatorial region through anomalous oceanic heat transport along the western boundary. The travel time of the trans‐Pacific signal in the subtropics appears to be responsible for the timescale of this mode. The AOGCM successfully simulated the second mode of SST with a major variation in the midlatitude North Pacific as in the observed SST. In the upper ocean heat content we found another distinct mode, which is characterized by a midlatitude‐subtropics dipole pattern migrating around the North Pacific subtropical gyre. However, the associated SST variation of this mode shows a poor correspondence in the dominant interdecadal modes for the observed SST.

熱帯における海洋・大気結合の10年・数10年変動モード

Homogeneous upper air data for 50 years (1949-1998) from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalyses project, sea surface temperatures and sea level pressure are used to bring out the three dimensional structure of two dominant decadal/multi-decadal variations in the tropics. The global three dimensional modes represent generalized forms of inter-decadal modes studied earlier only with surface data. In the vertical, both modes show approximate first baroclinic structures over the tropics. The Walker circulation associated with the multi-decadal mode has a wavenumber two structure in the zonal direction. It is shown that the magnitude of major ascending and descending motions associated with the multi-decadal Hadley and Walker circulations, are comparable to those associated with the dominant inter-annual mode. Implications of these large scale global circulations associated with the low frequency oscillations in modulating regional climate on a inter-annual time scale are discussed.

Eight centuries of north atlantic ocean atmosphere variability

Climate in the tropical North Atlantic is controlled largely by variations in the strength of the trade winds, the position of the Intertropical Convergence Zone, and sea surface temperatures. A high-resolution study of Caribbean sediments provides a subdecadally resolved record of tropical upwelling and trade wind variability spanning the past 825 years. These results confirm the importance of a decadal (12- to 13-year) mode of Atlantic variability believed to be driven by coupled tropical ocean-atmosphere dynamics. Although a well-defined interdecadal mode of variability does not appear to be characteristic of the tropical Atlantic, there is evidence that century-scale variability is substantial. The tropical Atlantic may also have been involved in a major shift in Northern Hemisphere climate variability that took place about 700 years ago.

Decadal climate variability in a coupled atmosphere‐ocean climate model of moderate complexity

In this study we determined characteristic temporal modes of atmospheric variability at the decadal and interdecadal timescales. This was done on the basis of 1000 year long integrations of a global coupled atmosphere‐ocean climate model of moderate complexity including the troposphere, stratosphere, and mesosphere. The applied model resolves explicitely the basic features of the large‐scale long‐term atmospheric and oceanic variables. The synoptic‐scale processes are described in terms of autocorrelation and crosscorrelation functions. The paper includes an extended description and validation of the model as well as the results of analyses of two 1000 year long model integrations. One model run has been performed with the fully coupled model of the atmosphere‐ocean system. The performed time‐frequency analyses of atmospheric fields reveal strong decadal and interdecadal modes with periods of about 9, 18, and 30 years. To quantify the influence of the ocean on atmospheric variations an additional run with seasonally varying prescribed sea surface temperatures has been carried out, which is characterized by strong decadal modes with periods of about 9 years. The comparison of both runs suggests that decadal variability can be understood as an inherent atmospheric mode due to the nonlinear dynamics of the large‐scale atmospheric circulation patterns whereas interdecadal climate variability has to be regarded as coupled atmosphere‐ocean modes.

Analytical Prototypes for Ocean–Atmosphere Interaction at Midlatitudes. Part II: Mechanisms for Coupled Gyre Modes*

A simple midlatitude coupled model for idealized ocean basins is used to investigate processes of ocean‐ atmosphere interaction and its role in interdecadal climate variability at midlatitudes. The ocean model consists of a linearized quasigeostrophic upper ocean layer and a sea surface temperature (SST) equation for an embedded surface mixed layer. The atmospheric response to the ocean is through wind stress and heat flux feedbacks associated with SST. Eigenvalue analysis of both coupled and uncoupled models presented here complements previous work on the stochastically forced system. Comparison of the eigenspectrum of coupled and uncoupled cases shows that coupling creates an oscillatory interdecadal mode whose properties are distinct from any other mode in the system. This mode exists whether the atmospheric feedbacks are weak or strong, and is stable even in the strong feedback case. The weak decay rate makes it possible for the mode to be maintained by atmospheric stochastic forcing. Analytic approximations to the dispersion relation show how the spatial structure of the atmospheric feedback tends to select a large-scale spatial pattern for this eigenmode. The oscillation involves westward Rossby wave propagation in the ocean with the atmosphere carrying information back eastward into the interior of the basin in response to SST anomalies produced by advection. SST modes are also found, which purely decay in most cases due to both local and nonlocal negative heat flux feedbacks. A case with large positive heat flux feedback can produce a purely growing SST mode but does not greatly impact the interdecadal mode.

Analytical Prototypes for Ocean–Atmosphere Interaction at Midlatitudes. Part I: Coupled Feedbacks as a Sea Surface Temperature Dependent Stochastic Process*

Effects of ocean‐atmosphere feedback processes and large-scale atmospheric stochastic forcing on the interdecadal climate variability in the North Atlantic and North Pacific Oceans are examined in a simple midlatitude ocean‐atmosphere model. In the ocean, the authors consider a linearized perturbation system with quasigeostrophic shallow-water ocean dynamics and a sea surface temperature (SST) equation for a surface mixed layer. The atmosphere is represented as stochastic wind stress and heat flux forcing. This includes a noise component that depends on SST, as well as an additive component that is independent of SST. Coupling is represented by the SST dependent stochastic process, in which SST influences the probability density function of the atmospheric noise both in shifting the mean and affecting the variance. It thus includes a multiplicative noise component. The model results in both oceans indicate that large-scale additive atmospheric stochastic forcing alone (the uncoupled case) can give coherent spatial patterns in the ocean and sometimes even a weak power spectral peak at interdecadal periods. Coupling due to the SST dependent stochastic process can produce a more distinct power-spectral peak relative to the uncoupled ocean. Moreover, the time and spatial scales of the interdecadal mode are insensitive to the standard deviation of the multiplicative noise. Thus a deterministic feedback limit can be used to simplify the coupled model for further investigation of the physical mechanisms of the interdecadal mode. In both uncoupled and coupled cases, the period of the interdecadal oscillation is determined by the zonal length scale of atmospheric wind stress and oceanic Rossby wave dynamics. The atmospheric spatial pattern sets the length scale of large-scale wave motion in the ocean. This wave propagates to the west due to oceanic Rossby wave dynamics and is dissipated at the western boundary. However, in the coupled case, the SST anomalies generated by geostrophic current can feed back to the atmosphere, which in turn brings some information back to the east and reexcites oceanic waves there. Although the magnitude of the feedback of SST on the atmosphere is much smaller than atmospheric internal variability, its effects are significant.

Modes of Interannual and Interdecadal Variability of Pacific SST

Abstract The multichannel singular spectrum analysis has been used to characterize the spatio–temporal structures of interdecadal and interannual variability of SST over the Pacific Ocean from 20°S to 58°N. Using the Comprehensive Ocean–Atmosphere Data Set from 1950 to 1993, three modes with distinctive spatio-temporal structures were found. They are an interdecadal mode, a quasi-quadrennial (QQ) oscillation with a period of 51 months, and a quasi-biennial (QB) oscillation with a period of 26 months. The interdecadal mode is a standing mode with opposite signs of SST anomalies in the North Pacific and in the tropical Pacific. The amplitude of this mode is larger in the central North Pacific than in the tropical Pacific. This mode contributes 11.4% to the total variance. It is associated with cooling in the central North Pacific and warming in the equatorial Pacific since around 1976–77. The QQ oscillation exhibits propagation of SST anomalies northeastward from the Philippine Sea and then eastward along 4...

Northern Hemispheric Interdecadal Variability: A Coupled Air-Sea Mode

A coupled air–sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean–atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific. The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal. The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air–sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration.

Northern Hemispheric Interdecadal Variability: A Coupled Air–Sea Mode

AbstractA coupled air–sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean–atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific.The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal.The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air–sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration.

On the role of ocean‐atmosphere interaction in midlatitude interdecadal variability

A simple midlatitude ocean‐atmosphere model is used to investigate possible roles of coupled feedback in interdecadal climate variability over the North Atlantic and North Pacific oceans. Stochastic forcing by atmospheric internal variability maintains variance in both uncoupled and coupled cases. Coupling contributes to creating an interdecadal mode with distinct spatial pattern and preferred time scale, seen as a broad spectral peak. A near‐analytic solution for the coupled interdecadal mode suggests that the most important parameter in determining the period is the zonal length scale of the atmospheric wind stress feedback over the region. Subject to this scale, the period is then determined by oceanic Rossby wave dynamics, which tends to give westward propagation in subsurface fields. The dipolar sea surface temperature (SST) anomalies are generated primarily by the advection of climatological SST by geostrophic current. Although the magnitude of the feedback of SST to atmospheric response is much smaller than atmospheric internal variability, its effects are significant.

Predicting decadal variations in Atlantic tropical cyclone numbers and Australian rainfall

Significant relationships on time scales longer than 5 years are demonstrated between northern Pacific sea surface temperatures, North Atlantic tropical cyclone numbers, and northeastern Australian rainfall. In agreement with previous work, the global nature of these correlations suggests that they are indications of an intrinsic interdecadal mode of oscillation in the global ocean‐atmosphere system. The persistence in time of the sea surface temperature variations may enable forecasts to be made of North Atlantic tropical cyclone numbers several years in advance. A similar capability for the prediction of north‐eastern Australian rainfall is also shown. Thus these relationships may be useful as an aid in long‐term planning decisions.

Multiple Equilibria, Natural Variability, and Climate Transitions in an Idealized Ocean–Atmosphere Model

An idealized coupled ocean–atmosphere is constructed to study climatic equilibria and variability. The model focuses on the role of large-scale fluid motions in the climate system. 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 two components exchange heat and freshwater fluxes synchronously. Coupled integrations are carried out over periods of several centuries to identify the equilibrium states of the ocean–atmosphere system. It is shown that there exist at least three types of equilibria, which are distinguished by whether they have upwelling or downwelling in the polar regions. Each of the coupled equilibria has a close analog in the ocean-only model with mixed boundary conditions. The oceanic circulation in the coupled model exhibits natural variability on interdecadal and longer timescales. The dominant interdecadal mode of variability is associated with the advection of oceanic temperature anomalies in the sinking regions. The sensitivity of the coupled model to climatic perturbations is studied. A rapid increase in the greenhouse gas concentrations leads to a collapse of the meridional overturning in the ocean. Introduction of a large positive surface freshwater anomaly in the high latitudes leads to a temporary suppression of the sinking motion, followed by a rapid recovery, due primarily to the high latitude cooling associated with the reduction of oceanic heat transport. In this evolution, the secondary roles played by the atmospheric heat transport and moisture transport in destabilizing the thermohaline circulation are compared, and the former is found to be dominant.

Global‐scale modes of surface temperature variability on interannual to century timescales

Using 100 years of global temperature anomaly data, we have performed a singular value decomposition of temperature variations in narrow frequency bands to isolate coherent spatio‐temporal “modes” of global climate variability. Statistical significance is determined from confidence limits obtained by Monte Carlo simulations. Secular variance is dominated by a globally coherent trend, with nearly all grid points warming in phase at varying amplitude. A smaller, but significant, share of the secular variance corresponds to a pattern dominated by warming and subsequent cooling in the high latitude North Atlantic with a roughly centennial timescale. Spatial patterns associated with significant peaks in variance within a broad period range from 2.8 to 5.7 years exhibit characteristic El Niño‐Southern Oscillation (ENSO) patterns. A recent transition to a regime of higher ENSO frequency is suggested by our analysis. An interdecadal mode in the 15‐to‐18 years period range appears to represent long‐term ENSO variability. This mode has a sizeable projection onto global‐average temperature, and accounts for much of the anomalous global warmth of the 1980s. A quasi‐biennial mode centered near 2.2‐years period and a mode centered at 7‐to‐8 years period both exhibit predominantly a North Atlantic Oscillation (NAO) temperature pattern. A potentially significant “decadal” mode centered on 11‐to‐12 years period also exhibits an NAO temperature pattern and may be modulated by the century‐scale North Atlantic variability.