An intimate coupling of ocean–atmospheric interaction over the extratropical North Atlantic and Pacific

The inter-basin teleconnection between the North Atlantic and the North Pacific ocean–atmosphere interaction is studied using a coupled ocean–atmosphere general circulation model. In the model, an idealized oceanic temperature anomaly is initiated over the Kuroshio and the Gulf Stream extension region to track the coupled evolution of ocean and atmosphere interaction, respectively. The experiments explicitly demonstrate that both the North Pacific and the North Atlantic ocean–atmosphere interactions are intimately coupled through an inter-basin atmospheric teleconnection. This fast inter-basin communication can transmit oceanic variability between the North Atlantic and the North Pacific through local ocean-to-atmosphere feedbacks. The leading mode of the extratropical atmospheric internal variability plays a dominant role in shaping the hemispheric-scale response forced by oceanic variability over the North Atlantic and Pacific. Modeling results also suggest that a century (two centuries) long observations are necessary for the detection of Pacific response to Atlantic forcings (Atlantic response to Pacific forcing).     

Limitations of time-slice experiments for predicting regional climate change over South Asia

While time-slice simulations with atmospheric general circulation models (GCMs) have been used for many years to regionalize climate projections and/or assess their uncertainties, there is still no consensus about the method used to prescribe sea surface temperature (SST) in such experiments. In the present study, the response of the Indian summer monsoon to increasing amounts of greenhouse gases and sulfate aerosols is compared between a reference climate scenario and three sets of time-slice experiments, consisting of parallel integrations for present-day and future climates. Different monthly mean SST boundary conditions have been tested in the present-day integrations: raw climatological SST derived from the reference scenario, observed climatological SST, and observed SST with interannual variability. For future climate, the SST forcing has been obtained by superimposing climatological monthly mean SST anomalies derived from the reference scenario onto the present-day SST boundary conditions. None of these sets of time-slice experiments is able to capture accurately the response of the Indian summer monsoon simulated in the transient scenario. This finding suggests that the ocean–atmosphere coupling is a fundamental feature of the climate system. Neglecting the SST feedback and variability at the intraseasonal to interannual time scales has a significant impact on the projected monsoon response to global warming. Adding interannual variability in the prescribed SST boundary conditions does not mitigate the problem, but can on the contrary reinforce the discrepancies between the forced and coupled experiments. The monsoon response is also shown to depend on the simulated control climate, and can therefore be sensitive to the use of observed rather than model-derived SSTs to drive the present-day atmospheric simulation, as well as to any approximation in the prescribed radiative forcing. While such results do not challenge the use of time-slice experiments for assessing uncertainties and understanding mechanisms in transient scenarios, they emphasize the need for high-resolution coupled atmosphere-ocean GCMs for dynamical downscaling, or at least for high-resolution atmospheric GCMs coupled with a slab or a regional ocean model.     

The role of ocean dynamics in producing decadal climate variability in the North Pacific

 Decadal time scale climate variability in the North Pacific has implications for climate both locally and over North America. A crucial question is the degree to which this variability arises from coupled ocean/atmosphere interactions over the North Pacific that involve ocean dynamics, as opposed to either purely thermodynamic effects of the oceanic mixed layer integrating in situ the stochastic atmospheric forcing, or the teleconnected response to tropical variability. The part of the variability that is coming from local coupled ocean/atmosphere interactions involving ocean dynamics is potentially predictable by an ocean/atmosphere general circulation model (O/A GCM), and such predictions could (depending on the achievable lead time) have distinct societal benefits. This question is examined using the results of fully coupled O/A GCMs, as well as targeted numerical experiments with stand-alone ocean and atmosphere models individually. It is found that coupled ocean/atmosphere interactions that involve ocean dynamics are important to determining the strength and frequency of a decadal-time scale peak in the spectra of several oceanic variables in the Kuroshio extension region off Japan. Local stochastic atmospheric heat flux forcing, integrated by the oceanic mixed layer into a red spectrum, provides a noise background from which the signal must be extracted. Although teleconnected ENSO responses influence the North Pacific in the 2–7 years/cycle frequency band, it is shown that some decadal-time scale processes in the North Pacific proceed without ENSO. Likewise, although the effects of stochastic atmospheric forcing on ocean dynamics are discernible, a feedback path from the ocean to the atmosphere is suggested by the results. Received: 23 January 2000 / Accepted: 10 January 2001     

Global monsoons in the mid-Holocene and oceanic feedback

The response of the six major summer monsoon systems (the North American monsoon, the northern Africa monsoon, the Asia monsoon, the northern Australasian monsoon, the South America monsoon and the southern Africa monsoon) to mid-Holocene orbital forcing has been investigated using a coupled ocean–atmosphere general circulation model (FOAM), with the focus on the distinct roles of the direct insolation forcing and oceanic feedback. The simulation result is also found to compare well with the NCAR CSM. The direct effects of the change in insolation produce an enhancement of the Northern Hemisphere monsoons and a reduction of the Southern Hemisphere monsoons. Ocean feedbacks produce a further enhancement of the northern Africa monsoon and the North American monsoon. However, ocean feedbacks appear to weaken the Asia monsoon, although the overall effect (direct insolation forcing plus ocean feedback) remains a strengthened monsoon. The impact of ocean feedbacks on the South American and southern African monsoons is relatively small, and therefore these regions, especially the South America, experienced a reduced monsoon regime compared to present. However, there is a strong ocean feedback on the northern Australian monsoon that negates the direct effects of orbital changes and results in a strengthening of austral summer monsoon precipitation in this region. A new synthesis is made for mid-Holocene paleoenvironmental records and is compared with the model simulations. Overall, model simulations produce changes in regional climates that are generally consistent with paleoenvironmental observations.     

Techniques for asynchronous and periodically synchronous coupling of atmosphere and ocean models

A periodically synchronous scheme suitable for coupling atmosphere and ocean models with high internal variability is presented. The performance of the scheme is tested by means of a simple zero-dimensional non-linear energy balance model with stochastic forcing. The equilibrium behaviour and the response to changes in the model parameters are analysed. The response experiments are similar to CO₂ doubling and transient CO₂ experiments. The best results are obtained using a method with weighted means of the air-sea fluxes which are calculated during the synchronously coupled periods.     

南海海域海-气耦合模式及其数值模拟试验

在ＮＣＡＲ区域气候模式ＲｅｇＧＭ２和普林斯顿海洋模式ＰＯＭ基础上发展适用于区域海－气相互作用研究的区域海－气耦合模式,模式采用同步耦合、海洋模式将海表温度提供给大气模式,大气模式为海洋模式提供太阳短波辐射、感热能量、潜热通量。海洋与大气模式每１５ｍｉｎ交换一次通量。耦合过程没有使用通量校正。使用该模式对中国南海区域１９９５年５－７月大气和海洋进行了模拟试验,将模拟结果与ＣＯＡＤＳ通量强迫的模拟结果     

Growing interfacially coupled oscillations of the ocean–atmosphere

A stability analysis of the coupled ocean–atmosphere is presented which shows that the potential energy (PE) of the upper layer of the ocean is available to generate coupled growing planetary waves. An independent analysis suggests that the growth of these waves would be maintained in the presence of oceanic friction. The growing waves are a consequence of relaxing the rigid lid approximation on the ocean, thus allowing an upward transfer of energy across the sea surface. Using a two and a half layer model consisting of an atmospheric planetary boundary layer, coupled with a two layer ocean comprising an active upper layer and a lower layer in which the velocity perturbation is vanishingly small, it is shown that coupled unstable waves are generated, which extract PE from the main thermocline. The instability analysis is an extension of earlier work [Tellus 44A (1992) 67], which considered the coupled instability of an atmospheric planetary boundary layer coupled with an oceanic mixed layer, in which unstable waves were generated which extract PE from the seasonal thermocline. The unstable wave is an atmospheric divergent barotropic Rossby wave, which is steered by the zonal wind velocity, and has a wavelength of about 6000 km, and propagates eastward at the speed of the deep ocean current. It is argued that this instability, which has a multidecadal growth time constant, may be generated in the Southern Ocean, and that its properties are similar to observations of the Antarctic Circumpolar Wave (ACW).     

Climate simulation of the twenty-first century with interactive land-use changes

To include land-use dynamics in a general circulation model (GCM), the physical system has to be linked to a system that represents socio-economy. This issue is addressed by coupling an integrated assessment model, IMAGE2.2, to an ocean–atmosphere GCM, CNRM-CM3. In the new system, IMAGE2.2 provides CNRM-CM3 with all the external forcings that are scenario dependent: greenhouse gas (GHGs) concentrations, sulfate aerosols charge and land cover. Conversely, the GCM gives IMAGE changes in mean temperature and precipitation. With this new system, we have run an adapted scenario of the IPCC SRES scenario family. We have chosen a single scenario with maximum land-use changes (SRES A2), to illustrate some important feedback issues. Even in this two-way coupled model set-up, land use in this scenario is mainly driven by demographic and agricultural practices, which overpowers a potential influence of climate feedbacks on land-use patterns. This suggests that for scenarios in which socio-economically driven land-use change is very large, land-use changes can be incorporated in GCM simulations as a one-way driving force, without taking into account climate feedbacks. The dynamics of natural vegetation is more closely linked to climate but the time-scale of changes is of the order of a century. Thus, the coupling between natural vegetation and climate could generate important feedbacks but these effects are relevant mainly for multi-centennial simulations.     

Synergistic feedbacks between ocean and vegetation on mid- and high-latitude climates during the mid-Holocene

Simulations with the IPSL atmosphere–ocean model asynchronously coupled with the BIOME1 vegetation model show the impact of ocean and vegetation feedbacks, and their synergy, on mid- and high-latitude (>40°N) climate in response to orbitally-induced changes in mid-Holocene insolation. The atmospheric response to orbital forcing produces a +1.2 °C warming over the continents in summer and a cooling during the rest of the year. Ocean feedback reinforces the cooling in spring but counteracts the autumn and winter cooling. Vegetation feedback produces warming in all seasons, with largest changes (+1 °C) in spring. Synergy between ocean and vegetation feedbacks leads to further warming, which can be as large as the independent impact of these feedbacks. The combination of these effects causes the high northern latitudes to be warmer throughout the year in the ocean–atmosphere-vegetation simulation. Simulated vegetation changes resulting from this year-round warming are consistent with observed mid-Holocene vegetation patterns. Feedbacks also impact on precipitation. The atmospheric response to orbital-forcing reduces precipitation throughout the year; the most marked changes occur in the mid-latitudes in summer. Ocean feedback reduces aridity during autumn, winter and spring, but does not affect summer precipitation. Vegetation feedback increases spring precipitation but amplifies summer drying. Synergy between the feedbacks increases precipitation in autumn, winter and spring, and reduces precipitation in summer. The combined changes amplify the seasonal contrast in precipitation in the ocean–atmosphere-vegetation simulation. Enhanced summer drought produces an unrealistically large expansion of temperate grasslands, particularly in mid-latitude Eurasia.     

Impact of ocean model resolution on CCSM climate simulations

The current literature provides compelling evidence suggesting that an eddy-resolving (as opposed to eddy-permitting or eddy-parameterized) ocean component model will significantly impact the simulation of the large-scale climate, although this has not been fully tested to date in multi-decadal global coupled climate simulations. The purpose of this paper is to examine how resolved ocean fronts and eddies impact the simulation of large-scale climate. The model used for this study is the NCAR Community Climate System Model version 3.5 (CCSM3.5)—the forerunner to CCSM4. Two experiments are reported here. The control experiment is a 155-year present-day climate simulation using a 0.5° atmosphere component (zonal resolution 0.625 meridional resolution 0.5°; land surface component at the same resolution) coupled to ocean and sea-ice components with zonal resolution of 1.2° and meridional resolution varying from 0.27° at the equator to 0.54° in the mid-latitudes. The second simulation uses the same atmospheric and land-surface models coupled to eddy-resolving 0.1° ocean and sea-ice component models. The simulations are compared in terms of how the representation of smaller scale features in the time mean ocean circulation and ocean eddies impact the mean and variable climate. In terms of the global mean surface temperature, the enhanced ocean resolution leads to a ubiquitous surface warming with a global mean surface temperature increase of about 0.2?°C relative to the control. The warming is largest in the Arctic and regions of strong ocean fronts and ocean eddy activity (i.e., Southern Ocean, western boundary currents). The Arctic warming is associated with significant losses of sea-ice in the high-resolution simulation. The sea surface temperature gradients in the North Atlantic, in particular, are better resolved in the high-resolution model leading to significantly sharper temperature gradients and associated large-scale shifts in the rainfall. In the extra-tropics, the interannual temperature variability is increased with the resolved eddies, and a notable increases in the amplitude of the El Ni?o and the Southern Oscillation is also detected. Changes in global temperature anomaly teleconnections and local air-sea feedbacks are also documented and show large changes in ocean–atmosphere coupling. In particular, local air-sea feedbacks are significantly modified by the increased ocean resolution. In the high-resolution simulation in the extra-tropics there is compelling evidence of stronger forcing of the atmosphere by SST variability arising from ocean dynamics. This coupling is very weak or absent in the low-resolution model.     

Dynamics and predictability of a low-order wind-driven ocean–atmosphere coupled model

The dynamics of a low-order coupled wind-driven ocean–atmosphere system is investigated with emphasis on its predictability properties. The low-order coupled deterministic system is composed of a baroclinic atmosphere for which 12 dominant dynamical modes are only retained (Charney and Straus in J Atmos Sci 37:1157–1176, 1980) and a wind-driven, quasi-geostrophic and reduced-gravity shallow ocean whose field is truncated to four dominant modes able to reproduce the large scale oceanic gyres (Pierini in J Phys Oceanogr 41:1585–1604, 2011). The two models are coupled through mechanical forcings only. The analysis of its dynamics reveals first that under aperiodic atmospheric forcings only dominant single gyres (clockwise or counterclockwise) appear, while for periodic atmospheric solutions the double gyres emerge. In the present model domain setting context, this feature is related to the level of truncation of the atmospheric fields, as indicated by a preliminary analysis of the impact of higher wavenumber (“synoptic” scale) modes on the development of oceanic gyres. In the latter case, double gyres appear in the presence of a chaotic atmosphere. Second the dynamical quantities characterizing the short-term predictability (Lyapunov exponents, Lyapunov dimension, Kolmogorov–Sinaï (KS) entropy) displays a complex dependence as a function of the key parameters of the system, namely the coupling strength and the external thermal forcing. In particular, the KS-entropy is increasing as a function of the coupling in most of the experiments, implying an increase of the rate of loss of information about the localization of the system on its attractor. Finally the dynamics of the error is explored and indicates, in particular, a rich variety of short term behaviors of the error in the atmosphere depending on the (relative) amplitude of the initial error affecting the ocean, from polynomial (at ² + bt ³ + ct ⁴) up to exponential-like evolutions. These features are explained and analyzed in the light of the recent findings on error growth (Nicolis et al. in J Atmos Sci 66:766–778, 2009).     

Techniques for asynchronous and periodically synchronous coupling of atmosphere and ocean models

Asynchronous and periodically-synchronous schemes for coupling atmosphere and ocean models are presented. The performance of the schemes is tested by simulating the climatic response to a step function forcing and to a gradually increasing forcing with a simple zero-dimensional non-linear energy balance model. Both the initial transient response and the asymptotic approach of the equilibrium state are studied. If no annual cycle is allowed the asynchronous coupling technique proves to be a suitable tool. However, if the annual cycle is retained, the periodically synchronous coupling technique reproduces the results of the synchronously coupled runs with smaller bias. In this case it is important that the total length of one synchronous period and one ocean only period is not a multiple of 6 months.     

Climate modification by future ice sheet changes and consequences for ice sheet mass balance

The future evolution of global ice sheets under anthropogenic greenhouse forcing and its impact on the climate system, including the regional climate of the ice sheets, are investigated with a comprehensive earth system model consisting of a coupled Atmosphere–Ocean General Circulation Model, a dynamic vegetation model and an ice sheet model. The simulated control climate is realistic enough to permit a direct coupling of the atmosphere and ice sheet components, avoiding the use of anomaly coupling, which represents a strong improvement with respect to previous modelling studies. Glacier ablation is calculated with an energy-balance scheme, a more physical approach than the commonly used degree-day method. Modifications of glacier mask, topographic height and freshwater fluxes by the ice sheets influence the atmosphere and ocean via dynamical and thermodynamical processes. Several simulations under idealized scenarios of greenhouse forcing have been performed, where the atmospheric carbon dioxide stabilizes at two and four times pre-industrial levels. The evolution of the climate system and the ice sheets in the simulations with interactive ice sheets is compared with the simulations with passively coupled ice sheets. For a four-times CO₂ scenario forcing, a faster decay rate of the Greenland ice sheet is found in the non-interactive case, where melting rates are higher. This is caused by overestimation of the increase in near-surface temperature that follows the reduction in topographic height. In areas close to retreating margins, melting rates are stronger in the interactive case, due to changes in local albedo. Our results call for careful consideration of the feedbacks operating between ice sheets and climate after substantial decay of the ice sheets.     

Climate drift in an ocean model coupled to a simple,perfectly matched atmosphere

A very simple, diffusive energy balance atmosphere is coupled to the GFDL ocean circulation model. This provides a useful tool for analyzing climate drift in the ocean model after coupling, and may be used to assess various schemes for minimizing such drift. In the experiment reported here, the atmosphere is constructed in such a way that it provides the ocean model at the moment of coupling with the same fluxes as during spinup. The experiment is therefore equivalent to coupling a perfectly flux-corrected atmosphere model, and is used to investigate the response of the ocean model under these conditions. In spite of the steady, passive, flux-corrected atmosphere, the ocean model drifts to a new equilibrium state after coupling. The transition takes about 2000 years; the new state is characterized by different sites of deep convection and resulting changes in high-latitude SST and global deep temperatures. The mechanism for the transition is an instability of the oceanic convection patterns under the new feedback, felt after coupling. A similar state transition of the ocean model may be triggered by the coupling shock in fully coupled GCMs. If this is so, the transition would contaminate the results of climate scenario experiments, and it would explain part of the residual drift observed in coupled models in spite of the use of flux corrections.     

A highly nonlinear coupled mode of decadal variability in a mid-latitude ocean–atmosphere model

This study examines mid-latitude climate variability in a model that couples turbulent oceanic and atmospheric flows through an active oceanic mixed layer. Intrinsic ocean dynamics of the inertial recirculation regions combines with nonlinear atmospheric sensitivity to sea-surface temperature (SST) anomalies to play a dominant role in the variability of the coupled system.Intrinsic low-frequency variability arises in the model atmosphere; when run in a stand-alone mode, it is characterized by irregular transitions between preferred high-latitude and less frequent low-latitude zonal-flow states. When the atmosphere is coupled to the ocean, the low-latitude state occurrences exhibit a statistically significant signal in a broad 5–15-year band. A similar signal is found in the time series of the model ocean's energy in this coupled simulation. Accompanying uncoupled ocean-only and atmosphere-only integrations are characterized by a decrease in the decadal-band variability, relative to the coupled integration; their spectra are indistinguishable from a red spectrum.The time scale of the coupled interdecadal oscillation is set by the nonlinear adjustment of the ocean's inertial recirculations to the high-latitude and low-latitude atmospheric forcing regimes. This adjustment involves, in turn, SST changes resulting in long-term ocean–atmosphere heat-flux anomalies that induce the atmospheric regime transitions.     

Ocean–atmosphere coupling and the boreal winter MJO

The influence of ocean–atmosphere coupling on the simulation and prediction of the boreal winter Madden–Julian Oscillation (MJO) is examined using the Seoul National University coupled general circulation model (CGCM) and atmospheric—only model (AGCM). The AGCM is forced with daily SSTs interpolated from pentad mean CGCM SSTs. Forecast skill is examined using serial extended simulations spanning 26 different winter seasons with 30-day forecasts commencing every 5 days providing a total of 598 30-day simulations. By comparing both sets of experiments, which share the same atmospheric components, the influence of coupled ocean–atmosphere processes on the simulation and prediction of MJO can be studied. The mean MJO intensity possesses more realistic amplitude in the CGCM than in AGCM. In general, the ocean–atmosphere coupling acts to improve the simulation of the spatio-temporal evolution of the eastward propagating MJO and the phase relationship between convection (OLR) and SST over the equatorial Indian Ocean and the western Pacific. Both the CGCM and observations exhibit a near-quadrature relationship between OLR and SST, with the former lagging by about two pentads. However, the AGCM shows a less realistic phase relationship. As the initial conditions are the same in both models, the additional forcing by SST anomalies in the CGCM extends the prediction skill beyond that of the AGCM. To test the applicability of the CGCM to real-time prediction, we compute the Real-time Multivariate MJO (RMM) index and compared it with the index computed from observations. RMM1 (RMM2) falls away rapidly to 0.5 after 17–18 (15–16) days in the AGCM and 18–19 (16–17) days in the CGCM. The prediction skill is phase dependent in both the CGCM and AGCM.     

Development of global coupled ocean-atmosphere general circulation models

It has long been believed that a climate model capable of realistically simulating many features of global climate, variability, and climate change must interactively represent the major components of the dynamically coupled climate system, particularly the atmosphere, ocean, and cryosphere. This effort traditionally has been constrained by computing power, our understanding of the observed system, and climate modeling capability. With the advent of supercomputers, improved understanding of global climate processes, and computationally efficient general circulation climate models, we have witnessed a rapid increase in the simulation of global climate by coupling together various representations of atmosphere, ocean, and sea ice. Beginning in the late 1960s and continuing through the early 1980s, general circulation models (GCMs) of the atmosphere, ocean, and sea ice were coupled and run asynchronously to produce credible simulations of the global climate. Systematic errors in these component models later led some modeling groups to use flux correction or flux adjustment, whereby either one or several of the variables at the air-sea interface are adjusted to bring the simulations in closer agreement with observations. Further advances in computing power and climate modeling techniques in the past few years have allowed global coupled ocean-atmosphere GCMs to be run synchronously (i.e., atmosphere and ocean communicate at least once each model day). Computing constraints, combined with the need for multidecadal climate integrations, still only allow relatively coarse-grid ocean GCMs to be coupled to correspondingly coarse-grid atmospheric models (on the order of 500 km × 500 km). However, results from this current generation of global, coupled GCMs have revealed interesting characteristics associated with ocean dynamics and global climate in experiments with gradual increases of carbon dioxide. Another somewhat surprising aspect of the global-coupled GCM simulations is the appearance of some features associated with the El Niño-Southern Oscillation. Along with concurrent efforts with other types of limited-domain, dynamical coupled models, this has led to the realization that inherent unstable coupled modes exist in the climate system that are the unique product of the interactive coupling of the atmosphere and the ocean. All of these efforts are leading to the next generation of coupled ocean-atmosphere GCMs. These models will run on even faster and larger-memory computers and will have higher-resolution atmosphere and ocean components, more accurate sea-ice formulations, improved cloud-radiation schemes, and increasingly realistic land-surface processes.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. DümenilThe National Center for Atmospheric Research is sponsored by the National Science Foundation     

一类非线性海气耦合波的研究

建立了一类非线性海气耦合波的模式,并用该模式作了解析研究,讨论了该类海气耦合波的存在条件,发现当海气耦合较强时分别存在以海洋为主导方面及以大气为主导方面的两支海气耦合波,而耦合较弱时则仅有前者存在。还求得了该模式中该类海气耦合波的椭圆余弦波解及孤立波解,并对以海洋为主导的海气耦合波的性质作了讨论,认为海气相互作用耦合也是产生大气季节内振荡的机制之一。     

Decadal oscillations in a simplified coupled model due to unstable interactions between zonal winds and ocean gyres

A simplified coupled ocean–atmosphere model, consisting of a one-layer bidimensional ocean model and a one-layer unidimensional energy balance atmospheric model [J. Clim. 13 (2000) 232] is used to study the unstable interactions between zonal winds and ocean gyres. In a specific range of parameters, decadal variability is found. Anomalies, quite homogeneous zonally, show small-scale wavelength in latitude: perturbations emerge and grow at the southern limb of the intergyre boundary and propagate southward before decaying. The wind stress anomalies are proportional to the meridional gradient of the atmospheric temperature anomalies: this ratio acts as a positive amplification factor, as confirmed by a parameter sensitivity analysis. Assuming zonally-averaged anomalies harmonic in the meridional direction, a very simple analytical model for the perturbations is derived, based on forced Rossby wave adjustment of the western boundary current and its associated anomalous heat transport: it accounts for the scale selection, the growth and the southward propagation of sea surface temperature anomalies in the subtropical gyre. The latter is not only due to the slow advection by the mean current, but to a prevailing mechanism of self-advecting coupled oceanic and atmospheric waves, out of phase in latitude. Relevance to the observational record is discussed.     

Coupled variability and air-sea interaction in the South Atlantic Ocean

A total of 52 years of data (1949–2000) from the NCEP/NCAR reanalysis are used to investigate mechanisms involved in forcing and damping of sea surface temperature (SST) variability in the South Atlantic Ocean. Organized patterns of coupled ocean–atmosphere variability are identified using EOF and SVD analyses. The leading mode of coupled variability consists of an SST pattern with a strong northeast–southwest gradient and an SLP monopole centered at 15°W, 45°S. The anomalous winds associated with this monopole generate the SST pattern through anomalous latent heat flux and mixed layer deepening. Other heat flux components and anomalous Ekman transport play only a secondary role. Once established, the SST pattern is attenuated through latent heat flux. The higher SST modes are also induced by anomalous winds and destroyed by latent heat flux. It thus appears that the coupled variability in the South Atlantic Ocean consists of atmospheric circulation anomalies that induce SST anomalies through anomalous latent heat fluxes and wind-induced mixed layer deepening. These SST anomalies are destroyed by latent heat flux with no detectable systematic feedback onto the atmospheric circulation. Atmospheric variability in the South Atlantic is found to be largely independent of that elsewhere, although there is a weak relation with ENSO (El Niño-Southern Oscillation).