Description: The dominant variability modes in the Tropics are investigated and contrasted with the anomalous situation observed during the last few years. The prime quantity analyzed is anomalous sea surface temperature (SST) in the region 30°S–60°N. Additionally, observed tropical surface wind stress fields were investigated. Further tropical atmospheric information was derived from a multidecadal run with an atmospheric general circulation model that was forced by the same SSTs. The tropical SST variability can be characterized by three modes: an interannual mode [the El Niño–Southern Oscillation (ENSO)], a decadal mode, and a trend or unresolved ultra-low-frequency variability. The dominant mode of SST variability is the ENSO mode. It is strongest in the eastern equatorial Pacific, but influences also the SSTs in other regions through atmospheric teleconnections, such as the Indian and North Pacific Oceans. The ENSO mode was strong during the 1980s, but it existed with very weak amplitude and short period after 1991. The second most energetic mode is characterized by considerable decadal variability. This decadal mode is connected with SST anomalies of the same sign in all three tropical oceans. The tropical Pacific signature of the decadal mode resembles closely that observed during the last few years and can be characterized by a horseshoe pattern, with strongest SST anomalies in the western equatorial Pacific, extending to the northeast and southeast into the subtropics. It is distinct from the ENSO mode, since it is not connected with any significant SST anomalies in the eastern equatorial Pacific, which is the ENSO key region. However, the impact of the decadal mode on the tropical climate resembles in many respects that of ENSO. In particular, the decadal mode is strongly linked to decadal rainfall fluctuations over northeastern Australia in the observations. It is shown that the anomalous 1990s were dominated by the decadal mode. Considerable SST variability can be attributed also to a linear trend or unresolved ultra-low-frequency variability. This trend that might be related to greenhouse warming is rather strong and positive in the Indian Ocean and western equatorial Pacific where it accounts for up to 30% of the total SST variability. Consistent with the increase of SST in the warm pool region, the trends over the tropical Pacific derived from both the observations and the model indicate a strengthening of the trade winds. This is inconsistent with the conditions observed during the 1990s. If the wind trends reflect greenhouse warming, it must be concluded that the anomalous 1990s are not caused by greenhouse warming. Finally, hybrid coupled ocean–atmosphere model experiments were conducted in order to investigate the sensistivity of ENSO to the low-frequency changes induced by the decadal mode and the trend. The results indicate that ENSO is rather sensitive to these changes in the background conditions.

Description: A primitive equation model of the equatorial Pacific Ocean was forced by realistic wind stress distributions over decades. Results were presented for a set of two experiments. In the first experiment the model was forced by an objectively analyzed wind field, while for the second experiment a subjectively analyzed wind field was used. The results indicate a strong sensitivity of the model to the choice of the wind fields. Especially, model results in the eastern Pacific show big differences between the two model runs. Taking the results of the second model run the performance of the model with respect to interannual variability is investigated. Sea level, temperature and zonal currents show pronounced interannual variations within the equatorial belt from 10°N to 10°S. Special attention is given to the simulation of the 1982/83 El Niño event. The model reproduces most of the basic features, which were observed during this El Niño event. In particular the deceleration of the equatorial undercurrent, the evolution of eastward surface currents and the zonal redistribution of heat associated with an eastward propagation of warm water are simulated by the model.

Description: An observational-based analysis of coupled variability in the equatorial Atlantic and its seasonality is presented. Regression analysis shows that the three elements of the Bjerknes positive feedback exist in the Atlantic and are spatially similar to those of the Pacific. The cross-correlation functions of the elements of the Bjerknes feedback are also similar and consistent with an ocean–atmosphere coupled mode. However, the growth rate in the Atlantic is up to 50% weaker, and explained variance is significantly lower. The Bjerknes feedback in the Atlantic is strong in boreal spring and summer, and weak in other seasons, which explains why the largest sea surface temperature anomalies (SSTAs) occur in boreal summer. Its seasonality is determined by seasonal variations in both atmospheric sensitivity to SSTA and SSTA sensitivity to subsurface temperature anomalies.

Description: Sea surface temperature (SST) observations in the North Atlantic indicate the existence of strong multidecadal variability with a unique spatial structure. It is shown by means of a new global climate model, which does not employ flux adjustments, that the multidecadal SST variability is closely related to variations in the North Atlantic thermohaline circulation (THC). The close correspondence between the North Atlantic SST and THC variabilities allows, in conjunction with the dynamical inertia of the THC, for the prediction of the slowly varying component of the North Atlantic climate system. It is shown additionally that past variations of the North Atlantic THC can be reconstructed from a simple North Atlantic SST index and that future, anthropogenically forced changes in the THC can be easily monitored by observing SSTs. The latter is confirmed by another state-of-the-art global climate model. Finally, the strong multidecadal variability may mask an anthropogenic signal in the North Atlantic for some decades.

Description: The importance of decadal climate variability (DCV) research is being increasingly recognized, including by the World Climate Research Program (WCRP) and the Intergovernmental Panel on Climate Change (IPCC). An improved understanding of DCV is very important because stakeholders and policymakers want to know the likely climate trajectory for the coming decades for applications to water resources, agriculture, energy, and infrastructure development. Responding to this demand, many climate modeling groups in the United States, Europe, Japan, and elsewhere are gearing up to assess the potential for decadal climate predictions. The magnitudes of regional DCV often exceed those associated with the trends resulting from anthropogenic changes. Therefore, differentiating between the two is also very important for planning, implementation, and national and international treaties.

Description: The interannual variability of the Indian Ocean SST is investigated by analyzing data from observations and an integration of a global coupled GCM (CGCM) ECHO-2. First, it is demonstrated that the CGCM is capable of producing realistic tropical climate variability. Second, it is shown that a considerable part of the interannual variability in Indian Ocean SST can be described as the response to interannual fluctuations over the Pacific related to ENSO. Although the Indian Ocean region also exhibits ENSO-independent interannual variability, this paper focuses on the ENSO-induced component only. Large-scale SST anomalies of the same sign as those observed in the eastern equatorial Pacific Ocean during ENSO extremes develop in the entire tropical and subtropical Indian Ocean with a time lag of about 4 months. This lead–lag relationship is found in both the observations and the CGCM. Using the CGCM output, it is shown that the ENSO signal is carried into the Indian Ocean mainly through anomalous surface heat fluxes.

Description: 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.

Description: To study the dynamics that may lead to decadal oscillations in the North Pacific a simple coupled model is developed. The ocean is based on the linear, potential vorticity equation for baroclinic planetary waves. The atmosphere is reduced to a nonlocal wind response to thermocline depth anomalies. The wind stress has a spatially fixed structure and its amplitude depends on the thermocline perturbation at one location or in a predefined index region. Such a simple coupled model produces decadal oscillations for suitable parameter choices. For realistic wind stress patterns, the patterns of oceanic variability are similar to those observed. It is determined by the speed of long Rossby waves at the coupling latitude. The period of the oscillation is rather insensitive to the coupling strength and amounts to approximately twice the time the Rossby wave needs to travel from the center of the wind stress curl anomaly to the coupling location. A stochastic component to the atmospheric forcing is incorporated by white noise added to the feedback. With such a forcing, typical oceanic spectra become red with a broad peak at decadal timescales superimposed.

Description: The interannual variability of the tropical Indian Ocean sea surface temperature (SST) is studied with observational data and a hierarchy of coupled general circulation models (CGCMs). Special attention is given to the question whether an oscillatory dipole mode exists in the tropical Indian Ocean region with centers east and west of 80°E. Our observational analyses indicate that dipole-like variability can be explained as an oscillatory mode only in the context of ENSO (El Nino/Southern Oscillation). A dipole-like structure in the SST anomalies independent of ENSO was found also. Our series of coupled model experiments shows that ocean dynamics is not important to this type of dipole-like SST variability. It is forced by surface heat flux anomalies that are integrated by the thermal inertia ofthe oceanic mixed layer, which reddens the SST spectrum.

Description: The response of the Max Planck Institutes ECHAM3 atmospheric general circulation model to a prescribed decade-long positive anomaly in sea surface temperatures (SSTs) over the North Atlantic is investigated. Two 10-yr realizations of the anomaly experiment are compared against a 100-yr control run of the model with seasonally varying climatological SST using a model spatial resolution of T42. In addition to the time-mean response, particular attention is paid to changes in intraseasonal variability, expressed in terms of North Atlantic?European weather regimes. The model regimes are quite realistic. Substantial differences are found in the 700-mb geopotential height field response between the two decadal realizations. The time-mean response in the first sample decade is characterized by the positive (zonal) phase of the North Atlantic oscillation (NAO); this response can be identified with changes in the frequency of occurrence of certain weather regimes by about one standard deviation. (Preliminary results of this numerical experiment were reported at the Atlantic Climate Variability Workshop held at the Lamont?Doherty Earth Observatory of Columbia University, Palisades, New York, 24?26 September 1997.) By contrast, the second SST anomaly decade shows a localized trough centered over the British Isles; it projects less strongly onto the models intrinsic weather regimes. The control run itself exhibits pronounced decade-to-decade variations in the weather regimes frequency of occurrence as well as in its NAO index. The two 10-yr anomaly experiments are insufficient, in length and number, to identify a robust SST response above this level of intrinsic variability.