Sea Surface Temperature Forcing Research Articles

Simulated U.S. Drought Response to Interannual and Decadal Pacific SST Variability

Abstract Idealized atmospheric general circulation model (AGCM) experiments by the U.S. Climate Variability and Predictability Program (CLIVAR) Drought Working Group were used in order to study the influence of natural modes of sea surface temperature (SST) variability in the Pacific on drought in the contiguous United States. The current study expands on previous results by examining the atmospheric response of three AGCMs to three different patterns of the idealized Pacific SST anomalies that operate on different time scales: low-frequency (decadal), high-frequency (interannual), and a pan-Pacific pattern that retains characteristics of interannual and decadal variability. While forcing patterns are generally similar in appearance, results indicate that differences in the relative amplitude of the equatorial and extratropical components of the SST forcing are sufficient to give rise to differing teleconnections, leading to regional differences in the amplitude and significance of the precipitation response. Results indicate that the differences in simulated drought response between AGCMs to different cool-phase (La Niña–like) SST patterns are determined by model sensitivity to changes in the relative amplitude of the equatorial and extratropical components of the SSTA forcing, the strength of the land–atmosphere coupling, and by the amplitude of internal atmospheric variability. Results indicate that the northwestern United States and Great Plains regions are particularly sensitive to the extratropical component of the SST forcing. Evidence is also found that when the cool-phase patterns of SST combine, as they have in recent years, constructive interference leads to an enhanced drought response over the Great Plains.

Sensitivity of Horn of Africa Rainfall to Regional Sea Surface Temperature Forcing

The Abdus Salam International Center for Theoretical Physics (ICTP) version 4.4 Regional Climate Model (RegCM4) is used to investigate the rainfall response to cooler/warmer sea surface temperature anomaly (SSTA) forcing in the Indian and Atlantic Oceans. The effect of SSTA forcing in a specific ocean basin is identified by ensemble, averaging 10 individual simulations in which a constant or linearly zonally varying SSTA is prescribed in individual basins while specifying the 1971–2000 monthly varying climatological sea surface temperature (SST) across the remaining model domain. The nonlinear rainfall response to SSTA amplitude also is investigated by separately specifying +1K, +2K, and +4K SSTA forcing in the Atlantic and Indian Oceans. The simulation results show that warm SSTs over the entire Indian Ocean produce drier conditions across the larger Blue Nile catchment, whereas warming ≥ +2K generates large positive rainfall anomalies exceeding 10 mm·day−1 over drought prone regions of Northeastern Ethiopia. However, the June–September rainy season tends to be wetter (drier) when the SST warming (cooling) is limited to either the Northern or Southern Indian Ocean. Wet rainy seasons generally are characterized by deepening of the monsoon trough, east of 40°E, intensification of the Mascarene high, strengthening of the Somali low level jet and the tropical easterly jet, enhanced zonal and meridional vertically integrated moisture fluxes, and steeply vertically decreasing moist static energy. The opposite conditions hold for dry monsoon seasons.

Influence of extratropical sea‐surface temperature on the Indian summer monsoon: an unexplored source of seasonal predictability

Based on extensive analysis of observations and a series of climate model experiments, here we establish that slow variations of northern hemispheric extratropical sea‐surface temperature (SST) anomalies can augment seasonal predictability of the south Asian monsoon. The SST conditions and performance of the south Asian monsoon during 2013 boreal summer months (June–September) led us to hypothesize that the strong extratropical SST anomalies in the North Pacific and North Atlantic in conjunction with weak tropical SST anomalies (weak La Niña) were responsible for the above‐normal rainfall over India during 2013. We also argue that the 2013 SST pattern and above‐normal monsoon condition are not unique but occurred on several occasions in the past. Further, we show that there is a complementary pattern of strong extratropical SST and weak tropical SST that is associated with below‐normal south Asian monsoon rainfall. We also show that the extratropical SST pattern in the Northern Hemisphere is associated with a low‐frequency interdecadal mode of variability indicating potential predictability associated with such extratropical SST forcing. Extensive experiments with an atmospheric general circulation model forced by such SST conditions elucidate the mechanism through which the extratropical SSTs influence the Indian monsoon. The SST anomalies affect the north–south temperature gradient and lead to a local displacement of the jet stream, setting up a quasi‐stationary wave. Such a stationary wave, in turn, affects the tropospheric temperature (TT) over southern Eurasia, influencing the north–south TT gradient in the region and thereby the Indian monsoon. Our discovery of this additional source of potential predictability together with the fact that the new‐generation coupled ocean–atmosphere models are capable of predicting the extratropical SST anomalies brightens the prospect of south Asian monsoon prediction.

Attributing Observed SST Trends and Subcontinental Land Warming to Anthropogenic Forcing during 1979–2005

AbstractAttribution studies conclude that it is extremely likely that most observed global- and continental-scale surface air temperature (SAT) warming since 1950 was caused by anthropogenic forcing, but some difficulties and uncertainties remain in attribution of warming in subcontinental regions and at time scales less than 50 years. This study uses global observations and CMIP5 simulations with various forcings, covering 1979–2005, and control runs to develop confidence intervals, to attribute regional trends of SAT and sea surface temperature (SST) to natural and anthropogenic causes.Observations show warming, significantly different from natural variations at the 95% confidence level, over one-third of all grid boxes, and averaged over 15 of 21 subcontinental regions and 6 of 10 ocean basins. Coupled simulations forced with all forcing factors, or greenhouse gases only, reproduce observed SST and SAT patterns. Uncoupled AMIP-like atmosphere-only (prescribed SST and atmospheric radiative forcing) simulations reproduce observed SAT patterns. All of these simulations produce consistent net downward longwave radiation patterns. Simulations with natural-only forcing simulate weak warming. Anthropogenic forcing effects are clearly detectable at the 5% significance level at global, hemispheric, and tropical scales and in nine ocean basins and 15 of 21 subcontinental land regions. Attribution results indicate that ocean warming during 1979–2005 for the globe and individual basins is well represented in the CMIP5 multimodel ensemble mean historical simulations. While land warming may occur as an indirect response to oceanic warming, increasing greenhouse gas concentrations tend to be the ultimate source of land warming in most subcontinental regions during 1979–2005.

Stochastically Generated North American Megadroughts

Abstract The importance of interannual-to-decadal sea surface temperature (SST) influences on drought in the United States is examined using a suite of simulations conducted with the T31×3 resolution version of the NCAR Community Earth System Model (CESM1.0.3). The model captures tropical Pacific teleconnections to North American precipitation reasonably well, although orographic features are somewhat enhanced at higher resolution. The contribution of SST anomalies is isolated by comparing two idealized, 1000-yr CESM1.0.3 experiments: a fully coupled control and an atmosphere-only (CAM4) run forced with the SST climatology from the control. Droughts are identified using the Palmer Drought Severity Index (PDSI), which is computed over four U.S. regions from the CESM1.0.3 experiments and compared with the North American Drought Atlas (NADA). The CESM1.0.3 reproduces the persistence of NADA droughts quite well, although the model underestimates drought severity. Within the CESM1.0.3 framework, SST forcing does not significantly affect drought intensity or frequency of occurrence, even for very persistent “megadroughts” of 15 yr or more in length. In both the CESM1.0.3 and NADA, with the exception of the Southeast United States, droughts in all regions have intensities, persistence lengths, and occurrence frequencies statistically consistent with a red noise null hypothesis. This implies that SST forcing is not the dominant factor in generating drought and therefore that many decadal megadroughts are caused by a combination of internal atmospheric variability and coupling with the land surface, with SST anomalies playing only a secondary role.

Stable components in extended-range forecasting during the continuous high temperature event in August 2013 over the mid-eastern China

Based on the continuous high temperature process over the mid-eastern China in August 2013, using the NCEP/NCAR (United States National Centers for Environmental Prediction/National Center for Atmospheric Research) daily average of 500 hPa height field, the wind field reanalysis data, and the NOAA (National Oceanic and Atmospheric Administration) reconstruction sea surface temperature (SST) data, through the selection method similar to the early adoption of SST forcing, band-pass filtering and empirical orthogonal function decomposition method to extract the 10-30 days of stable components, and through a stable component of diagnostic analysis, we investigate the mechanisms for sustaining and reducing high temperature process. Results show that by selecting and using the case that is the most similar to a pre-SST forcing 30-year climatology instead of the normal 30-year climatology (1981-2010), the steady-state component extracted climate proportion is reduced, and the proportion of anomalously stable components is significantly enhanced and the described influence strength and stability are improved significantly, which can more clearly show the extended maintenance mechanism of weather processes. It suggests that early consideration SST forcing in the extraction component is very necessary. Meanwhile, the analysis of extension of stable components shows that the process of maintaining and reducing high temperature is mainly caused by the combined effect of the Arctic Oscillation, continental high latitudes zonal circulation situation in Asia and the western Pacific subtropical high intensity and location.

Estimating the regional climate responses over river basins to changes in tropical sea surface temperature patterns

We investigate how to identify and assess teleconnection signals between anomalous patterns of sea surface temperature (SST) changes and climate variables related to hydrologic impacts over different river basins. The regional climate sensitivity to tropical SST anomaly patterns is examined through a linear relationship given by the global teleconnection operator (GTO, also generally called a sensitivity matrix or an empirical Green’s function). We assume that the GTO defines a multilinear relation between SST forcing and regional climate response of a target area. The sensitivities are computed based on data from a large ensemble of simulations using the NCAR Community Atmospheric Model version 3.1 (CAM 3.1). The linear approximation is evaluated by comparing the linearly reconstructed response with both the results from the full non-linear atmospheric model and observational data. The results show that the linear approximation can capture regional climate variability that the CAM 3.1 AMIP-style simulations produce at seasonal scales for multiple river basins. The linear method can be used potentially for estimating drought conditions, river flow forecasting, and agricultural water management problems.

Assessing Cold-Snap and Mortality Events in South Florida Coastal Ecosystems: Development of a Biological Cold Stress Index Using Satellite SST and Weather Pattern Forcing

Water temperature is considered both a controlling and lethal factor in coastal ecosystems, influencing behavioral and physiological responses in marine organisms. Abrupt weather events such as severe cold front passages and accompanied changes in weather conditions have led to sharp decreases in water temperatures, metabolic stress, and incidences of mortality in marine organisms. In this paper, we assess the weather-related factors associated with physical and biological response in South Florida systems through historical sea surface temperature (SST) from satellites and the use of a synoptic climatology spanning over 30 years. We utilize previous categorizations of sea-level pressure and newly developed categorizations of 850-mb temperature reanalysis data to define circulation and temperature patterns across the southeastern US and adjacent Gulf of Mexico. Systematic connections are seen between particular circulation and temperature patterns characteristic of enhanced north-to-south circulation and cold air outbreaks, SST, and turtle strandings data over the Florida Panhandle region for the period 2006–2013. Identified weather forcing variables associated with sharp SST decreases and turtle stuns are presented and assist in the formulation of a moving cold-stress index for South Florida coastal ecosystems. Results demonstrate the potential of using synoptic climatological analysis and derived indices for tracking and modeling changes in SST and other indicators related to biological health.

Understanding Recent Eastern Horn of Africa Rainfall Variability and Change

Abstract Observations and sea surface temperature (SST)-forced ECHAM5 simulations are examined to study the seasonal cycle of eastern Africa rainfall and its SST sensitivity during 1979–2012, focusing on interannual variability and trends. The eastern Horn is drier than the rest of equatorial Africa, with two distinct wet seasons, and whereas the October–December wet season has become wetter, the March–May season has become drier. The climatological rainfall in simulations driven by observed SSTs captures this bimodal regime. The simulated trends also qualitatively reproduce the opposite-sign changes in the two rainy seasons, suggesting that SST forcing has played an important role in the observed changes. The consistency between the sign of 1979–2012 trends and interannual SST–precipitation correlations is exploited to identify the most likely locations of SST forcing of precipitation trends in the model, and conceivably also in nature. Results indicate that the observed March–May drying since 1979 is due to sensitivity to an increased zonal gradient in SST between Indonesia and the central Pacific. In contrast, the October–December precipitation increase is mostly due to western Indian Ocean warming. The recent upward trend in the October–December wet season is rather weak, however, and its statistical significance is compromised by strong year-to-year fluctuations. October–December eastern Horn rain variability is strongly associated with El Niño–Southern Oscillation and Indian Ocean dipole phenomena on interannual scales, in both model and observations. The interannual October–December correlation between the ensemble-average and observed Horn rainfall 0.87. By comparison, interannual March–May Horn precipitation is only weakly constrained by SST anomalies.

Teleconnections of the tropical Atlantic and Pacific Oceans in a CMIP5 model ensemble

This study investigates the teleconnections between the tropical Atlantic and Pacific Oceans in 15 state-of-the-art fully coupled general circulation models and Earth system models without external SST forcing. In contrast to other studies, the teleconnection is considered in both directions—from the Pacific to the Atlantic and from the Atlantic to the Pacific. The model ensemble is generally able to simulate the propagation of atmospheric and oceanic signals to the adjacent ocean basin, generated by warm sea surface temperature (SST) anomalies in the tropical eastern oceans with Atlantic summer events lagging or leading Pacific boreal winter events. This is investigated by means of time-lagged composite analyses of different atmospheric parameters, including sea level pressure, wind, stream function, velocity potential, vertical air movement and divergent wind at several levels. However, the modelled inter-basin teleconnection and its correct frequency of occurrence depend on the strong warm SST biases in the Atlantic Benguela upwelling region and in the Pacific Ocean.

The worst North American drought year of the last millennium: 1934

AbstractDuring the summer of 1934, over 70% of western North America experienced extreme drought, placing this summer far outside the normal range of drought variability and making 1934 the single worst drought year of the last millennium. Strong atmospheric ridging along the West Coast suppressed cold season precipitation across the Northwest, Southwest, and California, a circulation pattern similar to the winters of 1976–1977 and 2013–2014. In the spring and summer, the drying spread to the Midwest and Central Plains, driven by severe precipitation deficits downwind from regions of major dust storm activity, consistent with previous work linking drying during the Dust Bowl to anthropogenic dust aerosol forcing. Despite a moderate La Niña, contributions from sea surface temperature forcing were small, suggesting that the anomalous 1934 drought was primarily a consequence of atmospheric variability, possibly amplified by dust forcing that intensified and spread the drought across nearly all of western North America.

Interdecadal variability of the mega-ENSO–NAO synchronization in winter

Mega-El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO), as two principal components of the global air-sea coupling system, may have synchronous or out-synchronous fluctuations during different epochs. Understanding such connection change is instrumental for climate prediction, particularly the decadal prediction. Results in this study show that mega-ENSO has experienced a notable inter-decadal change in its linkage with the winter NAO during the past 56 years: mega-ENSO was significantly correlated with the NAO during 1957–1981 (or synchronous epoch), while such correlation has broken down since 1982 (or out-synchronous epoch). This marked change might be attributed to a sea surface temperature (SST) forcing change in the North Atlantic, based on the observational and numerical evidences in this study. The synchronous epoch is concurrent with the anomalous tropical North Atlantic (TNA) SST forcing, whereas the out-synchronous epoch is associated with the anomalous extra-tropical North Atlantic (XNA) SST forcing. Two possible reasons may explain how the synchronous behaviors between mega-ENSO and the NAO were tied to the TNA SST anomaly (SSTA). There is a positive feedback between the TNA SSTA and the NAO-like atmosphere anomalies, which helps to “prolong” the NAO impacts from the developing phase through mature phase of mega-ENSO. Additionally, the TNA SSTA itself may induce a NAO-like atmosphere anomaly. Since 1982, the TNA SSTA has been replaced by the XNA SSTA and the latter primarily favors a NAO-neutral state in the atmosphere, which ends the synchronous epoch.

The response of midlatitude jets to increased CO2: Distinguishing the roles of sea surface temperature and direct radiative forcing

AbstractIn Coupled Model Intercomparison Project Phase 5 (CMIP5) models, the zonal‐mean tropospheric circulation shifts robustly poleward in the Southern Hemisphere extratropics in response to increased atmospheric CO2 concentrations. However, in the Northern Hemisphere (NH) extratropics, the circulation response to CO2 is largely absent in the zonal mean and is instead characterized by complex regional anomalies. This study decomposes the atmospheric circulation response to CO2 forcing in CMIP5 models into two components: a direct component due to CO2 radiative forcing and an indirect component associated with sea surface temperature (SST)‐mediated changes. The direct radiative forcing of CO2 drives a weak poleward jet shift in both hemispheres, whereas the indirect (SST) component of the CO2 forcing dominates the total response and drives a zonally asymmetric response in the NH. Hence, understanding the SST‐mediated component of atmospheric CO2 forcing appears crucial to unlocking the mechanisms that contribute to forced extratropical circulation changes.

Potential predictability of Northern America surface temperature in AGCMs and CGCMs

In this study, the potential predictability of the Northern America (NA) surface air temperature (SAT) was explored using an information-based predictability framework and two multiple model ensemble products: a one-tier prediction by coupled models (T1), and a two-tier prediction by atmospheric models only (T2). Furthermore, the potential predictability was optimally decomposed into different modes for both T1 and T2, by extracting the most predictable structures. Emphasis was placed on the comparison of the predictability between T1 and T2. It was found that the potential predictability of the NA SAT is seasonal and spatially dependent in both T1 and T2. Higher predictability occurs in spring and winter and over the southeastern US and northwestern Canada. There is no significant difference of potential predictability between T1 and T2 for most areas of NA, although T1 has higher potential predictability than T2 in the southeastern US. Both T1 and T2 display similar most predictable components (PrCs) for the NA SAT, characterized by the inter-annual variability mode and the long-term trend mode. The first one is inherent to the tropical Pacific sea surface temperature forcing, such as the El Nino-Southern Oscillation, whereas the second one is closely associated with global warming. In general, the PrC modes can better characterize the predictability in T1 than in T2, in particular for the inter-annual variability mode in the fall. The prediction skill against observations is better measured by the PrC analysis than by principal component analysis for all seasons, indicating the stronger capability of PrCA in extracting prediction targets.

Changes in global net radiative imbalance 1985-2012.

Combining satellite data, atmospheric reanalyses, and climate model simulations, variability in the net downward radiative flux imbalance at the top of Earth's atmosphere (N) is reconstructed and linked to recent climate change. Over the 1985–1999 period mean N (0.34 ± 0.67 Wm−2) is lower than for the 2000–2012 period (0.62 ± 0.43 Wm−2, uncertainties at 90% confidence level) despite the slower rate of surface temperature rise since 2000. While the precise magnitude of N remains uncertain, the reconstruction captures interannual variability which is dominated by the eruption of Mount Pinatubo in 1991 and the El Niño Southern Oscillation. Monthly deseasonalized interannual variability in N generated by an ensemble of nine climate model simulations using prescribed sea surface temperature and radiative forcings and from the satellite-based reconstruction is significantly correlated (r∼0.6) over the 1985–2012 period.

Climatology and Variability of Precipitation in the Twentieth-Century Reanalysis

Abstract The performance of the Twentieth-Century Reanalysis (20CR) in reproducing observed monthly mean precipitation over the global domain is compared to that of comprehensive reanalyses that also assimilate upper-air and satellite observations [the Climate Forecast System Reanalysis (CFSR), ECMWF Interim Re-Analysis (ERA-Interim), and NCEP–U.S. Department of Energy reanalysis (NCEP2)] and to that of an atmospheric general circulation model (GCM) ensemble simulation [Global Ocean Global Atmosphere (GOGA)] that is forced with observed sea surface temperature (SST). Wintertime rainfall variability in the midlatitude continents and storm tracks is captured with great accuracy, similar to the comprehensive reanalyses, but summertime rainfall is not, probably in consequence of the greater importance of convection in the summer season. Over the tropics, the accuracy of all reanalyses is much less than over the midlatitudes. Over tropical land, the performance of 20CR is better than NCEP2 and similar to ERA-Interim and CFSR, but over the tropical oceans the most recent reanalyses perform significantly better. Across the twentieth century, the clearest gain from the assimilation of a denser observational dataset is the expansion of the area of good skill. A comparison of the accuracy and ensemble spread in the 20CR and GOGA ensembles highlights regions where SST forcing is a stronger source of skill than data assimilation for 20CR. In contrast, for some tropical regions such as the Sahel, the assimilation of sea level pressure is effective in constraining precipitation values—but model biases in the teleconnections with global SST limit the performance of 20CR.

Relation of the South China Sea Precipitation Variability to Tropical Indo-Pacific SST Anomalies during Spring-to-Summer Transition

Abstract The present study investigates the relationship of South China Sea (SCS) precipitation to tropical Indo-Pacific sea surface temperature (SST) during April–June (AMJ), which is the transition season from spring to summer. It is revealed that SCS rainfall anomalies in AMJ are influenced by SST anomalies in the equatorial Pacific (EP), tropical Indian Ocean (TIO), and western North Pacific (WNP). Three types of SST-influenced cases are obtained based on different combinations of SST anomalies in the above three regions. When same-sign EP and TIO SST anomalies are accompanied by opposite WNP SST anomalies, both anomalous cross-equatorial flows from the southwestern TIO induced by negative SST anomalies there and an anomalous Walker circulation forced by negative EP SST anomalies contribute to enhanced convection over the SCS and the surrounding regions with additional contribution from positive WNP SST anomalies via a Rossby wave–type response. In the cases of combined effects of EP and WNP SST anomalies, above-normal SST in the WNP is a direct cause of above-normal SCS rainfall though the WNP SST anomalies are induced by EP SST forcing. In the cases of combined impacts of TIO and EP SST anomalies, the accompanying coastal Sumatra SST anomalies contribute to the SCS rainfall variability via an anomalous cross-equatorial vertical circulation. The negative SST anomalies near the Sumatra coast induce descent over the southeastern TIO and ascent over the SCS and WNP. Model experiments with an atmospheric model confirm the impacts of southern TIO and EP SST anomalies on AMJ rainfall variation over the SCS.

A model study of tropospheric impacts of the Arctic ozone depletion 2011

Record Arctic ozone loss in spring 2011 occurred in concert with record positive values of the tropospheric Northern Annular Mode (NAM) index raising the question about the role of stratospheric driver on this tropospheric climate event. A set of 50 years long simulations by atmospheric general circulation model European Centre/Hamburg version 5 (ECHAM5) is carried out and the responses of the model to observed anomalies in stratospheric ozone (O3) and sea surface temperatures (SST) separately and also the response to combined SST and O3 forcing (ALL) are analyzed. In all three experiments the response is characterized by a strengthening of stratospheric polar vortex in March–April. In the ALL experiment, this strengthening is followed by a significant, long-lasting shift of the tropospheric circulation toward a positive NAM phase and increased probability of occurrence of extremely positive NAM events. The combined effect of the O3 and SST forcings on the stratospheric circulation differs from the sum of the individual O3 and SST responses, most likely due to nonlinear effects, leading to a colder stratosphere in February–March. In the troposphere, the sum of the individual responses is comparable in magnitude to the ALL response, but the individual responses are delayed with respect to that in ALL. In summary, these results suggest that both ozone-induced stratospheric cooling and tropospheric forcing associated with the SST anomalies contributed to the record tropospheric climate anomalies observed in spring 2011.

On the Role of SST Forcing in the 2011 and 2012 Extreme U.S. Heat and Drought: A Study in Contrasts

Abstract This study compares the extreme heat and drought that developed over the United States in 2011 and 2012 with a focus on the role of sea surface temperature (SST) forcing. Experiments with the NASA Goddard Earth Observing System, version 5 (GEOS-5), atmospheric general circulation model show that the winter/spring response over the United States to the Pacific SST is remarkably similar for the two years despite substantial differences in the tropical Pacific SST. As such, the pronounced winter and early spring temperature differences between the two years (warmth confined to the south in 2011 and covering much of the continent in 2012) primarily reflect differences in the contributions from the Atlantic and Indian Oceans, with both acting to cool the east and upper Midwest during 2011, while during 2012 the Indian Ocean reinforced the Pacific-driven, continental-wide warming and the Atlantic played a less important role. During late spring and summer of 2011, the tropical Pacific SST forced a continued warming and drying over the southern United States, though considerably weaker than observed. Nevertheless, the observed 2011 anomalies fall well within the model’s intraensemble spread. In contrast, the observed rapid development of intense heat and drying over the central United States during June and July 2012 falls on the margins of the model’s intraensemble spread, with the response to the SST giving little indication that 2012 would produce record-breaking precipitation deficits and heat. A diagnosis of the 2012 observed circulation anomalies shows that the most extreme heat and drought was tied to the development of a stationary Rossby wave and an associated anomalous upper-tropospheric high maintained by weather transients.

Distinguishing the effects of internal and forced atmospheric variability in climate networks

Abstract. The fact that the climate on the earth is a highly complex dynamical system is well-known. In the last few decades great deal of effort has been focused on understanding how climate phenomena in one geographical region affects the climate of other regions. Complex networks are a powerful framework for identifying climate interdependencies. To further exploit the knowledge of the links uncovered via the network analysis (for, e.g., improvements in prediction), a good understanding of the physical mechanisms underlying these links is required. Here we focus on understanding the role of atmospheric variability, and construct climate networks representing internal and forced variability using the output of an ensemble of AGCM runs. A main strength of our work is that we construct the networks using MIOP (mutual information computed from ordinal patterns), which allows the separation of intraseasonal, intra-annual and interannual timescales. This gives further insight to the analysis of climatological data. The connectivity of these networks allows us to assess the influence of two main indices, NINO3.4 – one of the indices used to describe ENSO (El Niño–Southern oscillation) – and of the North Atlantic Oscillation (NAO), by calculating the networks from time series where these indices were linearly removed. A main result of our analysis is that the connectivity of the forced variability network is heavily affected by "El Niño": removing the NINO3.4 index yields a general loss of connectivity; even teleconnections between regions far away from the equatorial Pacific Ocean are lost, suggesting that these regions are not directly linked, but rather, are indirectly interconnected via El Niño, particularly at interannual timescales. On the contrary, on the internal variability network – independent of sea surface temperature (SST) forcing – the links connecting the Labrador Sea with the rest of the world are found to be significantly affected by NAO, with a maximum at intra-annual timescales. While the strongest non-local links found are those forced by the ocean, the presence of teleconnections due to internal atmospheric variability is also shown.