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R. T. Sutton and D. L. R. Hodson

Abstract

The influence of changing ocean conditions on the variability of climate in the North Atlantic region is studied by analyzing ensemble simulations with an atmospheric GCM forced with reconstructed sea surface temperature (SST) data for the period 1871–1999. The ocean influence on multidecadal variability is analyzed separately from the influence on interannual variability.

SST-forced variability on multidecadal timescales is shown to be dominated by a single mode that, in wintertime, resembles the North Atlantic Oscillation. The principal forcing for this mode is from variations in North Atlantic SST. In addition, however, evidence is found that SST variations in other ocean basins were influential during some sections of the time period studied, in particular during the most recent 50 yr.

Variations in North Atlantic climate on interannual timescales are influenced by the Pacific ENSO phenomenon and also by Atlantic SST. There appears to be competition, with differing outcomes in different regions, between these two influences. Furthermore, it is shown that during the period studied the relative importance of these influences varied; that is, the oceanic influence on North Atlantic climate was nonstationary. The consequences of these results for seasonal forecasting efforts are discussed.

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Rowan T. Sutton and Daniel L. R. Hodson

Abstract

Using experiments with an atmospheric general circulation model, the climate impacts of a basin-scale warming or cooling of the North Atlantic Ocean are investigated. Multidecadal fluctuations with this pattern were observed during the twentieth century, and similar variations—but with larger amplitude—are believed to have occurred in the more distant past. It is found that in all seasons the response to warming the North Atlantic is strongest, in the sense of highest signal-to-noise ratio, in the Tropics. However there is a large seasonal cycle in the climate impacts. The strongest response is found in boreal summer and is associated with suppressed precipitation and elevated temperatures over the lower-latitude parts of North and South America. In August–September–October there is a significant reduction in the vertical shear in the main development region for Atlantic hurricanes. In winter and spring, temperature anomalies over land in the extratropics are governed by dynamical changes in circulation rather than simply reflecting a thermodynamic response to the warming or cooling of the ocean.

The tropical climate response is primarily forced by the tropical SST anomalies, and the major features are in line with simple models of the tropical circulation response to diabatic heating anomalies. The extratropical climate response is influenced both by tropical and higher-latitude SST anomalies and exhibits nonlinear sensitivity to the sign of the SST forcing. Comparisons with multidecadal changes in sea level pressure observed in the twentieth century support the conclusion that the impact of North Atlantic SST change is most important in summer, but also suggest a significant influence in lower latitudes in autumn and winter.

Significant climate impacts are not restricted to the Atlantic basin, implying that the Atlantic Ocean could be an important driver of global decadal variability. The strongest remote impacts are found to occur in the tropical Pacific region in June–August and September–November. Surface anomalies in this region have the potential to excite coupled ocean–atmosphere feedbacks, which are likely to play an important role in shaping the ultimate climate response.

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J. R. Elliott, S. P. Jewson, and R. T. Sutton

Abstract

The El Niño–Southern Oscillation (ENSO) has far-reaching impacts on global climate via “teleconnections” associated with wavelike or other disturbances that are excited in the tropical Pacific. These teleconnections may influence the air–sea heat fluxes, either by altering the latent and sensible heat fluxes through a change in low-level wind speed or direction or by altering the degree of cloud cover and thus the radiation budget. The anomalous fluxes can generate sea surface temperature (SST) anomalies that can in turn feed back on the atmospheric circulation. These effects are explored for the 1997/98 ENSO event using a novel and powerful modeling technique in which a coupled ocean–atmosphere model (the U.K. Hadley Centre HadCM3 model) is forced to follow observed tropical Pacific SSTs using a strong thermal relaxation, while elsewhere the model is allowed to vary freely. This is an extension of previous studies in which the impact of ENSO was investigated using an atmospheric model coupled to an ocean mixed layer model. The authors focus on the impact of ENSO on the Atlantic Ocean. Model results are compared both with historical records of the Atlantic response to El Niño and with SST observations during the 1997/98 event. The model simulates well the warming of the tropical North Atlantic that is typical of El Niño events. In addition, it identifies a significant positive anomaly in the South Atlantic in the autumn of 1997/98 that was also observed and appears to be a feature of the Atlantic response to El Niño that has not previously been noted. The results suggest that many other large SST anomalies observed in the Atlantic during 1997/98 were not part of the response to El Niño.

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R. T. Sutton, P-P. Mathieu, M. Collins, and B. Dong
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R. T. Sutton, S. P. Jewson, and D. P. Rowell

Abstract

The tropical Atlantic region, unlike the tropical Pacific, is not dominated by any single mode of climate variability such as the El Niño–Southern Oscillation (ENSO). Rather, this region is subject to multiple competing influences of comparable importance. The nature and potential predictability of these various influences has been investigated by analysis of an ensemble of atmospheric GCM integrations forced with observed SST for the period December 1948–November 1993.

The dominant modes of internal atmospheric and SST-forced variability are determined. Internal variability in the tropical Atlantic region is dominated by the equatorward extension of extratropical patterns, especially the North Atlantic oscillation. Three different SST-forced signals are identified. These are (a) a remote response to ENSO, (b) a response to the so-called Atlantic Dipole SST pattern, and (c) a response to equatorial Atlantic SST anomalies. The spatial structure and seasonality of these different elements of climate variability are diagnosed and feedbacks onto the ocean are assessed. The evidence presented supports the possibility of ENSO-like variability in the equatorial Atlantic, but does not support the suggestion that the Atlantic Dipole is a coupled ocean–atmosphere mode of variability.

An important feature of this study is that the results include quantitative estimates of the comparative importance, in different regions and different seasons, of the various influences on tropical Atlantic climate variability. These estimates are used to assess the potential predictability of key climatic indices.

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Daniel L. R. Hodson, Jon I. Robson, and Rowan T. Sutton

Abstract

In the 1960s and early 1970s, sea surface temperatures in the North Atlantic Ocean cooled rapidly. There is still considerable uncertainty about the causes of this event, although various mechanisms have been proposed. In this observational study, it is demonstrated that the cooling proceeded in several distinct stages. Cool anomalies initially appeared in the mid-1960s in the Nordic Seas and Gulf Stream extension, before spreading to cover most of the subpolar gyre. Subsequently, cool anomalies spread into the tropical North Atlantic before retreating, in the late 1970s, back to the subpolar gyre. There is strong evidence that changes in atmospheric circulation, linked to a southward shift of the Atlantic ITCZ, played an important role in the event, particularly in the period 1972–76. Theories for the cooling event must account for its distinctive space–time evolution. The authors’ analysis suggests that the most likely drivers were 1) the “Great Salinity Anomaly” of the late 1960s; 2) an earlier warming of the subpolar North Atlantic, which may have led to a slowdown in the Atlantic meridional overturning circulation; and 3) an increase in anthropogenic sulfur dioxide emissions. Determining the relative importance of these factors is a key area for future work.

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P-P. Mathieu, R. T. Sutton, B. Dong, and M. Collins

Abstract

The predictability of winter climate over the North Atlantic–European (NAE) region during ENSO events is investigated. Rather than employing traditional composite analyses, the authors focus on the impacts of six individual events: three El Niño events and three La Niña events. The investigation is based on the analysis of ensemble simulations with an atmospheric GCM forced with prescribed sea surface temperatures (SST) for the period December 1985–May 2001, and on observations. Model experiments are used to separate the respective roles of SST anomalies in the Indo-Pacific basin and in the Atlantic basin.

A significant (potentially predictable) climate signal is found in the NAE region for all six ENSO events. However, there are notable differences in the impacts of individual El Niño and La Niña events. These differences arise not simply from atmospheric internal variability but also because the atmosphere is sensitive to specific features of the SST anomaly fields that characterize the individual events. The different impacts arise partly from differences in Indo-Pacific SST and partly from differences in Atlantic SST. SST anomalies in both ocean basins can influence tropical convection and excite a Rossby wave response over the North Atlantic. The evidence presented here for the importance of Atlantic Ocean conditions argues that, in the development of systems for seasonal forecasting, attention should not be focused too narrowly on the tropical Pacific Ocean.

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R. T. Sutton, G. D. McCarthy, J. Robson, B. Sinha, A. T. Archibald, and L. J. Gray

Abstract

Atlantic multidecadal variability (AMV) is the term used to describe the pattern of variability in North Atlantic sea surface temperatures (SSTs) that is characterized by decades of basinwide warm or cool anomalies, relative to the global mean. AMV has been associated with numerous climate impacts in many regions of the world including decadal variations in temperature and rainfall patterns, hurricane activity, and sea level changes. Given its importance, understanding the physical processes that drive AMV and the extent to which its evolution is predictable is a key challenge in climate science. A leading hypothesis is that natural variations in ocean circulation control changes in ocean heat content and consequently AMV phases. However, this view has been challenged recently by claims that changing natural and anthropogenic radiative forcings are critical drivers of AMV. Others have argued that changes in ocean circulation are not required. Here, we review the leading hypotheses and mechanisms for AMV and discuss the key debates. In particular, we highlight the need for a holistic understanding of AMV. This perspective is a key motivation for a major new U.K. research program: the North Atlantic Climate System Integrated Study (ACSIS), which brings together seven of the United Kingdom’s leading environmental research institutes to enable a broad spectrum approach to the challenges of AMV. ACSIS will deliver the first fully integrated assessment of recent decadal changes in the North Atlantic, will investigate the attribution of these changes to their proximal and ultimate causes, and will assess the potential to predict future changes.

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S. Venzke, M. R. Allen, R. T. Sutton, and D. P. Rowell

Abstract

Decadal fluctuations in the climate of the North Atlantic–European region may be influenced by interactions between the atmosphere and the Atlantic Ocean, possibly as part of a coupled ocean–atmosphere mode of variability. For such a mode to exist, a consistent atmospheric response to fluctuations in North Atlantic sea surface temperatures (SST) is required. Furthermore, this response must provide feedbacks to the ocean. Whether a consistent response exists, and whether it yields the required feedbacks, are issues that remain controversial. Here, these issues are addressed using a novel approach to analyze an ensemble of six integrations of the Hadley Centre atmospheric general circulation model HadAM1, all forced with observed global SSTs and sea-ice extents for the period 1949–93.

Characterizing the forced atmospheric response is complicated by the presence of internal variability. A generalization of principal component analysis is used to estimate the common forced response given the knowledge of internal variability provided by the ensemble. In the North Atlantic region a remote atmospheric response to El Niño–Southern Oscillation and a further response related to a tripole pattern in North Atlantic SST are identified. The latter, which is most consistent in spring, involves atmospheric circulation changes over the entire region, including a dipole pattern in sea level pressure often associated with the North Atlantic oscillation. Only over the tropical/subtropical Atlantic, however, does it account for a substantial fraction of the total variance. How the atmospheric response could feed back to affect the ocean, and in particular the SST tripole, is investigated. Several potential feedbacks are identified but it has to be concluded that, because of their marginal consistency between ensemble members, a coupled mode that relied on these feedbacks would be susceptible to disruption by internal atmospheric variability. Notwithstanding this conclusion, the authors’ results suggest that predictions of SST evolution could be exploited to predict some aspects of atmospheric variability over the North Atlantic, including fluctuations in spring of the subtropical trade winds and the higher latitude westerlies.

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Matthew B. Menary, Daniel L. R. Hodson, Jon I. Robson, Rowan T. Sutton, and Richard A. Wood

Abstract

The North Atlantic Ocean subpolar gyre (NA SPG) is an important region for initializing decadal climate forecasts. Climate model simulations and paleoclimate reconstructions have indicated that this region could also exhibit large, internally generated variability on decadal time scales. Understanding these modes of variability, their consistency across models, and the conditions in which they exist is clearly important for improving the skill of decadal predictions—particularly when these predictions are made with the same underlying climate models. This study describes and analyzes a mode of internal variability in the NA SPG in a state-of-the-art, high-resolution, coupled climate model. This mode has a period of 17 yr and explains 15%–30% of the annual variance in related ocean indices. It arises because of the advection of heat content anomalies around the NA SPG. Anomalous circulation drives the variability in the southern half of the NA SPG, while mean circulation and anomalous temperatures are important in the northern half. A negative feedback between Labrador Sea temperatures/densities and those in the North Atlantic Current (NAC) is identified, which allows for the phase reversal. The atmosphere is found to act as a positive feedback on this mode via the North Atlantic Oscillation (NAO), which itself exhibits a spectral peak at 17 yr. Decadal ocean density changes associated with this mode are driven by variations in temperature rather than salinity—a point which models often disagree on and which may affect the veracity of the underlying assumptions of anomaly-assimilating decadal prediction methodologies.

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