Any reduction in global mean near-surface temperature due to a future decline in solar activity is likely to be a small fraction of projected anthropogenic warming. However, variability in ...ultraviolet solar irradiance is linked to modulation of the Arctic and North Atlantic Oscillations, suggesting the potential for larger regional surface climate effects. Here, we explore possible impacts through two experiments designed to bracket uncertainty in ultraviolet irradiance in a scenario in which future solar activity decreases to Maunder Minimum-like conditions by 2050. Both experiments show regional structure in the wintertime response, resembling the North Atlantic Oscillation, with enhanced relative cooling over northern Eurasia and the eastern United States. For a high-end decline in solar ultraviolet irradiance, the impact on winter northern European surface temperatures over the late twenty-first century could be a significant fraction of the difference in climate change between plausible AR5 scenarios of greenhouse gas concentrations.
Initial-value predictability measures the degree to which the initial state can influence predictions. In this paper, the initial-value predictability of six atmosphere–ocean general circulation ...models in the North Pacific and North Atlantic is quantified and contrasted by analyzing long control integrations with time invariant external conditions. Through the application of analog and multivariate linear regression methodologies, average predictability properties are estimated for forecasts initiated from every state on the control trajectories. For basinwide measures of predictability, the influence of the initial state tends to last for roughly a decade in both basins, but this limit varies widely among the models, especially in the North Atlantic. Within each basin, predictability varies regionally by as much as a factor of 10 for a given model, and the locations of highest predictability are different for each model. Model-to-model variations in predictability are also seen in the behavior of prominent intrinsic basin modes. Predictability is primarily determined by the mean of forecast distributions rather than the spread about the mean. Horizontal propagation plays a large role in the evolution of these signals and is therefore a key factor in differentiating the predictability of the various models.
Three prominent quasi‐global patterns of variability and change are observed using the Met Office's sea surface temperature (SST) analysis and almost independent night marine air temperature ...analysis. The first is a global warming signal that is very highly correlated with global mean SST. The second is a decadal to multidecadal fluctuation with some geographical similarity to the El Niño–Southern Oscillation (ENSO). It is associated with the Pacific Decadal Oscillation (PDO), and its Pacific‐wide manifestation has been termed the Interdecadal Pacific Oscillation (IPO). We present model investigations of the relationship between the IPO and ENSO. The third mode is an interhemispheric variation on multidecadal timescales which, in view of climate model experiments, is likely to be at least partly due to natural variations in the thermohaline circulation. Observed climatic impacts of this mode also appear in model simulations. Smaller‐scale, regional atmospheric phenomena also affect climate on decadal to interdecadal timescales. We concentrate on one such mode, the winter North Atlantic Oscillation (NAO). This shows strong decadal to interdecadal variability and a correspondingly strong influence on surface climate variability which is largely additional to the effects of recent regional anthropogenic climate change. The winter NAO is likely influenced by both SST forcing and stratospheric variability. A full understanding of decadal changes in the NAO and European winter climate may require a detailed representation of the stratosphere that is hitherto missing in the major climate models used to study climate change.
In early 2016, we predicted that the annual rise in carbon dioxide concentration at Mauna Loa would be the largest on record. Our forecast used a statistical relationship between observed and ...forecast sea surface temperatures in the Niño 3.4 region and the annual CO₂ rise. Here, we provide a formal verification of that forecast. The observed rise of 3.4 ppm relative to 2015 was within the forecast range of 3.15±0.53 ppm, so the prediction was successful. A global terrestrial biosphere model supports the expectation that the El Niño weakened the tropical land carbon sink. We estimate that the El Niño contributed approximately 25% to the record rise in CO₂, with 75% due to anthropogenic emissions. The 2015/2016 CO₂ rise was greater than that following the previous large El Niño in 1997/1998, because anthropogenic emissions had increased. We had also correctly predicted that 2016 would be the first year with monthly mean CO₂ above 400 ppm all year round. We now estimate that atmospheric CO₂ at Mauna Loa would have remained above 400 ppm all year round in 2016 even if the El Niño had not occurred, contrary to our previous expectations based on a simple extrapolation of previous trends.
This article is part of a discussion meeting issue 'The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.
The causality of the link between Autumn Barents‐Kara (BK) sea ice and the winter North Atlantic Oscillation (NAO) is uncertain, given teleconnections stemming from the tropics may influence both the ...extra‐tropics and the Arctic. We explore the relationship between tropical rainfall and BK sea ice in autumn, by nudging the tropics to follow observed variability in otherwise free running ensemble simulations. Tropical forcing alone can skillfully reproduce a significant fraction of observed interannual NAO variability in late autumn. We also show that interannual variability in the NAO is strongly related to simulated BK sea ice. As a result, we are able to reproduce some of the observed link between tropical rainfall and autumn BK sea ice. However, only during the strong 1997 El Niño are clear tropical influences at high latitudes found. Large ensembles and strong tropical forcing are required to detect tropical forced variability in models at high latitudes.
Plain Language Summary
Regional variations in Arctic sea ice during autumn have been linked to large scale weather patterns over the Atlantic, which impact on European weather. Commonly, this can be attributed to the North Atlantic Oscillation (NAO). Whether the relationship with sea ice and the NAO is physical, or coincidental, is unclear, given that remote tropical weather can impact on both Arctic sea‐ice and the NAO. In this study, we explore how the tropics could impact sea ice in the Barents‐Kara (BK) seas during autumn, by constructing controlled experiments where we specify the state of the tropical atmosphere. We repeat this many times, to determine how much variability there is in this relationship. We find that there is a link between tropical weather and large‐scale weather patterns in the Atlantic, with a weak link found between tropical weather and BK sea‐ice. However, during years when tropical convection is particularly active in the east Pacific, we can see a much stronger impact on the Arctic during autumn. Our results indicate that the link between the tropics and Arctic may be too weak in models and is only detectable during years where the tropical variability is particularly strong.
Key Points
Tropical nudging experiments reproduce some of the observed interannual variability in autumn Barents‐Kara sea ice
Tropical nudging reproduces a significant fraction of observed autumn North Atlantic Oscillation (NAO) variability, and the anticorrelation of NAO with sea ice
Only during the strong El Niño of 1997 is the model able to reproduce the strong observed teleconnection from the tropics to sea ice
The predictability of the quasi‐biennial oscillation (QBO) is examined in initialized climate forecasts extending out to lead times of years. We use initialized retrospective predictions made with ...coupled ocean‐atmosphere climate models that have an internally generated QBO. We demonstrate predictability of the QBO extending more than 3 years into the future, well beyond timescales normally associated with internal atmospheric processes. Correlation scores with observational analyses exceed 0.7 at a lead time of 12 months. We also examine the variation of predictability with season and QBO phase and find that skill is lowest in winter. An assessment of perfect predictability suggests that higher skill may be achievable through improved initialization and climate modeling of the QBO, although this may depend on the realism of gravity wave source parameterizations in the models. Finally, we show that skilful prediction of the QBO itself does not guarantee predictability of the extratropical winter teleconnection that is important for surface winter climate prediction.
Key Points
The QBO is skilfully predicted in seasonal‐decadal forecast systems
Further improvements in predictions of the QBO are possible
The QBO winter surface teleconnection is reproduced with mixed success
The Quasi‐Biennial Oscillation (QBO) is the dominant mode of interannual variability in the tropical stratosphere, with easterly and westerly zonal wind regimes alternating over a period of about ...28 months. It appears to influence the Northern Hemisphere winter stratospheric polar vortex and atmospheric circulation near the Earth's surface. However, the short observational record makes unequivocal identification of these surface connections challenging. To overcome this, we use a multicentury control simulation of a climate model with a realistic, spontaneously generated QBO to examine teleconnections with extratropical winter surface pressure patterns. Using a 30‐hPa index of the QBO, we demonstrate that the observed teleconnection with the Arctic Oscillation (AO) is likely to be real, and a teleconnection with the North Atlantic Oscillation (NAO) is probable, but not certain. Simulated QBO‐AO teleconnections are robust, but appear weaker than in observations. Despite this, inconsistency with the observational record cannot be formally demonstrated. To assess the robustness of our results, we use an alternative measure of the QBO, which selects QBO phases with westerly or easterly winds extending over a wider range of altitudes than phases selected by the single‐level index. We find increased strength and significance for both the AO and NAO responses, and better reproduction of the observed surface teleconnection patterns. Further, this QBO metric reveals that the simulated AO response is indeed likely to be weaker than observed. We conclude that the QBO can potentially provide another source of skill for Northern Hemisphere winter prediction, if its surface teleconnections can be accurately simulated.
Key Points
QBO teleconnection with the winter Arctic Oscillation is likely to be real, but observations only weakly constrain its strength
Analysis using new deep QBO phase measure increases the strength and significance of the winter surface teleconnections
Teleconnections between the deep QBO and Arctic Oscillation simulated by the HadGEM3 climate model are likely weaker than those observed
The North Atlantic Oscillation (NAO) has a profound effect on winter climate variability around the Atlantic basin. Strengthening of the NAO in recent decades has altered surface climate in these ...regions at a rate far in excess of global mean warming. However, only weak NAO trends are reproduced in climate simulations of the 20th Century, even with prescribed climate forcings and historical sea‐surface conditions. Here we show that the unexplained strengthening of the NAO can be fully simulated in a climate model by imposing observed trends in the lower stratosphere. This implies that stratospheric variability needs to be reproduced in models to fully simulate surface climate variations in the North Atlantic sector. Despite having little effect on global mean warming, we show that downward coupling of observed stratospheric circulation changes to the surface can account for the majority of change in regional surface climate over Europe and North America between 1965 and 1995.
In the 2013/2014 winter, the eastern U.S. was exceptionally cold, the Bering Strait region was exceptionally warm, California was in the midst of drought, and the UK suffered severe flooding. It has ...been suggested that elevated sea surface temperatures (SSTs) in the tropical West Pacific (TWPAC) were partly to blame due to them producing a Rossby wave train that propagated into the extratropics. We find that seasonal forecasts with the tropical atmosphere relaxed toward a reanalysis give 2013/2014 winter mean anomalies with strong similarities to those observed in the Northern Hemisphere, indicating that low‐latitude anomalies had a role in the development of the extremes. Relaxing just the TWPAC produces a strong wave train over the North Pacific and North America in January, but not in the winter mean. This suggests that anomalies in this region alone had a large influence but cannot explain the extremes through the whole winter. We also examine the response to applying the observed TWPAC SST anomalies in two atmospheric general circulation models. We find that this does produce winter mean anomalies in the North Pacific and North America resembling those observed and that the tropical forcing of Rossby waves due to the applied SST anomalies appears stronger than that in reanalysis, except in January. Therefore, both experiments indicate that the TWPAC influence was important, but the true strength of the TWPAC influence is uncertain. None of the experiments indicate a strong systematic impact of the TWPAC anomalies on Europe.
Key Points
Low latitudes had an important role in the development of the 2013/2014 winter extratropical extremes
Relaxation experiments indicate the tropical West Pacific (TWPAC) had an influence in January only
Atmospheric GCM experiments indicate a larger role of the TWPAC, making its influence uncertain
The stratosphere can have a significant impact on winter surface weather on subseasonal to seasonal (S2S) timescales. This study evaluates the ability of current operational S2S prediction systems to ...capture two important links between the stratosphere and troposphere: (1) changes in probabilistic prediction skill in the extratropical stratosphere by precursors in the tropics and the extratropical troposphere and (2) changes in surface predictability in the extratropics after stratospheric weak and strong vortex events. Probabilistic skill exists for stratospheric events when including extratropical tropospheric precursors over the North Pacific and Eurasia, though only a limited set of models captures the Eurasian precursors. Tropical teleconnections such as the Madden‐Julian Oscillation, the Quasi‐Biennial Oscillation, and El Niño–Southern Oscillation increase the probabilistic skill of the polar vortex strength, though these are only captured by a limited set of models. At the surface, predictability is increased over the United States, Russia, and the Middle East for weak vortex events, but not for Europe, and the change in predictability is smaller for strong vortex events for all prediction systems. Prediction systems with poorly resolved stratospheric processes represent this skill to a lesser degree. Altogether, the analyses indicate that correctly simulating stratospheric variability and stratosphere‐troposphere dynamical coupling are critical elements for skillful S2S wintertime predictions.
Key Points
Tropospheric precursors of SSW events are better represented for the North Pacific than for Eurasia
Teleconnections from the tropics add probabilistic skill but are only represented by a few models
Weak and strong vortex events in the NH stratosphere can contribute to surface skill 3–4 weeks later
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