One of the most repeatable phenomena seen in the atmosphere, the quasi-biennial oscillation (QBO) between prevailing eastward and westward wind jets in the equatorial stratosphere (approximately 16 ...to 50 kilometers altitude), was unexpectedly disrupted in February 2016. An unprecedented westward jet formed within the eastward phase in the lower stratosphere and cannot be accounted for by the standard QBO paradigm based on vertical momentum transport. Instead, the primary cause was waves transporting momentum from the Northern Hemisphere. Seasonal forecasts did not predict the disruption, but analogous QBO disruptions are seen very occasionally in some climate simulations. A return to more typical QBO behavior within the next year is forecast, although the possibility of more frequent occurrences of similar disruptions is projected for a warming climate.
The Atlantic Multidecadal Oscillation (AMO) is a near‐global scale mode of observed multidecadal climate variability with alternating warm and cool phases over large parts of the Northern Hemisphere. ...Many prominent examples of regional multidecadal climate variability have been related to the AMO, such as North Eastern Brazilian and African Sahel rainfall, Atlantic hurricanes and North American and European summer climate. The relative shortness of the instrumental climate record, however, limits confidence in these observationally derived relationships. Here, we seek evidence of these links in the 1400 year control simulation of the HadCM3 climate model, which produces a realistic long‐lived AMO as part of its internal climate variability. By permitting the analysis of more AMO cycles than are present in observations, we find that the model confirms the association of the AMO with almost all of the above phenomena. This has implications for the predictability of regional climate.
Abstract
Numerous studies have established a link between tropical atmospheric conditions and northern midlatitude circulation mediated by Rossby wave propagation in winter. In recent years, research ...has also investigated tropical to midlatitude teleconnections in northern hemisphere summer. In this paper, we examine summer connections further by imposing observed tropical conditions in climate model simulations. We examine resulting changes in the representation of seasonal mean surface climate variables and mid-troposphere circulation in the northern hemisphere summer, identifying regions where model fidelity improves following the imposition of tropical conditions. We demonstrate robust connections between the tropics and mid-latitudes on the seasonal timescale, with these connections apparent in three mid-latitude regions, namely eastern North America, central Europe and northern Siberia. These regions are shown to be impacted by wave trains originating in specific regions of the subtropics associated with patterns of upper-level convergence. The results provide a clearer picture of tropical to extratropical teleconnections that affect summer mean climate in the northern midlatitudes.
Instrumental sea surface temperature records in the North Atlantic Ocean are characterized by large multidecadal variability known as the Atlantic multidecadal oscillation (AMO). The lack of strong ...oscillatory forcing of the climate system at multidecadal time scales and the results of long unforced climate simulations have led to the widespread, although not ubiquitous, view that the AMO is an internal mode of climate variability. Here, a more objective examination of this hypothesis is performed using simulations with natural and anthropogenic forcings from the Coupled Model Intercomparison Project phase 3 (CMIP3) database. Ensemble means derived from these data allow an estimate of the response of models to forcings, as averaging leads to cancellation of the internal variability between ensemble members. In general, the means of individual model ensembles appear to be inconsistent with observed temperatures, although small ensemble sizes result in uncertainty in this conclusion. Combining the ensembles from different models creates a multimodel ensemble of sufficient size to allow for a good estimate of the forced response. This shows that the variability in observed North Atlantic temperatures possess a clearly distinct signature to the climate response expected from forcings. The reliability of this finding is confirmed by sampling those models with low decadal internal variability and by comparing simulated and observed trends. In contrast to the inconsistency with the ensemble mean, the observations are consistent with the spread of responses in the ensemble members, suggesting they can be accounted for by the combined effects of forcings and internal variability. In the most recent period, the results suggest that the North Atlantic is warming faster than expected, and that the AMO entered a positive phase in the 1990s. The differences found between observed and ensemble mean temperatures could arise through errors in the observational data, errors in the models’ response to forcings or in the forcings themselves, or as a result of genuine internal variability. Each of these possibilities is discussed, and it is concluded that internal variability within the natural climate system is the most likely origin of the differences. Finally, the estimate of internal variability obtained using the model-derived ensemble mean is proposed as a new way of defining the AMO, which has important advantages over previous definitions.
Skillful seasonal prediction of northern hemisphere mid‐latitude winter climate has become a reality in the last decade, but its practical applications are hampered by the “signal‐to‐noise paradox,” ...in which predicted signal amplitudes are much smaller than expected from the correlation with observations. Various hypotheses for the paradox have been advanced. Here we test whether we can identify a contribution from extratropical processes. To do this, we use ensembles of numerical experiments in which the tropical dynamical state is relaxed toward observationally based reanalysis. This allows us to remove errors in the representation of the tropical sources of teleconnections to mid‐ and high‐latitudes. We find that a signal‐to‐noise paradox remains present in our relaxation experiments, implying that at least part of the origin of the paradox must arise from errors in simulating processes in the extratropics. This finding helps to narrow the search for its ultimate cause.
Plain Language Summary
Many economic sectors could benefit from skillful indications of mid‐latitude weather in the season ahead. For winter, at least, moderate skill has become a reality in recent years for some mid‐latitude regions. Despite this, these predictions suffer from a problem that limits their usefulness. Winter seasonal forecasts are reasonably successful in predicting the direction of the change in likelihood, for example, showing more chance of a cold winter when cold winters actually occur. However, the sizes of these shifts are much smaller than would be expected from this level of skill. This points to shortcomings in the climate models used to make the predictions. In this study, we test whether this is a result of errors in model processes occurring outside of the tropics. To do this, we artificially impose observed conditions in the tropical atmosphere of a seasonal prediction model to eliminate tropical errors that can influence mid‐latitude predictions. We find that the problem remains despite this intervention, implying its origin must, at least in part, be from errors originating in middle or high latitudes.
Key Points
Tropical relaxation is used to test whether the mid‐latitude “signal‐to‐noise paradox” can be removed by specifying the tropical state
This is found not to be the case, implying that at least part of the paradox is due to errors in simulating processes in the extratropics
The results have implications for narrowing the identification of the source of the paradox
European and North American winter weather is dominated by year‐to‐year variations in the North Atlantic Oscillation (NAO) which controls the direction and speed of the prevailing winds. An ability ...to forecast the time‐averaged NAO months to years ahead would be of great societal benefit, but current operational seasonal forecasts show little skill. However, there are several elements of the climate system that potentially influence the NAO and may therefore provide predictability for the NAO. We review these potential sources of skill, present emerging evidence that the NAO may be usefully predictable (with correlations exceeding 0.6) on seasonal time‐scales, and discuss prospects for improving skill and extending predictions to multi‐year time‐scales.
The authors estimate the change in extreme winter weather events over Europe that is due to a long-term change in the North Atlantic Oscillation (NAO) such as that observed between the 1960s and ...1990s. Using ensembles of simulations from a general circulation model, large changes in the frequency of 10th percentile temperature and 90th percentile precipitation events over Europe are found from changes in the NAO. In some cases, these changes are comparable to the expected change in the frequency of events due to anthropogenic forcing over the twenty-first century. Although the results presented here do not affect anthropogenic interpretation of global and annual mean changes in observed extremes, they do show that great care is needed to assess changes due to modes of climate variability when interpreting extreme events on regional and seasonal scales. How changes in natural modes of variability, such as the NAO, could radically alter current climate model predictions of changes in extreme weather events on multidecadal time scales is also discussed.
Skilful climate predictions of the winter North Atlantic Oscillation and Arctic Oscillation out to a few months ahead have recently been demonstrated, but the source of this predictability remains ...largely unknown. Here we investigate the role of the Tropics in this predictability. We show high levels of skill in tropical rainfall predictions, particularly over the Pacific but also the Indian and Atlantic Ocean basins. Rainfall fluctuations in these regions are associated with clear signatures in tropical and extratropical atmospheric circulation that are approximately symmetric about the Equator in boreal winter. We show how these patterns can be explained as steady poleward propagating linear Rossby waves emanating from just a few key source regions. These wave source ‘hotspots’ become more or less active as tropical rainfall varies from winter to winter but they do not change position. Finally, we show that predicted tropical rainfall explains a highly significant fraction of the predicted year‐to‐year variation of the winter North Atlantic Oscillation.
Winter forecasts and Rossby wave trains. Skilful extratropical signals occur in Met Office seasonal predictions as a result of successful predictions of year to year changes in tropical rainfall, associated changes in Rossby wave sources and poleward propagating stationary wave trains. The plot shows winter geopotential height anomalies (colour) and Rossby wave trains for zonal wave number 3 (black dots).
Variability in solar irradiance has been connected to changes in surface climate in the North Atlantic through both observational and climate modelling studies which suggest a response in the ...atmospheric circulation that resembles the North Atlantic Oscillation or its hemispheric equivalent the Arctic Oscillation. It has also been noted that this response appears to follow the changes in solar irradiance by a few years, depending on the exact indicator of solar variability. Here we propose and test a mechanism for this lag based on the known impact of atmospheric circulation on the Atlantic Ocean, the extended memory of ocean heat content anomalies, and their subsequent feedback onto the atmosphere. We use results from climate model experiments to develop a simple model for the relationship between solar variability and North Atlantic climate.
Key Points
Solar‐climate variability interacts with the ocean
This interaction causes a significant lag in the Atlantic climate response
A smaller lag implies ocean‐atm interaction may be weak in models
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