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CMIP5 Projections of Arctic Amplification, of the North American/North Atlantic Circulation, and of Their Relationship

Title
Publication TypeManual Entry
Year of Publication2015
AuthorsBarnes, Elizabeth A., and Lorenzo M. Polvani
Journal of Climate
Volume28
Issue13
Pagination5254 - 5271
PublisherAmerican Meteorological Society
0894-8755
Abstract

Recent studies have hypothesized that Arctic amplification, the enhanced warming of the Arctic region compared to the rest of the globe, will cause changes in midlatitude weather over the twenty-first century. This study exploits the recently completed phase 5 of the Coupled Model Intercomparison Project (CMIP5) and examines 27 state-of-the-art climate models to determine if their projected changes in the midlatitude circulation are consistent with the hypothesized impact of Arctic amplification over North America and the North Atlantic.Under the largest future greenhouse forcing (RCP8.5), it is found that every model, in every season, exhibits Arctic amplification by 2100. At the same time, the projected circulation responses are either opposite in sign to those hypothesized or too widely spread among the models to discern any robust change. However, in a few seasons and for some of the circulation metrics examined, correlations are found between the model spread in Arctic amplification and the model spread in the projected circulation changes. Therefore, while the CMIP5 models offer some evidence that future Arctic warming may be able to modulate some aspects of the midlatitude circulation response in some seasons, the analysis herein leads to the conclusion that the net circulation response in the future is unlikely to be determined solely?or even primarily?by Arctic warming according to the sequence of events recently hypothesized.AbstractRecent studies have hypothesized that Arctic amplification, the enhanced warming of the Arctic region compared to the rest of the globe, will cause changes in midlatitude weather over the twenty-first century. This study exploits the recently completed phase 5 of the Coupled Model Intercomparison Project (CMIP5) and examines 27 state-of-the-art climate models to determine if their projected changes in the midlatitude circulation are consistent with the hypothesized impact of Arctic amplification over North America and the North Atlantic.Under the largest future greenhouse forcing (RCP8.5), it is found that every model, in every season, exhibits Arctic amplification by 2100. At the same time, the projected circulation responses are either opposite in sign to those hypothesized or too widely spread among the models to discern any robust change. However, in a few seasons and for some of the circulation metrics examined, correlations are found between the model spread in Arctic amplification and the model spread in the projected circulation changes. Therefore, while the CMIP5 models offer some evidence that future Arctic warming may be able to modulate some aspects of the midlatitude circulation response in some seasons, the analysis herein leads to the conclusion that the net circulation response in the future is unlikely to be determined solely?or even primarily?by Arctic warming according to the sequence of events recently hypothesized.

URLhttps://doi.org/10.1175/JCLI-D-14-00589.1
Short TitleJ. Climate
Citation Key2565
Access
Community Notes

Key Points:

  • In this paper, the authors answer two questions separately: Can Arctic warming impact midlatitude weather and its extremes? Will Arctic warming impact midlatitude weather and its extremes?

    • They compare to this hypothesis of future changes when evaluating whether Arctic warming can/will alter the midlatitude circulation: “Enhanced Arctic warming, presumably caused by increasing greenhouse gases and potentially accelerated by sea ice loss, reduces the equator-to-pole temperature gradient at the surface. This causes 1) the midlatitude winds to decelerate, 2) the jet stream to slow down, and 3) the jet to shift equatorward [negative North Atlantic Oscillation (NAO)/Arctic Oscillation response]. Associated with these changes in the midlatitude flow, the large-scale Rossby waves 4) propagate more slowly and 5) amplify in the meridional direction, leading to 6) an increase in the frequency of blocking events, which are known to lead to extreme weather in the Northern Hemisphere midlatitudes.”

  • 27 CMIP5 models are used, and the key variables examined are monthly mean zonal wind and temperature, and daily mean 500 hPa geopotential height, zonal, and meridional winds.  The models use RCP8.5 for the radiative forcing scenario, and the focus is on the “long term” changes between 2076-2099 and 1980-2004, although some analysis of 2020-2044 is presented as well.  The area of focus is North America and the North Atlantic, because it is in this region that many claims have been made showing observational evidence that Arctic warming is influencing the midlatitude circulation here.

  • 6 metrics are used to evaluate the midlatitude circulation, 3 for the mean flow and 3 for the waves.  They are zonal wind, jet speed, jet position, wave speed, wave extent, and blocking.

  • The authors focus on lower tropospheric temperature (between 925 and 700 hPa), and not just the surface temperature, to define Arctic amplification.  This is because surface warming is confined to the lower troposphere, and does not reach the middle troposphere, where it is able to influence the lower latitude circulation.  

  • In winter, Arctic warming is much stronger than tropical lower or upper tropospheric warming, indicating large Arctic amplification.  In the summer, Arctic warming is still larger, but to a lesser degree.

  • In the near term, all the models in all seasons show Arctic amplification occuring, with the greatest warming occurring in fall and winter.  However, there is large disagreement on the projected changes in the midlatitude circulation. Therefore, Arctic amplification alone is not an adequate predictor of changes in the midlatitude circulation in the near term.  This applies whether one is looking at mean flow or wave metrics.

  • In the long term, all models in all seasons show Arctic amplification occurring.  The magnitude of Arctic amplification remains similar to the near term values, suggesting that, while are Arctic warms throughout the 21st century, its warming relative to the rest of the globe changes very little over time.  In this case, there is more agreement amongst models in the midlatitude response.

    • There is no model consensus on the sign of the changes in zonal wind and jet speed, but do indicate that the jet will shift poleward in all seasons except for winter by the end of the century.

    • For the wave metrics, the wave (phase) speed increases in the fall, and there is not other robust model agreement in the other seasons.  Models robustly agree that wave extent will decrease in spring and summer, with no consensus in winter. Finally, annual mean blocking frequencies robustly decrease.  

  • For winter (JFM), zonal wind changes in the models are strongly correlated with Arctic amplification, with 49% of the variation in zonal wind change being explained by the variation in Arctic amplification between models.  Models with larger Arctic amplification exhibit a weaker zonal wind in the future. This relationship also holds true for jet speed and position, but the correlations for these two metrics with Arctic amplification are much weaker.

  • While the negative correlation between zonal wind change and Arctic amplification is significant, the model spread in zonal wind change is large in all months, so the projected changes are not robust.

  • Arctic amplification is negatively correlated with jet speed in the spring and summer, but there is no consensus among the models in regards to the sign of the change.

  • Models with larger Arctic amplification are correlated with a larger equatorward shift in the jet in winter, but again, there is a lack of model consensus.  This lack of consensus is partially explained by the model spread in Arctic warming. The sign of the correlation is reversed between August and October, suggesting a more poleward jet in the summer, but there is still a lack of model agreement.

  • For the wave metrics in winter, only wave speed has a significant correlation with Arctic amplification.  Larger Arctic amplification is correlated with smaller increases in the wave speed.

  • Wave speed has a significant negative correlation with Arctic amplification in most months.  Although the correlation is negative, the net wave speed response in the models is positive.

  • Wave extent and blocking show little or no correlation with Arctic amplification throughout the year.

  • Overall, while every model in every season exhibits Arctic amplification by the end of the century, the projected response of the circulation is either the opposite of what has been hypothesized, or the spread among the models is too large to obtain a robust response.  Therefore, while there is evidence that Arctic amplification may modulate certain parts of the midlatitude circulation, there is little evidence that it will be the only, or even the dominant, driver of changes.

 

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