| NSF Org: |
AGS Division of Atmospheric and Geospace Sciences |
| Recipient: |
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| Initial Amendment Date: | May 27, 2014 |
| Latest Amendment Date: | May 27, 2014 |
| Award Number: | 1407360 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Eric DeWeaver
edeweave@nsf.gov (703)292-8527 AGS Division of Atmospheric and Geospace Sciences GEO Directorate for Geosciences |
| Start Date: | June 1, 2014 |
| End Date: | May 31, 2017 (Estimated) |
| Total Intended Award Amount: | $680,173.00 |
| Total Awarded Amount to Date: | $680,173.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
160 ALDRICH HALL IRVINE CA US 92697-0001 (949)824-7295 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
Department of Earth Syste Scienc Irvine CA US 92697-3100 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Climate & Large-Scale Dynamics |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Abstract
The Atlantic Multidecadal Oscillation (AMO) is a form of sea surface temperature (SST) variation in which SSTs over a large portion of the North Atlantic simultaneously warm or cool, with oscillations from warm to cold taking place over a period of 60 to 70 years. The AMO has been shown to influence precipitation over North America and Europe, and is linked to rainfall and drought over Northeast Brazil and the Sahel. The AMO is thought to be linked to the overturning circulation of the North Atlantic, so that slower overturning results in the cold phase of the AMO, while faster overturning leads to the warm phase.
While previous studies have focused on the impacts of the AMO on precipitation and drought during the summer season, work here addresses the possibility of an AMO influence on Northern Hemisphere atmospheric circulation and consequent weather and climate impacts during the winter. The particular effect hypothesized here is that the AMO influences the North Atlantic Oscillation (NAO), a large-scale fluctuation of sea level pressure and wind which affects the weather and climate of Europe and North America. The expected relationship is such that the warm phase of the AMO favors the negative phase of the NAO, which is characterized by a more meandering jet stream and increased incidence of cold air outbreaks. Research to test this relationship includes analysis of the 20th Century Reanalysis (20CR) dataset, in which atmospheric weather and circulation fields are reconstructed based on surface barometer readings from 1871 to 2010, along with SST records from the same time period. The observational analysis is augmented with numerical model simulations intended to capture the atmospheric response to imposed AMO SSTs, and these simulations are further compared to the archive of coupled climate model simulations prepared for the Coupled Model Intercomparison Project version 5 (CMIP5). The research will also consider the possibility that the AMO affects the NAO indirectly, as warm SSTs lead to reduced Arctic ice cover, which in turn affects atmospheric circulation.
A better understanding of the effects of the AMO on the NAO is desirable due to the impact of the NAO on the severity of winters in the US and Europe, particularly given that the slow variation of the AMO could imply some level of predictability for winter conditions. In addition to the societal benefit of the research results, the work will have broader impacts by supporting a graduate student, thereby providing for the future workforce in climate research. In addition, the researchers will participate in an outreach program titled "Climate Literacy Empowerment And iNquiry (CLEAN). The program is targeted at elementary and middle school children.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The Atlantic Multidecadal Variability (AMV) is a mode of variability in North Atlantic sea surface temperature (SST), consisting of basin-wide, alternating warm and cold conditions that has a period of 60-70 years as seen in SST observations (e.g., HadISST). Because of its long time scale, only 2 cycles of this mode are available from observations. We examined the potential role of this mode for multidecadal predictability of winter climate over the North Atlantic basin and nearby continents. For the first part of the project we focused on observations (20th Century reanalysis for daily atmospheric fields, using data starting in 1900). By using composite analysis, we found convincing evidence that the positive phase of the AMV precedes a trend towards the negative North Atlantic Oscillation (NAO) by 5-10 years, and the same for the negative AMV and the positive NAO. This modulation of the wintertime atmospheric circulation by the AMV signal is also detected in the multidecadal variability of the intraseasonal weather regimes over the North Atlantic sector.
Because of the shortness of the observational record, we performed numerical experiments with an atmospheric global climate model (AGCM, the NCAR model CAM5) forced with both the positive and negative AMV pattern. Results from the modeling experiments were supportive of the observational results, with the positive AMV promoting a negative winter NAO. However, this modeling study did not address the potential role of the stratosphere in the teleconnections and the possible role of thermodynamic coupling to the ocean. We therefore ran additional modeling experiments to address these points, with i) a high-top version of the AGCM (WACCM5) and ii) coupling the low-top AGCM to a slab ocean.
The results support our earlier findings, that indeed the positive AMV promotes the negative NAO in winter. The results were only marginally sensitive to the inclusion of the stratosphere and the thermodynamical coupling to the ocean, respectively. As part of this study, we also examined the relative role of the tropical vs. the extratropical part of the SST field corresponding to the AMV in forcing the response. We found that both parts were important to the response and in fact, reinforced each other, so that the combined response is greater than the response to the tropical forcing plus the response to the extratropical forcing individually (nonlinear response). The tropical response consists of a Rossby wave train from the tropical Atlantic into higher latitudes, then eddy feedback enhances the response in the extratropics.
A drawback of our modeling studies is that the full ocean dynamics is not represented. We therefore drew on already completed modeling studies from the archive of the 5th Coupled Multimodel Intercomparison Project (CMIP5), focusing on pre Industrial simulations (not forced with external forcing) that are run for at least 500 years. All of the models provide a forcing of the ocean by the wintertime NAO, but a multidecadal feedback by the AMV (from ocean to atmosphere) is not readily detected in most of the models. Overall, the internally generated AMV as simulated by the models is too weak. Only two models (GFDL-ESM2G and HadGEM2-ES) show a small lagged NAO signal that suggests a forcing role of the ocean on the decadal fluctuations of the atmosphere. Recent coupled modeling studies of AMOC variability suggest that the forcing of the ocean by the winter NAO is very much present in the models. However, the long-term NAO fluctuations are generally too weak, leading to underestimated ocean-atmosphere coupling on multidecadal time scales that may explain the lack of AMV feedback on the atmosphere. It may be that the atmospheric model may have to be run at very high resolution in order to force the ocean sufficiently to capture the coupled interaction in the North Atlantic. As more high resolution model simulations become available, we will be able to test the hypothesis. We have made a concerted effort in recent months to publicize this work at conferences, workshops and seminars to draw attention to this possibility.
Five different graduate students were associated with this project, including three women and one African American. Two of those have defended their PhD dissertation (two women), one works for the government, the other is a presidential postdoctoral fellow at the University of Michigan. A female undergraduate student who is now a graduate student at the University of Madison Wisconsin also contributed to this project.
Last Modified: 07/12/2017
Modified by: Gudrun Magnusdottir
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