
NSF Org: |
OPP Office of Polar Programs (OPP) |
Recipient: |
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Initial Amendment Date: | April 19, 2017 |
Latest Amendment Date: | April 19, 2017 |
Award Number: | 1643436 |
Award Instrument: | Standard Grant |
Program Manager: |
David Sutherland
OPP Office of Polar Programs (OPP) GEO Directorate for Geosciences |
Start Date: | May 1, 2017 |
End Date: | April 30, 2022 (Estimated) |
Total Intended Award Amount: | $387,742.00 |
Total Awarded Amount to Date: | $387,742.00 |
Funds Obligated to Date: |
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History of Investigator: |
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Recipient Sponsored Research Office: |
4333 BROOKLYN AVE NE SEATTLE WA US 98195-1016 (206)543-4043 |
Sponsor Congressional District: |
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Primary Place of Performance: |
1013 NE 40th Street Seattle WA US 98105-6698 |
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): | ANT Ocean & Atmos Sciences |
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): | 47.078 |
ABSTRACT
This project will use observations and coupled climate model simulations to examine the causes of sea ice variability. Sea ice in the Southern Ocean has increased in area over the observational record but researchers have yet to agree on the cause. Researchers suggests that changes in surface winds, upper-ocean freshening, or internal ocean/atmosphere variability could be the main driver for the increase in sea ice area. This project will determine how much of the change in sea ice area from year to year is due to oceanic, atmospheric, and radiative processes. Reconciling the observation-based understanding with model representations of sea ice variability will improve confidence in projections of future changes in Southern Ocean sea ice.
The goal of this proposal is to improve our understanding of the processes that drive Southern Ocean sea ice year-to-year variability and long term trends. This knowledge will provide insight into how Southern Ocean sea ice responded to greenhouse gas and ozone forcing in the past and how it will respond in the future. The energy budget of the coupled cryosphere/ocean/atmosphere climate system will be used as a framework to disentangle drivers and responses during sea ice loss events. The technique consists of: (i) calculating the coupled energy budget of the climate system at the monthly timescale, (ii) isolating the radiative impact of sea ice variability from the radiative impact of cloud variability in the observed satellite radiation record and (iii) analyzing the vertical structure of atmospheric energy transport to determine the vertical profile of energy transport into the atmospheric column. This framework will allow the investigators to distinguish whether ice loss events are triggered by oceanic processes, atmospheric dynamics, or radiative processes. Preliminary results show that a diversity of mechanisms can drive Southern Ocean sea ice variability in coupled climate models whereas observed sea ice variability appears to be dominated by atmospheric dynamics. The exploration of biases between models and observations in both the mean state and in specific processes will yield more accurate projections of the future of sea ice in the Southern Ocean.
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.
Sea ice extent in the Southern Ocean has shown a modest long-term increase over the satellite era (1979-present). In contrast, state of the art climate models unanimously simulate a reduction of sea ice extent over the Southern Ocean as would be expected from the build up of greenhouse gases and global warming. This project investigated mechanism of Southern Ocean sea ice loss in climate models and observations in search of the missing or poorly represented physics in the models that are responsible for the observational/model mismatch of the historical trend. Our work covered mechanisms of sea ice loss at time scales ranging from days to year-to-year variability to long-term forced trends. We found that models have too little sea ice variability at the time scale of weather events (less than 10 days). We leveraged this finding to probe mechanisms of model biases in sea ice variability across timescales in hopes that the physics responsible for well sampled day-to-day variability might also impact long-term trends.
Our project analyzed three primary mechanisms of sea ice loss and evaluated whether these processes are biased in climate models.
Radiative impact of sea ice loss: Sea ice melting exposes the darker ocean surface to the sun resulting in additional heating of the surface which can melt more ice. We evaluated the magnitude of this positive feedback using satellite radiation data and climate model output from over 20 models. We found that models differ by a factor of two in the radiative impact of sea ice loss due to differences in the thickness of clouds in the Southern Ocean. However, models are not, on average, biased relative to observations. Thus, the radiative impact of sea ice leads to inter-model diversity in the sensitivity of sea-ice loss to warming but does not lead to a model bias.
Impact of observed winds and sea surface temperatures in sea ice variability: We developed a novel technique to make the winds in a climate model match those observed over the 1979-2020 period. We found that the model with observed winds nearly replicated the observed year-to-year variability of Southern Ocean sea ice. The long-term sea ice trend of the model with observed winds showed smaller reductions than the model with free running winds but still showed a reduction in sea ice (a mismatch to the observed expansion). When we additionally forced the sea surface temperatures in the mid-latitudes of the Southern Ocean to match those observed (which show a modest cooling trend), the model nearly replicated the long-term expansion of the sea ice. These results indicate that the year-to-year variability of Southern Ocean sea ice is primarily forced by wind variability whereas both winds and sea surface temperature impact long-term trends in sea ice. The model bias toward long-term sea ice loss is primarily due to faster than observed warming of the Southern Ocean simulated by the models.
Impact of atmospheric heat transport on sea ice loss: We demonstrated that sea ice loss events in models are initiated by anomalous atmospheric heat transport into the region above the sea ice loss which heats the atmosphere and subsequently melts ice. The atmospheric heat transport peaks 10 days before the ice loss. During and after the ice loss, the open ocean serves as a source of energy to the atmosphere and additionally heats the atmosphere. As a result, the atmosphere removes energy from the region during and after the ice loss. The opposing atmospheric heat fluxes before and after the event average to a very small change in atmospheric heat transport. This result cautions against interpreting long term changes in atmospheric heat transport as a climate forcing and, instead, suggests that the long-term changes in atmospheric heat transport represent a delicate balance of remote forcing and feedback to the ice loss. In future work we hope to analyze whether models are biased in the amplitude of atmospheric heat transport events at short (less than 10 days) timescales and whether these biases could explain the model biases in sea ice variability at the timescale of weather events.
Last Modified: 06/08/2022
Modified by: Aaron Donohoe
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