finding 11.5 : key-finding-11-5

Atmospheric circulation patterns connect the climates of the Arctic and the contiguous United States. Evidenced by recent record warm temperatures in the Arctic and emerging science, the midlatitude circulation has influenced observed arctic temperatures and sea ice (high confidence). However, confidence is low regarding whether or by what mechanisms observed arctic warming may have influenced the midlatitude circulation and weather patterns over the continental United States. The influence of arctic changes on U.S. weather over the coming decades remains an open question with the potential for significant impact.



This finding is from chapter 11 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The midlatitude circulation has influenced observed arctic temperatures, supported by recent observational and model-based evidence as well as the physical understanding from emerging science. In turn, confidence is low regarding the mechanisms by which observed arctic warming has influenced the midlatitude circulation and weather patterns over the continental United States, due to the disagreement between numerous studies and a lack of understanding of the physical mechanism(s). Resolving the remaining questions requires longer data records and improved understanding and modeling of physics in the Arctic. The influence of arctic changes on U.S. weather over the coming decades remains an open question with the potential for significant impact.

Description of evidence base: The midlatitude circulation influences the Arctic through the transport of warm, moist air, altering the Arctic surface energy budget.aa901e05-668b-422e-9051-ef492dbdf0c2 9eef4c2e-2b3f-4386-af91-10e83338a398 289576a2-4f02-47e2-9a46-567482c10722 1e9f503b-a791-4267-acbe-11ae80ef29a8 The intrusion of warm, moist air from midlatitudes increases downwelling longwave radiation, warming the arctic surface and hindering wintertime sea ice growth.4a7689c2-e1b8-4ef9-a805-23e48a8ce4df 7a2bd7c3-58ef-47d0-9f3f-f758000eb8b7 Emerging research provides a new understanding of the importance of synoptic time scales and the episodic nature of midlatitude air intrusions.4a7689c2-e1b8-4ef9-a805-23e48a8ce4df 37084560-ef67-4df8-a7a5-297582db124c 39754a3f-cc3a-4cc9-8eac-23611de42eec The combination of recent observational and model-based evidence as well as the physical understanding of the mechanisms of midlatitude circulation effects on arctic climate supports this Key Finding.

In addition, research on the impact of arctic climate on midlatitude circulation is rapidly evolving, including observational analysis and modeling studies. Multiple observational studies provide evidence for concurrent changes in the Arctic and Northern Hemisphere large-scale circulation changes.36734ba2-a8b8-419b-99ad-775a948d2408 b7233b5f-ea63-454c-948f-3a14797c355b e94a833d-caff-4c43-9ebd-daa3c8bf17a7 Further, modeling studies demonstrate that arctic warming can influence the midlatitude jet stream and storm track.8686421f-6c0c-4608-bc94-a550fe22514f 625f0fff-b107-4dd0-95ae-0eba38aada7c b7233b5f-ea63-454c-948f-3a14797c355b 29ef1bb5-092f-4a1b-ab0e-1fdc5a0e2ae0 However, attribution studies indicate that the observed midlatitude circulation changes over the continental United States are smaller than natural variability and are therefore not detectable in the observational record.9eef4c2e-2b3f-4386-af91-10e83338a398 1e9f503b-a791-4267-acbe-11ae80ef29a8 a3628a75-8c8e-49e9-ab07-d3a24017225a e393f5f9-08a1-4c08-9ec6-8e445e988b83 This disagreement between independent studies using different analysis techniques and the lack of understanding of the physical mechanism(s) supports this Key Finding.

New information and remaining uncertainties: A major limiting factor is our understanding and modeling of natural climate variability in the Arctic. Longer data records and a better understanding of the physical mechanisms that drive natural climate variability in the Arctic are required to reduce this uncertainty. The inability of climate models to accurately capture interactions between sea ice and the atmospheric circulation and polar stratospheric processes limits our current understanding.

Assessment of confidence based on evidence: High confidence in the impact of midlatitude circulation on arctic changes from the consistency between observations and models as well as a solid physical understanding.

Low confidence on the detection of an impact of arctic warming on midlatitude climate is based on short observational data record, model uncertainty, and lack of physical understanding.

Provenance
This finding was derived from figure -.2: Confidence / Likelihood

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