finding 5.2 : key-finding-5-2

Recurring patterns of variability in large-scale atmospheric circulation (such as the North Atlantic Oscillation and Northern Annular Mode) and the atmosphere–ocean system (such as El Niño–Southern Oscillation) cause year-to-year variations in U.S. temperatures and precipitation (high confidence). Changes in the occurrence of these patterns or their properties have contributed to recent U.S. temperature and precipitation trends (medium confidence), although confidence is low regarding the size of the role of human activities in these changes.



This finding is from chapter 5 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Recurring modes of variability strongly affect temperature and precipitation over the United States on interannual timescales (high confidence) as supported by a very large number of observational and modeling studies. Changes in some recurring patterns of variability have contributed to recent trends in U.S. temperature and precipitation (medium confidence). The causes of these changes are uncertain due to the limited observational record and because models exhibit some difficulties simulating these recurring patterns of variability and their underlying physical mechanisms.

Description of evidence base: The Key Finding is supported by a large number of studies that diagnose recurring patterns of variability and their changes, as well as their impact on climate over the United States. Regarding year-to-year variations, a large number of studies based on models and observations show statistically significant associations between North Atlantic Oscillation/Northern Annular Mode and United States temperature and precipitation,e58558a8-6072-4c71-9e2a-ad3182f9d999 ed3d1a48-a7f4-4f31-a0c2-91e438ea459d eae7e119-f3d7-4ad5-9f47-5d99a2cbdd1d f5711fb8-ea3b-4564-8a60-0fabd66bd173 b35b1dec-2d93-42a8-8a85-ba1a2cfa76b9 33619644-54fd-43b3-9fa8-e7ac5ddec196 as well as El Niño–Southern Oscillation and related U.S. climate teleconnections.20e48aa7-625b-4527-8639-e0bcd469b79d 26e7ebff-78f5-4697-8203-cfc9586c6a0c d81a06f0-65ed-4df1-9b24-eb2874f944b7 e2259324-cd73-41c2-be96-a459a5021ae1 0eca3f20-4ffe-4d20-baaa-2d605941578d ff7033b4-13c2-4cf2-8ba2-c0b81affa2b4 4e933785-f080-4cc8-86a0-5259159292e0 Regarding recent decadal trends, several studies provide evidence for concurrent changes in the North Atlantic Oscillation/Northern Annular Mode and climate anomalies over the United States.99d2a237-b811-4c42-9bc3-cfd6cbb76504 56bdd0b7-8658-48c1-9814-24ea5a5c2fc9 655f721c-7b5f-4f69-a788-fa1ee8a2488e ed68531d-2082-4da6-8803-35890cb08222 Modeling studies provide evidence for a linkage between cooling trends of the tropical Pacific Ocean that resemble La Niña and precipitation changes in the southern United States.39a41162-bda6-4606-9561-37e3e1839913 6782b38a-17f4-40d2-9cff-da07da38f76a Several studies describe a decadal modification of ENSO.bbbae1eb-1f00-40e1-ae43-b7863655812f 20e157eb-e2b4-4201-8422-1a0dc8cd499f a3771226-1458-474e-ba3b-f046d4351558 7f094c4f-301c-422c-8fba-371b5502767c Modeling evidence is provided that such decadal modifications can be due to internal variability.7f094c4f-301c-422c-8fba-371b5502767c Climate models are widely analyzed for their ability to simulate recurring patterns of variability and teleconnections over the United States.ed3d1a48-a7f4-4f31-a0c2-91e438ea459d ecf2a293-f555-4632-8f7a-1fb12c9f098e a46eaad1-5c17-46f7-bba6-d3fee718a092 3c2004f1-4f51-4b84-8ec0-64a5e2c3a790 33619644-54fd-43b3-9fa8-e7ac5ddec196 2f2d945e-b0fe-43d7-a521-2a9219236862 Climate model projections are also widely analyzed to diagnose the impact of human activities on NAM/NAO, ENSO teleconnections, and other recurring modes of variability associated with climate anomalies.ed3d1a48-a7f4-4f31-a0c2-91e438ea459d 541fc57b-d7ad-4617-97de-2df91f99afc0 0cc23089-0001-453b-b3f7-99b53325f44b 94d52b54-b40c-4447-a525-b79062fcedf1

New information and remaining uncertainties: A key uncertainty is related to limited observational records and our capability to properly simulate climate variability on decadal to multidecadal timescales, as well as to properly simulate recurring patterns of climate variability, underlying physical mechanisms, and associated variations in temperature and precipitation over the United States.

Assessment of confidence based on evidence: There is high confidence that preferred patterns of variability affect U.S. temperature on a year-to-year timescale, based on a large number of studies that diagnose observational data records and long simulations. There is medium confidence that changes in the occurrence of these patterns or their properties have contributed to recent U.S. temperature and precipitation trends. Several studies agree on a linkage between decadal changes in the NAO/NAM and climate trends over the United States, and there is some modeling evidence for a linkage between a La Niña-like cooling trend over the tropical Pacific and precipitation changes in the southwestern United States. There is no robust evidence for observed decadal changes in the properties of ENSO and related United States climate impacts. Confidence is low regarding the size of the role of human influences in these changes because models do not agree on the impact of human activity on preferred patterns of variability or because projected changes are small compared to internal variability.

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

References :

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