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Abstract Atmospheric rivers (ARs) manifest as transient filaments of intense water vapor transport that contribute to synoptic‐scale extremes and interannual variability of precipitation. Despite these influences, the synoptic‐ to planetary‐scale processes that lead to ARs remain inadequately understood. In this study, North Pacific ARs within the November–April season are objectively identified in both reanalysis data and the Community Earth System Model Version 2, and atmospheric patterns preceding AR landfalls beyond 1 week in advance are examined. Latitudinal dependence of the AR processes is investigated by sampling events near the Oregon (45°N, 230°E) and southern California (35°N, 230°E) coasts. Oregon ARs exhibit a pronounced anticyclone emerging over Alaska 1–2 weeks before AR landfall that migrates westward into Siberia, dual midlatitude cyclones developing over southeast coastal Asia and the northeast Pacific, and a zonally elongated band of enhanced water vapor transport spanning the entire North Pacific basin that guides anomalous moisture toward the North American west coast. The precursor high‐latitude anticyclone corresponds to a significant increase in atmospheric blocking probability, suppressed synoptic eddy activity, and an equatorward‐shifted storm track. Southern California ARs also exhibit high‐latitude blocking but have an earlier‐developing and more intense northeast Pacific cyclone. Compared to reanalysis, Community Earth System Model Version 2 underestimates Northeast Pacific AR frequencies by 5–20% but generally captures AR precursor patterns well, particularly for Oregon ARs. Collectively, these results indicate that the identified precursor patterns represent physical processes that are central to ARs and are not simply an artifact of statistical analysis.
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Statistical Relationship between Atmospheric Rivers and Extratropical Cyclones and Anticyclones
Abstract Statistical relationships between atmospheric rivers (ARs) and extratropical cyclones and anticyclones are investigated on a global scale using objectively identified ARs, cyclones, and anticyclones during 1979–2014. Composites of circulation and moisture fields around the ARs show that a strong cyclone is located poleward and westward of the AR centroid, which confirms the close link between the AR and extratropical cyclone. In addition, a pronounced anticyclone is found to be located equatorward and eastward of the AR, whose presence together with the cyclone leads to strong horizontal pressure gradient that forces moisture to be transported along a narrow corridor within the warm sector of the cyclone. This anticyclone located toward the downstream equatorward side of the cyclone is found to be missing for cyclones not associated with ARs. These key features are robust in composites performed in different hemispheres, over different ocean basins, and with respect to different AR intensities. Furthermore, correlation analysis shows that the AR intensity is much better correlated with the pressure gradient between the cyclone and anticyclone than with the cyclone/anticyclone intensity alone, although stronger cyclones favor the occurrence of AR. The importance of the horizontal pressure gradient in the formation of the AR is also consistent with the fact that climatologically ARs are frequently found over the region between the polar lows and subtropical highs in all seasons.
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- Award ID(s):
- 1658218
- PAR ID:
- 10212474
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 33
- Issue:
- 18
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 7817 to 7834
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JCLI-D-19-0126.1
