Abstract
Variations of the North Atlantic subtropical high (NASH) western ridge and their implication to the Southeastern United States (SE US) summer precipitation were analyzed for the years 1948–2007. The results show that the movement of the NASH western ridge regulates both moisture transport and vertical motion over the SE US, especially in the last three decades, during which the ridge moved westward towards the American continent. When the NASH western ridge is located southwest (SW) of its mean climate position, excessive summer precipitation is observed due to an enhanced moisture transport. In contrast, when the western ridge is located in the northwest (NW), a precipitation deficit prevails as downward motion dominates the region. Composite analysis indicates that SW ridging results mainly from the NASH center’s intensification; whereas NW ridging is likely caused by stationary wave propagation from the eastern Pacific/US western coast. In recent decades, both the SW and NW ridge positions have been observed to increase in frequency. Our results suggest that the increase in the SW ridging consistently follows the NASH’s intensification associated with anthropogenic forcing as projected by coupled climate models. However, the increased frequency of NW ridging tends to follow the positive Pacific decadal oscillation (PDO) index. Thus, the enhanced variability in the SE US summer precipitation in recent decades might be a combined result of anthropogenic forcing and internal variability of the climate system. Results suggest that, as anthropogenic forcing continues to increase, the SE US will experience more frequent wet summers and an increase in the frequency of dry summers during positive PDO phases.
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Notes
The NCEP/NCAR reanalysis used in the study is due to following reasons. First, the relatively longer period data availability of the NCEP/NCAR reanalysis matches the precipitation data (1948–2007); second, NCEP/NCAR reanalysis reasonably represents the locations of the NASH center and its western ridge comparing with surface observational data (ICOADS); third, the NCEP/NCAR reanalysis does not show discrepancy from other reanalysis data such as the ERA-40 (1958–2002), JRA (1979–2007) for the NASH’s center’s intensity and the western ridge.
As convention, the 850 hPa geopotential height is usually plotted at 60-m intervals with the reference level 1,500 m. For the North Atlantic subtropical high, the 1,500-gpm line is far into the continent while 1,620-gpm isoline is still over the North Atlantic; The 1,560-gpm line is also closely related to the distribution of precipitation and vertical motion over the eastern coast of US (Fig. 1).
To avoid the possible spread of the western ridge among the CMIP3 models and to make results comparable, we used the climatological contour straddling the longitude of 86°W to represent the western edge of the NASH for each model under scenario 20C3M. This longitude is derived according to observed climatological NASH location based on the NCEP reanalysis data (Li et al. 2011).
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Acknowledgments
We thank the international modeling groups for providing their data for analysis, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) for collecting and archiving the model data, the JSC/CLIVAR Working Group on Coupled Modeling (WGCM) and their Coupled Model Intercomparison Project (CMIP) and Climate Simulation Panel for organizing the model data analysis activity, and the IPCC WG1 TSU for technical support. The IPCC Data Archive at Lawrence Livermore National Laboratory is supported by the Office of Science, US Department of Energy. We thank Drs. M. Susan Lozier, Ming Cai, Amy Clement, Paul A. Baker, Ana Barros, Robert E. Dickinson, and Jiansheng Ye and two anonymous reviewers for their insightful comments, and Dr. Alex Glass and Ms. Diane Bryson for editorial assistance. This work is supported by startup funds from Duke University and the Nicholas School Office of the Dean. Y. Kushnir was supported by NOAA Grant NA10OAR4310137.
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Li, L., Li, W. & Kushnir, Y. Variation of the North Atlantic subtropical high western ridge and its implication to Southeastern US summer precipitation. Clim Dyn 39, 1401–1412 (2012). https://doi.org/10.1007/s00382-011-1214-y
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DOI: https://doi.org/10.1007/s00382-011-1214-y