Abstract
The extratropical response to tropical remote forcing has been examined with so-called tropical ocean-global atmosphere experiments, which use prescribed sea surface temperature (SST) in the tropical Pacific and a slab mixed-layer ocean model elsewhere. In this study we have revisited this experimental design and found that the extratropical response is quite sensitive to the meridional extent of tropical prescribed SST domain. Even in the case of a prescribed annual cycle only (i.e., no ENSO), the differences in the prescribed SST regions lead to different atmospheric motions in the adjacent extratropics. When the tropical forcing includes ENSO, the sensitivity to the meridional domain is more prominent, especially during La Niña events. In La Niña, the prescribed SST is warmer than the simulated SST in the northern subtropics, and the warmer SST differences continue to 30°N. This broad SST differences accompany enhanced atmospheric meridional circulation that directly connects the tropics and extratropics within the Pacific basin. Moreover, the Rossby wave excitation also increases, so the effect of prescribed region difference is felt beyond the Pacific basin. On the other hand, the effect of ENSO sea surface temperature anomalie (i.e., ENSO experiment composite minus control experiment annual cycle, both of which have the same prescribed SST domain) is stronger in the broad tropical forcing experiment. However, the ENSO anomaly composite from own annual cycle is similar regardless of the meridional extent of forcing region, and commonly mimics the Northern Hemisphere El Niño composite of nature in the boreal winter season.
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Notes
The F test indicates that the variance of the M20C experiment around Antarctica is significantly larger than that of the M10C experiment (not shown) so we need to be cautious in interpreting the t test results.
The NCAR CCSM3 Slab Ocean Control data is obtained from the World Climate Research Programme’s (WCRP’s) Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset. The resolution of CCSM3 is T85.
The NCAR CAM3 AMIP data is obtained from WCRP’s CMIP3 multi-model dataset. Because the model data is only available from January 1978 to December 2000, four El Niño and four La Niña events in this period are used for the composite analysis. The resolution of CAM3 is T85.
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Acknowledgments
This manuscript is a part of George Mason University Ph.D. dissertation of DJ. DJ would like to thank dissertation committee members, Professor Tim DelSole, Edwin Schneider, and David Straus for comments and suggestions on the manuscript. We also thank two anonymous reviewers for improving the first draft of this paper. BPK acknowledges support from NOAA grants NA17RJ1226, NA080AR4320889 and NSF grants OCI0749165 and ATM0754341.
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Jin, D., Kirtman, B.P. The extratropical sensitivity to the meridional extent of tropical ENSO forcing. Clim Dyn 34, 935–951 (2010). https://doi.org/10.1007/s00382-009-0600-1
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DOI: https://doi.org/10.1007/s00382-009-0600-1