We thank the anonymous reviewer for insightful comments and Hannah M. Aird for editorial assistance. This work is supported by the NSF AGS 1147601 and NSF AGS 1147608.
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Units in degrees should be deleted in the figure caption as the movement of the western ridge is the normalized change.
The NASH western ridge is defined using both 850-hPa geopotential heights and streamfunction to further test the westward expansion of the western ridge in a warming climate. To avoid the possible spread of the western ridge among the IPCC AR4 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.
Compared to the unfiltered AMO index, high-frequency oscillations are damped when the filtered AMO index (using the Lanczos filter) is applied.