The Impact of the Annual Cycle on the North Pacific/North American Response to Remote Low-Frequency Forcing

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Abstract
The impact of the climatological seasonally varying 300-mb flow on the North Pacific/North American response to remote anomalous forcing is considered in the context of a linear barotropic model. WKB theory suggests that the total wavenumber of stationary Rossby waves over the Pacific increases from about 7 in January to 8.5 by June, with the reverse occurring during fall. This change is accompanied by monthly changes in the location and shape of the Rossby waveguide itself. Using a diagnostic tool called the influence function, it is shown that the most sensitive area of forcing for producing a large response over the United States shifts from the east Pacific in late winter to the west Pacific by late spring. As spring progresses, there is also a marked increase in the sensitivity to smaller-scale forcing in both of these regions, particularly the west Pacific. The amplitude of the forced response can potentially be larger in June than any other month of the year. These results suggest that the... Abstract The impact of the climatological seasonally varying 300-mb flow on the North Pacific/North American response to remote anomalous forcing is considered in the context of a linear barotropic model. WKB theory suggests that the total wavenumber of stationary Rossby waves over the Pacific increases from about 7 in January to 8.5 by June, with the reverse occurring during fall. This change is accompanied by monthly changes in the location and shape of the Rossby waveguide itself. Using a diagnostic tool called the influence function, it is shown that the most sensitive area of forcing for producing a large response over the United States shifts from the east Pacific in late winter to the west Pacific by late spring. As spring progresses, there is also a marked increase in the sensitivity to smaller-scale forcing in both of these regions, particularly the west Pacific. The amplitude of the forced response can potentially be larger in June than any other month of the year. These results suggest that the...

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