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

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 evolution of extreme springtime weather events over North America may depend critically upon the precise timing and geographical structure of forcing anomalies over both the east and the west Pacific. In this model, low-frequency variability within and downstream of the Rossby waveguide is sensitive to the annual cycle of the ambient flow. This suggests that the impact of the annual cycle must be taken into account in any complete theory of low-frequency variability. The impact is large enough to raise the possibility of significant interactions across timescales. In other words, it is possible for a steady forcing to produce an unsteady response and, equally, for an unsteady forcing to produce a seasonal-mean response. In such situations, particularly during the northern spring and fall seasons, investigating low-frequency anomalies as departures from three-month seasonal climatologies may lead to confusion and may not be useful.