The Impact of the Annual Cycle upon the North Pacific/North America response to Low Frequency Forcing

Matthew Newman and Prashant D. Sardeshmukh, NOAA-CIRES/Climate Diagnostics Center

Is low frequency variability over the North Pacific/North America region sensitive to month-to-month changes in the base state, or are these changes sufficiently minor that we can usefully consider anomalies as departures from seasonal means?

Or, are there certain times of the year and/or geographical locations for which this annual cycle is particularly important?

Examining Rossby wave propagation in the context of a barotropic model, we find that:

o  As spring progresses, North American height anomalies are increasingly sensitive to smaller scale forcing, particularly in the far west Pacific.
o  Data analysis and modeling studies during spring and fall may be complicated by the seasonal change of the geographical scale of forced waves.
o  Change of the base state due to the annual cycle may be important for any complete theory of low-frequency variability, possibly even during winter.

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