Residence time distributions of the Northern Annular Mode

摘要

The Northern Annular Mode (NAM) is a large-scale climatic anomaly pattern characterizing the atmospheric circulation in the polar and the mid-latitude regions. To help in understanding the basic physics underlying the NAM we determine and examine the probability distributions of residence times in the positive and negative phases of the NAM. These distributions are found to have only slightly different mean residence times but differ strongly in their tails. The difference in the tails of the distributions expresses a dominance of one phase of the NAM over the other during rarely occurring events such as the dominant positive NAM in the mid 1960s to the late 1990s. We investigate a possible influence of solar variability on the residence time distributions and find that at high solar activity the NAM spends slightly more time in its negative phase in the stratosphere and slightly less time (compared to the positive phase) in the troposphere. Much stronger influence on the NAM may occur during rare but prolonged changes in solar activity. This extends the conjecture that external forcing only affects the mean residence times ('occupation frequencies') of the dynamical states. An example is the dominance of the negative NAM during the 70-year long Maunder Minimum. The distributions are consistent with generation by a non-linear process of random transitions between two equilibrium states of a system with the addition of a small forced component. A simple two-well potential model provides an approximate description of this process elucidating its dynamics as well as the effect of external influences on the annular modes.