Nonlinear features of Northern Annular Mode variability

• Nonlinear features of daily NAM variability are quantified.
• Two measures reach the same conclusion.
• Nonlinear features in NAM variation are significant at lower pressure levels.

Nonlinear features of daily Northern Annular Mode (NAM) variability at 17 pressure levels are quantified by two different measures. One is nonlinear correlation, and the other is time-irreversible symmetry. Both measures show that there are no significant nonlinear features in NAM variability at the higher pressure levels, however as the pressure level decreases, the strength of nonlinear features in NAM variability becomes predominant. This indicates that in order to reach better prediction of NAM variability in the lower pressure levels, nonlinear features must be taken into consideration to build suitable models.