Enhancement of North Atlantic Oscillation by Transient Eddies

Time series of the intensity of North Atlantic transient eddy activity and a daily index of the North Atlantic Oscillation (NAO) are calculated by using the NCEP/NCAR reanalysis data. The results suggest that peaks in the intensity of eddy activity are followed by strengthening of the NAO mode, accompanied by a collapse in eddy energy. To determine whether the interaction between eddies and low-frequency flow results in the transformation of synoptic-scale energy to low-frequency-scale energy, the eddy vorticity flux is used to investigate the transfer of vorticity and energy. The results show that transient eddies can induce low-frequency anomalies and that vorticity transfer can enhance the anomaly, resulting in an increase in the NAO index. However, eddies, low-frequency flow, and the vorticity flux show continuous variations, meaning that they cannot attain equilibrium all the time. Accordingly, the authors analyze the total contribution of stochastic eddies to the process that induces an NAO-like response to external forcing. The steady solution suggests that eddy forcing can induce a response more similar to the NAO mode.