The dynamics of the North Atlantic Oscillation during the summer season

This study examines the dynamical characteristics of the North Atlantic Oscillation (NAO) life cycle for the summer season. The diagnostic techniques, which use NCEP/NCAR Reanalysis data, include composites of the 300 hPa stream function, a projection analysis with each term in the stream-function tendency equation, and composites of potential temperature on the 2 PVU potential-vorticity surface. For both phases, the NAO life cycles take about two weeks to complete. The NAO anomalies are found to be driven by both high-frequency (period < 10 days) and low-frequency (period > 10 days) transient eddy vorticity fluxes. The decay of the NAO is accomplished by both low-frequency transient eddy vorticity fluxes and horizontal divergence. The breaking of synoptic-scale waves is found to play a crucial role during the summer NAO life cycle. For the positive NAO phase, the southern centre of action of the dipole arises from anticyclonic wave-breaking, and the northern centre of action from anticyclonic wave-breaking followed by trough intensification. For the negative NAO phase, both centres of action develop from cyclonic wave-breaking. These characteristics are very similar to those for the NAO life cycle during the winter season, the primary differences being the weaker anomalies, shorter zonal and meridional scales, and less intense wave-breaking of the summer NAO.