The Long- and Short-Lived North Atlantic Oscillation Events in a Simplified Atmospheric Model

This study investigates the North Atlantic Oscillation (NAO) events with relatively long and short lifetimes based on an 8000-day perpetual-boreal-winter [December–February (DJF)] run result of the idealized Geophysical Fluid Dynamics Laboratory (GFDL) dynamical core atmospheric model. We identify the so-called long- and short-lived positive and negative NAO events from the 8000-day model output. The composite 300-hPa geopotential height anomalies show that the spatial patterns of the composite long-lived NAO events closely resemble the Northern Hemisphere annular mode (NAM) because the NAO dipole is accompanied with a statistically significant North Pacific meridional dipole (NPMD) at similar latitudes as that of the NAO dipole. The composite short-lived NAO events exhibit the locally confined canonical NAO. Twelve sets of modified initial-value experiments indicate that an absence (a presence) of the NPMD-type perturbations at the early stage of the long (short)-lived NAO events will decrease (increase) their intensities and naturally shorten (lengthen) their lifetimes. Thus, the preceding NPMD is an early factor that is conducive to the emergence of the long-lived NAO events in the model. We argue that through directly modulating the synoptic eddy forcing over the North Atlantic region, the preceding NPMD can gradually arouse the NAO-like circulation anomalies on the following days. That is the reason why the preceding NPMD can modulate the intensities and lifetimes of the NAO events.