Weak Atlantic-Pacific teleconnections as synchronized chaos

The relationship between blocking events in the Atlantic and Pacific sectors of the Northern Hemisphere mid-latitudes is investigated in a Vautard-Legras two-layer quasigeostrophic channel model with two sectors, each sector forced by a separate baroclinic jet. It is found that the exchange of medium-scale eddies tends to cause anticorrelation between blocking events in the two sectors, while the large-scale flow components tend to cause positive correlation. The net correlation in blocking is more positive when the jets are skewed latitudinally, a result that is confirmed in the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data and separately in a long run of a global circulation model (GCM). The anticorrelating effect of the eddy exchange follows from the tendency of two distinct, coextensive, chaotically vacillating channel flows to synchronize when their corresponding medium-scale eddy components are coupled (a physically unrealizable configuration), regardless of differences in initial conditions. In the Vautard-Legras model, blocking in one sector weakly inhibits blocking in the opposite sector. Generalized synchronization between two channels with forcing in different sectors implies that the two inhibition effects combine coherently, giving anticorrelation in blocking activity. The anticorrelation effect is small because of the physical distance between the sectors and the resulting long advective time scales. That the smallest-scale eddies need not be coupled to affect synchronization would follow from the existence of an inertial manifold that slaves the smallest scales to the larger scales in each channel. The paradigm of low-order chaos synchronization may be relevant to climate dynamics in a variety of situations where such inertial manifolds exist.