Drivers of an Ecologically-Relevant Summer North American Dipole


The teleconnection mechanisms associated with midlatitude climate dipoles are of high interest because of their potential broad impacts on ecological patterns and processes. A prominent example attracting increasing research interest is a summer (June-August) North American dipole (NAD), which drives continental-scale bird irruptions in the Boreal Forest (semiperiodic movements of large numbers of individual birds). Here the NAD is objectively defined as a second principal component of 500-hPa geopotential height and is linked to two mechanisms: 1) Rossby waves associated with Madden-Julian Oscillation (MJO) convection, and 2) a pan-Pacific stationary Rossby wave triggered by east Asian monsoonal convection. The MJO mechanism relates to anomalously frequent occurrence of MJO phases 1 or 6, which is captured by the leading principal component of daily summer MJO phases (PCM1, accounting for 46% of the phase variance). In “non-uniform” MJO summers defined as |PCM1| > 0.5, anomalously frequent phase 1 triggers positive NAD, and anomalously frequent phase 6 triggers negative NAD, yielding the correlation r (NAD, PCM1) = 0.55, p < 0.01. During “uniform” MJO summers defined as |PCM1| ≤ 0.5, the effect of east Asian precipitation anomalies (PEA) becomes apparent, and r (NAD, PEA) = 0.49, p < 0.01. The impacts of PEA are largely masked during non-uniform MJO summers, meaning this subset of summers lacks a significant correlation between the NAD and PEA. Our interpretation is that uniformly distributed MJO allows monsoonal convection over the mid-latitudes to modulate the NAD, whereas tropical convection anomalies associated with anomalously frequent MJO phases 1 and 6 overwhelm the extratropical teleconnection.