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 phase 1 or 6, which are captured by the leading principal component of daily summer MJO phases (PCM1; accounting for 46% of the phase variance). In “nonuniform” 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 nonuniform 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 midlatitudes to modulate the NAD, whereas tropical convection anomalies associated with anomalously frequent MJO phases 1 and 6 overwhelm the extratropical teleconnection.