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Abstract Climate associated ecological phenomena that occur approximately once per decade suggest the influence of decadal climate oscillations. However, the consistency and origins of such climate patterns in the Atlantic and Pacific regions is currently under debate. Here, we propose a probabilistic explanation for episodic ecological events based on the likelihood of multiple climate patterns converging in a particular phase combination. To illustrate, we apply this model to continental scale facultative migration of seed-eating finches out of the boreal forest. Thisirruptionphenomenon is triggered by seed crop failures stemming from two weakly correlated climate patterns occurring simultaneously in their positive phases—the North Atlantic Oscillation (NAO) and the North Pacific Oscillation (NPO). The joint probability of NAO and NPO both being positive (above upper tercile) is about $(1/3{)}^2\approx 0.11$, illustrating a simple probabilistic explanation for quasi-decadal finch irruption and potentially other episodic ecological events in regions affected by multiple climate modes. 

Atmospheric variability can impact biological populations by triggering facultative migrations, but the stability of these atmosphere-biosphere connections may be vulnerable to climate change. As an example, we consider the leading mode of continental-scale facultative migration of Pine Siskins, where the associated ecological mechanism is changes in resource availability, with a mechanistic pathway of climate conditions affecting mast seeding patterns in trees which in turn drive bird migration. The three summers prior to pine siskin irruption feature an alternating west-east mast-seeding dipole in conifer trees with opposite anomalies over western and eastern North America. The climate driver of this west-east mast-seeding dipole, referred to as the North American Dipole, occurs during summer in the historical record, but shifts to spring in response to future climate warming during this century in a majority of global climate models. Identification of future changes in the timing of the climate driver of boreal forest mast seeding have broadly important implications for the dynamics of forest ecosystems. 

Abstract 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 (PC M1 ; accounting for 46% of the phase variance). In “nonuniform” MJO summers, defined as |PC M1 | > 0.5, anomalously frequent phase 1 triggers positive NAD, and anomalously frequent phase 6 triggers negative NAD, yielding the correlation r (NAD, PC M1 ) = 0.55, p < 0.01. During “uniform” MJO summers, defined as |PC M1 | ≤ 0.5, the effect of East Asian precipitation anomalies P EA becomes apparent, and r (NAD, P EA ) = 0.49, p < 0.01. The impacts of P EA are largely masked during nonuniform MJO summers, meaning this subset of summers lacks a significant correlation between the NAD and P EA . 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. 

Abstract The summer North American dipole (NAD) is a pattern of climate variability linked to variations in boreal forest seed production and migration of seed-eating birds. This is a modeling investigation of two teleconnections identified as drivers of the NAD in prior observational work: 1) tropically sourced atmospheric Rossby waves associated with anomalies in the phase distribution of the Madden–Julian oscillation (MJO) (i.e., phases 1 and 6 are anomalously prominent), and 2) a pan-Pacific atmospheric Rossby wave linked to East Asian monsoonal (EAM) convection. Sea surface temperature (SST) boundary forcing experiments were conducted with the Community Earth System Model 2 (CESM2) to trigger convection patterns that align with those observed during EAM and nonuniform phase distributions of MJO. For the EAM case, an El Niño–like SST dipole pattern combined with cool southern Japan SST forcing produced a convection and jet stream shift anomaly over East Asia and the northern Pacific with a positive NAD pattern downstream over North America, similar to the observed pattern when precipitation over East Asia (P_EA) is relatively high. A companion experiment with only ENSO-like SST forcing also produced the NAD but featured a different structure over the Eurasian continent with a response resembling the summer east Atlantic (SEA) pattern over eastern North America and the eastern Atlantic. Simulation results suggest that the southern Japan SST forcing region has a secondary importance in triggering the NAD, producing only a somewhat NAD-like pattern by itself and only slightly improving the NAD produced by ENSO-like forcing. Simulations using SST forcing to induce seasonal convection anomalies with spatial patterns similar to anomalously frequent occurrence of MJO phase 1 (phase 6) produced circulation response patterns resembling the positive NAD (negative NAD).