The research, led by Shanshan Fu of Nanjing University of Information Science and Technology and the Dalian Meteorological Observatory, together with Zhiwei Zhu and Xiao Pan of the Ocean University of China and the Max Planck Institute for Meteorology, analyzed observational and reanalysis data spanning 1961 to 2024. The team identified three clearly separated phases in how the tripole relates to springtime surface air temperature across the Eurasian and North African landmass. In the first phase, from 1961 to 1986, a positive tripole, meaning warmer water in the subtropical and subpolar North Atlantic flanking a cooler band in between, produced warming over North Africa, the Middle East and the Indo-China Peninsula, along with cooling over Northern Europe. Those were the days when the tripole could be counted on as a reliable seasonal clue.

Then came the middle chapter. Between 1987 and 2005, the subtropical temperature responses largely collapsed. The warming signals over North Africa and the Middle East faded to statistical insignificance, while the cooling over Northern Europe stubbornly persisted. Forecasters and researchers who had built seasonal expectations on the tripole’s influence suddenly found the connection unreliable over much of the affected domain. In the third phase, from 2006 to 2024, the story shifted again: the warming over North Africa re-emerged, but a new and significant cooling signal appeared over Northeast Asia, a region that had not featured prominently in the tripole’s earlier influence map. The pattern, in other words, did not simply strengthen or weaken. It changed its geography.

What could cause a fixed oceanic pattern to reshuffle its downstream fingerprints so dramatically? The authors point to the propagation pathways and structures of the Rossby wave trains that the tripole excites. When sea surface temperature anomalies alter the heating of the overlying atmosphere, they launch planetary-scale waves that arc across the midlatitudes like ripples guided along a curved channel. Where those waves deposit their energy determines which regions warm and which cool. If the wave train takes a more zonal track hugging the subtropics, North Africa and the Middle East feel the effects. If it arcs poleward into higher latitudes, Europe and Northeast Asia come into play. The tripole provides the push, but the atmosphere decides where the push lands.

The decisive factor, according to the study, is the background mean zonal wind, essentially the prevailing east-west flow of the atmosphere through which the waves must travel. Rossby waves do not propagate through a static medium; their trajectories, group velocities and amplitudes depend on the structure of the ambient flow, particularly the position and strength of the jet streams that act as waveguides. When the background zonal wind shifts, the wave train bends with it, and the regions of wave-activity flux convergence, where the teleconnection’s temperature anomalies are ultimately expressed, migrate accordingly. The three phases identified in the observations correspond to three configurations of this mean flow, each steering the tripole’s atmospheric response along a different route.

The study goes one step further by asking what drives the mean flow changes themselves. The authors identify two contributors: intensified local transient-eddy activity and shifted tropical convection anomalies. Transient eddies, the swirling synoptic-scale disturbances that populate the storm tracks, exert a feedback on the time-mean flow, and changes in their intensity can reinforce or reshape the zonal winds. Meanwhile, shifts in tropical convection, the towering organized thunderstorm systems of the deep tropics, can alter the large-scale circulation remotely, nudging the midlatitude flow into a new configuration. Either pathway, operating in different phases, sufficed to explain the observed changes in the background winds that steered the wave trains.

To test whether these mean-flow changes were truly sufficient, rather than merely correlated, the team turned to a linear baroclinic model, a simplified numerical tool designed to isolate the role of the background flow in wave propagation. By imposing the observed mean-flow conditions from each phase and forcing the model with the tripole’s atmospheric signature, they found that the model reproduced the phase-dependent Rossby wave trajectories seen in the real atmosphere. The observed changes in the mean flow alone were enough to recreate the shifting pathways, without needing to invoke changes in the tripole itself. This result elevates the background mean flow from a plausible suspect to the key mechanism: it is the control knob that determines how the ocean’s influence is distributed across the continents.

The findings carry practical weight for seasonal climate prediction. Spring surface air temperature over Eurasia and North Africa affects agriculture, water resources, snowmelt timing and the likelihood of temperature extremes, and forecasters have long used North Atlantic sea surface temperature anomalies as precursors. A teleconnection relationship that holds in one decade may fail in the next, and this study explains why: the relationship is conditional on the state of the background flow, which evolves on interdecadal timescales under the influence of eddy activity and tropical convection. Skillful use of the tripole as a predictor therefore requires monitoring not just the ocean, but the atmospheric highway that carries its signal eastward.

More broadly, the work adds to a growing recognition that climate teleconnections are not fixed features of the Earth system but dynamic relationships that drift as the background circulation evolves. Similar interdecadal changes have been documented for other ocean-atmosphere links, including the influence of the tripole on the western North Pacific subtropical high and on East Asian rainfall, suggesting that the mean-flow modulation mechanism may be a general property of midlatitude teleconnections. As greenhouse warming continues to alter jets, storm tracks and tropical convection, the map of which regions are teleconnected to which ocean basins may keep redrawn itself. The tripole has not lost its voice; the atmosphere has simply changed the rooms in which it can be heard.

Subject of Research: How changes in background mean flow modulate the influence of the North Atlantic tripole sea surface temperature pattern on springtime surface air temperature over Eurasia and North Africa

Article Title: Changing influence of the North Atlantic tripole SST pattern on surface air temperature in Eurasia and North Africa modulated by the background mean flow

Article References: Fu, S., Zhu, Z., & Pan, X. (2026). Changing influence of the North Atlantic tripole SST pattern on surface air temperature in Eurasia and North Africa modulated by the background mean flow. Climate Dynamics, 64(10), Article 429. https://doi.org/10.1007/s00382-026-08391-x

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08391-x

Keywords: North Atlantic tripole, sea surface temperature, surface air temperature, Rossby wave train, background mean flow, Eurasia, North Africa, teleconnection, Climate Dynamics, jet stream, transient eddies, spring climate