Fig. 8: Coupled interactions for multi-year La Niña triggered by Australian wildfire smoke in 2019–2020.

Top: year 1 onset; (1) wildfire smoke crosses the Pacific where it reaches the eastern Pacific, (2) brightens clouds there that (3) reflect more solar radiation to cool SSTs that propagate into the equatorial eastern Pacific where (4) Bjerknes feedbacks spread the negative SST anomalies westward (background SST anomalies from Fig. 1); Bottom left: in years 2 and 3, Bjerknes feedbacks spread negative SST anomalies across the Pacific and contribute to (5) increasing SSTs and precipitation over the Maritime Continent and northern Australia. The consequent enhanced vertical motion there then produces stronger upper-level outflow, (6) a stronger anomalous Walker Circulation with enhanced upper-level convergence over the eastern subtropical Pacific, and (7) a stronger subtropical high and stronger trade winds to maintain cool SSTs. Coupled feedbacks associated with the negative phase of the IPO in the western tropical Pacific result from (8) negative precipitation and convective heating anomalies over the anomalously cool water, (9) a Gill-type response with anomalous highs (yellow ovals) to the north and south, easterly wind stress anomalies in the near equatorial western Pacific, and westerly wind stress anomalies near 25°N and 25°S (blue arrows); these wind stress anomalies result in anomalous Ekman pumping near 15°–20°N and 15°–20°S which increases off-equatorial western Pacific ocean heat content characteristic of extended negative IPO-type conditions (background SST anomalies from Fig. 4a); Bottom right: For 2-year La Niña events that transition to El Niño in year 3, a positive NPMM north of the equator in the tropical Pacific produces positive precipitation and negative SLP anomalies (red oval), anomalous upper-level outflow with upper-level convergence over the Maritime Continent to the west (red ribbon arrow) with relatively higher surface pressure there (solid blue oval). Meanwhile, positive anomalies of surface temperature and precipitation and negative surface pressure anomalies over Australia (solid red oval) sustain the anomalous Walker Circulation to the east (blue ribbon arrow) with stronger descent and an anomalously strong South Pacific High (blue oval) with stronger trades in the equatorial eastern Pacific. The larger negative SLP anomalies over Australia set up an anomalous meridional SLP gradient that contributes to anomalous westerly surface winds in the far western equatorial Pacific (black arrow) that leads to a transition to El Niño in the third year (background SST anomalies from Fig. 4b).