Exploratory MPAS Sensitivity Experiments on Rainforest Biogenic Salt Aerosols, Tropical Rainfall, and Poleward Moisture Transport

Brian Lue

Equatorial rainforest trees and their associated fungi emit hygroscopic potassium-salt (K-salt) aerosols that activate readily as cloud condensation nuclei and can accelerate warm rain — a biogenic, vegetation-linked pathway not represented as a dedicated process in major climate models. We ask whether this source measurably modulates the poleward transport of latent heat, using MPAS-Atmosphere ensembles with a prescribed-CCN methodology that holds rainforest CCN at observed pristine values (Pöhlker et al. 2012).

The response separates cleanly into three levels. At the source, adding K-salt roughly doubles Amazon cloud-droplet number in every ensemble member — the expected Twomey activation response, confirming the forcing is applied as designed. Downstream, the Amazon precipitation response bifurcates regime-dependently across years (invigoration or suppression), a clean multi-year demonstration of the aerosol–cloud precipitation duality. In the far field, the 30°N latent-heat-transport response is dominated by internal atmospheric variability: corrected January and July five-pair ensembles both return nulls (30°N = −12 ± 101 TW, p = 0.80), so single-realization-per-year ensembles cannot separate a forced signal from weather noise. We show that an initial-condition perturbation ensemble is the method that can, and report a completed pilot that resolves an apparent large single-year outlier (+159 TW) as a chaotic draw reproduced by no member — turning an apparent contradiction into a measurement of the internal-variability noise floor.

This version supersedes an earlier one whose headline 30°N reduction (−80 ± 22 TW, p = 0.0013) was traced to an initialization defect and is retracted. The corrected initialization pipeline, its four-zone verification, and a full corrigendum documenting the defect and its diagnosis are provided in the open code-and-data archive (github.com/bluesaltbarrier/blue-salt-barrier; Zenodo 10.5281/zenodo.21117113).