Unintended winter–early spring warming over North America caused by tropical and subtropical deployment of marine cloud brightening

Marine cloud brightening (MCB) aims to reduce global warming, but its regional impacts remain uncertain. Using the Community Earth System Model, we implement MCB over a portion of tropical and subtropical oceans under SSP2-4.5. Despite overall cooling, MCB induces significant warming over mid-eastern North America in winter-to-early spring. The response originates mainly in the tropical and subtropical eastern Pacific, where MCB cools the atmosphere by reflecting solar radiation. This change modifies extratropical circulations through Gill-type and Rossby wave responses. The circulation changes warm mid-eastern North America via two pathways: suppressed synoptic disturbance activity, which reduces cloud cover and increases downward shortwave radiation, and enhanced positive temperature advection. This MCB-induced warming resembles that associated with a strong La Niña event, with implications for snowpack and water resources. Our results highlight potential unintended risks of regional MCB and the need for a better understanding of process-level teleconnection response to climate intervention strategies.