Our results, based on Reanalysis data, suggest that the warming signal during the transitioning summer is the joint product from the decaying El Niño and the developing La Niña. The developing La Niña cools the sea surface temperature (SST) over the central to eastern tropical Pacific and weakens the deep convection there, while the decaying El Niño leaves negative precipitation anomalies over the western subtropical Pacific caused by the delayed warming of the tropical Indian Ocean. The teleconnections forced in these regions of suppressed deep convective propagate towards North America and their superposition results in a significant anomalous ridge over Northeast North America, and the resulting warming.These features are unique to the developing La Niñas transitioning from El Niños. For a La Niña without a preceding El Niño, the impacts of summer teleconnections on North America are much weaker and insignificant. These observational differences are supported by a stationary wave model with observed and idealized ENSO forcing. These results suggest that it is important to distinguish the two different types of La Niña to understand their remote impacts.
Further work on examining the ability of climate models with prescribed SST in reproducing the two different developing La Niñas will be conducted, in order to provide useful information for improving the skills of seasonal prediction – and subsequently benefiting agricultural managements – over North America.