Decadal Regime Shifts in the Pacific Ocean: Mechanisms, Hydroclimatic Imprints, and Predictability

A map of the difference in Pacific sea surface temperature (SST) between two decades might show a wedge-shaped region of relatively warm sea surface temperatures (SSTs) extending west and toward the equator from the coasts of North and South America, covering much of the tropical Pacific and flanked by cold SSTs in the higher latitudes of both hemispheres. This pattern and its negative, with a wedge of colder SSTs spanning the tropical Pacific, account for a large fraction of Pacific decadal SST variability, referred to as PDV. The PDV pattern is of interest due to its association with prolonged periods of drier and wetter conditions around the world. In particular more precipitation is found over the western and central US in decades featuring the warm Pacific wedge while the cold wedge favors drought. The association of warmer with wetter and cooler with drier is also found in South America, as might be expected given the equatorial symmetry of the wedge, and model simulations in which the PDV pattern is imposed can reproduce the wetter and drier conditions over the Americas. The PDV pattern has attracted considerable attention but our understanding of it is limited by the length of the observational record for SSTs, which goes back roughly a century and a half and thus only captures a handful of events.

Work under this award extends our understanding of the PDV pattern and its connections to continental hydroclimate using climate reconstructions for the last millennium based on tree rings and other proxy data. These include drought atlases for North America (The North American Drought Atlas, or NADA), Monsoon Asia (MADA), South American (SADA), and Europe and North Africa (the Old World Drought Atlas), among others, as well as the Paleo-Hydrodynamics Data Assimilation (PHYDA) and Last Millennium Reanalysis (LMR) products. These proxy-based datasets are complemented by analysis of simulations from the Paleoclimate Model Intercomparision Project (PMIP4) and the Community Earth System Model Last Millennium Ensemble (CESM LME). One question to be addressed is the extent to which the behavior of the PDV pattern is characterized by abrupt 'regime shifts' between its warm and cold phases. Abrupt shifts are a notable feature of the PDV pattern in the 20th century but gradual transitions might be more common if a longer period is examined. Another question is whether the PDV warming and cooling were as pronounced in earlier centuries as in the 20th century, and whether PDV impacts on continental hydroclimate were as pronounced. The research also considers the extent to which the PDV pattern is predictable, a question which is pursued through additional model simulations including initialized hindcast simulations from the Decadal Climate Prediction Project.

The work has broader impacts due to the societal and ecological impacts of prolonged pluvials and droughs. Given the similar PDV impacts over North and South America the project builds research connections to South America by hosting scientists from Argentina and Chile and convening a week-long workshop offering remote participation to a an online audience at several South American universities. The project also hosts a website for the dissemination of results. Project participants conduct a variety of education and outreach activities including presentations at the American Museum of Natural History, the Columbia Youth Summit, and local public schools. In addition, the project supports a graduate student, thereby supporting workforce development in this research area.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.