NSFGEO-NERC: Wider Impacts of Subpolar nortH atlantic decadal variaBility on the OceaN and atmospherE (WISHBONE)

University Corporation For Atmospheric Res, Boulder CO

Investigators

Abstract

This project is jointly funded by the National Science Foundation's Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with their own country. The Subpolar North Atlantic (SNA), which is the region of the Atlantic Ocean between 45N - 65N latitude, is a highly variable region. Surface temperatures and surface salinity here have varied on a range of timescales with those on decadal or longer being the dominant ones. This decadal timescale variability appears to form a key component of a larger climate mode, the Atlantic Multidecadal Variability, which has been linked to a broad range of important climate impacts, including rainfall in the North African and south Asian monsoons, floods and droughts over Europe and North America, and the number of hurricane formations. The SNA is also one of the most predictable places on Earth at decadal timescales, which suggests there is potential for improved predictions of regional climate and high-impact weather years ahead. However, the origins of this variability, and the processes controlling its impacts, are far from fully understood. There is significant evidence to suggest that anomalous heat loss from the subpolar North Atlantic Ocean to the atmosphere can instigate a cascade of changes across the North Atlantic basin in both the ocean and atmosphere. For example, changes in the SNA can change the strength of the ocean circulation to the south, affect the northward transport of heat and freshwater in the North Atlantic, and subsequently affect the upper ocean temperatures and salinity across the whole North Atlantic basin, and into the Arctic. Changes in the subpolar North Atlantic surface temperature are also thought to affect the atmospheric circulation (i.e., wind patterns) in both summer and winter. However, observational records are very short, and so there are significant problems with understanding causality, and considerable uncertainty about how well many of the important processes are represented in current climate models. This project (WISHBONE) will make use of new advanced climate simulations and forecast systems to improve our understanding of the impact of the subpolar North Atlantic on the wider North Atlantic basin. It will also test specific hypotheses related to understanding the specific role of heat loss over the subpolar North Atlantic in driving changes throughout the basin including the role of surface anomalies in driving wind patterns. The project will enhance international collaboration with the UK and science results would be of interest to the public in the US and the UK, as both regions are impacted by SNA variability. NCAR will engage in outreach activities to communicate key findings through public lectures, press releases, and university lectures amongst others. It is anticipated that this work will inform ongoing and future decadal prediction activities, such as those led by NCAR and the UK Met Office. The overarching objective of WISHBONE is to characterize the linkages between anomalous buoyancy forcing of the SNA and impacts on the wider North Atlantic coupled system on decadal timescales, and to determine the oceanic and atmospheric processes that control these impacts. WISHBONE will do this by focusing on the specific linkages and underlying processes. Hence, the specific objectives of the project are: 1) to determine and evaluate the oceanic pathways through which changes in the SNA impact on lower latitudes in the Atlantic basin; 2) to determine the impact of changes in the SNA on atmospheric circulation over the North Atlantic on a range of timescales, and to deduce the important processes involved; 3) to determine the oceanic and atmospheric pathways through which changes in the SNA impact on higher latitudes and the Arctic; 4) to assess how robust are the mechanisms and timescales in numerical simulations at different resolutions, and their consistency with observations; 5)to explore how the influence of the SNA is modified by anthropogenic forcing. The key time scale to be addressed is decadal, but in the context of interannual-to-centennial changes in the SNA. The primary hypothesis is that buoyancy forcing over the SNA is the primary controller of Atlantic Decadal-to-Multidecadal coupled variability due to its key role in shaping the Thermohaline Circulation (THC) across the North Atlantic basin and, subsequently, upper ocean changes that result in an atmospheric response. Specific hypotheses include: 1) subsurface density anomalies in the SNA propagate down the western boundary and affect the AMOC at lower latitudes and the Gulf Stream Extension (GSE); 2) SNA SSTs force changes in the speed and/or latitude of the atmospheric eddy-driven jet in summer and winter, contributing to decadal jet variability; 3) subsurface density anomalies in the SNA are a key ingredient for successful predictions of major changes in the SNA (e.g. the mid-1990s warming), and associated changes in phase of the AMV. 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.