National Science Foundation, Climate and Large-Scale Dynamics and Physical Oceanography

Decadal Variability in the North Atlantic Extra-Tropics: The Role of Coupling Between Atmospheric Blocking and the Atlantic Multidecadal Oscillation

Caroline Ummenhofer, Hyodae Seo, Young-Oh Kwon, Terry Joyce
Physical Oceanography Department
Woods Hole Oceanogrpahic Institution

Award: $500,000
Duration: 2014-2016

Project Summary.

Overview: We propose to investigate the origin of the observed co-variability in the extra-tropical North Atlantic ocean-atmosphere system on multi-decadal timescales, with particular emphasis on the interaction between changes in weather variability in the atmosphere and decadal variability in ocean circulation, using state-of-the-art coupled, atmosphere, and ocean climate model simulations, as well as observational/reanalysis products. This project will improve our understanding of the nature of the multi- decadal variability in North Atlantic atmospheric blocking, accompanying atmospheric circulation patterns, and their association with the Atlantic Multidecadal Oscillation (AMO). By examining the variations on two very distinct timescales, i.e. synoptic to intra-seasonal in the atmosphere and decadal in the ocean, and how they interact to generate low-frequency coupled variability in the North Atlantic, the proposed research will clarify the origin of the multi-decadal variability of the atmospheric blocking, the extent to which ocean-to-atmosphere feedback plays a role, and whether the AMO is a resulting coupled ocean-atmosphere mode of variability.

Intellectual Merit: This project will investigate interactions across a range of timescales of the three most important modes of North Atlantic variability, namely AMO, East Atlantic Pattern (EAP), and North Atlantic Oscillation (NAO): The AMO is the leading mode of the region’s SST; the NAO is the dominant mode of atmospheric variability in the North Atlantic sector, which exerts enormous influence on the North Atlantic ocean circulation; the EAP is a characteristic pattern of atmospheric blocking in the Atlantic sector, which not only influences mid-latitude mean climate, but is often associated with extreme events, such as heat waves and droughts. We will develop a mechanistic understanding of the relative role of the oceanic versus atmospheric forcing for the existence of the observed multi-decadal co-variability in the ocean-atmosphere system of the North Atlantic. Atmospheric blocking characteristics in the North Atlantic sector will be evaluated, along with the oceanic co-variability, across a range of timescales in reanalysis products and existing climate model simulations by the National Center for Atmospheric Research. While previous studies suggested that a realistic mean state of North Atlantic variability is crucial in simulating blocking in the North Atlantic sector, state-of-the-art coupled models still suffer from significant mean biases in their simulations, e.g. overly zonal North Atlantic Current. We therefore also propose a suite of uncoupled simulations with atmosphere and ocean models, in which a realistic mean state of the boundary conditions is constrained, to test whether (1) low-frequency variations in the blocking-related EAP and/or NAO originate from oceanic variability associated with the AMO and (2) EAP and/or NAO drive decadal ocean variability, including the AMO. To achieve our goal, the key lies in the simultaneous investigation of both atmospheric and oceanic components of the climate system, while previous research has often been conducted in separate research communities.

Broader Impacts: Given the impacts of North Atlantic variability on societies in Europe and North America through extreme weather events, such as severe extra-tropical storms, heat waves, and atmospheric rivers, improved understanding of decadal modulations in the ocean-atmosphere system of the North Atlantic is desirable. With extreme events often associated with sustained blocking episodes in North America and Europe, improved understanding of low-frequency variability in these phenomena have potential implications for decadal predictions and thus clear societal importance. We currently lack the understanding of the explicit link how low-frequency oceanic variability manifests in atmospheric conditions of importance to society, which is crucial for improved predictions. As such, the proposed research will benefit the climate community effort on decadal predictions. The project also provides an opportunity to educate the next generation of climate scientists, by (1) training a graduate student in the atmospheric, climate, oceanographic sciences, as well as state-of-the-art climate modeling; (2) entraining undergraduate students into the project by providing them with hands-on experiences in original research at no cost to the proposal through various undergraduate research programs, such as the Significant Opportunities in Atmospheric Research and Science (SOARS), the Woods Hole Oceanographic Institution (WHOI) Summer Student Fellow or the German Research Internships in Science and Engineering (RISE) program. Research findings will be disseminated to the general public through outreach efforts, such as WHOI Ocean Science Exhibit Center talks “Science Made Public” or visits by science teachers as part of the Woods Hole Science and Technology Education Partnership.