Award: OCE-1558960

Warm-core rings (WCRs) acting between western boundary currents and the continental shelf exert significant impact on the physical and biological environments of the slope seas and coastal oceans, which are major contributors to the global primary production. The overall goal of our proposed work is to better understand the physical-biological processes associated with the evolution of Gulf Stream warm-core rings and provide improved scientific understanding of the biogeochemical and ecosystem dynamics in the shelf-slope system of the Northwest Atlantic. The results from this project are disseminated through 6 peer-reviewed journal articles. Our work has been presented at multiple conferences, and seminars have been given at several research institutes and universities. Intellectual Merit: We investigated the relative roles of various physical processes in terms of vertical nutrient delivery, which is considered as one fundamental factor for satellite observed mesoscale chlorophyll patterns. We conducted targeted numerical modeling work to elucidate the importance of multiple physical processes in producing vertical nutrient transport. Our work showed that frictional decay alone is ineffective in raising isopycnals and transporting nutrients to the upper ocean. Instead, wind-forced vertical motions are more important in sustaining deep nutrient to the sunlit upper ocean. With representative wind forcing, the magnitude of vorticity gradient-induced Ekman pumping is not necessarily larger than current-induced counterpart on a time scale relevant to ecosystem response. Under realistic forcing conditions, strain deformation can perturb the ring to be non-circular and induce vertical velocities much larger than the Ekman vertical velocities. Our research also reveals the time-dependence nature of the vertical nutrient delivery. At time scales that are relevant to individual phytoplankton (hours to days), the magnitudes of nutrient flux by Ekman velocities and deformation-induced velocities are comparable. Over the life span of a typical warm core ring, surface current-induced Ekman pumping is the most effective mechanism in upper ocean nutrient enrichment because of its persistence in the center of anticyclones regardless of the direction of the wind forcing. Our research provides a representative view of vertical physical and biological processes in large anticyclones in the western boundary current system, elucidates the roles of several key biophysical mechanisms, and improves the understanding of biogeochemical processes and ecosystem dynamics in the Northwest Atlantic. We also examined how the near-surface chlorophyll evolves in both Gulf Stream warm-core rings and cold-core rings (CCRs). This is to address some of the long-standing questions regarding the roles of cyclonic and anti-cyclonic eddies in shaping the upper ocean ecosystem, including the contrasting results from some of the earlier studies, and the temporal variability of the contributions associated with eddies. Our analysis of multi-platform satellite observations shows that the averaged CHL anomaly (CHLA) within the rings exhibits both positive and negative linear trends during the evolution of the WCRs while negative trends dominate in CCRs. This difference is associated with a variety of physical processes occurring during the evolution process. Meanwhile, Short-term fluctuations of CHLA in WCRs and CCRs are closely correlated with mixed layer depth and sea surface temperature anomaly and highlight the complex interplay between multiple mechanisms. In addition, we find higher concentration CHL in some WCRs than that in CCRs during the same season, providing an alternative view of the characteristics of the surface ecosystem in Gulf Stream rings. This work systematically describes the surface chlorophyll variability beyond prior composite analysis, highlands the remarkable diversity in Gulf Stream rings, and challenges the conventional understanding of the role of anticyclonic versus cyclonic eddies in shaping the upper ocean ecosystem. Our work also showed that WCRs play an important role in facilitating the spawning of blue fin tuna in the Northwest Atlantic slope region because of its nature in sustaining persistent physical and biogeochemical ocean conditions suitable for spawning and larvae retention. In addition, WCRs are shown to be increasing important in driving shelf-ocean exchanges and in initiating extreme temperature anomalies, i.e., Marine Heatwaves on the Northwest Atlantic continental shelf. Broader Impacts: The improved understanding on nutrient dynamics from this project contributes to the stewardship of living marine resources, and better ecosystem management. The result from this project also contributes to the potential prediction of fish school distributions associated with GS WCRs and thus can benefit the commercial fishing industry, which the PIs have been in contact with through the Commercial Fisheries Research Foundation. This project supported two early-career scientists transitioning to principal investigators. Also, a Summer Student Fellow was recruited and trained at WHOI. In addition, a Guest Student is advised by the PI to work on topics related to the project. As part of this project, the legacy hard-copy data from the Warm Core Ring Program in the 1980s are digitized and submitted to the BCO-DMO to serve the broader scientific community. This process represents a community effort of advancing preservation, access, and usability of valuable legacy data for future science. Last Modified: 05/05/2021 Submitted by: Ke Chen