Collaborative Research: Investigating the Asymmetries and Temporally Varying Nature of Gulf Stream Ring Formation

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As the vigorously meandering Gulf Stream (GS) flows between Cape Hatteras and the Grand Banks, this western boundary current extension sheds anticyclonic warm core rings (WCRs) into the Slope Sea and cyclonic cold core rings (CCRs) into the Sargasso Sea. Long term observations indicate asymmetries in the generation of warm- and cold-core rings as well as a regime shift. This study will investigate several hypotheses for the processes behind this variability by fully exploiting existing observational datasets. Identifying the causes and consequences of seasonal and interannual variability in the GS and rings will help clarify the ocean’s role in climate variability and establish to what extent elements of the system are predictable. Reports from the commercial fishing industry note recent increases off New England in warm water species commonly associated with the GS, and hence likely related to WCR activity. Results from this work will be communicated at semi-annual meetings at the Commercial Fisheries Center of Rhode Island and also to the general public through regional and national media. Andres will continue her ongoing outreach efforts with an elementary school that serves a group traditionally underrepresented in STEM. This project will support a graduate student who will be trained in both advanced data analysis and dynamical aspects of the changes in the North Atlantic. The research will generate observation-based derived products (e.g., CCR census, track census, and a regional Argo-Rings census) which will be made available to others.Analysis of four decades of synoptic data based on Clark Analysis Charts demonstrates three characteristics of ring formation that are not yet understood dynamically: (i) the number of WCRs shed in a given year does not equal the number of CCRs (“spatial asymmetry”); (ii) after a striking WCR Regime Shift in 2000, more WCRs are usually formed each year during 2000-2018 than are CCRs; prior to this shift (during 1980-1999) more CCRs were formed than WCRs (“interannual asymmetry”); and (iii) the formation of CCRs peaks in winter/spring, whereas WCR formations peak in summer/fall (“seasonal asymmetry”). Based on these observed asymmetries, it is hypothesized that three different ring-types (“aneurisms”, “pinched-off rings”, and “large-ring splits”) are shed from the Stream, and that each type has distinct characteristics due to its respective formation processes. An additional hypothesis is that an observed spatial asymmetry of relative vorticity across the GS is dynamically linked to the observed asymmetry in the annual rates of WCR versus CCR formations. Finally, it is hypothesized that seasonal changes in stratification contribute to the difference in the seasonality of WCR versus CCR formations and that seasonal asymmetry in ring formation depends on spatial asymmetry in the stratification across the Stream. To investigate each of these asymmetries, existing datasets will be analyzed to test these three hypotheses. First the researchers will identify the conditions which give rise to formation of the different ring-types. Then they will quantify whether (and how) a set of state-space parameters changes and whether these changes are associated with the observed regime shift, in order to determine whether the GS System exhibits multiple equilibria. In addition to using the Clark Chart-derived WCR and CCR Censuses, ring-types and the system’s asymmetries will be explored with along-track satellite altimetry, Argo floats (trajectories and vertical property profiles) and existing regional datasets from the Line W Program (moored and shipboard measurements of velocity and water properties), the Oleander Project (temperature profiles and velocity sections) and the Pioneer Array (velocity, temperature, and salinity). Maps of satellite-measured sea surface height and sea surface temperature will provide spatial and temporal context. The analysis plan includes (i) examining all available along-track altimeter data from 1993-2020 along the GS path (75-55W); (ii) calculating conditional averages selected, for example, by sector, by ring-type, and by focusing on a specific year or season, (iii) characterizing individual ring events that are well-sampled, and (iv) examining the state-space for evidence of multiple equilibria using available and derived long-duration time-series of variables (including relative vorticity, stratification, transport, basin scale winds, sea surface height) to compare and contrast the regime change period of 1997-2003 with a recent possible shift period spanning 2016-2021.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.