A Census of the Warm‐Core Rings of the Gulf Stream: 1980–2017

Abstract

A census of Gulf Stream (GS) warm‐core rings (WCRs) is presented based on 38 years (1980–2017) of data. The census documents formation and demise times and locations, and formation size for all 961 WCRs formed in the study period that live for a week or more. A clear regime shift was observed around the Year 2000 and was reported by a subset of authors (Gangopadhyay et al., 2019, https://doi.org/10.1038/s41598-019-48661-9). The WCR formation over the whole region (75–55°W) increased from an average of 18 per year during Regime 1 (1980–1999) to 33 per year during Regime 2 (2000–2017). For geographic analysis formation locations were grouped in four 5° zones between 75°W and 55°W. Seasonally, WCR formations show a significant summer maxima and winter minima, a pattern that is consistent through all zones and both temporal regimes. The lifespan and size distribution show progressively more rings with higher longevity and greater size when formed to the east of 70°W. The average lifespan of the WCRs in all four zones decreased by 20–40% depending on zones and/or seasons from Regime 1 to Regime 2, while the size distribution remained unchanged across regimes. The ring footprint index, a first‐order signature of impact of the WCRs on the slope, increased significantly (26–90%) for all zones from Regime 1 to Regime 2, with the highest percent increase in Zone 2 (70–65°W). This observational study establishes critical statistical and dynamical benchmarks for validating numerical models and highlights the need for further dynamical understanding of the GS‐ring formation processes.

A Survival Analysis of the Gulf Stream Warm Core Rings

“…When WCR survival probabilities were compared between the two regimes, we found that the survival of WCRs has decreased from the previous regime (1980–1999) to the later regime (2000–2017). This result agrees with the reduced mean lifespan median values of WCRs in Regime 2 (see Gangopadhyay, Gawarkiewicz, Silva, et al, 2020, Tables 3 and 4). Two factors might have led to this result.…”

“…Within the GIS data frame, an analyst examined each chart following the WCR formation, continuity, and dissipation rules. A new ring formation is documented in three situations: (i) a typical GS crest forming a closed anticyclonic vortex and detaches from the stream in the slope water; (ii) an anticyclonic eddy forms off of another large anticyclonic eddy in the slope water; and (iii) an anticyclonic eddy further away from the stream coming into the domain through Zone 4 (Gangopadhyay, Gawarkiewicz, Silva, et al, 2020). The demise of a ring is decided when a ring was last documented (or seen) in the JC charts.…”

“…The time duration between a WCR formation and its demise is the lifespan or the total survival time of a WCR. It is the difference between the date of absorption (DOA) and the date of birth (DOB) of a WCR (refer to Table 1 of Gangopadhyay, Gawarkiewicz, Silva, et al, 2020). The master data set of these aforementioned annual WCR censuses, which consisted of 961 GS WCRs for the period 1980 to 2017 (Gangopadhyay & Gawarkiewicz, 2020), was used for the survival analysis.…”

“…The master data set of these aforementioned annual WCR censuses, which consisted of 961 GS WCRs for the period 1980 to 2017 (Gangopadhyay & Gawarkiewicz, 2020), was used for the survival analysis. A detailed description of the WCR census development is presented in Gangopadhyay, Gawarkiewicz, Silva, et al (2020).…”

“…The present study was carried out following the development of a 38‐yearlong census of the WCRs (Gangopadhyay & Gawarkiewicz 2020) and the results of a number of related recent studies (Gangopadhyay et al, 2019; Gangopadhyay, Gawarkiewicz, Silva, et al, 2020; Monim, 2017; Silva, 2019) using the census data. These systematic studies on the GS WCR formation and distribution characteristics highlighted the critical need of investigating the dependence of the longevity of WCRs on various factors including their locations of formation and demise, and the seasons and size at formation.…”