A Test of the Parsons–Veronis Hypothesis on the Separation of the Gulf Stream

The Parsons–Veronis model, based on a two-layer wind-driven ocean, predicts the latitude at which the western boundary current separates from the western boundary. It has been tested on the Gulf Stream using both satellite and in situ observations. The hypothesis attributes the difference in the thermocline depth from the eastern to the western side of the ocean and the corresponding northward geostrophic transport (with closed northern end) to the southward Ekman transport integrated across the basin. Twelve years (1977–88) of satellite sea surface temperature data and wind data [from the Fleet Numerical Oceanography Center (FNOC) wind database] have been used for this study. The satellite-derived Gulf Stream northern edges were used to determine the latitudes of separation (i.e., crossing the 2000-m isobath into deep water). Parsons\u27 model is sensitive to two “free” parameters, namely, the reduced gravity and the thermocline depth on the eastern side of the basin. Based on available CTD data and previous current meter studies, these free parameters are selected to establish a representative two-layer model for the midlatitude North Atlantic. When the Ekman drift is integrated over several years, the predicted separation latitude variability agrees with observations with unit slope within 95% confidence limits. The relevant time scale of integration is on the order of 3 years, somewhat less than the estimated time for long baroclinic planetary waves to cross the Atlantic. For this limited dataset, little improvement in the prediction is found for a larger number of years of averaging. More detailed and long-term investigation of this hypothesis should be made in future in context of other western boundary currents

Modeling the Gulf Stream System: How Far from Reality?

Analyses of a primitive equation ocean model simulation of the Atlantic Ocean circulation at 1/6 deg horizontal resolution are presented with a focus on the Gulf Stream region. Among many successful features of this simulation, this letter describes the Gulf Stream separation from the coast of North America near Cape Hatteras, meandering of the Gulf Stream between Cape Hatteras and the Grand Banks, and the vertical structure of temperature and velocity associated with the Gulf Stream. These results demonstrate significant improvement in modeling the Gulf Stream system using basin- to global scale ocean general circulation models. Possible reasons responsible for the realistic Gulf Stream simulation are discussed, contrasting the major differences between the present model configuration and those of previous eddy resolving studies

Seasonal evolution of oceanic upper layer processes in the northern Bay of Bengal following a single Argo float

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 46(10), (2019): 5369-5377, doi: 10.1029/2019GL082078.Seasonal evolution of the barrier layer (BL) and temperature inversion in the northern Bay of Bengal and their role on the mixed layer temperature (MLT) is examined using observations from a single Argo during December 2013 to July 2017. During fall, low salinity at surface generates BL in this region. It thickens to almost 80 m in winter enhanced by deepening of isothermal layer depth due to remote forcing. During winter, surface cooling lowers near‐surface temperature, and thus, the subsurface BL experiences a significant temperature inversion (~2.5 °C). This temperature inversion diffuses to distribute heat within ML and surface heating begins deep penetration of shortwave radiation through ML during spring. Hence, the ML becomes thermally well stratified, resulting in the warmest MLT. The Monin‐Obukhov length attains its highest value during summer indicating wind dominance in the ML. During spring and fall, upper ocean gains heat allowing buoyancy to dominate over wind mixing.A. S. and S. S. thank financial support from Space Application Centre (SAC), Indian Space Research Organization (ISRO), Government of India (Grant: SAC/EPSA/4.19/2016). This study was also supported by the first phase of Ministry of Earth Sciences (MoES), Government of India grant to establish a Bay of Bengal Coastal Observatory (BOBCO) at IITBBS (Grant: RP088). Authors acknowledged NCPOR Contribution number J ‐ 03/2019‐20 for this work. The authors are grateful to the reviewers and the Editor for constructive suggestions. The figures are generated using Matlab. The data source and availability are given in the Text S1.2019-10-2

Secular change and inter-annual variability of the Gulf Stream position, 1993–2013, 70°−55°W

The Gulf Stream (GS) is the northeastward-flowing surface limb of the Atlantic Ocean's meridional overturning circulation (AMOC) “conveyer belt” that flows towards Europe and the Nordic Seas. Changes in the GS position after its separation from the coast at Cape Hatteras, i.e., from 75°W to 50°W, may be key to understanding the AMOC, sea level variability and ecosystem behavior along the east coast of North America. In this study we compare secular change and inter-annual variability (IAV) of the Gulf Stream North Wall (GSNW) position with equator-ward Labrador Current (LC) transport along the southwestern Grand Banks near 52°W using 21 years (1993–2013) of satellite altimeter data. Results at 55°, 60°, and 65°W show a significant southward (negative) secular trend for the GSNW, decreasing to a small but insignificant southward trend at 70°W. IAV of de-trended GSNW position residuals also decreases to the west. The long-term secular trend of annual mean upper layer (200 m) LC transport near 52°W is positive. Furthermore, IAV of LC transport residuals near 52°W along the southwestern Grand Banks are significantly correlated with GSNW position residuals at 55°W at a lag of +1-year, with positive (negative) LC transport residuals corresponding to southward (northward) GSNW positions one year later. The Taylor-Stephens index (TSI) computed from the first principal component of the GSNW position from 79° to 65°W shows a similar relationship with a more distal LC index computed along altimeter ground track 250 located north of the Grand Banks across Hamilton Bank in the western Labrador Sea. Increased (decreased) sea height differences along ground track 250 are significantly correlated with a more southward (northward) TSI two years later (lag of +2-years). Spectral analysis of IAV reveals corresponding spectral peaks at 5–7 years and 2–3 years for the North Atlantic Oscillation (NAO), GSNW (70°−55°W) and LC transport near 52°W for the 1993–2013 period suggesting a connection between these phenomena. An upper-layer (200 m) slope water volume calculation using the LC IAV rms residual of +1.04 Sv near 52°W results in an estimated GSNW IAV residual of 79 km, or 63% of the observed 125.6 km (1.13°) rms value at 55°W. A similar upper-layer slope water volume calculation using the positive long-term, upper-layer LC transport trend accounts for 68% of the mean observed secular southward shift of the GSNW between 55° and 70°W over the 1993–2013 period. Our work provides additional observational evidence of important interactions between the upper layers of the sub-polar and sub-tropical gyres within the North Atlantic over both secular and inter-annual time scales as suggested by previous studies

Interannual and seasonal asymmetries in gulf stream ring formations from 1980 to 2019

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Silver, A., Gangopadhyay, A., Gawarkiewicz, G., Silva, E. N. S., & Clark, J. Interannual and seasonal asymmetries in gulf stream ring formations from 1980 to 2019. Scientific Reports, 11(1), (2021): 2207, https://doi.org/10.1038/s41598-021-81827-y.As the Gulf Stream separates from the coast, it sheds both Warm and Cold Core Rings between 75∘ and 55∘W. We present evidence that this ring formation behavior has been asymmetric over both interannual and seasonal time-scales. After a previously reported regime-shift in 2000, 15 more Warm Core Rings have been forming yearly compared to 1980–1999. In contrast, there have been no changes in the annual formation rate of the Cold Core Rings. This increase in Warm Core Ring production leads to an excess heat transfer of 0.10 PW to the Slope Sea, amounting to 7.7–12.4% of the total Gulf Stream heat transport, or 5.4–7.3% of the global oceanic heat budget at 30∘N. Seasonally, more Cold Core Rings are produced in the winter and spring and more Warm Core Rings are produced in the summer and fall leading to more summertime heat transfer to the north of the Stream. The seasonal cycle of relative ring formation numbers is strongly correlated (r = 0.82) with that of the difference in upper layer temperatures between the Sargasso and Slope seas. This quantification motivates future efforts to understand the recent increasing influence of the Gulf Stream on the circulation and ecosystem in the western North Atlantic.The authors acknowledge financial supports from NOAA (NA11NOS0120038), NSF (OCE-1851242), SMAST and UMass Dartmouth. GG was supported by NSF under grant OCE-1851261 and ONR under grant N00014-19-1-2646

An observed regime shift in the formation of warm core rings from the gulf stream

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Gangopadhyay, A., Gawarkiewicz, G., Silva, E. N. S., Monim, M., & Clark, J. An observed regime shift in the formation of warm core rings from the gulf stream. Scientific Reports, 9(1), (2019): 12319-019-48661-9, doi:10.1038/s41598-019-48661-9.We present observational evidence that a significant regime change occurred around the year 2000 in the formation of Warm Core Rings (WCRs) from the Gulf Stream (GS) between 75° and 55°W. The dataset for this study is a set of synoptic oceanographic charts available over the thirty-eight-year period of 1980–2017. The upward regime change shows an increase to 33 WCRs per year during 2000–2017 from an average of 18 WCRs during 1980 to 1999. A seasonal analysis confirms May-June-July as the peak time for WCR births in agreement with earlier studies. The westernmost region (75°-70°W) is least ring-productive, while the region from 65°W to 60°W is most productive. This regime shift around 2000 is detected in WCR formation for all of the four 5-degree wide sub-regions and the whole region (75°-55°W). This might be related to a reduction of the deformation radius for ring formation, allowing unstable meanders to shed more frequent rings in recent years. A number of possible factors resulting in such a regime shift related to the possible changes in reduced gravity, instability, transport of the GS, large-scale changes in the wind system and atmospheric fluxes are outlined, which suggest new research directions. The increase in WCRs has likely had an impact on the marine ecosystem since 2000, a topic worthy for future studies.The authors acknowledge financial supports from NOAA (NA11NOS0120038), NSF (OCE-0815679), SMAST and UMass Dartmouth. GG was supported by NSF under grant OCE-1657853 as well as a Senior Scientist Chair from WHOI. We have benefitted from many discussions on GS system behavior and variability with Tom Rossby, Charlie Flagg, Kathy Donohue, Randy Watts, Peter Cornillon, Magdalena Andres and on WCR identification with Jim Bisagni. The WCR data from Jenifer Clark (co-author) and Roger Pettipas were used to develop the original census. We wish to thank the Editor and two anonymous reviewers for their helpful comments and encouragement to a previous version which improved the focus of this manuscript

Forecasting the Gulf Stream Path using buoyancy and wind forcing over the North Atlantic

Fluctuations in the path of the Gulf Stream (GS) have been previously studied by primarily connecting to either the wind-driven subtropical gyre circulation or buoyancy forcing via the subpolar gyre. Here we present a statistical model for 1 year predictions of the GS path (represented by the GS northern wall—GSNW) between urn:x-wiley:21699275:media:jgrc24667:jgrc24667-math-0001W and urn:x-wiley:21699275:media:jgrc24667:jgrc24667-math-0002W incorporating both mechanisms in a combined framework. An existing model with multiple parameters including the previous year's GSNW index, center location, and amplitude of the Icelandic Low and the Southern Oscillation Index was augmented with basin-wide Ekman drift over the Azores High. The addition of the wind is supported by a validation of the simpler two-layer Parsons-Veronis model of GS separation over the last 40 years. A multivariate analysis was carried out to compare 1-year-in-advance forecast correlations from four different models. The optimal predictors of the best performing model include: (a) the GSNW index from the previous year, (b) gyre-scale integrated Ekman Drift over the past 2 years, and (c) longitude of the Icelandic Low center lagged by 3 years. The forecast correlation over the 27 years (1994–2020) is 0.65, an improvement from the previous multi-parameter model's forecast correlation of 0.52. The improvement is attributed to the addition of the wind-drift component. The sensitivity of forecasting the GS path after extreme atmospheric years is quantified. Results indicate the possibility of better understanding and enhanced predictability of the dominant wind-driven variability of the Atlantic Meridional Overturning Circulation and of fisheries management models that use the GS path as a metric

A census of the warm-core rings of the Gulf Stream: 1980-2017

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 125(8), (2020): e2019JC016033, doi:10.1029/2019JC016033.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.The authors acknowledge financial support from NOAA (NA11NOS0120038), NSF (OCE‐0815679 and OCE‐1851242), and SMAST and UMass Dartmouth. G. G. was supported by NSF under Grant OCE‐1657853 as well as a Senior Scientist Chair from WHOI. We have benefitted from many discussions on Gulf Stream and WCR with Magdalena Andres, Andre Schmidt, Paula Fratantoni, Jon Hare, Wendell Brown, Kathy Donohue, Tom Rossby, Peter Cornillon, and Randy Watts.2020-12-2

The changing nature of shelf-break exchange revealed by the OOI Pioneer Array

Author Posting. © The Oceanography Society, 2018. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 31, no. 1 (2018): 60–70, doi:10.5670/oceanog.2018.110.Although the continental shelf and slope south of New England have been the subject of recent studies that address decadal-scale warming and interannual variability of water mass properties, it is not well understood how these changes affect shelf-break exchange processes. In recent years, observations of anomalous shelf and slope conditions obtained from the Ocean Observatories Initiative Pioneer Array and other regional observing programs suggest that onshore intrusions of warm, salty waters are becoming more prevalent. Mean cross-shelf transects constructed from Pioneer Array glider observations collected from April 2014 through December 2016 indicate that slope waters have been warmer and saltier. We examine shelf-break exchange events and anomalous onshore intrusions of warm, salty water associated with warm core rings located near the shelf break in spring 2014 and winter 2017 using observations from the Pioneer Array and other sources. We also describe an additional cross-shelf intrusion of ring water in September 2014 to demonstrate that the occurrence of high-salinity waters extending across the continental shelf is rare. Observations from the Pioneer Array and other sources show warm core ring and Gulf Stream water masses intrude onto the continental shelf more frequently and penetrate further onshore than in previous decades.GG, WZ, RT, and MD were supported by the National Science Foundation under grant OCE-1657853. WZ was also supported by grant OCE-1634965. JP is grateful for the support of the Woods Hole Oceanographic Institution Summer Student Fellow Program. AMM was supported by a grant from the MacArthur Foundation. GG and AMM were also supported by a grant from the van Beuren Charitable Foundation for collection and analysis of hydrographic data collected by the CFRF Shelf Research Fleet