by Ayan Chaudhuri SMAST, UMASS Dartmouth

1. by Ayan Chaudhuri SMAST, UMASS Dartmouth Interannual Variability of Gulf Stream Warm-Core Rings in Response to the North Atlantic Oscillation

2. Overview • The Gulf Stream (GS) forms large amplitude meanders downstream of Cape Hatteras from baroclinic and barotropic instability processes. • Individual meander crests, if large enough (surface radii of 2-4 times the internal Rossby radius) can separate from the main GS current, loop back onto themselves and form warm-core rings (WCRs) [Saunders 1971, Csanady 1979]. Source: http://kingfish.coastal.edu/marine/gulfstream/p5.htm

3. Overview Shelf Water Entrainment • Common occurrence of Gulf Stream warm-core rings (WCRs) within the western North Atlantic’s (WNA) Slope Sea (SS) and their role in causing seaward entrainment of outer continental shelf water is well documented. • Most reports concerning WCRs and their associated shelf water entrainments have been based upon single surveys or time-series from individual WCRs. Long term impacts are unknown. Source: John Hopkins Remote Sensing Lab 05/16/97

4. Overview + - • NAO is the north south alternation of pressure difference of atmospheric masses centered near the quasi-permanent Azores (subtropical) high and Icelandic (subpolar) low • The North Atlantic Oscillation (NAO) may be driving the IAV observed in the mean position of the GS [Taylor and Stephens, 1998, Rossby andBenway, 2000] at an observed lag of 1-2 years. • The NAO is also well correlated to the Eddy Kinetic Energy (EKE) of the Gulf Stream. [Stammer and Wunsch, 1998, Penduff et al., 2004] Source: Dr. Martin Visbeck: http://www.ldeo.columbia.edu/NAO/

5. H1: Given that the state of the NAO is known to correlate with the GSNW position, IAV in GSNW mean position caused by the NAO has a direct influence on WCR activity. H2: Given that EKE estimates can be used as a proxy for baroclinic instability processes that generate WCRs, the NAO has a direct influence on the GSR EKE distribution, affecting WCR generation. H3: Higher occurrences of WCRs create more instances of shelf water entrainment. Hypotheses (1) Brachet et al., 2004 NAO->Wind Stress-> Circulation (2) Penduff et al., 2004 NAO-> Combination of Non-linear modes [+4-12 month lag] (1) Spall and Robinson, 1990 Baroclinic Instability modes->WCR formation (2) Stammer, 1998 EKE->Baroclinic instability GS EKE Shelf Water Entrainment WCR NAO GS Position (1) Taylor and Stephens, 1998 NAO->Wind Stress->Rossby Propagation [+2 year lag] (2) Rossby and Benway, 2000 NAO->Buoyancy Flux->Labrador Spilling [+1.5 year lag] (1) Richardson, 1980 GS->New England Seamounts (2) Teague and Hallock, 1990 Topography-> GS Meandering

6. 1989 1996 Data • Data consist of hand-digitized weekly frontal charts produced from satellite-derived sea surface temperature (SST) and charts produced by NOAA and the U.S. Navy. ([Drinkwater et al. 1994]). Dataset contains WCR location, GS position and Shelf Slope Front position from 1978-1999. • A total of 459 quality controlled WCRs show significant IAV from 1978-1999 with maximum occurrence of 31 WCRs in 1990 and minimum occurrence of 7 WCRs in 1978. • An average of 21 WCRs are seen to occur in a year. • A major dip in the North Atlantic Wintertime Index (NAOWI) was seen in 1996 WCR Occurrence 1975 2000 NAOWI 1989 1996

7. NAO, GS Position and WCRs NAO vs. WCRs R=0. 51 p-value = 0.0135 Correlation Coefficient (R) NAO vs. GS Position R= 0.56 p-value = 0.0113 GS Position vs. WCRs R= 0.54 p-value = 0.0120 • The lateral movement of the GS is most likely not forcing the rate of baroclinic instability of the GS and hence rate of WCR formation • The NAOWI is observed to be significantly correlated to WCR activity at a lag of under a year and maybe forcing WCR formation by means different from GS movement.

8. H1:Given that the state of the NAO is known to correlate with the GSNW position, IAV in GSNW mean position caused by the NAO has a direct influence on WCR activity. (ALTERNATE) H1:Given that the state of the NAO is known to correlate with the GSNW position, IAV in GSNW mean position caused by the NAO has a direct influence on WCR activity. H2: Given that EKE estimates can be used as a proxy for baroclinic instability processes that generate WCRs, the NAO has a direct influence on the GSR EKE distribution, affecting WCR generation. H3: Higher occurrences of WCRs create more instances of shelf water entrainment. Hypotheses GS EKE Shelf Water Entrainment WCR NAO GS Position

9. NAO, GS EKE and WCRs • The response of GSR EKE to variability in NAO-induced ocean conditions is studied by numerical simulation of the North Atlantic basin (NAB). • The domain is implemented using a 1/6o ROMS model. 50 vertical levels • Southampton Ocean Center (SOC) ocean-atmosphere atlas [Josey, 2001] derived forcing. • Model spun-up using climatological forcing and then annual year simulations run from 1980-1999. Model output stored at 3 day intervals. Yearly time-series of 120 points • Model validation done by comparing results with Penduff et al. [2004] and TOPEX/POSIEDON data. where

10. NAO, GS EKE and WCRs NAO vs. GS EKE R= 0.52 p-value = 0.0127 GS Position vs. GS EKE R= 0.46 p-value = 0.0542 • GS EKE Anomalies are calculated by removing the global mean EKE from annual GS EKE averages from 1980-1999 • The NAO is observed to be significantly correlated to GS EKE at 0-year lag and thus provide a strong indication that GS EKE variability is the most probable mechanism affecting the annual occurrences of WCRs. • No significant correlation could be established between GS position and GS EKE

11. H1:Given that the state of the NAO is known to correlate with the GSNW position, IAV in GSNW mean position caused by the NAO has a direct influence on WCR activity. (ALTERNATE) H2: Given that EKE estimates can be used as a proxy for baroclinic instability processes that generate WCRs, the NAO has a direct influence on the GSR EKE distribution, affecting WCR generation. H3: Higher occurrences of WCRs create more instances of shelf water entrainment. H2: Given that EKE estimates can be used as a proxy for baroclinic instability processes that generate WCRs, the NAO has a direct influence on the GSR EKE distribution, affecting WCR generation. (NULL) Hypotheses GS EKE WCR SWE NAO GSNW

12. Shelf Water Entrainment Ellipse-Fitting Model (EM) • The positions of all observed WCR edges located in the SS during 1978 -1999 and between 75° and 50°W digitized at Bedford Institute of Oceanography (BIO) • Data only has positions of rings • Surface or subsurface momentum or tracer observations for the WCRs are not available • Key characteristics like WCR center position, radius and orientations are determined by analyzing the WCR observations using an ellipse-fitting feature model proposed by Glenn et al. [1990] and implemented by Gangopadhyay et al. [1997] • Swirl velocities will be computed by finite differencing WCR orientations (q) obtained from the feature model time series. Vi = [(qi-qi-1)/(ti-ti-1)] * Ri (After: Gangopadhyay et al. 1997, Figure 10 (a))

13. Shelf Water Entrainment (a) (b) • WCR observations are averaged over 1o latitude X 1o longitude bins based on WCR center positions obtained from Ellipse-Fitting Model • (a) Mean WCR Occurrence • (b) First WCR Occurrence • (c) Mean WCR Swirl Velocities (c)

14. Shelf Water Entrainment Ring Entrainment Model (RM) • Stern [1987] suggests that after a WCR is formed, it achieves steady state, such that, its potential vorticity (PV) is conserved. However, over time as WCRs become slower and smaller, the PV balance is disrupted. • The PV imbalance is compensated by lateral entrainment or detrainment of ambient water in order to re-establish steady state. • A 3-D Quasi-Geostrophic Potential Vorticity (QGPV) Model was proposed as follows: PV =/f - h’/Hm where,  is the WCR relative vorticity, f is the planetary vorticity, Hm is the WCR mean vertical thickness and h’ is the deviation from the time-averaged mean thickness (Hm). • Entrainment would occur when the gradient of relative vorticity is dominant, while detrainment would occur when the gradient of isopycnal thickness is dominant.

15. Shelf Water Entrainment Ring Entrainment Model (RM) • Since subsurface data for most of the WCRs are not available, the three-dimensional model cannot be used in this proposed study. • Observations support the notion that WCR radius can be assumed to be a good proxy to WCR depth or thickness. • A 3-D QGPV Model is transformed to a 2-D QGPV as follows: PV = /f - r’/Rm  = V/R + dV/dR  is relative vorticity for WCRs [Csanady, 1979], where V is the swirl velocity of the WCR, f = 2 sin is the planetary vorticity, Rm is mean radius of the WCR and r’ is the deviation from the mean radius (Rm) in time. • The 2-D model identifies WCRs that entrain ambient water which may not necessary be from the shelf. The model also provides a deformation radius scale which is used to estimate volume transport • Distance of a WCR to the position of the SSF (considered the outer boundary of continental waters) determines whether the ambient water entrained is derived from the outer continental shelf. After: Olson 1985, Figure 3(g)) Ring 82-B

16. Shelf Water Entrainment MAB GB SS GSL TGB • Higher incidences of shelf water entrainment occurring off GB and SS and comparatively lower incidences are seen off the MAB, GSL and TGB, reflecting spatial variability of WCR activity. • Nearly 65% of the WCRs entrained ambient water • Nearly 55% of the WCRs entrained shelf water

17. NAO vs. Volume Flux R= 0.51 p-value = 0.0217 Shelf Water Entrainment • Volume fluxes derived from deformation radius of entrainment and assuming a constant streamer depth of 50m [Bisagni, 1976] • Mean Annual Volume flux due to shelf water entrainment is 0.75 Sv (23700 km3/year) • Near-zero volume flux from 1978-1980 due to large sample size of 7 days during this period. • Correlation suggest higher WCR occurrences bring about higher WCR induced shelf water fluxes.

18. H1:Given that the state of the NAO is known to correlate with the GSNW position, IAV in GSNW mean position caused by the NAO has a direct influence on WCR activity. (ALTERNATE) H2: Given that EKE estimates can be used as a proxy for baroclinic instability processes that generate WCRs, the NAO has a direct influence on the GSR EKE distribution, affecting WCR generation. (NULL) H3: Higher occurrences of WCRs create more instances of shelf water entrainment. H3: Higher occurrences of WCRs create more instances of shelf water entrainment.(NULL) Hypothesis GSEKE WCR SWE NAO GSNW

19. RESULTS • The lateral movement of the GS is most likely not forcing the rate of baroclinic instability of the GS and hence rate of WCR formation • As baroclinic instability is a major eddy source term throughout the ocean, especially for western boundary currents, and leads to the occurrence of eddies and rings, this study suggests that high (low) phases in the state of the NAO exhibit higher (lower) EKE in the GSR, possibly providing a greater (lesser) source of baroclinic instability to the GS front, resulting in higher (lower) occurrences of WCRs. • Higher (lower) occurrences of WCRs during high (low) NAO years results in higher (lower) incidences of shelf water volume transport.

20. ACKNOWLEDGEMENTS • Dr. K. Drinkwater, Institute for Marine Research, Bergen, Norway • R. Pettipas, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada • Dr. Avijit Gangopadhyay, SMAST • Dr. J. J. Bisagni, SMAST • Dr. Stephen Frasier, UMASS Amherst • This work is being supported by the NASA’s Interdisciplinary Science (IDS) Program, under grant number NNG04GH50G.