Shelf Water Entrainment by Gulf Stream Warm-Core Rings ... · INTRODUCTION The common occurrence of...
Transcript of Shelf Water Entrainment by Gulf Stream Warm-Core Rings ... · INTRODUCTION The common occurrence of...
INTRODUCTION
The common occurrence of anticyclonic Gulf Stream warmcore rings (WCRs) within the western North Atlantic’s Slope Sea (SS) and their role in causing seaward entrainment of outer continental shelf water into the SS along their eastern margins, have been well documented (Fig. 1). However, most reports concerning WCRs and their associated shelf water entrainments have been based upon single surveys or timeseries from individual WCRs. To date, estimates of annual shelf water volume entrained into the SS by WCRs and its interannual variability (IAV) have not been made. Recognizing that WCRs significantly affect the hydrographic, physical and biogeochemical environment of the continental shelf and slope region we address the following questions: (1) How much shelf water is transported into the Slope Sea due to streamer entrainment by WCRs? (2) Does WCR activity affect the volume transport of shelf water into the Slope Sea? (3) Is there any interannual variability in the volume transport?
DATA AND METHODS Positions of all observable WCRs between 1978 to 1999 within 75° and 50°W are obtained from hand digitized satellite images and U. S. Navy charts produced at Bedford Institute of Oceanography (Fig. 2). The observations are analyzed using an ellipsefitting feature model [Gangopadhyay et. al. 1997] to determine key characteristics like ring center position and radius, which are then used to compute swirl velocity by finite differencing ring orientations (θ) obtained from the feature model time series (Fig. 3). Model uncertainty is determined by computing error of fit for each observation and propagated through subsequent calculations. A 2D Ring Entrainment Model (REM) is used to estimate streamer entrainment events for individual ring observations and subsequent temporal integration results in shelf water volume transports. REM is based on a variation of the quasigeostrophic potential vorticity equation proposed by Stern [1987]. The model equation is as follows:
PV = ζ/f r’/Rm (1) ζ = V/R + dV/dR [Csanady, 1979] is relative vorticity for WCRs, where V is the swirl velocity of the ring, f = 2W sinφ is the planetary vorticity, Rm is mean radius of ring and r’ is deviation from the mean radius (Rm) in time. The potential vorticity in this case is dimensionless and ∆PV=0 is considered steady state. If ∆PV becomes greater than 0 during the lifetime of the ring, it implies ζ/f term (Rossby Number) in (1) dominates the r’/Rm term, thus causing instability. To revert back to steady state the ring will need to increase r’ i.e. increase its radius or entrain ambient water. Proximity of a WCR to the position of the shelfslope front determines whether the ambient water entrained is derived from the outer continental shelf. Conversely, if ∆PV becomes less than 0, it implies r’/Rm term will dominate the Rossby number of the ring, and will have to decrease its radius or detrain water to regress back to steady state.
RESULTS & CONCLUSIONS WCR entrainment of shelf water events are obtained using REM. A regional analysis of entrainment events shows (Fig. 4) that maximum entrainment occurs off the Georges Bank (GB) region (70°65°W) and minimum entrainment happens off the Gulf of St. Lawrence (GSL) region (60°55°W). Garfield and Evans [1987] determined shelf water entrainment off the GB region to be 69 ± 20% on examination of 7 years of satellite and drogue data. Our analysis presents a more conservative estimate (50.60 ± 8.57%) over 22 years of observations. REM provides the length scale of streamers that pull out water from the shelf. Since this scale is the most dominant for calculating volume, reasonable assumptions of the width (12 km from Bisagni [1983]) and depth (60m from Schlitz [2003]) are made to derive shelf water volume fluxes. Individual streamer fluxes are temporally integrated to provide annual estimates of shelf water volume transport (Fig. 5A). WCR observations were recorded at every 7 days till 02/04/81 and subsequently every 3 days. Since streamers usually have lifespans lesser than 7 days, volume transports during 1978 to 1980 are grossly underestimated and hence neglected. Annual shelf water volume transport fluxes show significant positive correlation (pvalue = 0.0001, r = 0.9331) with WCR activity (Fig. 5B) at zerolag (Fig. 5D). Volume transport calculations show considerable interannual variability (Fig. 5A). On annual to decadal time scales, the single largest factor affecting the circulation of the North Atlantic basin is the North Atlantic Oscillation (NAO). Correlations between the North Atlantic Oscillation Wintertime Index (NAOWI) and ring activity are seen to be significantly positively correlated (pvalue = 0.0135, r = 0.6122) with maximum correlation occurring at zerolag (Fig. 5E). Significant correlation between NAOWI and ring activity and recognizing the profound affect of ring activity on shelf water volume transports suggests a direct coupling between the NAO forcing and shelf water volume fluxes. Cross correlation between NAOWI and shelf water transport show maximum significant positive correlation (pvalue=0.054, r= 0.6221) at a lag of 1 year (Fig. 5C).
Figure 1. Satellite imagery of Sea Surface Temperature (SST) for the Northwest Atlantic (NWA) region on 07/04/2001 (Source: John Hopkins APL). The Gulf Stream (GS) (A) can be seen as a turbulent jet separating the warm Sargasso Sea (B) from the colder continental shelf (C). The Slope Sea (SS) (D) is shown as an intermediate region engulfing two warm core rings (E,F).
ACKNOWLEDGEMENTS The authors would like to thank Dr. K. Drinkwater, Institute for Marine Research, Bergen, Norway and R. Pettipas, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada, for providing digitized Gulf Stream north wall, shelfslope front data and warmcore ring positional data. This work is being supported by the NASA’s Interdisciplinary Science (IDS) Program, under grant number NNG04GH50G.
University of MassachusettsUniversity of MassachusettsSchool for Marine Science and TechnologySchool for Marine Science and Technology
New Bedford, Massachusetts, USANew Bedford, Massachusetts, USA([email protected], [email protected], ([email protected], [email protected],
[email protected])[email protected])
Shelf Water Entrainment by Gulf Stream Warm-Core Rings Between 75° and 50°WShelf Water Entrainment by Gulf Stream Warm-Core Rings Between 75° and 50°WAyan ChaudhuriAyan Chaudhuri, James J. Bisagni and Avijit Gangopadhyay, James J. Bisagni and Avijit Gangopadhyay
Figure 3. Computed swirl velocities averaged spatially into onedegree latitude and longitude bins between 75o and 50oW based upon WCR center positions from 1978 to 1999. Global mean WCRedge swirl velocity calculated from all observations is 105.72 ± 0.7 km/day (122.36 ± 0.81 cm/sec).. Binned swirl velocities are in general agreement with the known WCR formation mechanism; displaying higher velocities in the vicinity of the Gulf Stream North Wall and successively lower velocities as WCRs age and propagate southwestward through the SS, impinging along the outer continental shelf.
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Figure 2. WCR Observations (19781999) at 1 degree resolution between 75o and 50oW, based upon WCR center positions. Maximum ring activity is around Georges Bank region (70° 65°W).
Figure 4. Regional entrainment statistics based on computations from REM. The spatial domain is divided into 5 main regions, namely, MidAtlantic Bight (MAB), Georges Bank (GB), Scotian Shelf (SS), Gulf of St. Lawrence (GSL) and Tail of Grand Banks (TGB). Error estimates are shown for ambient and shelf water entrainment
Figure 5. The lagged correlations (C,D,E) are computed after discarding shelf water volume estimates from 1978 to 1980 due to underestimation.
REFERENCES Bisagni J.J, 1983: Lagrangian Current Measurements within the Eastern Margin of a WarmCore Gulf Stream Ring, JPO, 13(4), 11771190
Csanady G. T., 1979: The birth and death of a warm core ring, JGR, 84(C2), 777780 Gangopadhyay A., Robinson A. R., Arango H. G., 1997: Circulation and Dynamics of the Western North Atlantic. Part I: Multiscale Feature Models, Journal of Atmosphere and Oceanic Technology, Vol (12), 13141332. Garfield III N., Evans D., 1987: Shelf Water Entrainment by Gulf Stream WarmCore Rings, JGR,92(C12), 1300313012. Schlitz R., 2003: Interaction of Shelf Water with Warm Core Rings, Focusing in the kinematics and statistics of Shelf Water Entrained within Streamers, NOAA Technical Memorandum NMFSNE170 Stern M., 1987: Largescale Lateral Entrainment and Detrainment at the edge of a Geostrophic Shear Layer, JPO, 17(10), 16801687
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