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Interannual Variability of the Shelf-Slope Front Position Between 75  and 50  W

Interannual Variability of the Shelf-Slope Front Position Between 75  and 50  W James J. Bisagni and Hyun-Sook Kim University of Massachusetts, Dartmouth School for Marine Science and Technology New Bedford, Massachusetts, USA. Supported by the National Science Foundation

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Interannual Variability of the Shelf-Slope Front Position Between 75  and 50  W

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  1. Interannual Variability of the Shelf-Slope Front Position Between 75 and 50 W James J. Bisagni and Hyun-Sook Kim University of Massachusetts, Dartmouth School for Marine Science and Technology New Bedford, Massachusetts, USA Supported by the National Science Foundation US-GLOBEC NW Atlantic Georges Bank Program

  2. Talk outline: Motivation for the work Observational data used Methodology Compare study results with other recent work in the MAB Conclude and speculate

  3. Motivation:(See: Rossby, 1999, “On gyre interactions”, DSR-II, 46:139-164) Rossby speculates that seasonal and interannual variability (IAV) of the Gulf Stream North Wall (GSNW) position results from time-varying “spilling” of Labrador shelf water (LSW) into eastern slope waters: 1) Seasonal increase in volume transport and spillage of LSW during fall deepens the slope water pycnocline, forcing the GSNW into a more southerly position by spring, with maximum Gulf Stream transport by June. ===> Represents a seasonal “back-door” thermohaline interaction between the subtropical and subpolar gyres. 2) Changes in transport and spillage of LSW over interannual time scales may result in observed IAV of the GSNW position. We present an analysis of IAV of the shelf-slope front (SSF) between the Tail of the Grand Banks (TGB) and Cape Hatteras (CH) that may allow us to begin testing the “back-door” gyre-interaction hypothesis between LSW and the GSNW over interannual time scales.

  4. TGB UMass Dartmouth Shelf Slope Front (SSF) CH Gulf Stream North Wall (GSNW) 2 SST (C) 32 Weekly “warmest-pixel” sea surface temperature (SST) satellite image centered on 18 May 1998

  5. Latitudinal positions of the SSF were hand-digitized along 26 longitude lines between 75° to 50°W from twice-weekly SST analysis charts by workers at Bedford Institute of Oceanography (BIO) for a 29-year period (1973-2001).

  6. The 20-Year Long-Term Mean SSF Position (1973-1992)

  7. TGB SSF GSNW CH Long-term mean SSF (dashed line) and GSNW (solid line) from 20 years (1973-1992) of digitized EOFA & Gulf Stream charts. (After Drinkwater et al., 1994)

  8. Seasonal Variability of the SSF Position (1973-1992)

  9. Long-term monthly-mean SSF position anomalies averaged between 75°W and 50°W , positive (negative) SSF position anomalies are located shoreward (seaward) of the 20-year mean SSF position. (After Drinkwater et al., 1994)

  10. ~70W TGB CH Aug Oct Dec   Feb   Apr   Jun   Aug Offshore Onshore Distance-time plot of seasonal variability of long-term monthly-mean SSF position anomalies for 1973-1992 (Bisagni et al., submitted)

  11. (-5.3) (-2.9) (-41.7) (-23.7) (-55.0) (-40.6) (-34.6) (-47.7) Position of maximum seaward SSF distance anomaly (trough) between December and July. Parentheses ( ) indicate amplitude of SSF distance anomaly in km. Also shown is the least-squares functional linear regression fit (solid line).

  12. IAV Study Methodology: 1)Compute long-term monthly-mean SSF positions along each of 26 lines of longitude between 75°W and 50°W using individual monthly-mean SSF position data for the 20-year (1973-1992) period of Drinkwater et al., (1994). 2) Subtract the 20-year long-term monthly-mean SSF positions from corresponding individual monthly-mean SSF position data at each longitude over the full 29-year (1973-2001) period of record: ==> month-by-month IAV of SSF position anomalies from 1973 through 2001 3) Compute annual mean SSF position anomalies from 1973 through 2001 using month-by-month SSF position anomalies from each year: ==> year-by-year IAV of SSF position anomalies from 1973-2001

  13. Not enough data Onshore Offshore Longitude-time plot of IAV of SSF position anomalies,1973-2001

  14. Total PVE ~92% First 8 eigenvalues from a “standard” EOF decomposition of the “raw” (gappy) data. (Errors computed given N independent realizations)

  15. Offshore Onshore Longitude-time plot of “reconstructed” IAV of SSF position anomalies,1973-2001 using EOF modes 1 through 4

  16. Mode 1 Temporal Amplitude Mode 1 Spatial Amplitude  Mode 1 Spatial Phase Pattern Mode 1 Time Sequence of Phase Westward Phase Propagation CEOF analysis can reveal “propagating” features (Horel, 1984) Complex EOF (CEOF) method allows analysis of “reconstructed” dataset: Strong Interannual Variability C = ~ 2.5 cm s-1

  17. Middle-Atlantic Bight-wide shelf water volume anomalies from 1975-2000. (After Mountain, 2003) Interannual variability of SSF position anomalies from 1975-2000 averaged between 70 and 75 W longitude.

  18. GSNW SSF Offshore Onshore Longitude-time plots of IAV of annual mean GSNW and SSF position anomalies,1973-2001

  19. Summary & Conclusions • Reanalysis of satellite-derived surface SSF positions measured between 50 and 75W allows resolution of IAV of the surface SSF position between the TGB and CH. • Longitude-time analysis reveals alternating bands of offshore (late-1970s, late-1980s, late-1990s) and onshore (early-1980s, early-1990s, early-2000s) annual mean SSF anomaly values, exhibiting a period of about 10 years. • Similar to SSF seasonal variability, annual mean SSF anomaly amplitudes are largest east of 60 W and of O(100 km) for years when data are available, although this magnitude appears to be somewhat larger than seasonal variability within the region. 4) Complex EOF analysis of IAV of annual SSF position anomalies demonstrates a series of westward-traveling“waves” (~1-2 cm s-1) between the TGB and CH resulting in the pattern of alternating bands of shoreward and seaward SSF position anomalies. 5) Preliminary comparison between IAV of annual anomalies of surface SSF position and shelfwater volume for the Middle Atlantic Bight suggests a direct relationship. 6) Preliminary examination of IAV of annual GSNW surface position anomalies exhibits some similar banding structure, although data appear “noisy” relative to SSF data.

  20. Speculation We can speculate that the eastward-increasing IAV of surface SSF position (towards the TGB may result from strong IAV of input waters from the Labrador Sea, possibly from both inshore and offshore branches of the Labrador Current as suggested previously by Chapman and Beardsley (1989).

  21. Acknowledgements Special thanks to K. Drinkwater and R. Pettipas, BIO, for providing digitized frontal data. Thanks also to A. Chaudhuri, University of Massachusetts, Dartmouth, for assisting with the data processing, and to the many other colleagues who continue to provide interesting discussions and comments. This work is supported by the National Science Foundation US-GLOBEC NW Atlantic Georges Bank Program

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