1. SAB Chlorophyll Variability Local vs. Remote Forcing
Chuck McClain
Sergio Signorini
U.S. ECoS Team Meeting
NOAA/NMFS, Narragansett, RI
March 30-31, 2006

2. (Summarized from Menzel et al. [1993])
Inner Shelf (0-20m)
Multi-inlet coastline connecting low-lying coastal marshes to the ocean.
Circulation influenced by river discharge, tides, and winds.
Tidal transport and mixing provides two-way exchange of materials between estuaries and the ocean.
Chl-a concentrations range from 1 to 25 mg m-3 and reflect a decreasing abundance of phytoplankton in the seaward direction.
Production is driven primarily by the rates at which nutrients are recycled and/or re-suspended from the sediments.
Mid and Outer Shelves (20-40m and 40m-60m)
Circulation highly variable due to combined influence of tides, winds, and Gulf Stream intrusions. Subsurface intrusions of NACW occur mostly from May to August.
More event-scale variability in plankton abundance and production than in the inner shelf. Plankton densities may change 10-fold or more within days.
Flow convergence near the Charleston Bump is the preferred means of offshore export of carbon.
SAB Ocean Processes
(Summarized from Menzel et al. [1993])

3. POC in the SAB - Clark Algorithm vs. Observed
Example for May 14, 2000
POC Algorithm (g C/L)
Apr Jul Sep Dec
Mean
Observed POC (g C/L)

4. SAB Chl-a & POC Time Series: 1997-2004
Jan-Feb 98 Event
Intense El-Niño Period
0-20m
Sep-Oct 99 Event
Hurricanes Dennis,
Floyd, and Irene
Mar-Apr 03 Event
End of 02-03
Moderate El-Niño
20-40m
POC/Chl-a
POC
Chlorophyll-a
40-60m
Note POC/Chl-a
trend to higher
values offshore
60-500m

5. Seasonal Variability of Optical Depth (Z90) from SeaWiFS 9km Mapped Images
Z90=1/K490=Depth at which 90% of downwelling irradiance is attenuated

6. SeaWiFS-derived Seasonal Cycle of Chl-a (OC4v4), POC (Clark), and POC/Chl-a Ratio
Offshore: deeper Chl-a and POC maxima
Offshore: less river discharge influence
Observed at BATS

7. SeaWiFS-derived GSM01 Chl and adg(443) Monthly Composites
for February 98 (high river discharge) and August 98 (low river discharge)
Feb
Aug
SAB regional Means of Chl-a (mg/m3) and adg (m-1) for
February and August 98 and their Feb/Aug ratios

8. And Chlorophyll from GABEX-II (July-August 1981)
Georgia Bight Summer Gulf Steam Intrusions – St. Augustine Transects of Temperature
And Chlorophyll from GABEX-II (July-August 1981)

9. Seasonal Composites of Aqua SST
Seasonal Variability of Winds, SST, Nitrate, MLD, SSHA, and Chlorophyll
White triangles
SST, NO3, MLD
White Circles
SSHA, OC4v4 Chl-a
Off-Shelf Seasonal Climatology 1 2
GS transport: lower in spring (1), higher in fall (2)
Seasonal Composites of Aqua SST
and QuikScat Winds

10. Time Series of SSHA (inverted) and Chl-a at the 3 Shelf Break Sites
Time Series of Sea Level (TP SSHA) and Chl-a (SeaWiFS) in the SAB Shelf Break
Time Series of SSHA (inverted) and Chl-a
at the 3 Shelf Break Sites
SeaWiFS Chl-a Climatology and Locations of
Chl-a and SSHA Time Series (red crosses)

11. Seasonal and Interannual Variability of North Atlantic Subtropical Gyre (NASG) Size and Strength

12. Seasonal and Interannual Variability of North Atlantic Subtropical Gyre (NASG) Size and Strength

13. Seasonal and Interannual Variability of North Atlantic Subtropical Gyre (NASG) Size and Strength
Blue line=September Red line=March

14. Correlation Between Forcing and Response
Local (River Discharge) vs. Large-Scale (NASG) Forcing of SAB Chl-a Variability
River Flow vs. SAB Chl r2
Inner Shelf
Mid Shelf
Outer Shelf
Shelf Slope
Mean NASG Chl vs. SAB Chl
Inner Shelf
Mid Shelf
Outer Shelf
Shelf Slope
Correlation Between Forcing and Response

15. Summary and Conclusions
Local and remote forcing effects on Chl variability are hard to separate, but river runoff, mixed layer depth seasonal changes, and displacement of GS axis are major forcing factors
River discharge variability is the major source of interannual Chl and CDOM variability on the inner and middle SAB shelves
POC/Chl-a ratio increases offshore towards more oligotrophic waters and seasonal cycle reverses
Seasonal changes of NASG size induces GS axis lateral translation along the outer shelf and slope with an impact on the local surface Chl-a and POC variability
SSH and Chl-a are highly correlated along the shelf break and slope as a result of GS transport variability
GS transports max amount of water in the fall and a minimum in the spring, in phase with the north-south shifts of its position (Kelly and Gille 1990; Zlotnicki 1991; Kelly 1991; Hogg and Johns 1995). This is in agreement with our findings based on T/P SSHA data.

16. Backup Slides

17. Seasonal Chlorophyll and SST Variability within 8 Georgia Bight Regions

20. ROMS
ROMS