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

(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])

POC in the SAB - Clark Algorithm vs. Observed Example for May 14, 2000 POC Algorithm (g C/L) Apr Jul Sep Dec 1998 745 329 117 195 1999 322 249 153 181 2000 380 153 138 113 2001 395 260 180 135 2002 189 273 160 204 2003 974 302 170 187 Mean 501 261 153 169 99-02 321 233 157 159 Observed POC (g C/L) 1973 157 167 1974 327 236

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

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

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

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  

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)

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

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)

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

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

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

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 0.71 Mid Shelf 0.53 Outer Shelf 0.36 Shelf Slope 0.28 Mean NASG Chl vs. SAB Chl Inner Shelf 0.12 Mid Shelf 0.27 Outer Shelf 0.42 Shelf Slope 0.52 Correlation Between Forcing and Response

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.

Backup Slides

Seasonal Chlorophyll and SST Variability within 8 Georgia Bight Regions

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