Bisagni Report

### US-GLOBEC: Satellite-Derived Sea Surface Temperature and Ocean Color Variability on Southern Georges Bank

    INVESTIGATORS:

    Dr. J. J. Bisagni
    NOAA/NMFS
    Northeast Fisheries Science Center
    28 Tarzwell Drive
    Narragansett, RI 02882
    (401) 782-3313
    (401) 782-3201 (Fax)
    bisagni@fish1.gso.uri.edu

    Dr. J. A. Yoder
    University of Rhode Island
    Graduate School of Oceanography
    South Ferry Road
    Narragansett, RI 02882
    (401) 782-6864
    (401) 792-6728 (Fax)
    yoder@biosat.gso.uri.edu

    J. E. O'Reilly
    NOAA/NMFS
    Northeast Fisheries Science Center
    28 Tarzwell Drive
    Narragansett, RI 02882
    (401) 782-3267
    (401) 782-3201 (Fax)
    oreilly@fish1.gso.uri.edu

    GRANT PERIOD:

    October 1, 1994 - September 30, 1995

### ABSTRACT:

Our US-GLOBEC Northwest Atlantic/Georges Bank project is testing the hypothesis that non-seasonal variation of sea surface temperature (SST) and phytoplankton abundance on Georges Bank, over periods of several days-few weeks, may be caused by changes in upwelling of colder, nutrient-rich water across the seasonal thermocline and tidally-mixed front as a result of variations in tidal stirring and wind mixing. We are testing this hypothesis through time series analyses of historical, multi-year satellite- derived SST and ocean color, tidal current speed, and wind stress measured on southern Georges Bank during the late-1970's to mid- 1980's. Analyses of satellite-derived SST data from late winter- spring 1992 and 1993 have been completed to determine interannual SST variability on southern Georges Bank related to inflow of cold, low salinity water from the southwestern Scotian Shelf. Processing of multi-year historical SST and ocean color data is being completed using the latest available coefficients, to correct for the effects of the Earth's atmosphere. Analyses of satellite-derived SST data from late winter 1992 and 1993 show large interannual differences in the amount of cold Scotian Shelf water in the Georges Bank region possibly due to the combined effects of interannual variability in upstream river discharge from the St. Lawrence River and local wind stress. The relevance of our work is rooted in the sensitivity of Georges Bank to climate change through climate-induced variations in the flux of new nitrogen into the euphotic zone caused by changes in density stratification. For example, a decrease in the magnitude of the North Atlantic wind field due to climate change, as suggested by Manabe and Wetherald (1980), might result in decreased mixing, increased stratification and decreased nitrogen availability to Georges Bank waters from April-October, the period of sustained, elevated phytoplankton production and greatest abundance of zooplankton and ichthyoplankton. Furthermore, increased seasonal buoyancy input to Georges Bank surface waters, caused by higher air temperatures as a result of global warming (Manabe and Stouffer, 1980), together with the non-linear dependence of density upon temperature in the equation of state for sea water, might magnify this effect. Through a better understanding of the processes controlling the flux of new nitrogen onto Georges Bank, the effects of global climate change on the production of phytoplankton, zooplankton and fish may be better understood.

### STATEMENT OF OBJECTIVES:

Our objectives are:

- To test the hypothesis that non-seasonal variation of sea surface temperature (SST) and phytoplankton abundance on Georges Bank, for periods of several days to a few weeks, may be caused by changes in upwelling of colder, nutrient-rich sub-thermocline water across the seasonal thermocline and tidally-mixed front as a result of variations in tidal stirring and wind mixing (Bisagni and Sano, 1993).

- To determine interannual SST variability on southern Georges Bank between late winter-spring 1992 and 1993 related to differences in the amount and/or properties of cold, low salinity water flowing from the southwestern Scotian Shelf onto Georges Bank.

- To make near real time satellite-derived SST images available to US-GLOBEC research vessels and principal investigators.

### STATEMENT OF WORK:

Our methodology includes:

- Assembly and validation of multi-year SST and ocean color time-series from southern Georges Bank, derived from historical Advanced Very High Resolution Radiometer (AVHRR) and Coastal Zone Color Scanner (CZCS) data collected during the late-1970's to mid- 1980's from NOAA's polar orbiting satellites and the NIMBUS-7 satellite respectively.

- Comparison of multi-year, historical satellite-derived SST and ocean color time-series from southern Georges Bank with tidal stirring and wind mixing time series using statistical techniques and existing analytical models.

- Mapping satellite-derived SST data from southern Georges Bank during late winter-spring 1992 and 1993 using optimal interpolation procedures.

- Comparison of SST variability between late winter-spring 1992 and 1993 with interannual variability of upstream discharge from the St. Lawrence River and local wind stress.

AVHRR and CZCS satellite data have been provided by the University of Rhode Island Graduate School of Oceanography's Oceanographic Remote Sensing Laboratory through the joint NOAA/University of Rhode Island Cooperative Marine Education and Research Program. Satellite data analysis has been completed using software developed at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. Wind stress and tidal currents are provided by the US Navy's Fleet Numerical Oceanography Center model winds and a harmonic tide model, respectively, for grid points located on Georges Bank. River discharge data are provided by Environment Canada.

### SUMMARY OF KEY FINDINGS:

- We have determined optimum coefficients for atmospheric correction of CZCS data for the Georges Bank-Gulf of Maine region using chlorophyll pigment data collected from ships, resulting in more accurate satellite-derived pigment values (Bisagni et al., 1994). A manuscript describing these results is being prepared for submission to Continental Shelf Research.

- We have processed all valid full-resolution CZCS data within our project's study domain (Fig. 1) for final assembly of multi-year CZCS ocean color time series for comparison with tidal stirring and wind mixing time series.

  Figure 1. Study domain for AVHRR and CZCS satellite data used in this project. Also shown is the 200-m isobath (dashed).

- We have processed all valid full-resolution AVHRR data within our project's study domain (Fig. 1) for 1987-1988 using the NOAA/NASA Pathfinder atmospheric correction coefficients and automated cloud-screening for final assembly of multi-year SST time series for comparison with tidal stirring and wind mixing time series. Processing of AVHRR data from earlier years is currently underway.

- We have produced equally-spaced cloud-free SST maps from AVHRR data collected during late winter-spring 1992 and 1993 using optimal interpolation (OI) to determine interannual SST variability on southern Georges Bank (Ruhsam et al., 1994). Analyses of OI SST maps and data collected from ships during late winter-spring 1992 and 1993 show significant differences in SST (Figs. 2a, 2b, 3a, 3b, 4) and near-surface salinity (Fig. 5) between years related to the amount and/or properties of water flowing onto Georges Bank from the southwestern Scotian Shelf. This interannual variability may be related to differences in St. Lawrence River discharge between the spring seasons preceding 1992 and 1993 (spring 1991 and 1992) in agreement with earlier work from the Scotian Shelf (Smith, 1989).A manuscript describing these results has been submitted to Deep-Sea Research.

  Figure 2a. Optimally interpolated SST for year day 60 1992 (March 1, 1992). Also shown is the 200-m isobath (black line).

  Figure 2b. Optimally interpolated SST for year day 60 1993 (March 2, 1993). Also shown is the 200-m isobath (black line).

  Figure 3a. Optimally interpolated SST for year day 120 1992 (April 30, 1992). Also shown is the 200-m isobath (black line).

  Figure 3b. Optimally interpolated SST for year day 120 1993 (May 1, 1993). Also shown is the 200-m isobath (black line).

  Figure 4. Volume rendering of SST less than or equal to 2.5 deg. C (red) from all optimally interpolated maps produced for 1992 (upper panel) and 1993 (lower panel) using an oblique view of the SST region shown in Figs. 2 and 3. Also shown is the 200-m isobath (black line). This figure shows a large decrease in the amount of SST values less than or equal to 2.5 deg. C during 1993 relative to 1992.

  Figure 5. Near-surface temperatures and salinities from all hydrographic stations conducted on southern Georges Bank during cruise AL9205 (upper panel) in late May 1992 and AL9306 (lower panel) in late May 1993 from the NOAA ship Albatross IV. Station locations were nearly identical but separated by 1 year, thus showing that near surface waters were significantly warmer with higher salinities during 1993 relative to 1992.

  ### DATA ACCESS:

  Contemporary AVHRR SST images (October 1, 1993 - present) for the study domain (Fig. 1) will be made available on the World Wide Web in .GIF file format. Retrospective AVHRR SST and CZCS pigment images will be stored on 4-mm DAT tape due to limited disk space.

  ### REFERENCES:

  Bisagni, J. J. and M. H. Sano, 1993. Satellite observations of sea surface temperature variability on southern Georges Bank. Cont. Shelf Res., v. 13, p. 1045-1064.

  Bisagni, J. J., C. M. Wolfteich, J. E. O'Reilly, J. A. Yoder, A. H. Barnard and T. S. Moore, 1994. Determination of optimum aerosol optical thickness ratios for atmospheric correction of Coastal Zone Color Scanner data. Eos, v. 75, p.375.

  Manabe, S. and R. J. Stouffer, 1980. Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere. J. Geophys. Res., v. 85, p. 5529-5554.

  Manabe, S. and R. T. Wetherald, 1980. On the distribution of climate change resulting from an increase in CO2 content of the atmosphere. J. Atmos. Sci., v. 37, p. 99-118.

  Ruhsam, C. M., J. J. Bisagni, J. P. Manning, W. J. Williams and R. C. Beardsley, 1994. Interannual sea surface temperature variability on the southern flank of Georges Bank: Spring 1992 and 1993. Eos, v. 75, p. 67.

  Smith P. C., 1989. Seasonal and interannual variability of current, temperature and salinity off southwest Nova Scotia. Can. J. Fish. Aquat. Sci., v. 46, p. 4-20.