SeaWiFS estimates of surface chlorophyll concentrations are reported for the region of the U.S. JGOFS study in the Southern Ocean (similar to 170 degrees W, 60 degrees S). Elevated chlorophyll was observed at the Southern Ocean fronts, near the edge of the seasonal ice sheet, and above the Pacific-Antarctic Ridge. The elevated chlorophyll levels associated with the Pacific-Antarctic Ridge are surprising since even the crest of the ridge is at depths \u3e 2000 m. This elevated phytoplankton biomass is likely the result of mesoscale physical-biological interactions where the Antarctic Circumpolar Current (ACC) encounters the ridge. Four cruises surveyed this region between October 1997 and March 1998, as part of the U.S. JGOFS. Satellite-derived chlorophyll concentrations were compared with in situ extracted chlorophyll measurements from these cruises. There was good agreement (r(2) of 0.72, from a linear regression of shipboard vs, satellite chlorophyll), although SeaWiFS underestimated chlorophyll concentrations relative to the ship data

Abbott, MR

al, et

Moore, JK

Richman, RG

Smith, Walker O., Jr.

English

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W&M ScholarWorks VIMS Articles Virginia Institute of Marine Science 5-15-1999 SeaWiFS satellite ocean color data from the Southern Ocean JK Moore MR Abbott RG Richman Walker O. Smith Jr. College of William and Mary et al Follow this and additional works at: https://scholarworks.wm.edu/vimsarticles  Part of the Oceanography Commons Recommended Citation Moore, JK; Abbott, MR; Richman, RG; Smith, Walker O. Jr.; and al, et, "SeaWiFS satellite ocean color data from the Southern Ocean" (1999). VIMS Articles. 1407. https://scholarworks.wm.edu/vimsarticles/1407 This Article is brought to you for free and open access by the Virginia Institute of Marine Science at W&M ScholarWorks. It has been accepted for inclusion in VIMS Articles by an authorized administrator of W&M ScholarWorks. For more information, please contact scholarworks@wm.edu. GEOPHYSICAL RESEARCH LETTERS, VOL. 26, NO. 1 O, PAGES 1465-1468, MAY 15, 1999 SeaWiFS satellite ocean color data from the Southern Ocean J. Keith Moore l, Mark R. Abbott l, James G. Richman l, Walker O. Smith 2,Timothy J. Cowles l, Kenneth H. Coale 3, Wilford D. Gardner 4,Richard T. Barber • Abstract. SeaWiFS estimates of surface chlorophyll concentrations are reported for the region of the U.S. JGOFS study in the Southern Ocean (~ 170 øW, 60 øS). Elevated chlorophyll was observed at the Southern Ocean fronts, near the edge of the seasonal ice sheet, and above the Pacific- Antarctic Ridge. The elevated chlorophyll evels associated with the Pacific-Antarctic Ridge are surprising since even the crest of the ridge is at depths > 2000 m. This elevated phytoplankton biomass is likely the result of mesoscale physical-biological interactions where the Antarctic Circumpolar Current (ACC) encounters the ridge. Four cruises surveyed this region between October 1997 and March 1998, as part of the U.S. JGOFS. Satellite-derived chlorophyll concentrations were compared with in situ extracted chlorophyll measurements from these cruises. There was good agreement (r 2 of 0.72, from a linear regression of shipboard vs. satellite chlorophyll), although SeaWiFS underestimated chlorophyll concentrations relative to the ship data. Introduction The u.s. JGOFS (Joint Global Ocean Flux Study) site along 170 øW is an ambitious effort to improve our understanding of the role ocean biota play in the global carbon cycle and climate system. Four cruises surveyed the region along 170 øW between October 1997 and March 1998. The Antarctic Polar Front (PF) was the primary focus of these cruises. Two other major Southern Ocean fronts are found in this region. The Subantarctic Front (SAF) is north of the PF, and the Southern Antarctic Circumpolar Current Front (SACCF) is south of the PF [Orsi et al., 1995]. Studies were carried out to evaluate carbon fluxes and their critical controlling processes. Here we present SeaWiFS (Sea- viewing Wide Field-of-view Sensor) surface chlorophyll data for the JGOFS region, which provides a larger spatial context for interpreting the shipboard observations. •College ofOceanic and Atmospheric Sciences, Oregon State University 2Virginia Institute ofMarine Sciences 3Moss Landing Marine Laboratories •Texas A& M University, Department of Oceanography 5Duke University, NSOE Marine Laboratory Copyright 1999 by the American Geophysical Union. Paper number 1999GL900242. 0094-8276/99/1999GL900242505.00 Results The mean SeaWiFS-derived surface chlorophyll concentrations (composite averages of the version 2, Level 3 daily files obtained from NASA) during the JGOFS cruises are shown in Figure 1 (areas in black had no chlorophyll data due to cloud or ice cover). Figure 1E shows the topography of the region [Smith and Sandwell, 1994], the mean path of the PF [Moore et al., 1999], and the mean paths of the SAF and SACCF [Orsi et al., 1995]. The Pacific-Antarctic Ridge dominates the topography of the region. Elevated chlorophyll levels are seen primarily in three areas: near the receding ice edge, at the major fronts, and above the Pacific-Antarctic Ridge. A seasonal progression of phytoplankton biomass is also apparent, with low chlorophyll concentrations during early austral spring (Figure 1A), a bloom period (Figure lB & 1C), and low values during fall (Figure 1D). Remarkable in Figure 1 are the elevated chlorophyll concentrations above the Pacific-Antarctic Ridge relative to surrounding waters. This pattern was extremely consistent, evident to some extent during each cruise. This topographic effect is surprising since the crest of the ridge is at depths of 2000 m or more (Figure 1E). These phytoplankton blooms may be caused by mesoscale physical-biological interactions where the ACC encounters the ridge. Two physical processes, meander-induced upwelling at the Southern Ocean fronts and increased eddy activity, are likely to increase nutrient input from below the surface layer (including micronutrients) at the ridge. Mesoscale meandering of the PF increases where it encounters large topographic features [Moore et al., 1999]. Surface drifters released in January 1998 indicate that mesoscale meandering is intensified near the ridge (Figure 2). Note that the tracks closest o the ridge crest have sharp turns and meanders with wavelengths of ~ 50-100 km, while the tracks farthest from the ridge crest exhibit broad, slow, meandering with wavelengths of ~ 300-500 km. Mesoscale meandering causes localized areas of upwelling•downwelling, which in turn can have strong effects on ocean biota [Flierl and Davis, 1993]. Large amounts of relative vorticity are input to the water column where ocean depth changes rapidly, such as along the Pacific-Antarctic Ridge, through the shrinking/stretching of vortex lines. This relative vorticity is likely dissipated through nonlinear processes uch as eddy actions. Elevated eddy mixing would increase nutrient flux to the surface layer, including micronutrients uch as iron, from sub-surface waters. This increased eddy mixing would occur both within the PF and the SACCF, and in open ocean waters away from the fronts. Phytoplankton blooms were observed along the Pacific-Antarctic Ridge with the Coastal Zone Color Scanner [Sullivan et al., 1993]. We have observed similar 1465 1466 MOORE ET AL.: SEAWIFS OCEAN COLOR IN THE SOUTHERN OCEAN 55øS 60 ø' 4.76 1.97 0.82 0.34 0.14 0.06 0.03 Chlorophyll (mg/m**3) le. 70•800 170 ø 160 ø 150øW A, Survey I (10/20-11/22) • ,• ß ,. 55øS 70•80 o 170 ø 160 ø 150øW B. Process I (12/3-1/3) 55oS i,I it 41000 t• 2OOO 3OOO 40O0 >5000 Depth (m) - SAF ' PF 150øW C. Survey II (1/8-2/7) E. Topography D. Process II (2/14-3/18) SACCF Figure 1. Mean surface chlorophyll concentration for each of the four JGOFS cruises are displayed (IA, lB, IC, and ID). Also shown is the topography of the region [Smith and Sandwell, 1994], the mean path of the Antarctic Polar Front (PF) [Moore et al., 1999], and the mean paths of the Subantarctic Front (SAF) and the Southern Antarctic Circumpolar Current Front (SACCF) [Orsi et al., 1995] (IE). increases in chlorophyll concentration in SeaWiFS imagery where the ACC encounters other large topographic features, such as Kerguelen Plateau, the North Scotia Ridge, and the Southeast Indian Ridge. During the Survey I cruise (10/20-11/22/97), the sea ice extended to- 63 øS (Figure IA). The mean SeaWiFS- derived chlorophyll concentration during this cruise over the region was 0.24 mg/m 3 (the region is defined as the area from 180-150 øW by 55-70 øS). Along 170 øW, the mean SeaWiFS chlorophyll concentration was 0.21 mg/m 3 (mean values for along 170 øW include the area 170.5-169.5 øW by 55-70 øS). Higher chlorophyll values are seen near the ice edge, in the vicinity of the PF, and above the Pacific-Antarctic Ridge from-155-150 øW (Figure IA & IE). The high chlorophyll concentrations (> 1.0 mg/m 3)just west of the JGOFS line from -180-173 øW did not appear until the last week of the cruise period. Mixed layer depths were generally quite deep (at times > 150 m). Towards the end of the cruise, mixed layers depths of 60-80 m were observed. By the time of the Process I cruise (12/3/97-1/3/98), the sea ice had retreated and a bloom was present at the PF across the whole region (Figure lB & 1E). Mean chlorophyll concentration for the region had increased to 0.48 mg/m 3,and along 170øW to 0.50 mg/m 3. Elevated chlorophyll values north of 55 øS were in the approximate location of the SAF (Figure lB & IE). A large phytoplankton bloom near the retreating ice edge was visible from -180-165 øW (Figure lB). This bloom was situated in the vicinity of the SACCF above the Pacific-Antarctic Ridge and may be the result of the combined influences of topography and sea ice retreat (Figure lB & IE). The retreating ice edge provides micronutrients such as iron (in dust accumulated over the winter) to the water column [Martin, 1990; Sedwick and DiTullio, 1997] and improves the irradiance-mixing regime for phytoplankton [Smith and Nelson, 1985]. This cruise was marked by shallow mixed layers (< 30m) with low salinity surface water. During the Survey II cruise (1/8-2/7/98), the sea ice had retreated below 70 øS (Figure I C). Regional mean chlorophyll concentration had declined to 0.32 mg/m -•, and along 170øW was 0.36 mg/m 3. Elevated chlorophyll in the vicinity of the PF and the SAF was seen east of the JGOFS study site between -165-150 øW (Figure 1C). A phytoplankton bloom was visible from - 170-167 øW between -63-65 øS. This bloom was sampled during the Survey II cruise. It was located directly above the Pacific-Antarctic Ridge (Figure I C & IE) and was not continuous with the blooms farther south, which are likely associated with the retreating ice edge. This bloom appears to be a continuation MOORE ET AL.' SEAWIFS OCEAN COLOR IN THE SOUTHERN OCEAN 1467 of the bloom initiated during the Process I cruise; it had dissipated by the first week of February. Mixed layer depths remained shallow during this cruise. During the Process II cruise (2114-3118198), regional mean chlorophyll concentration had declined to 0.19 mg/m 3, and along 170 øW to 0.18 mg/m 3 (Figure 1D). Elevated chlorophyll values were again seen above the Pacific- Antarctic Ridge (Figure 1D & 1E). This was a persistent pattern, seen in all weekly images over the cruise period. Mixed layer depths were generally deeper than during the preceding cruises, often exceeding 80 m depth north of the PF, with some values south of the PF < 50 m. Shipboard extracted chlorophyll measurements were compared with co-located satellite data collected within 24 hours of the ship sample. Both underway and station shipboard chlorophyll samples were used. For station data where samples from different depths were available, we estimated optical depth with a chlorophyll-based approximation f Kp• [Morel, 1988]. Shipboard samples from the upper two optical depths of the water column were averaged for comparison with satellite data. The ship samples were compared with the version 2 chlorophyll data from NASA. The Level 3 standard mapped images (9-km resolution) and Local Area Coverage (LAC, 1-km resolution) files were used. The LAC data was processed using the global chlorophyll algorithm and the SEADAS software. The comparison of satellite-derived chlorophyll concentrations with the shipboard chlorophyll measurements reveals generally good agreement (Figure 3). SeaWiFS tended to underestimate chlorophyll relative to shipboard measurements, as did the previous generation ocean color satellite in this region [Sullivan et al., 1993]. A Model II linear regression of shipboard vs. satellite chlorophyll gives the equation Chls.•t = 0.52(Chlship) - 0.03, with r2 = 0.72 where n = 84 (95% confidence interval for the slope was 0.47 to 0.64 and for the intercept -0.0066 to -0.066). The generally good agreement between satellite-derived and shipboard 55ø$ 60 ø 2000 3000 4000 65 ø >5000 ...... Depth(m) 70•80 ø 170 ø 160 ø 150øW Figure 2. The tracks followed by surface drifters released during January 1998 as part of the Survey II cruise are shown over the topography of the region [topography from Smith and Sandwell, 1994]. 1.8 -• 1.6 '• 1.4 1.2 0.8 0.6 0.4 0.2 [ i i i i i i i i i i i i i ! i [ i I 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Shipboard Chlorophyll (mg/m 3) Figure 3. A comparison of satellite-derived estimates of surface chlorophyll concentration with shipboard measurements of extracted chlorophyll is displayed. Satellite data was collected within 24 hours of the co-located ship data. measurements of chlorophyll indicates that SeaWiFS is accurately distinguishing between areas of high and low biomass and, thus, reliably captures the mesoscale features described in this paper. Conclusions There was good agreement between the SeaWiFS surface chlorophyll concentrations along 170 øW and the regional average during all four JGOFS cruises. Thus the JGOFS line along 170 øW was broadly representative of this sector of the Southern Ocean. Three areas of elevated chlorophyll concentrations were identified from the satellite imagery: at the receding ice edge, near the Southern Ocean fronts, and above the Pacific-Antarctic Ridge. Each of these high biomass areas was studied intensively at some point during the JGOFS cruises. Synthesis of the JGOFS results with satellite and modeling studies hould greatly improve our understanding of the role of marine biota in the carbon cycle of the Southern Ocean and in the larger global climate system. Acknowledgments. The authors would like to thank the SeaWiFS project (Code 970.2) and the Distributed Active Archive Center (Code 902) at the Goddard Space Flight Center, Greenbelt, MD 20771, for the production and distribution of these data, respectively. These activities are sponsored by NASA's Earth Science Enterprise Program. The authors would also like to thank the captain and crew of the Revelle and all who participated inthe gathering and analysis of the shipboard chlorophyll measurements. This work was supported by a NASA Earth Science Fellowship (J.K.M), NASA Earth Observing System Grant NAG5-4947 (M.R.A.) and NSF Office of Polar Programs Grant 953-0507-OPP (M.R.A.). This is JGOFS contribution number 505. 1468 MOORE ET AL.: SEAWIFS OCEAN COLOR IN THE SOUTHERN OCEAN References Flierl, G.R. & C.S. Davis. Biological effects of Gulf Stream meandering. J. Mar. Res. 51,529-560 (1993). Martin, J.H. Glacial-Interglacial CO2 change: The Iron Hypothesis. Paleoceanogr. 5, 1-13 (1990). Moore, J.K., Abbott, M.R., & J.G. Richman. Location and dynamics of the Antarctic Polar Front from satellite sea surface temperature data. J. Geophys. Res., 104, 3059-3073 (1999). Morel, A. Optical modeling of the upper ocean in relation to its biogenous matter content (Case I waters). J. Geophys Res.93, 10,749-10,768 (1988). Orsi, A.H., Whitworth III, T., & W.D. Nowlin Jr. On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep Sea Res., Part I, 42, 641-673 (1995). Sedwick, P.N. & G.R. DiTullio. Regulation of algal blooms in Antarctic Shelf waters by the release of iron from melting sea ice. Geophys. Res. Lett., 24, 2515-2518. Smith Jr., W.O., & D.M. Nelson. Phytoplankton bloom produced by a receding ice edge in the Ross Sea: Spatial coherence with the density field. Science 227, 163-166 (1985). Smith, W.H.F. & D.T. Sandwell. Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry. J. Geophys. Res. 99, 21803-21824 (1994). Sullivan, C.W., Arrigo, K.R., McClain, C.R., Comiso, J.C, & J. Firestone. Distributions of Phytoplankton Blooms in the Southern Ocean. Science 262, 1832-1837 (1993). J.K. Moore, M. Abbott, J. Richman, and T. Cowles, College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin. Building, Corvallis, OR 97331-5503 (email: jmoore@oce.orst.edu, mark@oce.orst.edu). W. Smith,Virginia institute of Marine Sciences, Greate Road, Box 1346, Gloucester Pt., VA 23062 K. Coale, Moss Landing Marine Laboratories, P.O. Box 450, Moss Landing, CA 95039-0450 W. Gardner, Texas A & M University, Department of Oceanography, College Station, TX 77843-3146 R. Barber, Duke University, NSOE Marine Laboratory, 135 Duke Marine Lab Road, Beufort, NC 28516 (Received November 11, 1998; revised February 26,1999; accepted March 22, 1999) 

SeaWiFS satellite ocean color data from the Southern Ocean

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SeaWiFS satellite ocean color data from the Southern Ocean

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