A Satellite Study of the Biological Significance of
Eddies in the Drake Passage and
A thesis submitted in partial satisfaction of the
requirements for the degree Master of Science
in Oceanography
by
Xi Chen
Committee in charge:
B. Gregory Mitchell, Chair
Robert Pinkel
David Sandwell
Dariusz Stramski
1999
Copyright
Xi Chen, 1999
All rights reserved
The thesis of Xi Chen is approved:
Chair
1999
Signature Page.......................................................................................................... iii
A. Background
B. SeaWiFS
chl-a distributions
C. Oceanographic
characteristics of the region
D. Iron
distributions and sources
F. Interaction
between bathymetry and ocean dynamics.
B. Drifter
data
F Geographic latitude
f Coriolis parameter =2W SinF =planetary vorticity
H Water depth
f/H Planetary potential vorticity (PPV)
Figure 2. Southern Ocean krill distribution based on
BIOMASS Program results............... 32
Figure 3A. Bathymetry of
Figure 3B. January 1998 and 1999 composite SeaWiFS chl-a for region in 3A................... 33
Figure 3C. Ocean slope variability determined from satellite altimetry for region in 3A 33
Figure 4. Chl-a
in Drake Passage –
Figure 5A. Bathymetry
for Drake Passage –
Figure 5B. 2-year composite SeaWiFS chl-a for same region as 5A.................................... 34
Figure 6A. Sea surface temperature GAC image for April 1990 with drifter track 35
Figure 6B. Sea surface temperature GAC image for March 1996 with drifter track.............. 35
Figure 6C. Sea surface temperature GAC image for July 1996 with drifter track 35
Figure 7A. Satellite sea surface temperature time-series for 3 locations along 58°S 36
Figure 7B. Satellite sea surface
temperature for October 1998 for same region as 5A.......... 36
Figure 7C. SeaWiFS surface chl-a for October 1998 for same region as 5A 36
Figure 8A. Bathymetry near 58°S and 54°W 37
Figure 8B. January 1992 sea surface temperature near 58° S and 54°W............................... 37
Figure 9. Conceptual diagrams of ocean current flow over bathymetric features................. 37
I want to thank Greg Mitchell and JL Swan for their assistance in preparing my thesis. I thank Sarah Gille and John Moisan for their helpful discussions, Peter Niiler for surface drifter data and my thesis committee for their helpful comments and encouragement. I want to thank all the people in the Scripps Photobiology Group for their years of support: Scott Cheng, Elizabeth Frame, Mati Kahru, Austin Lee, Hyojik Lee, Todd Loeber, Tiffany Moisan, Jessica Nolan, Rick Reynolds, JL Swan and John Wieland.
NASA JCOSS Fellowship Grant #NGT-542 and NASA Ocean Biology grant # NAG5-6559, which helped support this work.
1992 B.E.,
1992 – 1995 Graduate
1995 M.S.,
1995 - 1997 Assistant
Researcher, Division of Satellite Remote Sensing,
1997-1999 Graduate Student Researcher,
1999 M.S.,
PUBLICATIONS
Chen, X., Automatic Tracking and Receiving Microcomputer
System for Polar-orbiting Satellites, Marine Forecasts, No. 3 1996, Vol. 13
Chen, X. and B.G. Mitchell. Design Criteria and Preliminary
Applications Algorithms for an Autonomous Profiling Physical-Optical System
(K_SOLO), accepted by AGU 2000 Ocean Science Meeting at
FIELDS OF STUDY
Major Field: Engineering and Ocean Science
Studies in Applied Ocean Science.
Dr. B. Gregory Mitchell and Professor Dariusz Stramski
Scripps
Institution of Oceanography,
Studies in Satellite Remote Sensing.
Studies in Electrical Engineering.
Professor Changyue Ouyang
Abstract of the Thesis
A Satellite Study of the Biological Significance of
Eddies in the Drake Passage and
by
Xi Chen
Master of Science in Oceanography
B. Gregory Mitchell, Chair
Phytoplankton biomass in pelagic waters of the Southern Ocean is generally
low although there are high concentrations of major inorganic nutrients and
favorable environmental conditions for phytoplankton growth. Synoptic maps of chlorophyll-a (chl-a) concentrations for the entire Southern Ocean collected by the
SeaWiFS ocean color satellite reveal great spatial and temporal variability in
chl-a concentrations during the
austral spring and summer in pelagic Antarctic waters. While most of the pelagic waters south of the
Polar Front (PF) have very low chl-a values, the
The Southern Ocean, defined as all marine waters south of the Subtropical Convergence (STC), is the largest oceanic area with high inorganic macro‑nutrient concentrations and relatively low chl-a (HNLC; see also references in Chisholm and Morel 1991). The paradox of why phytoplankton biomass is generally so low in Antarctic waters during summer has baffled researchers for decades (El‑Sayed 1987). Many researchers have attempted to explain the low phytoplankton biomass as a function of temperature, solar radiation, microelement concentrations, grazing pressure, or effects of deep mixing of the upper water column.
Since the first demonstration by Martin et al. (1990), other researchers have confirmed that iron (Fe) concentrations are low in Antarctic pelagic waters and considerably higher in Antarctic coastal waters. Fe enrichment experiments of natural populations which result in increased chl-a concentrations compared to controls (e.g, de Baar et al. 1990; Buma et al. 1991; Helbling et al. 1991; Martin et al. 1990) support the hypothesis that Fe limits biomass in Antarctic pelagic waters when there is sufficient light; in winter, insufficient light leads to low biomass. These experiments have demonstrated that with sufficient Fe and light, phytoplankton biomass can increase considerably above ambient concentrations, and macronutrients can be completely consumed within relatively short periods of time (days-weeks). Surface light
is abundant for months, so the
inability of phytoplankton to consume the available macronutrients during the
summer is generally considered to be caused by limitation of Fe. Still, there is large variability in surface
chl-a within pelagic waters of the Southern Ocean. If Fe is the main nutrient limiting biomass
accumulation, then its distribution and rates of supply to the euphotic zone
must also be variable within this important region that comprises approximately
20% of the global oceans and has a key role in global biogeochemical
processes. This research focuses on
physical mechanisms that may contribute to variable rates of Fe supply in the
Drake Passage region and the persistence of the large gradient in chl-a which is low to the west and high to the east of
SeaWiFS chl-a distributions
Ship surveys consistently have indicated that pelagic Antarctic waters have low chl-a values, and that waters in the PFZ and in the Sub-Antarctic zone show higher chl-a values (e.g., Hays et al. 1984; Froneman et al. 1995). These data are consistent with the satellite-derived maps for chl-a in the Southern Ocean (Figure 1A), which shows elevated chl-a concentrations to the north of the PF. While chl-a values for the Southern Ocean are generally low, there is much variability during January in surface chl-a concentrations in Antarctic pelagic waters south of the PF (Figure 1A). Since January is the period of maximal solar radiation and ocean thermal stratification, this is the season when one might expect maximum phytoplankton biomass (if the system were light limited). Conspicuous differences evident in Figure 1 are: (i) Persistent regions of blue water with surface chl-a concentrations less than 0.2 mg m-3 during mid‑summer for vast regions of the Antarctic Circumpolar Current (ACC) south of the Antarctic Polar Front (PF). The largest contiguous blue water zone (BWZ) extends from east of the Ross Sea to north of the South Shetland Islands and into the Drake Passage.
The seasonal changes in SeaWiFS-derived monthly mean surface chl-a concentrations in five regions at 60°S indicated in Figure 1A are illustrated in Figure 1B. All regions were south of the PF except region #5, which was in the Polar Front Zone but on the north side of the PF. Data for region #1 (south of the PF and located in the BWZ) and region #3 show that certain regions of the ACC simply do not develop significant blooms at any time of the year. Values of chl-a in October in the various BWZ regions are equivalent to other regions, but rather than increasing to a mid‑summer maximum, their chl-a decreases after November and remains low through the summer and autumn. By contrast chl-a peaks in December for regions #2, #4 and #5. In January, BWZ waters have surface chlorophyll values of approximately 0.1 mg m-3. The BWZ regions are evident in the 6-year mean CZCS data (Sullivan et al. 1993). OCTS had similar temporal coverage as SeaWiFS and the BWZ regions are evident in that data during the 1996-1997 season (data not shown). The data in Figure 1B and OCTS data (not shown) indicate that the BWZ regions persist through the austral spring‑summer season, and recur each year in the same regions. While these satellite maps only provide information on chl-a concentrations of the upper mixed layer (UML), the decline in surface pigments in BWZs from early spring through summer, rather than a mid-summer peak observed in other regions, implies that surface biomass for BWZs is more severely limited by some essential nutrient(s), since light, temperature and upper ocean stratification increase from spring to summer. By January the difference between surface chl-a in region #1 and #2 represents the extremes observed in the pelagic waters of the Southern Ocean south of the PF. The high phytoplankton biomass in region #2 - the western Scotia Sea - must be supported by elevated fluxes of Fe which is now accepted to be the limiting nutrient for the high nutrient, low chlorophyll (HNLC) regions of the Southern Ocean.
Because the mean flow is from west to east in the Drake Passage, there must be mechanisms that modify the extremely blue water that enters the central Drake Passage, and transforms it into high chl-a water a very short distance to the east. While there is general agreement that Fe limits biomass of phytoplankton in the Southern Ocean, there is relatively little information about why there is so much variability in the distribution of chl-a in the region. This study focuses on physical mechanisms that may contribute to the regional variability in chl-a in the pelagic zones of Drake Passage (areas #1, #2 and #5 in Figures 1A & 1B). This area contains regions with the minimum and maximum annual cycle of chl-a observed for open waters of the ACC south of the STC. As shown in Figure 2, the eastern part of this study region, in particular the zone northeast of Elephant Island, is among the regions with the highest krill biomass in the Southern Ocean. The transformation of BWZ water with extraordinarily low chl-a in the west of the study region into the high chl-a water to the east which can sustain high krill biomass has not been explained. This study focuses on physical mechanisms that may contribute to this extreme gradient in biological response.
Oceanographic characteristics of the region
The Antarctic Circumpolar Current (ACC) is associated with 3 fronts, the Sub Antarctic Front (SAF) on the north, Polar Front (PF) in the middle and Southern ACC Front (SACCF) in the south. There are large surface temperature gradients across these fronts, with warm water in the north and cold water in the south.