REMOTE SENSING AND MODELING OF PHYSICAL-BIOLOGICAL COUPLING IN THE EASTERN TROPICAL PACIFIC
J. P. Ryan, P. G. Strutton, and F. P. Chavez, MBARI     Y. Chao, NASA/JPL     F. Chai, U. Maine    P. S. Polito, INPE, Brazil

  

The eastern tropical Pacific is globally important in living resources and biogeochemistry. Intense primary productivity in the coastal waters off Peru translates into the largest single-species fishery. Denitrification in subsurface waters of the Peruvian upwelling system impacts nutrient content of upwelled waters, and due to offshore transport may influence nutrient distributions over a large region of the eastern tropical Pacific. The productivity rate is lower in the equatorial upwelling region due to iron limitation. This inhibits photosynthetic uptake of carbon dioxide over the vast equatorial upwelling tongue, making this region the largest natural source of CO2 to the atmosphere.

Although our understanding of the equatorial Pacific and coastal Peru upwelling systems individually is conceptually advanced, little effort has been focussed on quantifying links between them. Biogeochemical processes in these regions, while notably different, are influenced by similar interannual and seasonal forcing. Phenomena such as Rossby and Kelvin waves generated in one region can propagate into the other. Transports between the ecosystems exchange physical, chemical and biological properties. We are exploring processes and linkages through satellite remote sensing, in situ ship and mooring observations, and coupled biological-physical modeling.



A CONCEPTUAL MODEL


Thermocline (nutricline) depth strongly influences recruitment of nutrients into the euphotic zone. Its depth is affected by wind-driven transport / upwelling, wave forcing, heating, and mixing.

The El Niño / Southern Oscillation (ENSO) dominates interannual variability. ENSO-related variation in thermocline depth, winds, and ocean circulation strongly impact the ecosystem.

Iron is a critical limiting nutrient (Martin, 1990; Coale et al., 1996). The Equatorial Undercurrent (EUC) supplies iron to the central and eastern equatorial Pacific. ENSO affects EUC transport.

A MULTI-SENSOR PROCESS STUDY

Between March and December of 1998, large blooms of phytoplankton occurred across 10,000 km of the equatorial Pacific (Chavez et al., 1999; Murtugudde et al., 1999). Using a multi-sensor approach, we have further studied physical-biological coupling of these blooms:

The time series at the right shows mean chlorophyll concentrations between 2°N and 2°S. Blooms are defined as more than twice the mean between 160°E and 100°W during 9/1997 through 10/2000.

Physical conditions and processes that influence euphotic zone nutrient supply and hence bloom formation include thermocline (nutricline) depth, upwelling (wind-driven and along-isopycnal) and turbulent vertical mixing. Thermocline depth is closely related to sea surface height (SSH) in the equatorial Pacific. It is less closely related to the depth of the EUC (source waters for iron).

TOPEX/POSEIDON provides regular global coverage of SSH for studying thermocline depth variation in relation to satellite ocean color derived chlorophyll. Using finite impulse response filtering, we decomposed the total SSH signal into componenents ranging across a wide range of scales, from basin-scale/interannual to mesoscale/intraseasonal.

Bloom boundaries are contoured in white. The components of variation accounting for most (87%) of the SSH variation were clearly related to the blooms.

Bloom 2 began simultaneously across ~2500 km, then propagated eastward ~4500 km. Satellite and in situ observations show that bloom propagation was not related to eastward propagation of enhanced nutrient supply to the euphotic zone by local vertical processes. Rather, bloom advection in the EUC is indicated. Bloom propagation of 66 km day-1 east of 140°W was calculated from the SeaWiFS bloom contour. A zonal current speed of 63±16 km day-1 during this period results from the average of daily ADCP currents at 110°W, 0°N between 40 and 80 m. Additionally, the mean rate of increase in chlorophyll concentration was more than 2 times greater in the region of bloom propagation east of 140°W (0.32 mg m-3 day-1) than in the region of genesis west of 140°W (0.15 mg m-3
day-1). This is consistent with supply of a seed population via eastward advection.

In addition to its great scale, bloom 2 developed an extraordinary wavelike distribution. Meridional advection by the currents of tropical instability waves (TIWs) strongly influenced the spatial distribution of this largest-scale bloom. The figure below shows biological and physical variation averaged between 1°N and 2°N during bloom 2. This physical-biological structure extended to at least 4° poleward of the equator.

Oceanic and atmospheric anomalies of TIWs comprise an active area of research that will impact understanding of this ecosystem as a whole (Strutton et al., 2001; Polito et al., 2001).

PROCESSES: WEST  VERSUS EAST

In the west (bloom 1; 165°E), where the thermocline was deeper, the bloom developed more gradually and weakly. The bloom paralleled EUC shoaling and intensification of the trade winds. Turbulent vertical mixing and wind-driven upwelling were important local processes. In contrast, in the east (bloom 2; 140°W), blooming developed more rapidly. The nutricline and EUC shoaled directly into the euphotic zone, and the rapid bloom rise and fall mirrored along-isopycnal upwelling (blue line).

LINKAGES

EOF decomposition of SeaWiFS chlorophyll shows coherent variation in the Peruvian upwelling system and different regions of the equatorial Pacific.

Coupled Model Study

We are employing coupled physical-biological modeling to study processes and linkages in the eastern tropical Pacific. The model results below show a snapshot of surface total phytoplankton biomass.

Acknowledgements
 Support for this work is from the host institutions of the authors and their ongoing research grants. We thank NASA Goddard, NASA JPL, and NASDA for provision of satellite data, and the TAO Project Office, Dr. Michael J. McPhaden, Director, for provision of TAO observations. This work now continues under NASA grant NAG5-10639.

References
  • Chavez, F. P., Strutton, P. G., Friederich, G. E., Feely, R. A., Feldman, G. C., Foley, D. G., & McPhaden, M. J. (1999). Biological and chemical response of the equatorial Pacific Ocean to the 1997-98 El Niņo. Science, 286, 2126-2131.
  • Coale, K. S., Fitzwater, S. E., Gordon, R. M., Johnson, K. S., & Barber, R. T. (1996). Control of community growth and export production by upwelled iron in the equatorial Pacific. Nature, 379, 621-624.
  • Martin, J. H. (1990). Glacial-interglacial CO2change: The iron hypothesis, Paleoceanography, 5, 1-13.
  • Murtugudde, R. G., Signorini, S. R., Christian, J. R., Busalacchi, A. J., McClain C. R., & Picaut, J. (1999). Ocean color variability of the tropical Indo-Pacific basin observed by SeaWiFS during 1997-1998. Journal of Geophysical Research, 104, 18351-18366.
  • Strutton, P. G., Ryan, J. P., & Chavez, F. P. (2001). Enhanced chlorophyll at tropical instability wave fronts in the equatorial Pacific. Geophysical Research Letters, 28, 2005-2008.
  • Polito, P. S., Ryan, J. P., Liu, W. T., & Chavez, F. P. (2001). Oceanic and atmospheric anomalies of tropical instability waves. Geophysical Research Letters, in press.