Peter G. Strutton

Ocean circulation off east Antarctica affects ecosystem structure and sea-ice extent

Sea ice and oceanic boundaries have a dominant effect in structuring Antarctic marine ecosystems. Satellite imagery and historical data have identified the southern boundary of the Antarctic Circumpolar Current as a site of enhanced biological productivity. Meso-scale surveys off the Antarctic peninsula have related the abundances of Antarctic krill (Euphausia superba) and salps (Salpa thompsoni) to inter-annual variations in sea-ice extent. Here we have examined the ecosystem structure and oceanography spanning 3,500u2009km of the east Antarctic coastline, linking the scales of local surveys and global observations. Between 80° and 150°u2009E there is a threefold variation in the extent of annual sea-ice cover, enabling us to examine the regional effects of sea ice and ocean circulation on biological productivity. Phytoplankton, primary productivity, Antarctic krill, whales and seabirds were concentrated where winter sea-ice extent is maximal, whereas salps were located where the sea-ice extent is minimal. We found enhanced biological activity south of the southern boundary of the Antarctic Circumpolar Current rather than in association with it. We propose that along this coastline ocean circulation determines both the sea-ice conditions and the level of biological productivity at all trophic levels.

Biological-physical coupling in the Central Equatorial Pacific during the onset of the 1997–98 El Niño

Perturbations to phytoplankton biomass associated with the onset of the 1997–98 El Nino event are described and explained using physical and bio-optical data from moorings in the central equatorial Pacific. The physical progression of El Nino onset is depicted, from reversal of the trade winds in the western equatorial Pacific, through eastward propagation of equatorially trapped Kelvin waves and advection of waters from the nutrient-poor western equatorial warm pool. Fluctuations in chlorophyll and quantum yield of fluorescence are tightly coupled to thermocline variations.

Unusual large-scale phytoplankton blooms in the equatorial Pacific

Unusual large-scale accumulations of phytoplankton occurred across 10,000 km of the equatorial Pacific during the 1998 transition from El Nino to La Nina. The forcing and dynamics of these phytoplankton blooms were studied using satellite-based observations of sea surface height, temperature and chlorophyll, and mooring-based observations of winds, hydrography and ocean currents. During the bloom period, the thermocline (nutricline) was anomalously shallow across the equatorial Pacific. The relative importance of processes that enhanced nutrient flux into the euphotic zone differed between the western and eastern regions of the blooms. In the western bloom region, the important vertical processes were turbulent vertical mixing and wind-driven upwelling. In contrast, the important processes in the eastern bloom region were wave-forced shoaling of nutrient source waters directly into the euphotic zone, along-isopycnal upwelling, and wind-driven upwelling. Advection by the Equatorial Undercurrent spread the largest bloom 4500 km east of where it began, and advection by meridional currents of tropical instability waves transported the bloom hundreds of kilometers north and south of the equator. Many processes influenced the intricate development of these massive biological events. Diverse observations and novel analysis methods of this work advance the conceptual framework for understanding the complex dynamics and ecology of the equatorial Pacific.

Enhanced chlorophyll associated with tropical instability waves in the equatorial Pacific

High resolution mooring time series are used to quantify significant chlorophyll anomalies associated with tropical instability waves (TIWs) in the equatorial Pacific. Distinct peaks characterized by very high chlorophyll (up to 3.5 mg m -3 ) are observed in association with TIW cold cusps. These high-chlorophyll peaks appear to differ with respect to scale and intensity from those previously observed at subductive fronts. The physical processes responsible for the observed chlorophyll distributions are not mutually exclusive, and include advection, horizontal mixing, enhanced upwelling and concentration of biomass at fronts. Given the potentially large spatial extent of these high chlorophyll bands, their importance as regions of increased productivity and CO 2 uptake is discussed.

'BROKE' a biological/oceanographic survey off the coast of East Antarctica (80-150°E) carried out in January-March 1996

A large-scale biological/oceanographic survey (BROKE - Baseline Research on Oceanography, Krill and the Environment) was conducted off east Antarctica in the Austral summer of 1995/96. The prime focus was on describing the distribution and abundance of Antarctic krill (Euphausia superba) and on determining possible sources of Antarctic bottom water in the region. A range of other studies were also carried out on BROKE including: measurements of primary production, surveys of Zooplankton and phytoplankton community structure, and sighting surveys of seabirds and cetaceans. The large dataset collected over an area of 873xa0000xa0km2 has led to a greater understanding of the marine ecosystem off East Antarctica and the environmental features that determine productivity in this region.

Seasonal and interannual CO2 fluxes for the central and eastern equatorial Pacific Ocean as determined from fCO2‐SST relationships

[1]xa0In order to determine high-resolution variations of CO2 distributions in the equatorial Pacific, we have developed seasonal and interannual fCO2-SST relationships from shipboard data. The data were gathered onboard NOAA ships from 1992 through 2001. The cruises during the 10-year period included 89 transects of the equatorial Pacific between 95°W and 165°E, and spanned two El Nino events (1992–1994 and 1997–1998). Data were collected during the equatorial warm season (January–June) and cool season (July–December) as well as during all phases of the ENSO cycle, making it possible to examine the interannual and seasonal variability of the fCO2-SST relationship. There is a significant difference between the regression lines for El Nino versus non-El Nino data sets. During both non-El Nino and El Nino periods we observed seasonal differences in the fCO2-temperature relationship. With respect to the non-El Nino period, the seasonal regression lines have lower root mean square (rms) deviations than the composite non-El Nino regression line, and the slopes are significantly different at the 95% confidence level. The slope for the cool season is less negative than the slope for the warm season, suggesting higher biological productivity occurs during the latter half of the year. The derived fCO2-SST relationships have been combined with satellite-based temperature data to provide a composite time-space map of fCO2 in the central and eastern equatorial Pacific and corresponding fluxes for the period between 1985 and 2001. The mean flux for the 16-year record is 0.3 ± 0.1 PgC yr−1 for an area that covers approximately half of the Pacific equatorial belt.