Sergio R. Signorini

Ocean color variability of the tropical Indo‐Pacific basin observed by SeaWiFS during 1997–1998

High-quality ocean color data (chlorophyll) provided by the Sea-viewing Wide Field of view Sensor (SeaWiFS) satellite were analyzed for the first complete year of coverage (October 1997 to September 1998) in the tropical Indo-Pacific basin. This period coincides with the peak of one of the strongest El Nino events during December 1997 and the La Nina of 1998 that appeared dramatically in less than a month as a sea surface temperature (SST) change of over 6°C in the central equatorial Pacific during June 1998. The tropical Indian Ocean also underwent a highly anomalous series of events with negative SST anomalies (SSTA) of over 3°C in the eastern equatorial and coastal regions during October-December 1997 and warm SSTA in the west that peaked at over 2°C during February 1998. The ocean color variability is interpreted using other satellite data such as sea level from TOPEX/Poseidon and also in terms of the dynamics and thermodynamics of the region from simulations with an ocean general circulation model. The El Nino-related reductions in equatorial production and the off-equatorial increase in biological activity, and their basin scale evolution is clearly seen for the first time. Persistent northerly wind anomalies resulted in a northward shift of the equatorial divergence and the upwelling Kelvin wave which signalled the end of the 1997–1998 El Nino. The anomalous surface chlorophyll associated with this Kelvin wave was also clearly shifted north of the equator by nearly 300 km and appeared more than a month before the negative sea level anomalies seen by TOPEX/Poseidon. On the equator near 165°E, the disappearance of the barrier layer appeared to coincide with a localized bloom that occurred in response to the easterly wind bursts over the western Pacific that lasted from December 1997 through the boreal summer. The ecosystem response to the cold La Nina conditions is clearly seen as elevated chlorophyll during the boreal summer of 1998 in the equatorial Pacific cold tongue region. In the Indian Ocean, an anomalous phytoplankton bloom was observed by SeaWiFS during October-December 1997 coincident with the anomalous upwelling in the eastern equatorial region and off the coast of Sumatra. A stronger than normal northeast monsoon is seen as higher than climatological values of surface chlorophyll. The open ocean Ekman pumping and the shoaling of the thermocline near 60°E and 10°S and the eastward extension of mixed layer entrainment in the same latitude band is seen as a region of higher biological activity during the boreal summer.

Satellite ocean-color observations of the tropical Pacific Ocean

The Coastal Zone Color Scanner (CZCS) data set provided some insights into biological processes in the equatorial Pacific, but the sampling was too sparse to address questions on temporal and spatial variability. Since late 1996, the Ocean Color–Temperature Sensor (OCTS), the Polarization Detection Environmental Radiometer (POLDER), the Sea-viewing Wide Field-of-View Sensor (SeaWiFS), and the Moderate-Resolution Imaging Spectroradiometer (MODIS) have provided a nearly continuous record of biological processes in this region for the first time. This study summarizes the SeaWiFS observations of the tropical Pacific from September 1997 through March 2000, with particular emphasis on equatorial and mesoscale variability, the influence of biological processes on penetrating irradiance, and the performance of primary production algorithms in this region. Specific mesoscale phenomena described are the phytoplankton blooms along the west coast of Central America, in the vicinity of the Costa Rica dome, and south of the equator. The coastal Central American and Costa Rica dome blooms result from orographically steered coastal winds and Ekman divergence, respectively. An unusual bloom event occurred south of the equator and persisted for several months in 1999; specific mechanisms that would have sustained the bloom could not be identified. Also, the time-evolution of the equatorial bloom during the May–August 1998 transition from El Ni * no to La Ni * na is discussed. Again, no concise and broadly accepted explanation of the bloom’s genesis and migration has yet emerged. During this transition, the monthly mean diffuse attenuation coefficient decreased by a factor of 3 at some locations along the equator. This change in water transparency, coupled with large changes in mixed-layer depth, resulted in significant changes in surface layer heating rates that were substantiated with field observations. Finally, certain primary production algorithms designed to use remotely sensed chlorophyll-a concentrations are evaluated. None of the algorithms capture the observed variability in primary production, and all appear to underestimate the total primary production of the tropical Pacific. r 2002 Elsevier Science Ltd. All rights reserved.

Biological and physical signatures in the tropical and subtropical Atlantic

The variability of sea surface chlorophyll concentration in the tropical and subtropical Atlantic during the first year of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) imagery is examined. An Ocean General Circulation Model (OGCM) is used, along with TOPEX/Poseidon dynamic height observations and global gridded wind stress data sets, to explain the physical forcing of surface ocean color signals. Regions of high surface chlorophyll are strongly correlated with mesoscale and large-scale physical processes such as the strong upwelling off the west coast of Africa, the relatively high oceanic production within the Guinea Dome region, and the generation and propagation of large anticyclonic eddies along the coast of South America, north of the equator. The major river outflows (Amazon, Orinoco, and Congo) have strong signatures with plumes of apparently high Chl a in excess of 10 mg m-3 near their deltas. The fall bloom in the eastern tropical Atlantic observed by the Coastal Zone Color Scanner (CZCS) was absent in 1997, whereas a bloom was observed in this region in July-September 1995, which was not observed by the CZCS. We attribute these apparent anomalies to the projection of the 1997-1998 El Nino event into the tropical Atlantic basin; these signals are correlated with sea surface temperature anomalies known to be associated with ENSO. The SeaWiFS images show that there are seasonal blooms within the hydrographic provinces of the Guinea and Angola domes. These hydrographic provinces are characterized by the dynamic uplift of the thermocline at the North Equatorial Current southern boundary (Guinea Dome) and the Benguela Current eastern boundary (Angola Dome). Within these domes, the Ekman pumping and transport are significant due to the strong trade winds at the surface. The Ekman drift plays a major role in the spreading of surface blooms. The spreading of the oceanic bloom at 12N, 30W, the Congo River plume, and the areal extent of the upwelling blooms off the coast of Africa, parallels the strength and extent of the Ekman surface drift. Upwelling, when broadly defined to include large scale vertical excursions of the thermocline, explains virtually all of the surface chlorophyll observations in excess of 0.5 mg m-3, except in the river plumes. I

Seasonal and interannual variability of phytoplankton, nutrients, TCO2, pCO2, and O2 in the eastern subarctic Pacific (ocean weather station Papa)

A coupled, one-dimensional ecosystem/carbon flux model is used to simulate the seasonal and interannual variability of phytoplankton, nutrients, TCO2, O2, and pCO2 at ocean weather station Papa (OWS P at 50°N, 145°W). The 23-year interannual simulation (1958–1980) is validated with available data and analyzed to extend seasonal and interannual variations beyond the limited observational records. The seasonal cycles of pCO2 and sea-air CO2 flux are controlled by a combination of thermodynamics, winds, and biological uptake. There is ingassing of CO2 during the fall-winter months when SSTs are colder and wind forcing is vigorous, while there is a much smaller ingassing of CO2 during the summer when sea surface temperatures are warmer and wind speeds are reduced. Biological production plays a major role in maintaining the air-sea equilibrium. An abiotic simulation showed that OWS P would be a source of atmospheric CO2 (1.41 mol C m−2 yr−1) if the biological sink of CO2 were removed. The peak net community production in summer compensates for the increased temperature effect on pCO2, which prevents large outgassing in summer. Oxygen anomalies relative to the temperature-determined saturation value show that there is a seasonal cycle of air-sea flux, with ingassing in winter and outgassing in summer. The net surface oxygen flux is positive (0.8 mol m−2 yr−1), indicating that OWS P is a source of oxygen to the atmosphere. The average primary production is 167 g C m−2 yr−1. The 1960–1980 (1958 and 1959 spin-up years removed) mean carbon flux is −1.8 mol C m−2 yr−1, indicating that the ocean at OWS P is a sink of atmospheric carbon. The sea-air CO2 flux ranges from −1.2 to −2.3 mol C m−2 yr−1 during the 21-year simulation period. This finding emphasizes the need for long-term observations to accurately determine carbon flux budgets. A series of sensitivity experiments indicate that the seasonal variability and overall (21 years) mean of TCO2, pCO2, ΔpCO2, and air-sea CO2 flux are strongly dependent on the gas transfer formulation adopted, the total alkalinity near the surface, and the bottom (350 m) value adopted for TCO2. The secular atmospheric pCO2 upward trend is manifested in the TCO2 concentration within the upper 100 m by an increase of 15 mmol m−3 in 20 years, consistent with observations at other locations [Winn et al., 1998; Bates, 2001].

Environmental factors controlling the Barents Sea spring-summer phytoplankton blooms

[1] We provide an analysis of the seasonal change of the physical forcing factors and their impact on the timing and intensity of phytoplankton blooms in the Barents Sea, with emphasis on the different functional groups that can be distinguished (coccolithophores and other phytoplankton groups) using satellite remote sensing algorithms. Our analyses are based on an integration of satellite derived products and historical hydrographic data. There is a significant phase shift between the peaks of the diatom bloom in May to the coccolithophore bloom in August. Light and nutrient variability, driven by the large seasonal changes of solar irradiance and mixed layer depth in the Barents Sea, are the major environmental factors controlling these phytoplankton blooms. Based on previous field campaigns that identified Emiliana huxleyi as the most predominant species of coccolithophores in the Barents Sea, we conclude that the high concentration of calcite retrieved by ocean color satellites in summer is a result of bloom-forming coccolithophores of this species. A strong correlation between calcite concentration and wind speed squared (r 2 = 0.8) was found during the peak of the bloom. We suggest that this correlation is an indication of CO2 fertilization resulting from regionally enhanced uptake of atmospheric CO2. Citation: Signorini, S. R., and C. R. McClain (2009), Environmental factors controlling the Barents Sea spring-summer phytoplankton blooms, Geophys. Res. Lett., 36, L10604, doi:10.1029/2009GL037695.

A simulation of biological processes in the equatorial Pacific Warm Pool at 165°E

A 9-year simulation (1984–1992) of biological processes in the equatorial Pacific Warm Pool is presented. A modified version of the four-component (phytoplankton, zooplankton, nitrate, and ammonium) ecosystem model by McClain et al. [1996] is used. Modifications include use of a spectral model for computation of photosynthetically available radiation and inclusion of fecal pellet remineralization and ammonium nitrification. The physical parameters (horizontal and vertical velocities and temperature) required by the ecosystem model were derived from an improved version of the Gent and Cane [1990] ocean general circulation model [Murtugudde and Busalacchi, 1998]. Surface downwelling spectral irradiance was estimated using the clear-sky models of Frouin et al. [1989] and Gregg and Carder [1990] and cloud cover information from the International Satellite Cloud Climatology Project. The simulations indicate considerable variability on interannual timescales in all four ecosystem components. In particular, surface chlorophyll concentrations varied by an order of magnitude with maximum values exceeding 0.30 mg m−3 in 1988, 1989, and 1990 and pronounced minima during 1987 and 1992. The deep chlorophyll maximum (DCM) ranged between 75 and 125 m with values occasionally exceeding 0.40 mg m−3. With the exception of periods during 1984 and 1988, surface nitrate was always near depletion. Ammonium exhibited a subsurface maximum just below the DCM with concentrations as high as 0.5 mg-atN m−3, where “at” is atoms. Total integrated annual primary production varied between 40 and 250 gC m−2 yr−1 with an annual average of 140 gC m−2 yr−1. Finally, the model is used to estimate the mean irradiance at the base of the mixed layer, i.e., the penetration irradiance, which was 18 W m−2 over the 9-year period or about 8% of the incident total irradiance. The average mixed layer depth was 42 m.