Daily, seasonal and interannual variability of sea-surface carbon and nutrient concentration in the equatorial Pacific Ocean

Abstract

We present measurements of the partial pressure of CO 2 in seawater ( pCO 2), total dissolved inorganic carbon concentration (ΣCO 2) and alkalinity made during the JGOFS Survey I (TT007, February–March 1992) and Survey II (TT011, August–September 1992) expeditions. JGOFS data are compared with data from the Hawaii-Tahiti Shuttle Experiment (HTSE, February 1979–June 1980). The Survey I and II expeditions took place during and after the El Niño event of 1992, while HTSE occurred during mild El Niño to near-climatological conditions. The Survey I and II sea-surface temperatures (SSTs) are among the warmest and coldest, respectively, in the combined JGOFS and HTSE dataset, and sea-surface concentrations of the biological tracers, NO 3 and pCO 2, from JGOFS bracketed the HTSE data with lower concentrations during Survey I and higher values during Survey II. However, the air-sea contrast in pCO 2 was diminished in 1992 due to rising atmospheric values. The variability of sea-surface concentrations of biological tracers seems to be controlled primarily by the physical structure of the water column. In a comparison of HTSE and JGOFS data (decadal timescale), or Survey I and II data (seasonal/ENSO timescale), the concentrations of the tracers on constant-density (isopycnal) surfaces is nearly time invariant, so that the variation in sea-surface concentrations is controlled by the outcropping of isopycnal surfaces. On the timescale of the station occupation (diurnal to a few days), variation in replicate measurements of pCO 2 is correlated with variation in density, again indicating physical rather than biological control of pCO 2 variability. These findings make an interesting contrast to JGOFS North Atlantic Bloom Experiment results. [Chipman et al. (1993) Deep-Sea Research II, 40, 151–169], where recent local biological forcing was found to dominate chemical variability. The implication of this finding is that a physical understanding of the equatorial Pacific circulation may be sufficient to make predictions of short-term variability in air-sea pCO 2 fluxes in this region. Some minor exceptions to the rule of physical control of sea-surface chemical properties include a freshwater cap just south of the equator that follows the sea surface, rather than any density surface, and Si, which appears to vary seasonally independently of the other biological tracers NO 3, CO 2, and O 2. Based on the relationship between the alkalinity and ΣCO 2 in the upper 400 m of the water column, about 13% of the ΣCO 2 increase with depth is due to the dissolution of CaCO 3, 65% to the oxidation of biogenic debris and 23% to the increasing solubility of CO 2 in colder waters. The relationship between ΣCO 2 and O 2 in equatorial surface waters during Survey II indicates a vigorous circulation with an overturning timescale of only a few days.