Chapter 1 Long-Term Trend of the Partial Pressure of CO2 in Surface Waters and Sea−Air CO2 Flux in the Equatorial Pacific

Abstract

Measurements of partial pressure of CO 2 in surface waters ( p CO 2 sw ) and overlying air ( p CO 2 air ) were made intermittently in the central and western equatorial Pacific from January 1987 to January 2003. We estimated the long-term trend of the p CO 2 sw in the high nutrient low chlorophyll (HNLC) region and the western Pacific warm pool. The spatial distribution of p CO 2 sw in the HNLC region could be expressed as a linear function of sea surface temperature (SST) and concentration of macronutrients ([NO 2 − ]+[NO 3 − ]), and in the western Pacific warm pool as a function of SST and sea surface salinity (SSS). By using an average SST (27.4 °C) and concentration of nitrate and nitrite (3.9 μmol/kg) in the HNLC region and the average SST (29.6 °C) and SSS (34.29) in the western Pacific warm pool between 1987 and 2003, we obtained p CO 2 sw values for respective cruises. The growth rate of p CO 2 sw due to increases in atmospheric CO 2 was calculated to be 1.4±0.5 μatm/yr in the HNLC region and 1.3±0.3 μatm/yr in the western Pacific warm pool. The sea−air CO 2 flux in the equatorial Pacific since 1998 was evaluated by using underway p CO 2 sw data measured by Japan Meteorological Agency, Meteorological Research Institute (JMA/MRI), National Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory (NOAA/PMEL), and NOAA, Atlantic Oceanographic and Meteorological Laboratory (NOAA/AOML). From 1998 to 2003 the sea−air CO 2 flux in the equatorial Pacific (5°N–10°S, 140°E–90°W) showed lowest flux in January/February 1998 (0.1±0.1 Pg C/yr, 1997/98 El Niño), and highest (0.9±0.4 Pg C/yr) in January/February 2001, suggesting significant interannual variations in sea−air CO 2 flux in the equatorial Pacific. In October 2002−January 2003, which was within a weak El Niño period, the CO 2 flux in the equatorial Pacific was 0.5±0.3 Pg C/yr, almost same as that of the non-El Niño period. In this period, sea−air CO 2 flux in the central and western equatorial Pacific decreased considerably to the same level of January/February 1998, but that in the eastern equatorial Pacific remained fairly constant.