Introduction

By exchanging CO2 with the atmosphere, ocean plays an important role in determining the atmospheric CO2 level that has been increasing due to human activities (IPCC, 2001). The CO2 flux between the sea and the overlying air (F) can be estimated by the product of the gas transfer velocity expressed as a function of wind speed (k; Wanninkhof, 1992; Wanninkhof and McGillis, 1999), the solubility of CO2 (s; Weiss, 1974), and the difference in partial pressure (or fugacity) of CO2 between the sea surface water and the air (ApCO2 - pCO2w-pCOfir):

The pCOs2w actually determines whether the ocean acts as a source for atmospheric CO2 or a sink, because it varies significantly as compared with that of air (see e.g., Takahashi et al., 2002).

The equatorial Pacific is a well-documented source for atmospheric CO2 in which the sea—air CO2 flux has been estimated to vary between 0.02 and 0.96 Pg-C/yr, depending on changes in physical and biological processes driven by the El Nino—southern oscillation (ENSO) phenomena (Feely et al., 2002). As described by Le Borgne et al. (2002), the equatorial Pacific consists of two regions that have distinct carbon system dynamics: the high nutrient low chlorophyll (HNLC) region and the western Pacific warm pool. The HNLC region extends from the eastern edge of the western Pacific warm pool to the coast of south America. Since the 1980s temporal and spatial variations in pCO2w in the equatorial Pacific have been examined (see e.g., Feely et al., 2002; Takahashi et al., 2003). According to Feely et al. (2002), the prominent feature of pCO2w in the eastern equatorial Pacific can be described as follows. Shoaling thermocline to the east and local upwelling and the Peruvian upwelling bring waters containing high CO2 concentration to the surface. The northward-flowing Peru Current entrains water with high pCO2w into the south equatorial current (SEC). These lead to the highest pCO2w in the eastern equatorial Pacific HNLC region. A steep gradient of pCO2w occurs between the SEC and the north equatorial counter-current (NECC) with pCO2w nearly equal to that of air. The boundary between the SEC and NECC ranges from 0.5° or less from the equator in the east to 5°N at 140°W. The boundaries of the regions vary due to the phase of the ENSO cycle and the presence of the tropical instability waves.

The pCO2w in the western Pacific warm pool, characterized by high sea surface temperature (SST>28.5 °C), low sea surface salinity (SSS<34.5), and low concentration of macronutrients, tend to be slightly supersaturated with respect to that of atmospheric CO2 (Inoue et al., 1996, 2001; Feely et al., 2002; Le Borgne et al., 2002). In the central and western equatorial Pacific, the longitudinal distributions of pCO2w/SSS vary depending on the geographical position of the boundary between the HNLC region and western Pacific warm pool. Inoue et al. (1996) reported that the longitude with high gradient of pCO2w/SSS (the eastern edge of warm pool) was related to the southern oscillation index (SOI; Le Borgne et al., 2002; Ishii et al., 2003). The western Pacific warm pool migrates eastward during the El Nino event (Picaut et al., 1996; Delcroix et al., 1998; Johnson et al., 2000), which leads to a decrease in pCO2w to near equilibrium with respect to the air. Boutin et al. (1999) reported that western Pacific warm pool moving east—west exerts strong control on the sea—air CO2 flux in the equatorial Pacific.

By using atmospheric inversions and ocean models, Le Quere et al. (2003) report that ocean and land sinks are estimated to be, respectively, 0.3 (0.1—0.6) and 0.7 (0.4—0.9) Pg C/yr larger in the 1990s than in the 1980s. Oceanic regions where long-term variations in sea—air CO2 flux occur should be determined in order to understand the changing global carbon cycle; examining spatial distributions of pCO2w over decades is of particular importance in order to elucidate the long-term variations in oceanic uptake of anthropogenic CO2. In the equatorial Pacific, there have been a few studies of the long-term trend of pCO2w, which is mainly caused by the large interannual variations in pCO2w described above. In the eastern equatorial Pacific, Feely et al. (1999) estimated the long-term trend of pCO2w in the center of upwelling area near the equator (1.277 0.18 matm/yr). In the

120'E 140'E 160"E 180* 160'W 140W 120'W 100'W 80*W

120'E 140'E 160"E 180* 160'W 140W 120'W 100'W 80*W

120'E 140'E 160*E 180* 160'W 140'W 120'W 100'W 80'W

Figure 1: Equatorial Pacific where air—sea CO2 flux was examined (5°N-10°S, 140oE-80°W, surrounded by solid line). The long-term trend of pCO2w (dashed line) was examined in the western Pacific warm pool (A) and the HNLC region (B).

120'E 140'E 160*E 180* 160'W 140'W 120'W 100'W 80'W

Figure 1: Equatorial Pacific where air—sea CO2 flux was examined (5°N-10°S, 140oE-80°W, surrounded by solid line). The long-term trend of pCO2w (dashed line) was examined in the western Pacific warm pool (A) and the HNLC region (B).

southern subtropical zone along 150°W, Feely et al. (2002) reported the growth rate of pCO2w of 1.070.3 matm/yr, similar to that found by Feely et al. (1999) in the center of upwelling area. In the subtropics of the western North and South Pacific, however, the long-term trend of pCO2w (Inoue et al., 1995, 1999) is similar. Recently, Takahashi et al. (2003) determined a change in the pCO2w growth rate in the central and western equatorial Pacific between 1980s and 1990s attributed it to the changes in the phase of the Pacific decadal oscillation (PDO) that occurred between 1988 and 1992. They reported that before the PDO phase shift the in situ pCO2w changed at a mean rate of 5 7 3 matm per decade in the western Pacific warm pool and —9713 matm per decade in the central equatorial Pacific and after the PDO phase shift 3474 matm per decade in the western Pacific warm pool and 1877 matm per decade in the central equatorial Pacific.

In this work, first we will examine the long-term trend of pCO2w in the central and western equatorial Pacific (Fig. 1) by using the data measured by the MRI/JMA group over the period from January 1987 to March 2003 (Table 1), and then variations in sea—air CO2 flux in the equatorial Pacific (Fig. 1, 5°N-10°S, 140°E-90°W) by combining data taken after 1998 with underway pCO2w (fCO2w) data (http://www.pmel.noaa.gov/uwpco2/, http:// www.aoml.noaa.gov/ocd/oaces/mastermap.html) taken by the CO2 groups of the National Oceanic and Atmospheric Administration, the Pacific Marine Environmental Laboratory (NOAA/PMEL) and the Atlantic Oceanographic and Meteorological Laboratory (NOAA/AOML).

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