The results of atmospheric and oceanic CO
2 observations conducted by the Japan Meteorological Agency in the western North Pacific are presented for the period 1990-1993, covering an El Niño/Southern Oscillation (ENSO) event (onset: spring, 1991; withdrawal: summer, 1992). The atmospheric CO
2 concentration over the region south of 30°N increased drastically between 1990 and 1991 during the winter (4.0ppmv) and summer (4.5ppmv), although the values are not seasonally adjusted. Over the other two years of observations, the growth rates were smaller or at times negative. Oceanic CO
2, which is expressed in units of the mole fraction (ppmv) in dry air equilibrated with seawater, showed a significant increase, especially in low latitudes during both seasons of the years 1991-1993, compared to the 1990 values.
Oceanic CO
2, normalized at a constant temperature, also revealed a significant increase, with larger and smaller magnitudes for winter and summer, respectively. This implies that the increased summer oceanic CO
2 results mostly from changes in surface seawater temperature, while for that in winter, other factors, unknown at present, are more related to the increased oceanic CO
2.
Calculated values of ΔpCO
2 and CO
2 flux at the air-sea interface reveal that in winter, the region north of 10°N acts as a sink of CO
2, with a maximum net flux of --10.0mmol·m
-2·d
-1. The region south of 10°N, however, at times becomes a weak source of CO
2, with a maximum net flux of 2.4mmol·m
-2·d
-1. In summer, the western North Pacific becomes a weak source or is almost in equilibrium with atmospheric CO
2.
The increase in winter oceanic CO
2 or ΔpCO
2, as related to the 1991/92 ENSO event, is not as distinct as in the 1982/83 ENSO event, although lower temperatures and higher salinity in the surface seawater were commonly found during both the events.
The response of the CO
2 flux in the tropical western North Pacific to the 1991/92 ENSO event was rather small compared to the magnitude of the rate changes obtained in previous results for the central or eastern tropical Pacific.
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