Interannual variability of carbon dioxide drawdown by subantarctic surface water near New Zealand

Abstract

The ocean-atmosphere flux of carbon dioxide in subantarctic surface water (SASW) east of New Zealand has been determined using data from bi-monthly cruises on a time series transect for 8 years. The 60 km long transect extends from the coast (45.770°S 170.720°E) to a station at 45.833°S 171.500°E. Sea surface temperature, salinity, nutrient concentrations and pCO2 have been measured at a frequency of about once every 2 months from January 1998 until December 2005. Measured pCO2 exhibits a seasonal cycle with a maximum in late winter/spring, and a minimum in late summer/autumn, a mean 356 μatm, and an amplitude of 9 μatm. The magnitude of ΔpCO2 (the air-sea concentration gradient) has increased over the 8 years, primarily due to the increase in atmospheric CO2 concentration. The air-sea flux of CO2 was determined from wind speed data and ΔpCO2. The uptake of atmospheric CO2 by SASW in the study area changed from + 1 and +82 mmol m−2 in 1998 and 1999 respectively (ocean as source) to −870 and −510 mmol m−2 in 2004 and 2005 (ocean as sink). These values are substantially less in magnitude than the value obtained from the Takahashi et al. (Deep-Sea Res II, 2009) flux climatology.