Abstract
Despite the well-recognized importance in understanding the long term impact of anthropogenic release of atmospheric CO 2 (its partial pressure named as p CO 2 air) on surface seawater p CO 2 ( p CO 2 sw), it has been difficult to quantify the trends or changing rates of p CO 2 sw driven by increasing atmospheric CO 2 forcing ( p CO 2 sw atm_forced ) due to its combination with the natural variability of p CO 2 sw ( p CO 2 sw nat_forced ) and the requirement of long time series data records. Here, using a novel satellite-based p CO 2 sw model with inputs of ocean color and other ancillary data between 2002 and 2019, we address this challenge for a mooring station at the Hawaii Ocean Time-series Station in the North Pacific subtropical gyre. Specifically, using the developed p CO 2 sw model, we differentiated and separately quantified the interannual-decadal trends of p CO 2 sw nat_forced and p CO 2 sw atm_forced . Between 2002 and 2019, both p CO 2 sw and p CO 2 air show significant increases at rates of 1.7 ± 0.1 μatm yr –1 and 2.2 ± 0.1 μatm yr –1 , respectively. Correspondingly, the changing rate in p CO 2 sw nat_forced is mainly driven by large scale forcing such as Pacific Decadal Oscillation, with a negative rate (-0.5 ± 0.2 μatm yr –1 ) and a positive rate (0.6 ± 0.3 μatm yr –1 ) before and after 2013. The p CO 2 sw atm_forced shows a smaller increasing rate of 1.4 ± 0.1 μatm yr –1 than that of the modeled p CO 2 sw, varying in different time intervals in response to the variations in atmospheric p CO 2 . The findings of decoupled trends in p CO 2 sw atm_forced and p CO 2 sw nat_forced highlight the necessity to differentiate the two toward a better understanding of the long term oceanic absorption of anthropogenic CO 2 and the anthropogenic impact on the changing surface ocean carbonic chemistry.