Seasonality and Long-Term Trends in the Oceanic Carbon Cycle near Hawaii (Station ALOHA)

Predictions of the future behavior of the oceanic carbon cycle require a detailed understanding of the response of this cycle to meteorological perturbations. One of the strongest perturbations is the seasonal cycle, which has a pronounced impact on the upper ocean carbon cycle. We investigate this cycle in the subtropical gyre on the basis of a long-term upper ocean time series records from station ALOHA near Hawai'i. The samples were taken from 1988 until present and analyzed by the Carbon Dioxide Research Group at the Scripps Institution of Oceanography. The time series for dissolved organic carbon (DIC), the ocean surface partial pressure of CO₂ (pCO₂), the 13C/¹²C ratio of DIC (δ¹³C) as well as mixed layer temperature (T) and depth show distinct seasonal cycles. The seasonal changes of oceanic pCO₂ values are mainly determined by the annual cycle of T. They are smaller than their atmospheric counterparts during almost the entire time series implying a net uptake of CO₂ from the atmosphere. To examine the role and importance of the processes acting on the carbon cycle at ALOHA, we employed a diagnostic box model (based on the setup described by Gruber et al., 1998). Our results indicate that net community production (NCP) is the only consistent sink term for DIC in the upper ocean carbon budget. In spring and summer, NCP removes 31.4μmol~kg^-1 from the surface mixed layer. This is balanced only in parts by air-sea gas-exchange that adds 10.8μmol~kg^-1 leading to the observed drawdown of DIC. In fall and winter the weak net removal of 15.9μmol~kg^-1 by NCP is more than compensated by the combined addition of 24.9μmol~kg^-1 by air-sea gas-exchange, entrainment, and horizontal transport. These results indicate that the seasonal dynamics of the upper ocean carbon cycle are the result of a complex interaction of various processes, implying that this cycle likely doesn't respond in a simple manner to climatic perturbations. This is highlighted by the observation that the long-term oceanic pCO₂ trend is about two thirds stronger than the atmospheric pCO₂ increase without an obvious link to long-term trend in climate at this site.