Biogeochemical Responses and Feedbacks to the Climate Forcing in the Tropical Pacific Ocean Over the Past 60 Years

The tropical Pacific Ocean has experienced significantly decadal changes in physical fields over the last 60 years, which is associated with the Pacific Decadal Oscillation (PDO). Particularly, observational studies have identified two possible regime shifts in physical processes around 1977 and 1998. However, little is known about their impacts on the equatorial Pacific biogeochemistry and ecosystem structure. Moreover, what are the possible feedbacks of the tropical Pacific biological and chemical changes to the global climate is not known?

Here, we employ a basin-scale physical-biogeochemical model to study the responses of tropical Pacific ecosystem, biogeochemistry and carbon cycle to climate forcing over 1950-2013. We compare model results for three periods: 1950-1975, 1978-1995 and 1998-2013. Our results indicate significant changes in physics and biogeochemistry associated with the 1977 and 1998 PDO shifts. There was reduction in carbon uptake during 1978-1995, relative to other two periods, however, which is smaller than expected, due to phytoplankton photoadaption that leads to increased carbon to chlorophyll ratio in the upper euphotic zone. Photoadaption also results in clearer waters near the surface, allowing more solar radiation to penetrate to depth which enhances sub-surface production, and leading leads to less heating near the ocean surface. Our studies suggest that under a warm climate, the tropical Pacific Ocean releases less CO₂, and absorbs more heat, creating negative feedbacks to the atmosphere.