Climate-driven variability in tropical Pacific productivity and carbon cycling

The equatorial Pacific ocean spans a third of Earth’s circumference and is a key component of the global carbon cycle as the largest ocean region of CO₂ outgassing. There is considerable interannual to decadal variability here due to the El Niño-Southern Oscillation (ENSO) and Tropical Pacific Decadal Variability (TPDV). Low frequency variability here has considerable implications for physical and biogeochemical processes such as winds, temperature, stratification, subsurface transport, air-sea CO₂ flux, and new production. Phytoplankton are an important ecological indicator at the base of the aquatic foodweb, as they consume upwelled carbon and nutrients in the sunlit surface ocean. Chlorophyll-α concentration, as a proxy for phytoplankton biomass, is readily observed from satellite ocean colour sensors, and algorithms validated using in situ shipboard measurements can produce derived products such as net primary production – an important part of the carbon cycle. New production, the amount of phytoplankton net primary production driven by upwelled nitrate, plays a significant role in modulating air-sea CO₂ fluxes as the biological carbon pump removes carbon from the surface ocean. However, there are key uncertainties around phytoplankton, the carbon cycle, and low frequency variability in the equatorial Pacific. These include 1) the accuracy of satellite ocean colour measurements, 2) spatial and temporal relationships between biogeochemical processes such as new production and air-sea CO_{2 flux, and 3) the impact of difficult to measure processes such as subsurface transports on the overall equatorial Pacific} carbon budget. With these research gaps in mind, the recurring themes and the overall aims of this thesis are to 1) assess the accuracy of satellite ocean colour estimates and a climate reanalysis product CAFE60, 2) quantify the mean states of chlorophyll-a, new production, air-sea CO₂ flux and meridional subsurface transports of C_anth and their impacts on the equatorial Pacific carbon cycle using these data products, and 3) interrogate and describe the impact of ENSO, TPDV and satellite-era decadal trends on these biogeochemical processes.

We first assess the accuracy of existing satellite ocean colour algorithms, and find that three sensors: SeaWiFS, MODIS Aqua, and MERIS underestimate ‐ equatorial Pacific chlorophyll-a by 5.8%, 14%, and 2% respectively. We developed and released a data product for regionally tuned sensor specific coefficients and blending w ‐ indows for the OCI and OCx algorithms to reduce systematic biases in the equatorial Pacific. These updated estimates increase chlorophyll-α concentrations in open water and decrease concentrations around island and warm‐pool regions, with implications for our understanding of ENSO driven carbon fluxes and net primary productivity.

Secondly, we use available observations and interpolated data products to investigate the relationships between air-sea CO2 flux and new production, as well as physical drivers of biogeochemical variability including sea surface temperature and wind speed. We find that changes in wind speed, temperature and ENSO frequency have altered the surface carbon budget over the 1998 to 2020 satellite era. For example, air-sea CO₂ flux and ΔpCO_{2 increased in the cold tongue (45 mmolC m}⁻² _yr⁻²_{, 1.5 μatm yr}⁻¹_{, respectively) but decreased elsewhere, while new} production decreased everywhere. The western Pacific occasionally became a weak carbon sink, depending on ENSO phase, and this sink was strongest at 165°E during central Pacific “Modoki” El Niño events, which are becoming more frequent.

Thirdly, we asses and use the CAFE60 climate reanalysis product to investigate subsurface transport, uptake, and accumulation of anthropogenic carbon (C_anth) through the equatorial Pacific and teleconnections with the subtropics, particularly in the southern hemisphere. C_anth is a passive tracer independent of biology, and we demonstrate that it is useful to further explore the carbon cycle. We found that 72% of C_anth accumulating in 5°N to 5°S, 120°E to 80°W resides in the surface 300 m, and the subtropical cell efficiently transports C_anth between the subtropics and equator. We show that the Equatorial Undercurrent (EUC) has shoaled and shifted slightly south between 2000 and 2020. The southern interior and western boundary branches of the STC have slowed, however the surface divergence has increased. There is more C_anth diverging out of the surface equatorial Pacific than what enters the through the western boundary currents and ocean interior transport. Furthermore, increasing C_anth concentrations means there is still C_anth transport into the equator region despite negative mass transport trends. These findings have implications for climate-carbon feedbacks and the future uptake of C_anth in the ocean.

The work presented here focuses on the mean state and low frequency variability of phytoplankton, new production, air-sea CO₂ flux and subsurface Canth transports. The thesis finishes with a discussion that summarises the key findings of each chapter, and brings these findings into a wider context.