Decoupling of Temporal and Spatial Variability of Greenhouse Gases in a Coastal Upwelling Zone Using High Frequency Ship-Board Measurements and Numerical Analysis

Upwelling zones associated with eastern boundary currents are regions of intense biological productivity and biogeochemical cycling of climate active gases such as carbon dioxide, methane and nitrous oxide. Sea-air fluxes of these gases have important climate implications, pointing towards a need for understanding the sources of these gases and how they may evolve with future climate change. Coastal upwelling regions exhibit strong temporal and spatial variability in dissolved gas concentrations driven by physical forcing (advection, turbulent mixing and sea-air flux) and biogeochemical processes (primary productivity and microbial activity), and this variability is difficult to quantify based on low-resolution, discrete sampling. To address this challenge, we used high spatial resolution measurements of pCO2, CH4, N2O to map surface water variability of these gases in the California upwelling system, coupled with numerical model output to examine the underlying physical transport mechanism driving observed distributions. Continuous underway measurements revealed a distinct latitudinal gradient in CH4, N2O and pco2 levels in different upwelling plumes, with the highest concentrations (1900% CH4 saturation, 200% N2O saturation and ΔpCO2 of 475 μatm) observed in an upwelling plume north of Cape Blanco, and significantly lower values in the more southerly upwelling region of Cape Mendocino, (average values of 500% CH4 saturation, 120% N2O saturation and ΔpCO2 of 0 μatm). Using backward water mass trajectory analysis, we determined that variability in surface water concentrations primarily reflects differences in the source waters feeding upwelling in the northern and southern regions, rather than surface processes associated with the ‘aging’ of upwelling plumes. Results from our analysis reveal the utility of combining high resolution measurements with numerical models to disentangle coupled spatial and temporal variability in dynamic coastal waters.